Magnetic recording media, magnetic tape cartridges, and magnetic recording / recovery devices

Abstract

The present invention provides a magnetic recording medium that has a non-magnetic support and a first magnetic layer that includes a ferromagnetic powder and a binding agent, the magnetic recording medium also having a second magnetic layer between the non-magnetic support and the first magnetic layer, the second magnetic layer including a ferromagnetic powder that has a coercivity Hc of no more than 50 Oe and an average particle size of no more than 50 nm and a binding agent. The present invention also provides a magnetic tape cartridge and a magnetic recording reproduction device that include the magnetic recording medium.

Description

[Technical Field]

[0001] The present invention relates to a magnetic recording medium, a magnetic tape cartridge, and a magnetic recording and playback device. [Background technology]

[0002] Magnetic recording media are broadly classified into two types: thin-film metal type and coated type. Thin-film metal type magnetic recording media have a magnetic layer of a thin metal film formed by sputtering or the like (see, for example, Patent Document 1). In contrast, coated type magnetic recording media have a magnetic layer containing ferromagnetic powder together with a binder. [Prior art documents] [Patent Documents]

[0003] [Patent Document 1] Patent No. 6531764 specification [Overview of the project] [Problems that the invention aims to solve]

[0004] There are two recording methods for magnetic recording: perpendicular recording and in-plane recording. Hard disk drives (HDDs), which are representative examples of magnetic recording and playback devices equipped with metal thin-film magnetic recording media, generally employ the perpendicular recording method. Furthermore, Japanese Patent No. 6531764 (Patent Document 1) discloses that recording was performed using the perpendicular recording method on a metal thin-film magnetic recording media having a magnetic layer formed using a sputtering apparatus (see paragraph 0102 of Patent Document 1).

[0005] In contrast, for coated magnetic recording media, the in-plane recording method has traditionally been the dominant recording method. However, the vertical recording method is desirable for further high-density recording because it offers the potential for a dramatic improvement in recording density. Furthermore, it is desirable for magnetic recording media to exhibit excellent electromagnetic conversion characteristics.

[0006] In view of the above, one aspect of the present invention aims to provide a coated magnetic recording medium that can exhibit excellent electromagnetic conversion characteristics by magnetic recording in a vertical recording method. [Means for solving the problem]

[0007] One aspect of the present invention is as follows: [1] A non-magnetic support comprising a first magnetic layer containing ferromagnetic powder and a binder, A magnetic recording medium further having a second magnetic layer between the non-magnetic support and the first magnetic layer, the second magnetic layer comprising a ferromagnetic powder having a coercivity Hc of 50 Oe or less and an average particle size of 50 nm or less, and a binder. [2] The number distribution A of the equivalent circle diameters of multiple bright areas in the binarized secondary electron image obtained by imaging the surface of the first magnetic layer with a scanning electron microscope at an accelerating voltage of 5kV is as follows (1)~(3): (1) There are 10,000 to 30,000 bright areas with an equivalent circular diameter of 1 nm to 50 nm. (2) There are 7,000 to 25,000 bright areas with an equivalent circular diameter of 51 nm to 100 nm. (3) There are 1,000 to 3,000 bright areas with an equivalent circular diameter of 101 nm or more. Satisfying the conditions, and The number distribution B of the equivalent circle diameters of multiple dark regions in the binarized secondary electron image obtained by imaging the surface of the first magnetic layer described above with a scanning electron microscope at an accelerating voltage of 2kV is as follows (4)~(6): (4) There are 200 to 50,000 dark regions with an equivalent diameter of 1 nm to 50 nm. (5) There are 200 to 25,000 dark regions with an equivalent diameter of 51 nm to 100 nm. (6) There are 0 to 2000 dark regions with an equivalent diameter of 101 nm or more. A magnetic recording medium as described in [1] that satisfies the following conditions. [3] The magnetic recording medium according to [1] or [2], wherein the vertical aspect ratio of the magnetic recording medium is 0.60 or greater. [4] The magnetic recording medium according to any one of [1] to [3], wherein the coercivity Hc of the ferromagnetic powder contained in the second magnetic layer is 10 Oe or more and 50 Oe or less. [5] The magnetic recording medium according to any one of [1] to [4], wherein the average particle size of the ferromagnetic powder contained in the second magnetic layer is 5 nm or more and 50 nm or less. [6] The magnetic recording medium according to any one of [1] to [5], wherein the ferromagnetic powder contained in the second magnetic layer is spinel ferrite powder. [7] The magnetic recording medium according to any one of [1] to [6], wherein the ferromagnetic powder contained in the first magnetic layer is hexagonal barium ferrite powder. [8] The magnetic recording medium according to any one of [1] to [6], wherein the ferromagnetic powder contained in the first magnetic layer is hexagonal strontium ferrite powder. [9] The magnetic recording medium according to any one of [1] to [6], wherein the ferromagnetic powder contained in the first magnetic layer is ε-iron oxide powder.

[10] A magnetic recording medium according to any one of [1] to [9], further comprising a non-magnetic layer containing non-magnetic powder and a binder between the non-magnetic support and the second magnetic layer.

[11] A magnetic recording medium according to any one of [1] to

[10] , further comprising a back coat layer containing non-magnetic powder and a binder on the surface side of the non-magnetic support opposite to the surface side having the first magnetic layer and the second magnetic layer.

[12] A magnetic recording medium described in any of [1] to

[11] , which is a magnetic tape. A magnetic tape cartridge containing the magnetic recording medium described in

[13]

[12] . A magnetic recording and playback device containing a magnetic recording medium as described in any of

[14] [1] to

[12] . [Effects of the Invention]

[0008] According to one aspect of the present invention, a coated magnetic recording medium capable of exhibiting excellent electromagnetic conversion characteristics through recording in a vertical recording method can be provided. Furthermore, according to another aspect of the present invention, a magnetic tape cartridge and a magnetic recording / recovery device including such a magnetic recording medium can be provided. [Brief explanation of the drawing]

[0009] [Figure 1] An example of the arrangement of data bands and servo bands is shown. [Figure 2] This shows an example of a servo pattern arrangement for an LTO (Linear Tape-Open) Ultrium format tape. [Modes for carrying out the invention]

[0010] [Magnetic recording medium] A magnetic recording medium according to one aspect of the present invention comprises a non-magnetic support and a first magnetic layer comprising a ferromagnetic powder and a binder. The magnetic recording medium further comprises a second magnetic layer between the non-magnetic support and the first magnetic layer, comprising a ferromagnetic powder having a coercivity Hc of 50 Oe or less and an average particle size of 50 nm or less, and a binder.

[0011] The magnetic recording medium described above has the first magnetic layer and the second magnetic layer described above. The first magnetic layer is a layer on which data is recorded by magnetic recording, that is, a layer that can function as a recording layer. Data is recorded on the recording layer by applying a magnetic field from a magnetic head to magnetize the ferromagnetic powder particles in the recording layer. The in-plane recording method is a recording method in which the direction of the magnetic field applied to the recording layer for magnetization is controlled to be horizontal with respect to the surface of the recording layer. The in-plane recording method is also generally called the horizontal recording method or the longitudinal recording method. In contrast, the perpendicular recording method is a recording method in which the direction of the magnetic field applied to the recording layer for magnetization is controlled to be perpendicular with respect to the surface of the recording layer. In this invention and specification, "perpendicular" does not necessarily mean only strictly perpendicular, but includes a range of error that is normally permissible in the art to which this invention belongs. The range of error can mean, for example, a range of less than ±10° from strictly perpendicular. The same applies to "horizontal". In vertical recording, a further magnetic layer, generally called a soft-magnetic underlayer (SUL), is typically required to collect the magnetic field, which is sent perpendicularly from the magnetic head to the surface of the recording layer, through the recording layer and back to the recording head. In the above magnetic recording medium, the second magnetic layer can be a so-called soft magnetic layer, and is a layer that can function as the soft-magnetic underlayer. The inventors surmise that the ferromagnetic powder contained in the second magnetic layer that can function as a soft-magnetic underlayer in the above magnetic recording medium has a coercivity Hc of 50 Oe or less and an average particle size of 50 nm or less, which contributes to the above magnetic recording medium, being a coated magnetic recording medium, exhibiting excellent electromagnetic conversion characteristics (hereinafter also simply referred to as "electromagnetic conversion characteristics") through vertical recording. More specifically, the inventors believe that the coercivity Hc of the ferromagnetic powder contained in the second magnetic layer being 50 Oe or less contributes to suppressing noise originating from the second magnetic layer. Furthermore, the inventors speculate that the average particle size of the ferromagnetic powder contained in the second magnetic layer being 50 nm or less may contribute to suppressing the reduction in output. The inventors believe this is because the small average particle size of the ferromagnetic powder contained in the second magnetic layer, which is 50 nm or less, contributes to suppressing the roughening of the second magnetic layer. However, the present invention is not limited to the speculation described herein. Also, as stated above, the above magnetic recording medium is suitable as a magnetic recording medium for vertical recording. However, this does not preclude the possibility of performing magnetic recording on the above magnetic recording medium using an in-plane recording method.

[0012] The magnetic recording medium described above will be explained in more detail below.

[0013] <Second magnetic layer> (Various physical properties of ferromagnetic powders) Coercive force Hc In the present invention and this specification, the coercivity Hc of a ferromagnetic powder can be measured by a vibrating sample magnetometer. In the present invention and this specification, the coercivity Hc is a value measured at a measurement temperature of 25°C ± 1°C. The measurement temperature is the temperature of the ferromagnetic powder at the time of measurement. By setting the ambient temperature around the ferromagnetic powder to be measured to the measurement temperature, temperature equilibrium is achieved, thereby setting the temperature of the ferromagnetic powder to be measured to the measurement temperature. For the unit of coercivity Hc, Oe (Oersted), 1 [Oe] = 10⁻¹⁰ 3 It is / 4π[A / m].

[0014] The ferromagnetic powder contained in the second magnetic layer can be a ferromagnetic powder known as soft magnetic powder, and its coercivity Hc is 50 Oe or less. From the viewpoint of further improving electromagnetic conversion characteristics, the coercivity Hc of the ferromagnetic powder contained in the second magnetic layer is preferably 45 Oe or less, and more preferably in the order of 40 Oe or less, 35 Oe or less, 30 Oe or less, 25 Oe or less, and 20 Oe or less. The coercivity Hc of the ferromagnetic powder contained in the second magnetic layer can be, for example, 1 Oe or more, 3 Oe or more, or 5 Oe or more. A low coercivity Hc of the ferromagnetic powder contained in the second magnetic layer is preferable from the viewpoint of further improving electromagnetic conversion characteristics. The ferromagnetic powder having the above coercivity Hc can be, for example, spinel ferrite powder. In the present invention and this specification, "spinel ferrite powder" refers to a ferromagnetic powder in which the crystal structure of spinel ferrite is detected as the main phase by X-ray diffraction analysis. In the present invention and this specification, "main phase" refers to the structure to which the highest intensity diffraction peak in the X-ray diffraction spectrum obtained by X-ray diffraction analysis belongs. For example, if the highest intensity diffraction peak in the X-ray diffraction spectrum obtained by X-ray diffraction analysis is attributed to the crystal structure of spinel ferrite, it shall be determined that the crystal structure of spinel ferrite has been detected as the main phase. If only a single phase is detected by X-ray diffraction analysis, this detected structure shall be considered the main phase.

[0015] Average particle size In the present invention and this specification, unless otherwise specified, the average particle size of various powders such as ferromagnetic powders shall be the value measured using a transmission electron microscope by the following method. The powder is photographed using a transmission electron microscope at a magnification of 100,000x, and the resulting image is printed on photographic paper or displayed on a screen to obtain a total magnification of 500,000x, thereby obtaining a photograph of the particles that make up the powder. From the obtained photographs of the particles, the target particles are selected, and their contours are traced with a digitizer to measure the size of the particles (primary particles). Primary particles are defined as independent particles that do not aggregate. The above measurements are performed on 500 randomly selected particles. The arithmetic mean of the particle sizes of these 500 particles is taken as the average particle size of the powder. As the transmission electron microscope, for example, a Hitachi H-9000 transmission electron microscope can be used. The particle size can be measured using known image analysis software, for example, Carl Zeiss KS-400 image analysis software. Unless otherwise specified, the average particle sizes shown in the examples described later are values ​​measured using a Hitachi H-9000 transmission electron microscope and Carl Zeiss KS-400 image analysis software. In the present invention and this specification, "powder" means a collection of multiple particles. For example, ferromagnetic powder means a collection of multiple ferromagnetic particles. Furthermore, a collection of multiple particles is not limited to a form in which the particles constituting the collection are in direct contact, but also includes forms in which binders, additives, etc., described later, are interposed between the particles. The word "particle" is sometimes used to refer to powder.

[0016] In the present invention and this specification, unless otherwise specified, the size of the particles constituting the powder (particle size) is determined by the shape of the particles observed in the above particle photograph. (1) In the case of needle-shaped, spindle-shaped, columnar (however, the height is greater than the longest diameter of the base), etc., the length of the long axis constituting the particle is expressed as the long axis length, (2) In the case of a plate or column (provided that the thickness or height is less than the longest diameter of the plate or base), it shall be expressed by the longest diameter of the plate or base. (3) If the shape is spherical, polyhedral, irregular, etc., and the major axis constituting the particle cannot be determined from the shape, it shall be represented by the equivalent diameter of a circle. The equivalent diameter of a circle refers to the diameter obtained by the circular projection method.

[0017] Furthermore, the average needle-shape ratio of the powder refers to the arithmetic mean of the values ​​obtained for the 500 particles by measuring the length of the short axis of each particle, i.e., the short axis length, in the above measurement, and determining the (long axis length / short axis length) value for each particle. Here, unless otherwise specified, the short axis length refers to the length of the short axis constituting the particle in the above definition of particle size (1), the thickness or height in the case of (2), and in the case of (3), since there is no distinction between the long axis and the short axis, (long axis length / short axis length) is considered to be 1 for convenience. Unless otherwise specified, when the particle shape is specific, for example, in the case of definition (1) above, the average particle size is the average major axis length, and in the case of definition (2), the average particle size is the average plate diameter. In the case of definition (3), the average particle size is the average diameter (also called the average particle size or average particle diameter).

[0018] The average particle size of the ferromagnetic powder contained in the second magnetic layer is 50 nm or less, preferably 45 nm or less from the viewpoint of further improving electromagnetic conversion characteristics, and more preferably 40 nm or less, 35 nm or less, 30 nm or less, and 25 nm or less, in that order. The average particle size of the ferromagnetic powder contained in the second magnetic layer can be, for example, 3 nm or more, 5 nm or more, or 10 nm or more. A smaller average particle size of the ferromagnetic powder contained in the second magnetic layer is preferable from the viewpoint of further improving electromagnetic conversion characteristics.

[0019] With regard to the measurement of coercivity Hc and average particle size, if a powder sample is available, the above measurements can be performed on such a powder sample. Alternatively, sample powder can be taken from a magnetic recording medium for various measurements. As a method for taking a powder sample from a magnetic recording medium, for example, the method described in paragraph 0015 of Japanese Patent Application Publication No. 2011-048878 can be employed.

[0020] The coercivity Hc and average particle size of ferromagnetic powder can be controlled, for example, by the preparation conditions of the ferromagnetic powder. This will be discussed further later.

[0021] (Method for preparing ferromagnetic powder) Below, a glass crystallization method is described as an example of a method for preparing spinel ferrite powder that can be used as ferromagnetic powder for the second magnetic layer. However, the ferromagnetic powder contained in the second magnetic layer is not limited to spinel ferrite powder obtained by this preparation method.

[0022] The glass crystallization method generally includes the following steps: (1) A process of melting a raw material mixture containing at least spinel ferrite forming components and glass forming components to obtain a molten product (melting process); (2) A process of rapidly cooling the molten material to obtain an amorphous material (amorphization process); (3) A process of heat-treating an amorphous material to obtain a crystalline product containing spinel ferrite particles precipitated by the heat treatment and crystallized glass components (crystallization process); (4) A step to collect spinel ferrite particles from the crystalline material (particle collection step).

[0023] The above process will be explained in more detail below.

[0024] melting process The raw material mixture used in the glass crystallization method to obtain spinel ferrite powder contains spinel ferrite-forming components and glass-forming components. Examples of glass-forming components include oxides containing atoms that become constituent atoms of the glass components described later. When using the glass crystallization method to obtain spinel ferrite powder that can be used as ferromagnetic powder for the second magnetic layer, for example, B2O3 and XO components can be used as glass-forming components. Examples of X atoms include alkaline earth metal atoms such as calcium atoms (Ca), strontium atoms (Sr), and barium atoms (Ba). By using B2O3 and XO components in combination, a glass component represented by the composition formula: XB2O4 can be formed as the glass component described later. In the glass crystallization method, each component contained in the raw material mixture exists as an oxide or as various salts that can be converted into oxides during processes such as melting. In the present invention and this specification, "B2O3 component" is used to mean B2O3 itself and various salts such as H3BO3 that can be converted into B2O3 during the process. This point is also true for the other components.

[0025] Examples of spinel ferrite-forming components in the raw material mixture include oxides containing atoms that constitute the crystal structure of spinel ferrite. An example of spinel ferrite is the one represented by the chemical formula AFe2O4. Examples of A atoms include divalent metal atoms such as nickel (Ni), manganese (Mn), zinc (Zn), and copper (Cu). A divalent metal atom refers to a metal atom that can form a divalent cation as an ion. Specific examples of spinel ferrite-forming components include the Fe2O3 component and the AO component. Examples of the AO component include the NiO component, MnO component, ZnO component, and CuO component.

[0026] The content of each component in the raw material mixture is not particularly limited. The inventors investigated the coercivity Hc of spinel ferrite powder and found that, for example, in spinel ferrite powder containing nickel and zinc atoms as A atoms, the higher the zinc atom content, the lower the coercivity Hc tended to be. Therefore, increasing the content of the ZnO component in a raw material mixture containing NiO and ZnO components may lead to a lower coercivity Hc of the spinel ferrite powder. The raw material mixture can be prepared by weighing and mixing the various components. Next, the raw material mixture is melted to obtain a molten product. The melting temperature can be set according to the composition of the raw material mixture, and is usually 1000-1500°C. The melting time can be set appropriately to ensure that the raw material mixture is sufficiently melted.

[0027] Amorphization process Next, an amorphous material is obtained by rapidly cooling the resulting molten material. This rapid cooling can be carried out in the same way as the rapid cooling process usually performed to obtain an amorphous material by the glass crystallization method, and can be carried out by known methods such as pouring the molten material onto a rapidly rotating water-cooled twin roller and rapidly cooling it by rolling.

[0028] Crystallization process After the rapid cooling described above, the resulting amorphous material is heat-treated. This heat treatment allows for the precipitation of spinel ferrite particles and crystallized glass components. The particle size of the precipitated spinel ferrite particles can be controlled by the heating conditions. Increasing the heating temperature for crystallization (crystallization temperature) tends to increase the particle size of the precipitated spinel ferrite particles, and as a result, the average particle size of the prepared spinel ferrite powder tends to increase. Furthermore, for spinel ferrite powder, if the composition of the raw material mixture is similar, the coercivity Hc tends to increase as the average particle size increases. Considering the above points, it is preferable to control the heating conditions so that spinel ferrite powder with an average particle size and coercivity Hc within the above range is obtained. In one embodiment, the crystallization temperature is preferably in the range of 600°C to 690°C. Also, the heating time for crystallization (holding time at the above crystallization temperature) is, in one embodiment, for example, 0.1 to 24 hours, preferably 0.15 to 8 hours.

[0029] Particle collection process The crystalline material obtained by heat-treating an amorphous material contains spinel ferrite particles and crystallized glass components. Therefore, when the crystalline material is treated with acid, the crystallized glass components surrounding the spinel ferrite particles are dissolved and removed, allowing the spinel ferrite particles to be collected. Prior to the above acid treatment, it is preferable to perform coarse grinding to improve the efficiency of the acid treatment. Coarse grinding may be carried out by either a dry or wet method. The conditions for coarse grinding can be set according to known methods. The acid treatment for particle collection can be carried out by methods commonly used in glass crystallization methods, such as heated acid treatment. After that, spinel ferrite powder can be obtained by post-treatment such as classification (e.g., centrifugation, decantation, magnetic separation, etc.), washing with water, and drying, as needed.

[0030] The above describes one example of a method for preparing spinel ferrite powder. However, the ferromagnetic powder contained in the second magnetic layer of the magnetic recording medium is not limited to that obtained by the above preparation method.

[0031] (Binder) The above-described magnetic recording medium is a coated magnetic recording medium, and the second magnetic layer contains a binder. The binder is one or more resins. Various resins commonly used as binders for coated magnetic recording media can be used as the binder. For example, as the binder, a resin selected from polyurethane resin, polyester resin, polyamide resin, vinyl chloride resin, acrylic resin copolymerized with styrene, acrylonitrile, methyl methacrylate, etc., cellulose resin such as nitrocellulose, epoxy resin, phenoxy resin, polyvinyl acetal, polyvinyl alkylal resin such as polyvinyl butyral can be used alone or in mixture of multiple resins. Among these, polyurethane resin, acrylic resin, cellulose resin, and vinyl chloride resin are preferred. These resins may be homopolymers or copolymers. These resins can also be used as binders in the first magnetic layer, non-magnetic layer, and / or back coat layer described later. For details on the binders mentioned above, please refer to paragraphs 0028 to 0031 of Japanese Patent Publication No. 2010-24113. The average molecular weight of the resin used as a binder can be, for example, 10,000 or more and 200,000 or less as a weight-average molecular weight.

[0032] The second magnetic layer comprises the ferromagnetic powder and binder mentioned above, and may contain one or more additives. For further details of the second magnetic layer, such as the additives that may be included in the second magnetic layer, known technologies relating to magnetic layers generally called soft magnetic underlayers, known technologies relating to magnetic layers generally called recording layers, and / or known technologies relating to non-magnetic layers can be applied. Furthermore, for the second magnetic layer, the following descriptions relating to the first magnetic layer and / or non-magnetic layers can also be referenced.

[0033] The second magnetic layer described above can be provided directly on the surface of the non-magnetic support, or indirectly via the non-magnetic layer.

[0034] <First magnetic layer> In the above magnetic recording medium, the first magnetic layer is a layer that can function as a recording layer. From the viewpoint of further improving electromagnetic conversion characteristics, it is preferable that the number distribution A on the surface of the first magnetic layer satisfies (1) to (3) described above, and / or that the number distribution B satisfies (4) to (6) described above. In the above magnetic recording medium, it is preferable that the number distribution A satisfies (1) to (3) described above, and the number distribution B satisfies (4) to (6) described above.

[0035] The scanning electron microscope used in this invention and specification to determine the number distribution A and number distribution B, respectively, is a field emission scanning electron microscope (FE-SEM). For example, a Hitachi FE-SEM S4800 can be used as the FE-SEM, and this FE-SEM was used in the embodiments described later. As the magnetic recording medium to be measured, an unused magnetic recording medium that is not attached to a magnetic recording and playback device is used. For example, magnetic tapes are usually distributed in magnetic tape cartridges. For example, as the magnetic tape to be measured, a magnetic tape taken from an unused magnetic tape cartridge that is not attached to a magnetic recording and playback device is used. Furthermore, when determining the number distribution A and number distribution B, the coating treatment on the surface of the first magnetic layer is not performed before acquiring the SEM image. Each imaging is performed by selecting an unimaged area on the surface of the first magnetic layer. The SEM image captured is a secondary electron image. The equivalent diameter of the circle should be calculated in 1nm increments, rounding to one decimal place and truncating any decimal places beyond the second decimal place. When determining the number distribution A, in measuring the number of bright areas, bright areas that are only partially included in the binarized image and whose remaining portion is outside the binarized image will be excluded from the measurement. When determining the number distribution B, in measuring the number of dark areas, dark areas that are only partially included in the binarized image and whose remaining portion is outside the binarized image will be excluded from the measurement. Furthermore, in the present invention and this specification, "surface of the first magnetic layer" is synonymous with the surface of the first magnetic layer of the magnetic recording medium.

[0036] (Measurement method for the number distribution A) In the present invention and this specification, "number distribution A" is a number distribution measured by the following method. A scanning electron microscope (FE-SEM) is used to capture a secondary electron image of the surface of the first magnetic layer of the magnetic recording medium under measurement. The imaging conditions are: acceleration voltage of 5kV, working distance of 5mm, and magnification of 10,000x. During imaging, an unimaged area of ​​the surface of the first magnetic layer is selected, and the focus is adjusted under the above imaging conditions to capture a secondary electron image. The parts that indicate size, etc. (micron bars, cross marks, etc.) are removed from the captured image, and a secondary electron image with a resolution of 960 pixels × 1280 pixels is obtained. The above procedure is performed 100 times at different locations on the surface of the first magnetic layer of the magnetic recording medium to be measured. The resulting secondary electron image is imported into image processing software and binarized using the following procedure. For example, the free software ImageJ can be used as image analysis software. Binarization separates the image into bright areas (white parts) and dark areas (black parts). The thresholds for binarizing the secondary electron image obtained above are set to a lower limit of 210 gradations and an upper limit of 255 gradations. Binarization is performed using these two thresholds. After binarization, noise component removal is performed using image analysis software. Noise component removal can be performed, for example, by the following method: In the image analysis software ImageJ, select the Despeckle noise reduction process to remove noise components. The resulting binarized image is then analyzed using image analysis software to determine the number of highlight regions (i.e., white areas) and the area of ​​each highlight region. From the areas of the highlight regions obtained here, the equivalent diameter of each highlight region is calculated. Specifically, the equivalent diameter L is calculated from the obtained area A using the formula (A / π)^(1 / 2)×2=L. Here, the operator "^" represents exponentiation. The above steps are performed on the binarized images (100 images) obtained above. Thus, the distribution A of the number of items can be determined.

[0037] (Measurement method for the number distribution B) In the present invention and this specification, "number distribution B" is a number distribution measured by the following method. A scanning electron microscope (FE-SEM) is used to capture a secondary electron image of the surface of the first magnetic layer of the magnetic recording medium under measurement. The imaging conditions are as follows: acceleration voltage of 2kV, working distance of 5mm, and magnification of 10,000x. During imaging, an unimaged area of ​​the surface of the first magnetic layer is selected, and the focus is adjusted under the above imaging conditions to capture a secondary electron image. The parts that indicate size, etc. (micron bars, cross marks, etc.) are removed from the captured image, and a secondary electron image with a pixel count of 960 pixels × 1280 pixels is obtained. The above procedure is performed 100 times at different locations on the surface of the first magnetic layer of the magnetic recording medium to be measured. The resulting secondary electron image is imported into image processing software and binarized using the following procedure. For example, the free software ImageJ can be used as image analysis software. The thresholds for binarizing the secondary electron image obtained above are set with a lower limit of 0 gradations and an upper limit of 75 gradations. Binarization is performed using these two thresholds. After binarization, noise component removal is performed using image analysis software. Noise component removal can be performed, for example, by the following method: In the image analysis software ImageJ, select the Despeckle noise reduction process to remove noise components. In the resulting binarized image, image analysis software is used to determine the number of dark regions (i.e., black areas) and the area of ​​each dark region. From the areas of the dark regions obtained here, the equivalent diameter of each dark region is calculated. Specifically, the equivalent diameter L is calculated from the obtained area A using the formula (A / π)^(1 / 2)×2=L. The above steps are performed on the binarized images (100 images) obtained above. In this way, the count distribution B is obtained.

[0038] (Quantity distribution A and quantity distribution B) In the magnetic recording medium described above, the number distribution A obtained by the method described above is as follows (1) to (3): (1) There are 10,000 to 30,000 bright areas with an equivalent circular diameter of 1 nm to 50 nm. (2) There are 7,000 to 25,000 bright areas with an equivalent circular diameter of 51 nm to 100 nm. (3) There are 1,000 to 3,000 bright areas with an equivalent circular diameter of 101 nm or more. It is preferable that the following conditions be met.

[0039] Furthermore, in the magnetic recording medium described above, the number distribution B obtained by the method described above is as follows (4)~(6): (4) There are 200 to 50,000 dark regions with an equivalent diameter of 1 nm to 50 nm. (5) There are 200 to 25,000 dark regions with an equivalent diameter of 51 nm to 100 nm. (6) There are 0 to 2000 dark regions with an equivalent diameter of 101 nm or more. It is preferable that the following conditions be met.

[0040] The first magnetic layer can be formed, for example, using a magnetic layer forming composition containing one or more non-magnetic powders in addition to ferromagnetic powder. The inventors believe that, regarding the number distribution A and number distribution B obtained by the method described above, number distribution A can serve as an indicator of the state of existence of the non-magnetic powder (hereinafter also referred to as "abrasive") contained in the first magnetic layer on the surface of the first magnetic layer in order to impart abrasiveness to the surface of the first magnetic layer. Furthermore, the inventors believe that number distribution B can serve as an indicator of the state of existence of the non-magnetic powder (hereinafter also referred to as "filler") contained in the first magnetic layer on the surface of the first magnetic layer in order to form appropriate protrusions on the surface of the first magnetic layer in order to control the friction characteristics. Controlling number distribution A and number distribution B as described above can contribute to further improvement of electromagnetic conversion characteristics. This is presumed to be because it is preferable for number distribution A and number distribution B to be within the above range from the viewpoint of achieving both a reduction in spacing loss and an improvement in the stability of the contact state between the surface of the first magnetic layer and the magnetic head.

[0041] Regarding (1) above, the number of bright areas with an equivalent circle diameter of 1 nm to 50 nm is 10,000 to 30,000. The number of such bright areas is preferably 15,000 or more, and more preferably 20,000 or more. Furthermore, the number of such bright areas is preferably 28,000 or less, and more preferably 25,000 or less.

[0042] Regarding (2) above, the number of bright areas with an equivalent circle diameter of 51 nm to 100 nm is 7,000 to 25,000. The number of such bright areas is preferably 8,000 or more, and more preferably 9,000 or more. Furthermore, the number of such bright areas is preferably 24,000 or less, and more preferably 23,000 or less.

[0043] Regarding (3) above, the number of bright regions with an equivalent circle diameter of 101 nm or more is between 1,000 and 3,000. The number of such bright regions is preferably 1,500 or more, and more preferably 2,000 or more. Furthermore, the number of such bright regions is preferably 2,800 or less, and more preferably 2,500 or less. In one embodiment, the number of the above bright regions may be less than 3,000. Also, in one embodiment, the number of the above bright regions may be more than 1,000.

[0044] Regarding (4) above, the number of dark regions with an equivalent circle diameter of 1 nm to 50 nm is between 200 and 50,000. The number of such dark regions is preferably 1,000 or more, more preferably 2,000 or more, and even more preferably 3,000 or more. Furthermore, the number of such dark regions is preferably 40,000 or less, and more preferably 30,000 or less. In one embodiment, the number of the above dark regions can exceed 1,000.

[0045] Regarding (5) above, the number of dark regions with an equivalent circle diameter of 51 nm to 100 nm is between 200 and 25,000. The number of such dark regions is preferably 250 or more, and more preferably 300 or more. Furthermore, the number of such dark regions is preferably 20,000 or less, and more preferably 15,000 or less.

[0046] Regarding (6) above, the number of dark regions with an equivalent circle diameter of 101 nm or more is between 0 and 2000. The number of such dark regions is preferably 10 or more, and more preferably 20 or more. Furthermore, the number of such dark regions is preferably 1500 or less, and more preferably 1000 or less. In one embodiment, the number of the above dark regions can be less than 200.

[0047] The number distributions A and B can be controlled by the type of components added to the first magnetic layer forming composition used to form the first magnetic layer, and by the method of preparing such composition (e.g., dispersion method, classification method, etc.). Specific examples of control methods can also be found in the examples described later.

[0048] (Ferromagnetic powder) As the ferromagnetic powder contained in the first magnetic layer, one or more known ferromagnetic powders used in the magnetic layer generally called the recording layer in various magnetic recording media can be used. Using a ferromagnetic powder with a small average particle size is preferable from the viewpoint of improving recording density. From this viewpoint, the average particle size of the ferromagnetic powder contained in the first magnetic layer is preferably 50 nm or less, more preferably 45 nm or less, even more preferably 40 nm or less, even more preferably 35 nm or less, even more preferably 30 nm or less, even more preferably 25 nm or less, and still even more preferably 20 nm or less. On the other hand, from the viewpoint of magnetization stability, the average particle size of the ferromagnetic powder contained in the first magnetic layer is preferably 5 nm or more, more preferably 8 nm or more, even more preferably 10 nm or more, even more preferably 15 nm or more, and even more preferably 20 nm or more.

[0049] Hexagonal ferrite powder A preferred specific example of the ferromagnetic powder contained in the first magnetic layer is hexagonal ferrite powder. For details on hexagonal ferrite powder, see, for example, paragraphs 0012 to 0030 of Japanese Patent Publication No. 2011-225417, paragraphs 0134 to 0136 of Japanese Patent Publication No. 2011-216149, paragraphs 0013 to 0030 of Japanese Patent Publication No. 2012-204726, and paragraphs 0029 to 0084 of Japanese Patent Publication No. 2015-127985.

[0050] In the present invention and this specification, "hexagonal ferrite powder" refers to a ferromagnetic powder in which a hexagonal ferrite crystal structure is detected as the main phase by X-ray diffraction analysis. For example, if the highest intensity diffraction peak in the X-ray diffraction spectrum obtained by X-ray diffraction analysis is attributed to the hexagonal ferrite crystal structure, it shall be determined that the hexagonal ferrite crystal structure has been detected as the main phase. The hexagonal ferrite crystal structure contains at least iron atoms, divalent metal atoms, and oxygen atoms as constituent atoms. Divalent metal atoms are metal atoms that can become divalent cations as ions, and examples include strontium atoms, barium atoms, alkaline earth metal atoms such as calcium atoms, and lead atoms. In the present invention and this specification, hexagonal strontium ferrite powder refers to powder in which the main divalent metal atom contained is strontium, and hexagonal barium ferrite powder refers to powder in which the main divalent metal atom contained is barium. The main divalent metal atom refers to the divalent metal atom that accounts for the largest proportion of the divalent metal atoms contained in this powder, on an atomic percentage basis. However, rare earth atoms are not included in the divalent metal atoms relating to the hexagonal ferrite described above. In this invention and specification, "rare earth atoms" are selected from the group consisting of scandium atoms (Sc), yttrium atoms (Y), and lanthanide atoms. Lanthanide atoms are selected from the group consisting of lanthanum atoms (La), cerium atoms (Ce), praseodymium atoms (Pr), neodymium atoms (Nd), promethium atoms (Pm), samarium atoms (Sm), europium atoms (Eu), gadolinium atoms (Gd), terbium atoms (Tb), dysprosium atoms (Dy), holmium atoms (Ho), erbium atoms (Er), thulium atoms (Tm), ytterbium atoms (Yb), and lutetium atoms (Lu).

[0051] Below, we will describe hexagonal strontium ferrite powder, a form of hexagonal ferrite powder, in more detail.

[0052] The activation volume of the hexagonal strontium ferrite powder is preferably 800 to 1600 nm.3 is within the range. The micronized hexagonal strontium ferrite powder showing the activation volume within the above range is suitable for producing a magnetic tape exhibiting excellent electromagnetic conversion characteristics. The activation volume of the hexagonal strontium ferrite powder is preferably 800 nm 3 or more, for example, 850 nm 3 or more. Also, from the viewpoint of further improving the electromagnetic conversion characteristics, the activation volume of the hexagonal strontium ferrite powder is 1500 nm 3 or less, more preferably 1400 nm 3 or less, still more preferably 1300 nm 3 or less, even more preferably 1200 nm 3 or less, yet even more preferably 1100 nm 3 or less is still yet even more preferably. The same applies to the activation volume of the hexagonal barium ferrite powder.

[0053] The "activation volume" is a unit of magnetization reversal and is an index indicating the magnetic size of particles. The activation volume described in the present invention and this specification and the anisotropy constant Ku described later are measured using a vibrating sample magnetometer at magnetic field sweep rates of 3 minutes and 30 minutes in the coercive force Hc measurement section (measurement temperature: 23°C ± 1°C), and are values obtained from the following relational expression between Hc and the activation volume V. Regarding the unit of the anisotropy constant Ku, 1 erg / cc = 1.0×10 -1 J / m 3 is. Hc = 2Ku / Ms{1 - [(kT / KuV)ln(At / 0.693)] 1 / 2} [In the above formula, Ku: anisotropy constant (unit: J / m 3 ), Ms: saturation magnetization (unit: kA / m), k: Boltzmann constant, T: absolute temperature (unit: K), V: activation volume (unit: cm 3 ), A: spin precession frequency (unit: s -1 ), t: magnetic field reversal time (unit: s)]

[0054] As an indicator of the reduction of thermal fluctuations, or in other words, the improvement of thermal stability, the anisotropy constant Ku can be cited. The hexagonal strontium ferrite powder is preferably 1.8 × 10⁻⁶ 5 J / m 3 It can have a Ku of the above, and more preferably 2.0 × 10 5 J / m 3 It can have a Ku content of the above. Also, the Ku content of hexagonal strontium ferrite powder is, for example, 2.5 × 10⁻⁶. 5 J / m 3 The following values ​​are possible. However, since a higher Ku value is preferable as it indicates higher thermal stability, the values ​​are not limited to those exemplified above.

[0055] Hexagonal strontium ferrite powder may or may not contain rare earth atoms. When hexagonal strontium ferrite powder contains rare earth atoms, it is preferable that the rare earth atoms are present at a concentration of 0.5 to 5.0 atomic percent (bulk concentration) per 100 atomic percent of iron atoms. In one embodiment, hexagonal strontium ferrite powder containing rare earth atoms may exhibit a segregation of rare earth atoms in the surface layer. In the present invention and this specification, "rare earth atom surface layer segregation" means that the rare earth atom content relative to 100% of iron atoms in a solution obtained by partially dissolving hexagonal strontium ferrite powder with acid (hereinafter referred to as "rare earth atom surface layer content" or simply "surface layer content" with respect to rare earth atoms) is different from the rare earth atom content relative to 100% of iron atoms in a solution obtained by completely dissolving hexagonal strontium ferrite powder with acid (hereinafter referred to as "rare earth atom bulk content" or simply "bulk content" with respect to rare earth atoms), Rare earth atom surface content / Rare earth atom bulk content > 1.0 This means that the ratio is satisfied. The rare earth atom content of hexagonal strontium ferrite powder described later is synonymous with the rare earth atom bulk content. In contrast, partial dissolution using acid dissolves the surface layer of the particles constituting the hexagonal strontium ferrite powder, so the rare earth atom content in the solution obtained by partial dissolution is the rare earth atom content in the surface layer of the particles constituting the hexagonal strontium ferrite powder. When the rare earth atom surface layer content satisfies the ratio "rare earth atom surface layer content / rare earth atom bulk content > 1.0", it means that in the particles constituting the hexagonal strontium ferrite powder, rare earth atoms are concentrated in the surface layer (i.e., there are more of them in the surface layer than in the interior). In this invention and specification, the surface layer means a part of the region extending from the surface to the interior of the particles constituting the hexagonal strontium ferrite powder.

[0056] When hexagonal strontium ferrite powder contains rare earth atoms, the rare earth atom content (bulk content) is preferably in the range of 0.5 to 5.0 atomic percent relative to 100 atomic percent of iron atoms. It is believed that containing rare earth atoms at the bulk content within the above range, and having the rare earth atoms unevenly distributed on the surface of the particles constituting the hexagonal strontium ferrite powder, contributes to suppressing the decrease in regeneration output during repeated regeneration. This is presumed to be because the anisotropy constant Ku can be increased by containing rare earth atoms at the bulk content within the above range, and having the rare earth atoms unevenly distributed on the surface of the particles constituting the hexagonal strontium ferrite powder. The higher the value of the anisotropy constant Ku, the more it is possible to suppress the occurrence of a phenomenon called thermal fluctuation (in other words, to improve thermal stability). By suppressing the occurrence of thermal fluctuation, the decrease in regeneration output during repeated regeneration can be suppressed. It is speculated that the uneven distribution of rare earth atoms on the surface of hexagonal strontium ferrite powder particles contributes to stabilizing the spin of iron (Fe) sites within the crystal lattice of the surface layer, thereby increasing the anisotropy constant Ku. Furthermore, it is presumed that using hexagonal strontium ferrite powder with rare earth atoms unevenly distributed on the surface as the ferromagnetic powder for the first magnetic layer contributes to suppressing wear on the surface of the first magnetic layer due to sliding with the magnetic head. In other words, it is presumed that hexagonal strontium ferrite powder with rare earth atoms unevenly distributed on the surface may also contribute to improving the running durability of the magnetic recording medium. This is presumed to be because the uneven distribution of rare earth atoms on the surface of the particles constituting the hexagonal strontium ferrite powder contributes to improved interaction between the particle surface and organic substances (e.g., binders and / or additives) contained in the first magnetic layer, and as a result, the strength of the first magnetic layer is improved. From the viewpoint of suppressing the decrease in regeneration output during repeated regeneration and / or further improving running durability, the rare earth atom content (bulk content) is more preferably in the range of 0.5 to 4.5 atomic percent, even more preferably in the range of 1.0 to 4.5 atomic percent, and even more preferably in the range of 1.5 to 4.5 atomic percent.

[0057] The above bulk content is the content obtained by completely dissolving the hexagonal strontium ferrite powder. In this invention and specification, unless otherwise specified, the content of atoms refers to the bulk content obtained by completely dissolving the hexagonal strontium ferrite powder. The hexagonal strontium ferrite powder containing rare earth atoms may contain only one type of rare earth atom, or it may contain two or more types of rare earth atoms. When two or more types of rare earth atoms are included, the above bulk content is determined for the sum of the two or more types of rare earth atoms. This also applies to other components in this invention and specification. That is, unless otherwise specified, one type of component may be used, or two or more types may be used. When two or more types are used, the content or content refers to the sum of the two or more types.

[0058] When hexagonal strontium ferrite powder contains rare earth atoms, the rare earth atoms included may be one or more of any type of rare earth atom. From the viewpoint of suppressing a decrease in regeneration output during repeated regeneration, preferred rare earth atoms include neodymium atoms, samarium atoms, yttrium atoms, and dysprosium atoms, with neodymium atoms, samarium atoms, and yttrium atoms being more preferred, and neodymium atoms being even more preferred.

[0059] In hexagonal strontium ferrite powder having a rare-earth atom surface segregation, the rare-earth atoms only need to be segregated in the surface layer of the particles constituting the hexagonal strontium ferrite powder, and the degree of segregation is not limited. For example, in hexagonal strontium ferrite powder having a rare-earth atom surface segregation, the ratio of the rare-earth atom surface content obtained by partial dissolution under the dissolution conditions described later to the rare-earth atom bulk content obtained by total dissolution under the dissolution conditions described later, "surface content / bulk content," is greater than 1.0 and can be 1.5 or greater. A "surface content / bulk content" greater than 1.0 means that in the particles constituting the hexagonal strontium ferrite powder, rare-earth atoms are segregated in the surface layer (i.e., there are more of them in the surface layer than in the interior). Furthermore, the ratio of the surface content of rare earth atoms obtained by partial dissolution under the dissolution conditions described later to the bulk content of rare earth atoms obtained by total dissolution under the dissolution conditions described later, "surface content / bulk content," can be, for example, 10.0 or less, 9.0 or less, 8.0 or less, 7.0 or less, 6.0 or less, 5.0 or less, or 4.0 or less. However, in hexagonal strontium ferrite powder having a rare earth atom surface distribution bias, the rare earth atoms only need to be biased towards the surface of the particles constituting the hexagonal strontium ferrite powder, and the above "surface content / bulk content" is not limited to the upper or lower limits exemplified.

[0060] The partial and total dissolution of hexagonal strontium ferrite powder is described below. For hexagonal strontium ferrite powder existing as a powder, the sample powders to be partially and completely dissolved are taken from the same lot of powder. On the other hand, for hexagonal strontium ferrite powder contained in the first magnetic layer of a magnetic recording medium, a portion of the hexagonal strontium ferrite powder extracted from the first magnetic layer is subjected to partial dissolution, and another portion is subjected to total dissolution. The extraction of hexagonal strontium ferrite powder from the magnetic layer can be carried out, for example, by the method described in paragraph 0032 of Japanese Patent Application Publication No. 2015-91747. Partial dissolution, as described above, refers to a state where the hexagonal strontium ferrite powder is dissolved to the extent that residual particles can be visually confirmed in the liquid at the end of the dissolution process. For example, partial dissolution can dissolve 10 to 20% by mass of the particles constituting the hexagonal strontium ferrite powder, with the total particles being 100% by mass. On the other hand, total dissolution, as described above, refers to a state where the hexagonal strontium ferrite powder is dissolved to the extent that no residual particles can be visually confirmed in the liquid at the end of the dissolution process. The above-mentioned partial dissolution and surface layer content measurement are performed, for example, by the following method. However, the dissolution conditions such as the amount of sample powder described below are examples only, and any dissolution conditions that enable partial and total dissolution can be arbitrarily adopted. A container (e.g., a beaker) containing 12 mg of sample powder and 10 mL of 1 mol / L hydrochloric acid is held on a hot plate at a set temperature of 70°C for 1 hour. The resulting solution is filtered through a 0.1 μm membrane filter. Atomic analysis of the resulting filtrate is performed using an inductively coupled plasma (ICP) analyzer. In this way, the surface content of rare earth atoms relative to 100% iron atoms can be determined. If multiple types of rare earth atoms are detected by atomic analysis, the total content of all rare earth atoms is taken as the surface content. This is also the case when measuring bulk content. On the other hand, the measurement of total dissolution and bulk content is performed, for example, by the following method. A container (e.g., a beaker) containing 12 mg of sample powder and 10 mL of 4 mol / L hydrochloric acid is held on a hot plate at a set temperature of 80°C for 3 hours. Afterward, the bulk content relative to 100 atomic percent of iron can be determined by performing the same procedure as described above for partial dissolution and surface layer content measurement.

[0061] From the perspective of improving the playback output when reproducing data recorded on a magnetic recording medium, it is desirable for the ferromagnetic powder contained in the magnetic layer that can function as a recording layer in the magnetic recording medium to have a high mass magnetization σs. In this regard, hexagonal strontium ferrite powder containing rare earth atoms but lacking surface segregation of rare earth atoms tended to have a significantly lower σs compared to hexagonal strontium ferrite powder that does not contain rare earth atoms. In contrast, hexagonal strontium ferrite powder having surface segregation of rare earth atoms is considered preferable in order to suppress such a large decrease in σs. In one embodiment, the σs of hexagonal strontium ferrite powder is 45 A·m 2 It can be 47 A·m or more per kg. 2 It can also be more than / kg. On the other hand, σs is 80 A·m from the viewpoint of noise reduction. 2 Preferably less than / kg, at 60A·m 2 It is more preferable that it be less than or equal to / kg. σs can be measured using a known measuring device capable of measuring magnetic properties, such as a vibrating sample magnetometer. In the present invention and this specification, unless otherwise specified, the mass magnetization σs is the value measured at a magnetic field strength of 15 kOe.

[0062] Regarding the constituent atom content (bulk content) of hexagonal strontium ferrite powder, the strontium atom content can be in the range of, for example, 2.0 to 15.0 atomic percent per 100 atomic percent of iron atoms. In one form, hexagonal strontium ferrite powder may contain only strontium atoms as the divalent metal atom. In another form, hexagonal strontium ferrite powder may contain one or more other divalent metal atoms in addition to strontium atoms. For example, it may contain barium atoms and / or calcium atoms. When other divalent metal atoms besides strontium atoms are included, the barium atom content and calcium atom content in the hexagonal strontium ferrite powder can be in the range of, for example, 0.05 to 5.0 atomic percent per 100 atomic percent of iron atoms.

[0063] The known crystal structures of hexagonal ferrites include magnetoplumbite (also called "M-type"), W-type, Y-type, and Z-type. Hexagonal strontium ferrite powder may have any of these crystal structures. The crystal structure can be confirmed by X-ray diffraction analysis. Hexagonal strontium ferrite powder may show a single crystal structure or two or more crystal structures by X-ray diffraction analysis. For example, in one form, hexagonal strontium ferrite powder may show only the M-type crystal structure by X-ray diffraction analysis. For example, M-type hexagonal ferrite is AFe 12 O 19It is represented by the following compositional formula: Here, A represents a divalent metal atom, and if the hexagonal strontium ferrite powder is of type M, A is either only a strontium atom (Sr), or if A contains multiple divalent metal atoms, then as described above, strontium atoms (Sr) make up the largest proportion on an atomic percentage basis. The divalent metal atom content of hexagonal strontium ferrite powder is usually determined by the type of crystal structure of the hexagonal ferrite and is not particularly limited. The same applies to the iron atom content and oxygen atom content. Hexagonal strontium ferrite powder contains at least iron atoms, strontium atoms and oxygen atoms, and may also contain rare earth atoms. Furthermore, hexagonal strontium ferrite powder may or may not contain atoms other than these atoms. As an example, hexagonal strontium ferrite powder may contain aluminum atoms (Al). The aluminum atom content can be, for example, 0.5 to 10.0 atomic percent relative to 100 atomic percent of iron atoms. From the viewpoint of suppressing a decrease in regeneration output during repeated regeneration, the hexagonal strontium ferrite powder preferably contains iron atoms, strontium atoms, oxygen atoms, and rare earth atoms, and the content of atoms other than these atoms is preferably 10.0 atomic percent or less, more preferably in the range of 0 to 5.0 atomic percent, and may even be 0 atomic percent, relative to 100 atomic percent of iron atoms. That is, in one embodiment, the hexagonal strontium ferrite powder does not need to contain atoms other than iron atoms, strontium atoms, oxygen atoms, and rare earth atoms. The above content expressed in atomic percent is obtained by converting the content of each atom (unit: mass%) obtained by completely dissolving the hexagonal strontium ferrite powder into an atomic percent value using the atomic weight of each atom. Furthermore, in the present invention and this specification, "does not contain" for a certain atom means that the content measured by an ICP analyzer after complete dissolution is 0 mass%. The detection limit of an ICP analyzer is typically 0.01 ppm (parts per million) or less by mass. The term "does not contain" above is used to include the presence of substances in amounts below the detection limit of the ICP analyzer.Hexagonal strontium ferrite powder can, in one form, be bismuth-free (Bi).

[0064] metal powder A preferred specific example of ferromagnetic powder is ferromagnetic metal powder. For details on ferromagnetic metal powder, see, for example, paragraphs 0137-0141 of Japanese Patent Publication No. 2011-216149 and paragraphs 0009-0023 of Japanese Patent Publication No. 2005-251351.

[0065] ε-Iron oxide powder A preferred specific example of the ferromagnetic powder contained in the first magnetic layer is ε-iron oxide powder. In the present invention and this specification, "ε-iron oxide powder" refers to a ferromagnetic powder in which the crystalline structure of ε-iron oxide is detected as the main phase by X-ray diffraction analysis. For example, if the highest intensity diffraction peak in the X-ray diffraction spectrum obtained by X-ray diffraction analysis is attributed to the crystalline structure of ε-iron oxide, it is determined that the crystalline structure of ε-iron oxide has been detected as the main phase. Methods for producing ε-iron oxide powder include methods from goethite and the reverse micelle method. All of the above production methods are publicly known. Furthermore, for methods of producing ε-iron oxide powder in which some of the Fe is substituted with substitution atoms such as Ga, Co, Ti, Al, and Rh, see, for example, J. Jpn. Soc. Powder Metallurgy Vol. 61 Supplement, No. S1, pp. S280-S284, J. Mater. Chem. C, 2013, 1, pp. 5200-5206, etc. However, the method for producing ε-iron oxide powder that can be used as ferromagnetic powder in the first magnetic layer of the above-mentioned magnetic recording medium is not limited to the method described herein.

[0066] The activation volume of ε-iron oxide powder is preferably 300 to 1500 nm. 3 The activation volume is within the range described above. Micronized ε-iron oxide powder exhibiting an activation volume within the above range is suitable for the production of magnetic tapes that exhibit excellent electromagnetic conversion properties. The activation volume of the ε-iron oxide powder is preferably 300 nm. 3That's all, for example, 500nm 3 It can also be the above. Furthermore, from the viewpoint of further improving electromagnetic conversion characteristics, the activation volume of ε-iron oxide powder is 1400 nm. 3 The following is more preferable: 1300nm 3 It is even more preferable that the following occur: 1200 nm 3 It is even more preferable that the following conditions be met: 1100 nm 3 The following is even more preferable.

[0067] The anisotropy constant Ku can be cited as an indicator of the reduction of thermal fluctuations, or in other words, the improvement of thermal stability. The ε-iron oxide powder is preferably 3.0 × 10⁻⁶ 4 J / m 3 It can have a Ku of the above, and more preferably 8.0 × 10 4 J / m 3 It can have the above amount of Ku. Also, the amount of Ku in ε-iron oxide powder is, for example, 3.0 × 10⁻⁶. 5 J / m 3 The following values ​​are possible. However, a higher Ku value indicates higher thermal stability, which is preferable, so the values ​​are not limited to those exemplified above.

[0068] From the perspective of increasing the playback output when reproducing data recorded on a magnetic recording medium, it is desirable for the mass magnetization σs of the ferromagnetic powder contained in the magnetic layer that can function as a recording layer in the magnetic recording medium to be high. In this regard, in one embodiment, the σs of ε-iron oxide powder is 8 A·m 2 It can be 12 A·m or more / kg. 2 It can also be more than / kg. On the other hand, the σs of ε-iron oxide powder is 40 A·m from the viewpoint of noise reduction. 2 Preferably less than / kg, 35A·m 2 It is more preferable that the amount be less than or equal to / kg.

[0069] The content (filling rate) of ferromagnetic powder in the first magnetic layer is preferably in the range of 50 to 90% by mass, and more preferably in the range of 60 to 90% by mass, relative to the total mass of the first magnetic layer. A high filling rate of ferromagnetic powder in the first magnetic layer is preferable from the viewpoint of improving recording density.

[0070] (Binder) The first magnetic layer contains ferromagnetic powder and a binder. For details regarding the binder in the first magnetic layer, refer to the previous description concerning the binder in the second magnetic layer.

[0071] (Hardening agent) The first magnetic layer forming composition may also contain a curing agent along with a resin that can be used as a binder. In one form, the curing agent may be a thermosetting compound that undergoes a curing reaction (crosslinking reaction) upon heating, and in another form, it may be a photocurable compound that undergoes a curing reaction (crosslinking reaction) upon light irradiation. As the curing reaction progresses during the process of forming the first magnetic layer, at least a portion of the curing agent may be included in the first magnetic layer in a state where it has reacted (crosslinked) with other components such as a binder. This also applies to layers formed using compositions that contain a curing agent when those compositions are used to form other layers. Preferred curing agents are thermosetting compounds, and polyisocyanates are preferred. For details on polyisocyanates, refer to paragraphs 0124 to 0125 of Japanese Patent Application Publication No. 2011-216149. The curing agent can be used in the first magnetic layer forming composition in an amount of, for example, 0 to 80.0 parts by mass per 100.0 parts by mass of the binder, preferably 50.0 to 80.0 parts by mass from the viewpoint of improving the strength of the first magnetic layer.

[0072] (Additives) The first magnetic layer may contain one or more additives as needed. An example of an additive is the curing agent mentioned above. Other additives that can be included in the magnetic layer include non-magnetic powders, lubricants, dispersants, dispersion aids, antifungal agents, antistatic agents, antioxidants, etc. For example, regarding lubricants, see paragraphs 0030-0033, 0035, and 0036 of Japanese Patent Application Publication No. 2016-126817. A lubricant may also be included in the non-magnetic layer described later. For lubricants that can be included in the non-magnetic layer, see paragraphs 0030, 0031, and 0034-0036 of Japanese Patent Application Publication No. 2016-126817. For dispersants, see paragraphs 0061 and 0071 of Japanese Patent Application Publication No. 2012-133837.

[0073] As dispersants that can be added to the first magnetic layer forming composition, known dispersants for improving the dispersibility of ferromagnetic powders, such as carboxyl group-containing compounds and nitrogen-containing compounds, can also be mentioned. For example, the nitrogen-containing compound may be any of the following: a primary amine represented by NH2R, a secondary amine represented by NHR2, or a tertiary amine represented by NR3. In the above, R represents any structure constituting the nitrogen-containing compound, and multiple Rs may be the same or different. The nitrogen-containing compound may also be a compound (polymer) having multiple repeating structures in its molecule. It is thought that the nitrogen-containing compound can act as a dispersant because the nitrogen-containing portion of the nitrogen-containing compound functions as an adsorption site on the particle surface of the ferromagnetic powder. Examples of carboxyl group-containing compounds include fatty acids such as oleic acid. In the case of carboxyl group-containing compounds, it is thought that the carboxyl group functions as an adsorption site on the particle surface of the ferromagnetic powder, which is why the carboxyl group-containing compound can act as a dispersant. It is also preferable to use carboxyl group-containing compounds and nitrogen-containing compounds in combination. The amount of these dispersants used can be set as appropriate.

[0074] A dispersant may be added to the composition for forming a non-magnetic layer. For dispersants that can be added to the composition for forming a non-magnetic layer, refer to paragraph 0061 of Japanese Patent Application Publication No. 2012-133837.

[0075] Abrasive As mentioned earlier, the number distribution A is considered to be an indicator of the presence of abrasives on the surface of the first magnetic layer. Therefore, the number distribution A can be controlled by the type of non-magnetic powder added as an abrasive. As an abrasive, a non-magnetic powder with a Mohs hardness greater than 8 is preferred, and a non-magnetic powder with a Mohs hardness of 9 or higher is more preferred. The maximum value of Mohs hardness is 10. The abrasive can be an inorganic powder or an organic powder. The abrasive can be an inorganic or organic oxide powder or a carbide powder. Examples of carbides include boron carbide (e.g., B4C) and titanium carbide (e.g., TiC). Diamond can also be used as an abrasive. In one form, the abrasive is preferably an inorganic oxide powder. Specifically, examples of inorganic oxides include alumina (e.g., Al2O3), titanium oxide (e.g., TiO2), cerium oxide (e.g., CeO2), zirconium oxide (e.g., ZrO2), etc., with alumina being preferred among them. The Mohs hardness of alumina is approximately 9. For alumina powder, see paragraph 0021 of Japanese Patent Publication No. 2013-229090. In addition, specific surface area can be used as an indicator of the particle size of the abrasive. The larger the specific surface area, the smaller the particle size of the primary particles constituting the abrasive can be considered to be. As an abrasive, the specific surface area measured by the BET (Brunauer-Emmett-Teller) method (hereinafter referred to as "BET specific surface area") is 14 m². 2 It is preferable to use abrasives of 1 / g or more. Furthermore, from the viewpoint of dispersibility, a BET specific surface area of ​​40 m² is preferable. 2It is preferable to use an abrasive of less than / g. The abrasive content in the first magnetic layer is preferably 1.0 to 20.0 parts by mass, and more preferably 1.0 to 15.0 parts by mass, per 100.0 parts by mass of ferromagnetic powder. As the abrasive, only one type of non-magnetic powder may be used, or two or more types of non-magnetic powders with different compositions and / or physical properties (e.g., size) may be used. When two or more types of non-magnetic powders are used as abrasives, the abrasive content refers to the total content of those two or more types of non-magnetic powders. The same applies to the content of various components in the present invention and this specification. It is preferable to disperse the abrasive separately from the ferromagnetic powder (separate dispersion), and more preferable to disperse it separately from the filler described later (separate dispersion). When preparing the composition for forming the first magnetic layer, it is preferable to prepare two or more dispersions with different components and / or dispersion conditions as abrasive dispersions (hereinafter also referred to as "abrasive liquid") in order to control the number distribution A.

[0076] Dispersants can also be used to adjust the dispersion state of abrasive dispersions. Examples of compounds that can function as dispersants to improve the dispersibility of abrasives include aromatic hydrocarbon compounds having a phenolic hydroxyl group. A "phenolic hydroxyl group" refers to a hydroxyl group directly bonded to an aromatic ring. The aromatic ring contained in the above aromatic hydrocarbon compound may be monocyclic, polycyclic, or fused. From the viewpoint of improving the dispersibility of abrasives, aromatic hydrocarbon compounds containing a benzene ring or a naphthalene ring are preferred. Furthermore, the above aromatic hydrocarbon compound may have substituents other than a phenolic hydroxyl group. Examples of substituents other than a phenolic hydroxyl group include halogen atoms, alkyl groups, alkoxy groups, amino groups, acyl groups, nitro groups, nitroso groups, and hydroxyalkyl groups, with halogen atoms, alkyl groups, alkoxy groups, amino groups, and hydroxyalkyl groups being preferred. A single molecule of the above aromatic hydrocarbon compound may contain one, two, three, or more phenolic hydroxyl groups.

[0077] A preferred form of an aromatic hydrocarbon compound having a phenolic hydroxyl group is a compound represented by the following formula 100.

[0078] [ka] [In formula 100, X 101 ~X 108 Two of these are hydroxyl groups, and the other six each independently represent a hydrogen atom or a substituent.

[0079] In the compound represented by formula 100, the substitution positions of the two hydroxyl groups (phenolic hydroxyl groups) are not particularly limited.

[0080] The compound represented by formula 100 is X 101 ~X 108 Two of these are hydroxyl groups (phenolic hydroxyl groups), and the other six each independently represent a hydrogen atom or a substituent. Also, X 101 ~X 108 Of these, all but the two hydroxyl groups may be hydrogen atoms, and some or all of them may be substituents. Examples of substituents include those described above. In addition to the two hydroxyl groups, one or more phenolic hydroxyl groups may be included. From the viewpoint of improving the dispersibility of the abrasive, X 101 ~X 108 It is preferable that, with the exception of two of the hydroxyl groups, the other hydroxyl groups are not phenolic hydroxyl groups. That is, the compound represented by formula 100 is preferably dihydroxynaphthalene or a derivative thereof, and more preferably 2,3-dihydroxynaphthalene or a derivative thereof. 101 ~X 108 Preferred substituents represented by include halogen atoms (e.g., chlorine atoms, bromine atoms), amino groups, alkyl groups having 1 to 6 carbon atoms (preferably 1 to 4 carbon atoms), methoxy and ethoxy groups, acyl groups, nitro and nitroso groups, and -CH2OH groups.

[0081] Furthermore, for dispersants to improve the dispersibility of abrasives, please also refer to paragraphs 0024 to 0028 of Japanese Patent Publication No. 2014-179149.

[0082] A dispersant to improve the dispersibility of abrasives can be used, for example, when preparing an abrasive solution (or for each abrasive solution if multiple abrasive solutions are prepared), in a ratio of, for example, 0.5 to 20.0 parts by mass per 100.0 parts by mass of abrasive, and preferably in a ratio of 1.0 to 10.0 parts by mass.

[0083] filler As mentioned earlier, the number distribution B is considered to be an indicator of the state of existence of non-magnetic powder (filler) contained in the first magnetic layer on the magnetic layer surface in order to form appropriate protrusions on the surface of the first magnetic layer in order to control the frictional properties. Therefore, the number distribution B can be controlled by the type of non-magnetic powder added as filler. Carbon black can be given as one form of filler. The BET specific surface area of ​​carbon black is 10 m². 2 It is preferable that it be 15m or more / g 2 It is more preferable that the concentration is 50 m² or higher. The BET specific surface area of ​​carbon black is 50 m² from the viewpoint of ease of improving dispersibility. 2 It is preferable that the amount be less than or equal to 40m 2It is more preferable that the amount be less than or equal to / g. Another form of filler is colloidal particles. From the viewpoint of availability, inorganic colloidal particles are preferred, inorganic oxide colloidal particles are more preferred, and silica colloidal particles (colloidal silica) are even more preferred. In the present invention and this specification, "colloidal particles" means particles that, when 1 g is added per 100 mL of at least one organic solvent containing methyl ethyl ketone, cyclohexanone, toluene or ethyl acetate, or a mixed solvent containing two or more of the above solvents in any mixing ratio, disperse without settling and yield a colloidal dispersion. The average particle size of the colloidal particles can be, for example, 30 to 300 nm, and is preferably 40 to 200 nm. The filler content in the first magnetic layer is preferably 0.5 to 20.0 parts by mass, and more preferably 0.5 to 15.0 parts by mass, per 100.0 parts by mass of ferromagnetic powder. The filler is preferably subjected to dispersion treatment separately from the ferromagnetic powder, and more preferably separately from the abrasive. When preparing a composition for forming a magnetic layer, it is preferable to prepare two or more dispersions of fillers (hereinafter also referred to as "filler solution") with different components and / or dispersion conditions in order to control the number distribution B.

[0084] From the viewpoint of improving the dispersibility of carbon black, in one form when preparing the filler solution, a compound having an ammonium salt structure of an alkyl ester anion represented by the following formula 1 can be used. Note that "alkyl ester anion" can also be called "alkyl carboxylate anion".

[0085] [ka]

[0086] In Formula 1, R represents an alkyl group having 7 or more carbon atoms or a fluorinated alkyl group having 7 or more carbon atoms, and Z + This represents an ammonium cation.

[0087] Furthermore, from the viewpoint of improving the dispersibility of carbon black, in one embodiment, two or more components capable of forming the above-mentioned salt structure compound can be used when preparing the filler solution. As a result, at least a portion of these components can form the above-mentioned salt structure compound when preparing the filler solution.

[0088] Unless otherwise specified, the groups described below may or may not have substituents. Furthermore, for groups with substituents, "number of carbon atoms" means the number of carbon atoms excluding the substituent unless otherwise specified. In the present invention and this specification, examples of substituents include alkyl groups (e.g., alkyl groups having 1 to 6 carbon atoms), hydroxyl groups, alkoxy groups (e.g., alkoxy groups having 1 to 6 carbon atoms), halogen atoms (e.g., fluorine atoms, chlorine atoms, bromine atoms, etc.), cyano groups, amino groups, nitro groups, acyl groups, carboxyl groups, salts of carboxyl groups, sulfonic acid groups, salts of sulfonic acid groups, and the like.

[0089] The following provides a more detailed explanation of Equation 1.

[0090] In Formula 1, R represents an alkyl group having 7 or more carbon atoms or a fluorinated alkyl group having 7 or more carbon atoms. A fluorinated alkyl group has a structure in which some or all of the hydrogen atoms constituting the alkyl group are substituted with fluorine atoms. The alkyl group or fluorinated alkyl group represented by R may have a linear structure, a branched structure, or a cyclic alkyl group or fluorinated alkyl group, but a linear structure is preferred. The alkyl group or fluorinated alkyl group represented by R may have substituents or be unsubstituted, but it is preferred to be unsubstituted. The alkyl group represented by R is, for example, C n H 2n+1 It can be represented by -, where n is an integer greater than or equal to 7. Also, the alkyl fluoride represented by R is, for example, C n H 2n+1The alkyl group represented by - may have a structure in which some or all of the hydrogen atoms constituting the alkyl group are substituted with fluorine atoms. The alkyl group or fluorinated alkyl group represented by R has 7 or more carbon atoms, preferably 8 or more, more preferably 9 or more, even more preferably 10 or more, even more preferably 11 or more, even more preferably 12 or more, and even more preferably 13 or more. Furthermore, the alkyl group or fluorinated alkyl group represented by R has 20 or fewer carbon atoms, more preferably 19 or fewer, and even more preferably 18 or fewer.

[0091] In equation 1, Z + * represents an ammonium cation. The ammonium cation has, in detail, the following structure. In this invention and specification, the asterisk (*) in a formula representing a part of a compound represents the structure of that part and the bond position with an adjacent atom.

[0092] [ka]

[0093] Nitrogen cation of ammonium cation N + and the oxygen anion O in Equation 1 - These can form a salt crosslinking group, creating an ammonium salt structure of an alkyl ester anion represented by formula 1. The presence of a compound having an ammonium salt structure of an alkyl ester anion represented by formula 1 in the first magnetic layer can be confirmed by analyzing the magnetic recording medium using X-ray photoelectron spectroscopy (ESCA; Electron Spectroscopy for Chemical Analysis), infrared spectroscopy (IR; infrared spectroscopy), etc.

[0094] In one form, Z +The ammonium cation represented by can be obtained, for example, by the nitrogen atom of a nitrogen-containing polymer becoming a cation. A nitrogen-containing polymer means a polymer that contains nitrogen atoms. In this invention and specification, the terms "polymer" and "polymer" are used to encompass both homopolymers and copolymers. Nitrogen atoms can be included in one form as atoms constituting the main chain of the polymer, and in another form as atoms constituting the side chain of the polymer.

[0095] One form of nitrogen-containing polymer is polyalkyleneimines. Polyalkyleneimines are ring-opening polymers of alkyleneimines, and are polymers having multiple repeating units represented by the following formula 2.

[0096] [ka]

[0097] In Equation 2, the nitrogen atom N that makes up the main chain is a nitrogen cation N + And so Z in equation 1 + An ammonium cation represented by [formula] can be obtained. Then, with an alkyl ester anion, it can form an ammonium salt structure, for example, as shown below.

[0098] [ka]

[0099] The following provides a more detailed explanation of Equation 2.

[0100] In formula 2, R 1 and R 2 Each of these independently represents a hydrogen atom or an alkyl group, and n1 represents an integer greater than or equal to 2.

[0101] R 1 or R 2Examples of alkyl groups represented by include alkyl groups having 1 to 6 carbon atoms, preferably alkyl groups having 1 to 3 carbon atoms, more preferably a methyl group or an ethyl group, and even more preferably a methyl group. 1 or R 2 The alkyl group represented by is preferably an unsubstituted alkyl group. 1 and R 2 The combinations include a form in which one is a hydrogen atom and the other is an alkyl group, a form in which both are hydrogen atoms, and a form in which both are alkyl groups (identical or different alkyl groups), with the form in which both are hydrogen atoms being preferred. As an alkylene imine that yields a polyalkylene imine, the structure with the fewest number of carbon atoms constituting the ring is ethyleneimine, and the number of carbon atoms in the main chain of the alkylene imine (ethyleneimine) obtained by ring-opening of ethyleneimine is 2. Therefore, n1 in formula 2 is 2 or more. n1 in formula 2 can be, for example, 10 or less, 8 or less, 6 or less, or 4 or less. The polyalkylene imine may be a homopolymer containing only the same structure as the repeating structure represented by formula 2, or it may be a copolymer containing two or more different structures as the repeating structure represented by formula 2. The number-average molecular weight of the polyalkylene imine that can be used to form a compound having an ammonium salt structure of an alkyl ester anion represented by formula 1 can be, for example, 200 or more, preferably 300 or more, and more preferably 400 or more. Furthermore, the number-average molecular weight of the polyalkyleneimine can be, for example, 10,000 or less, preferably 5,000 or less, and more preferably 2,000 or less.

[0102] In the present invention and this specification, average molecular weight (weight-average molecular weight and number-average molecular weight) refers to the value obtained by measuring by gel permeation chromatography (GPC) and converting it to standard polystyrene equivalent. Unless otherwise specified, the average molecular weights shown in the examples described below are values ​​obtained by converting the values ​​measured using GPC under the following measurement conditions to standard polystyrene equivalent (polystyrene equivalent value). GPC device: HLC-8220 (manufactured by Tosoh Corporation) Guard Column: TSKguardcolumn Super HZM-H Columns: TSKgel Super HZ 2000, TSKgel Super HZ 4000, TSKgel Super HZ-M (manufactured by Tosoh Corporation, 4.6mm (inner diameter) x 15.0cm, three types of columns connected in series) Eluent: Contains tetrahydrofuran (THF) and stabilizer (2,6-di-t-butyl-4-methylphenol). Eluent flow rate: 0.35mL / min Column temperature: 40℃ Inlet temperature: 40℃ Refractive Index (RI) measurement temperature: 40℃ Sample concentration: 0.3% by mass Sample injection volume: 10 μL

[0103] Another form of nitrogen-containing polymer is polyallylamine. Polyallylamine is a polymer of allylamine, having multiple repeating units represented by the following formula 3.

[0104] [ka]

[0105] In formula 3, the nitrogen atom N constituting the amino group of the side chain is a nitrogen cation N + And so Z in equation 1 + An ammonium cation represented by [formula] can be obtained. Then, with an alkyl ester anion, it can form an ammonium salt structure, for example, as shown below.

[0106] [ka]

[0107] The weight-average molecular weight of the polyallylamine that can be used to form a compound having an ammonium salt structure of an alkyl ester anion represented by Formula 1 can be, for example, 200 or more, preferably 1,000 or more, and more preferably 1,500 or more. Furthermore, the weight-average molecular weight of the above polyallylamine can be, for example, 15,000 or less, preferably 10,000 or less, and more preferably 8,000 or less.

[0108] The presence of compounds having an ammonium salt structure of an alkyl ester anion represented by Formula 1, specifically compounds with structures derived from polyalkylene imines or polyallylamines, in the first magnetic layer can be confirmed by analyzing the surface of the first magnetic layer using time-of-flight secondary ion mass spectrometry (TOF-SIMS) or the like.

[0109] Compounds having an ammonium salt structure of an alkyl ester anion represented by Formula 1 can be salts of a nitrogen-containing polymer and one or more fatty acids selected from the group consisting of fatty acids having 7 or more carbon atoms and fluorinated fatty acids having 7 or more carbon atoms. The nitrogen-containing polymer that forms the salt can be one or more nitrogen-containing polymers, for example, a nitrogen-containing polymer selected from the group consisting of polyalkylene imines and polyallylamines. The fatty acids that form the salt can be one or more fatty acids selected from the group consisting of fatty acids having 7 or more carbon atoms and fluorinated fatty acids having 7 or more carbon atoms. Fluorinated fatty acids have a structure in which some or all of the hydrogen atoms constituting the alkyl group bonded to the carboxyl group COOH in the fatty acid are replaced with fluorine atoms. For example, the salt formation reaction can easily proceed by mixing the nitrogen-containing polymer and the above fatty acids at room temperature. Room temperature is, for example, about 20-25°C. In one embodiment, the salt formation reaction can be carried out by using one or more nitrogen-containing polymers and one or more of the above fatty acids as components of the filler solution and mixing them in the filler solution preparation step. Furthermore, in one embodiment, before preparing the filler solution, one or more nitrogen-containing polymers and one or more of the above fatty acids are mixed to form a salt, and this salt is then used as a component of the filler solution to prepare it. When mixing the nitrogen-containing polymers and the above fatty acids to form an ammonium salt of the alkyl ester anion represented by Formula 1, the nitrogen atoms constituting the nitrogen-containing polymer may also react with the carboxyl groups of the above fatty acids to form the following structure, and forms including such a structure are also included in the above compound.

[0110] [ka]

[0111] Examples of the above fatty acids include fatty acids having the alkyl group described earlier as R in Formula 1, and fluorinated fatty acids having the fluorinated alkyl group described earlier as R in Formula 1.

[0112] The mixing ratio of the nitrogen-containing polymer used to form the compound having the ammonium salt structure of the alkyl ester anion represented by Formula 1 to the above fatty acids is preferably 10:90 to 90:10, more preferably 20:80 to 85:15, and even more preferably 30:70 to 80:20, as the mass ratio of nitrogen-containing polymer to the above fatty acids. Furthermore, when preparing the filler solution (for each filler solution when preparing multiple filler solutions), the compound having the ammonium salt structure of the alkyl ester anion represented by Formula 1 can be used in an amount of, for example, 1.0 to 20.0 parts by mass, and preferably 1.0 to 10.0 parts by mass, per 100.0 parts by mass of carbon black. Furthermore, when preparing the filler solution (for each filler solution when preparing multiple filler solutions), 0.1 to 10.0 parts by mass of nitrogen-containing polymer can be used per 100.0 parts by mass of carbon black, and preferably 0.5 to 8.0 parts by mass of nitrogen-containing polymer. The above fatty acids can be used in amounts of, for example, 0.05 to 10.0 parts by mass per 100.0 parts by mass of carbon black, and it is preferable to use 0.1 to 5.0 parts by mass.

[0113] The first magnetic layer described above can be provided, for example, directly on a second magnetic layer formed on a non-magnetic support.

[0114] <Nonmagnetic layer> Next, the non-magnetic layer will be described. The magnetic recording medium described above may have a second magnetic layer directly on the surface of a non-magnetic support, or it may have a second magnetic layer on the surface of a non-magnetic support via a non-magnetic layer containing non-magnetic powder. The non-magnetic powder used in the non-magnetic layer may be an inorganic powder or an organic powder. Carbon black can also be used. Examples of inorganic powders include metals, metal oxides, metal carbonates, metal sulfates, metal nitrides, metal carbides, and metal sulfides. These non-magnetic powders are commercially available and can also be manufactured by known methods. For details, see paragraphs 0146 to 0150 of Japanese Patent Application Publication No. 2011-216149. For carbon black that can be used in the non-magnetic layer, see paragraphs 0040 and 0041 of Japanese Patent Application Publication No. 2010-24113. The content (filling rate) of non-magnetic powder in the non-magnetic layer is preferably in the range of 50 to 90% by mass, and more preferably in the range of 60 to 90% by mass, relative to the total mass of the non-magnetic layer.

[0115] The non-magnetic layer contains a binder and may also contain one or more additives. Regarding other details of the binder, additives, etc., of the non-magnetic layer, known technology relating to non-magnetic layers can be applied. Furthermore, regarding, for example, the type and content of the binder, the type and content of the additives, known technology relating to magnetic layers can also be applied.

[0116] In the present invention and this specification, the non-magnetic layer includes a substantially non-magnetic layer containing a small amount of ferromagnetic powder, for example as an impurity or intentionally, along with the non-magnetic powder. A substantially non-magnetic layer is defined as a layer having a remanent magnetic flux density of 10 mT or less, a coercivity of 7.96 kA / m(100 Oe) or less, or a layer having a remanent magnetic flux density of 10 mT or less and a coercivity of 7.96 kA / m(100 Oe) or less. It is preferable that the non-magnetic layer has no remanent magnetic flux density and coercivity.

[0117] <Backcoat layer> In one embodiment, the magnetic recording medium may have a back coat layer containing non-magnetic powder and a binder on the surface side of the non-magnetic support opposite to the surface side having the first and second magnetic layers. In another embodiment, the magnetic recording medium may be a magnetic recording medium without a back coat layer. When the magnetic recording medium has a back coat layer, the non-magnetic powder in the back coat layer is preferably either carbon black or inorganic powder, or both. The back coat layer contains a binder and may also contain one or more additives. Regarding the binders and various additives that may be included in the back coat layer, known technology relating to back coat layers can be applied, as can known technology relating to the formulations of magnetic layers and / or non-magnetic layers. For example, paragraphs 0018 to 0020 of Japanese Patent Application Publication No. 2006-331625 and lines 65 to 38 of column 5 of U.S. Patent No. 7,029,774 can be referenced regarding back coat layers.

[0118] <Nonmagnetic support> Next, we will explain non-magnetic supports (hereinafter also simply referred to as "supports"). Examples of non-magnetic supports include known materials such as biaxially oriented polyethylene terephthalate, polyethylene naphthalate, polyamide, polyamide-imide, and aromatic polyamide. Among these, polyethylene terephthalate, polyethylene naphthalate, and polyamide are preferred. These supports may be pre-treated with corona discharge, plasma treatment, adhesive treatment, heat treatment, etc.

[0119] <Various thicknesses> The thickness of the non-magnetic support is preferably 3.0 to 5.0 μm. The thickness of the first magnetic layer can be optimized depending on the saturation magnetization of the magnetic head used, the head gap length, the bandwidth of the recording signal, etc., for example, it is 0.01 μm to 0.15 μm, preferably 0.02 μm to 0.12 μm, and more preferably 0.03 μm to 0.1 μm, from the viewpoint of high-density recording. The first magnetic layer only needs to be one layer, and the first magnetic layer may be separated into two or more layers having different magnetic properties, and known configurations for multilayer magnetic layers can be applied. When separated into two or more layers, the thickness of the first magnetic layer is the total thickness of these layers. The same applies to the second magnetic layer. The thickness of the second magnetic layer is, for example, 0.1 to 1.5 μm, and preferably 0.1 to 1.0 μm. The thickness of the non-magnetic layer is, for example, 0.1 to 1.5 μm, and preferably 0.1 to 1.0 μm. The thickness of the backcoat layer is preferably 0.9 μm or less, and more preferably 0.1 to 0.7 μm. The various thicknesses mentioned above can be determined, for example, by the following method. After exposing the cross-section of the magnetic recording medium in the thickness direction using an ion beam, the exposed cross-section is observed using a scanning electron microscope or a transmission electron microscope. Various thicknesses can be determined as the arithmetic mean of the thicknesses obtained at any two locations during the cross-sectional observation. Alternatively, various thicknesses can be determined as design thicknesses calculated from manufacturing conditions, etc.

[0120] <Manufacturing process> (Preparation of compositions for each layer) The process of preparing a composition for forming a first magnetic layer, a second magnetic layer, a non-magnetic layer, or a back coat layer typically includes at least a kneading step, a dispersion step, and mixing steps provided before or after these steps as needed. Each individual step may be divided into two or more stages. Components used in the preparation of each layer-forming composition may be added at the beginning or in the middle of any of the steps. As a solvent, one or more of the various solvents commonly used in the manufacture of coated magnetic recording media can be used. For solvents, see, for example, paragraph 0153 of Japanese Patent Application Publication No. 2011-216149. Individual components may also be added in two or more separate steps. For example, a binder may be added in separate steps: a kneading step, a dispersion step, and a mixing step for viscosity adjustment after dispersion. Known manufacturing techniques can be used in various steps to manufacture the above magnetic tape. In the kneading step, it is preferable to use a kneader with strong kneading force, such as an open kneader, a continuous kneader, a pressure kneader, or an extruder. For details of the kneading process, please refer to Japanese Patent Publication No. 1-106338 and Japanese Patent Publication No. 1-79274. Known dispersers can be used. Filtration may be performed by known methods at any stage in preparing each layer-forming composition. Filtration can be performed, for example, by filter filtration. As filters used for filtration, for example, filters with a pore size of 0.01 to 3 μm (e.g., glass fiber filters, polypropylene filters, etc.) can be used.

[0121] The abrasive solution used in preparing the first magnetic layer forming composition is preferably prepared by dispersing it separately from the ferromagnetic powder and filler. The dispersion state of the abrasive in the abrasive solution can be adjusted by whether or not a dispersant is used to improve the dispersibility of the abrasive, the amount of such dispersant used, the processing conditions of the dispersion treatment such as bead dispersion, the processing conditions of the classification treatment such as centrifugation, etc. Adjusting the dispersion state of the abrasive is preferable for controlling the number distribution A. Preferably, the abrasive solution is prepared separately from the ferromagnetic powder and filler as one or more types of abrasive solutions containing an abrasive, a solvent, and preferably a binder, and can be used in preparing the magnetic layer forming composition. Commercially available equipment can be used for the dispersion treatment and classification treatment. The conditions for performing these treatments are not particularly limited and should be set according to the type of equipment used, etc., so that the number distribution A satisfies (1) to (3) described above.

[0122] Furthermore, it is preferable that the filler solution used in the preparation of the first magnetic layer forming composition be prepared by dispersing it separately from the ferromagnetic powder and abrasive. The dispersion state of the filler in the filler solution can be adjusted by the presence or absence of components for improving the dispersibility of the filler, the amount of such components used, the processing conditions of dispersion treatments such as bead dispersion, and the processing conditions of classification treatments such as centrifugation. In one embodiment, one or more nitrogen-containing polymers and one or more of the fatty acids described above are used as components of the filler solution, and the salt formation reaction can be carried out by mixing these in the filler solution preparation step. Alternatively, in one embodiment, one or more nitrogen-containing polymers and one or more of the above fatty acids can be mixed to form a salt before preparing the filler solution, and then this salt can be used as a component of the filler solution to prepare the filler solution. Adjusting the dispersion state of the filler is preferable for controlling the number distribution B. The filler solution can preferably be prepared separately from the ferromagnetic powder and abrasive as one or more types of abrasive solutions containing a filler, a solvent, and preferably a binder, and used in the preparation of the magnetic layer forming composition. Commercially available equipment can be used for stirring, dispersion, and classification. The conditions for these processes are not particularly limited and should be set according to the type of equipment used, etc., so that the number distribution B satisfies the conditions (4) to (6) described above.

[0123] Regarding the dispersion treatment of the first magnetic layer forming composition, in one embodiment, the dispersion treatment of the ferromagnetic powder is performed in two stages. In the first stage of dispersion treatment, coarse aggregates of the ferromagnetic powder are broken up, and then a second stage of dispersion treatment can be performed in which the collision energy applied to the ferromagnetic powder particles by collision with dispersed beads is smaller than that of the first stage of dispersion treatment. It is believed that this dispersion treatment can achieve both improved dispersibility of the ferromagnetic powder and suppression of chipping (partial loss of particles). This is also preferable for controlling the vertical angular ratio, which will be described later.

[0124] An example of the two-stage dispersion process described above is a dispersion process that includes a first step of obtaining a dispersion by dispersing ferromagnetic powder, a binder, and a solvent in the presence of first dispersion beads, and a second step of dispersing the dispersion obtained in the first step in the presence of second dispersion beads having a smaller bead diameter and density than the first dispersion beads. The above dispersion process will be further explained below.

[0125] To improve the dispersibility of the ferromagnetic powder in the first magnetic layer forming composition, it is preferable to perform the first and second steps described above as a dispersion treatment before mixing the ferromagnetic powder with other powder components. For example, it is preferable to perform the first and second steps described above as a dispersion treatment of a liquid (magnetic liquid) containing ferromagnetic powder, a binder, a solvent, and optionally added additives, before mixing with an abrasive and a filler.

[0126] The bead diameter of the second dispersion bead is preferably 1 / 100 or less of the bead diameter of the first dispersion bead, and more preferably 1 / 500 or less. The bead diameter of the second dispersion bead can also be, for example, 1 / 10000 or more of the bead diameter of the first dispersion bead. However, it is not limited to this range. For example, the bead diameter of the second dispersion bead is preferably in the range of 80 to 1000 nm. On the other hand, the bead diameter of the first dispersion bead can be, for example, in the range of 0.2 to 1.0 mm. In this invention and specification, the bead diameter is a value measured by the same method as the method for measuring the average particle size of the powder described above.

[0127] The second step described above is preferably carried out under conditions in which the second dispersion beads are present in an amount of 10 times or more the amount of the ferromagnetic hexagonal ferrite powder, and more preferably in an amount of 10 to 30 times the amount. On the other hand, it is preferable that the amount of first dispersed beads in the first stage is also within the above range.

[0128] The second dispersion beads are beads having a lower density than the first dispersion beads. "Density" is obtained by dividing the mass of the dispersion beads (unit: g) by the volume (unit: cm 3 ). The measurement is performed by the Archimedes method. The density of the second dispersion beads is preferably 3.7 g / cm 3 or less, more preferably 3.5 g / cm 3 or less. The density of the second dispersion beads may be, for example, 2.0 g / cm 3 or more, or may be less than 2.0 g / cm 3 . Preferred second dispersion beads in terms of density include diamond beads, silicon carbide beads, silicon nitride beads, etc., and preferred second dispersion beads in terms of density and hardness include diamond beads. On the other hand, as the first dispersion beads, dispersion beads having a density exceeding 3.7 g / cm 3 are preferred, dispersion beads having a density of 3.8 g / cm 3 or more are more preferred, and dispersion beads having a density of 4.0 g / cm 3 or more are even more preferred. The density of the first dispersion beads may be, for example, 7.0 g / cm 3 or less, or may exceed 7.0 g / cm 3 . As the first dispersion beads, it is preferable to use zirconia beads, alumina beads, etc., and it is more preferable to use zirconia beads.

[0129] The dispersion time is not particularly limited and may be set according to the type of disperser used, etc.

[0130] (Coating step) The second magnetic layer can be formed, for example, by directly coating a composition for forming the second magnetic layer on the surface of a non-magnetic support, or by applying a multilayer coating sequentially or simultaneously with a composition for forming a non-magnetic layer. The first magnetic layer can be formed, for example, by applying a multilayer coating of a composition for forming the first magnetic layer sequentially or simultaneously with a composition for forming the second magnetic layer. The backcoat layer can be formed by applying a backcoat layer forming composition to the surface of the non-magnetic support opposite to the surface having (or subsequently having) a first magnetic layer, a second magnetic layer, or a further non-magnetic layer. For details on the coating process for forming each layer, refer to paragraph 0066 of Japanese Patent Publication No. 2010-231843.

[0131] (Other processes) For other processes for manufacturing magnetic recording media, known techniques can be applied. For details on these processes, see, for example, paragraphs 0067 to 0070 of Japanese Patent Publication No. 2010-231843. For example, the coated layer of the first magnetic layer forming composition can be subjected to orientation treatment in an orientation zone while the coated layer is wet. Various known techniques, including those described in paragraph 0052 of Japanese Patent Application Publication No. 2010-24113, can be applied to the orientation treatment. For example, vertical orientation treatment can be performed by known methods such as using opposite-polarity opposing magnets. In the orientation zone, the drying rate of the coated layer can be controlled by the temperature and airflow of the drying air and / or the transport speed in the orientation zone. Alternatively, the coated layer may be pre-dried before being transported to the orientation zone.

[0132] A magnetic recording medium according to one aspect of the present invention may be a tape-shaped magnetic recording medium (magnetic tape) or a disk-shaped magnetic recording medium (magnetic disk). The magnetic tape is housed in, for example, a magnetic tape cartridge, and the magnetic tape cartridge is mounted on a magnetic recording and playback device. For example, a long roll of magnetic tape raw material obtained through various processes can be cut (slit) to the width of the magnetic tape to be wound onto the magnetic tape cartridge using a known cutting machine. The above width is determined according to standards, for example, 1 / 2 inch. 1 inch = 12.65 mm.

[0133] (Servo pattern) A servo pattern can be formed on a magnetic recording medium by known methods to enable head tracking in a magnetic recording and playback device. "Formation of a servo pattern" can also be referred to as "recording of a servo signal." The formation of a servo pattern will be explained below using magnetic tape as an example.

[0134] Servo patterns are typically formed along the longitudinal direction of the magnetic tape. Examples of control methods that utilize servo signals (servo control) include timing-based servo (TBS), amplitude servo, and frequency servo.

[0135] As indicated in ECMA (European Computer Manufacturers Association) - 319 (June 2001), magnetic tapes conforming to the LTO (Linear Tape-Open) standard (commonly called "LTO tapes") employ a timing-based servo system. In this timing-based servo system, the servo pattern is composed of multiple pairs of non-parallel magnetic stripes (also called "servo stripes") arranged continuously along the longitudinal direction of the magnetic tape. A servo system is a system that performs head tracking using servo signals. In this invention and specification, "timing-based servo pattern" refers to a servo pattern that enables head tracking in a timing-based servo system. As described above, the reason why the servo pattern is composed of pairs of non-parallel magnetic stripes is to inform the servo signal reading element of its position as it passes over the servo pattern. Specifically, the above pairs of magnetic stripes are formed so that their spacing changes continuously along the width direction of the magnetic tape, and the servo signal reading element can determine the relative position between the servo pattern and the servo signal reading element by reading this spacing. This relative position information enables tracking of the data track. Therefore, multiple servo tracks are typically set up on the servo pattern, aligned with the width of the magnetic tape.

[0136] A servo band consists of a servo pattern that runs continuously along the longitudinal direction of the magnetic tape. Typically, multiple servo bands are provided on a magnetic tape. For example, in an LTO tape, there are five. The area between two adjacent servo bands is the data band. A data band consists of multiple data tracks, each corresponding to a servo track.

[0137] In one embodiment, as shown in Japanese Patent Publication No. 2004-318983, each servo band has embedded information indicating the servo band number (also called "servo band ID (identification)" or "UDIM (Unique DataBand Identification Method) information"). This servo band ID is recorded by shifting a specific pair of servo stripes within a servo band so that its position is displaced relative to the longitudinal direction of the magnetic tape. Specifically, the method of shifting a specific pair of servo stripes is changed for each servo band. As a result, the recorded servo band ID is unique for each servo band, so that a servo band can be uniquely identified by reading it with a servo signal reading element.

[0138] Furthermore, one method for uniquely identifying a servo band is the staggered method, as described in ECMA-319 (June 2001). In this staggered method, a group of non-parallel magnetic stripes (servo stripes) arranged continuously along the longitudinal direction of the magnetic tape are recorded in a way that they are shifted along the longitudinal direction of the magnetic tape for each servo band. Since the combination of this shift between adjacent servo bands is unique across the entire magnetic tape, it is possible to uniquely identify a servo band when reading the servo pattern with two servo signal reading elements.

[0139] Furthermore, each servo band typically contains embedded information indicating its position along the longitudinal direction of the magnetic tape (also known as "LPOS (Longitudinal Position) information"), as described in ECMA-319 (June 2001). This LPOS information, like the UDIM information, is recorded by shifting the positions of a pair of servo stripes along the longitudinal direction of the magnetic tape. However, unlike the UDIM information, the same signal is recorded for each servo band in this LPOS information.

[0140] It is also possible to embed information other than the UDIM and LPOS information mentioned above into the servo bands. In this case, the embedded information may be different for each servo band, like the UDIM information, or it may be common to all servo bands, like the LPOS information. Furthermore, methods other than those described above can be used to embed information in the servo band. For example, a predetermined code may be recorded by thinning out a predetermined pair from a group of servo stripes.

[0141] A servo pattern forming head is called a servo light head. A servo light head typically has a pair of gaps corresponding to the pair of magnetic stripes mentioned above, for each servo band. Typically, a core and a coil are connected to each pair of gaps, and by supplying current pulses to the coils, the magnetic field generated in the core can create a leakage magnetic field in the pair of gaps. When forming a servo pattern, by inputting current pulses while running a magnetic tape over the servo light head, the magnetic patterns corresponding to the pair of gaps are transferred to the magnetic tape, thereby forming the servo pattern. The width of each gap can be appropriately set according to the density of the servo pattern to be formed. For example, the width of each gap can be set to 1 μm or less, 1 to 10 μm, 10 μm or more, etc.

[0142] Before forming a servo pattern on a magnetic tape, it is usually demagnetized (erased). This erasing process can be performed by applying a uniform magnetic field to the magnetic tape using a DC or AC magnet. There are two types of erasing: DC (Direct Current) erasing and AC (Alternating Current) erasing. AC erasing is performed by gradually reducing the strength of the magnetic field while reversing the direction of the magnetic field applied to the magnetic tape. On the other hand, DC erasing is performed by applying a unidirectional magnetic field to the magnetic tape. There are two further methods of DC erasing. The first method is horizontal DC erasing, which applies a unidirectional magnetic field along the longitudinal direction of the magnetic tape. The second method is vertical DC erasing, which applies a unidirectional magnetic field along the thickness direction of the magnetic tape. The erasing process may be performed on the entire magnetic tape or on each servo band of the magnetic tape.

[0143] The direction of the magnetic field of the formed servo pattern is determined according to the direction of the erase. For example, when a magnetic tape is horizontally DC erased, the servo pattern is formed such that the direction of the magnetic field is opposite to the direction of the erase. This makes it possible to increase the output of the servo signal obtained by reading the servo pattern. As shown in Japanese Patent Application Publication No. 2012-53940, when a magnetic pattern using the above gap is transferred to a vertically DC erased magnetic tape, the servo signal obtained by reading the formed servo pattern is in the shape of a single-pole pulse. On the other hand, when a magnetic pattern using the above gap is transferred to a horizontally DC erased magnetic tape, the servo signal obtained by reading the formed servo pattern is in the shape of a double-pole pulse.

[0144] <Vertical squareness ratio> In one embodiment, the vertical aspect ratio of the magnetic recording medium is preferably, for example, 0.60 or more. The upper limit of the aspect ratio is, in principle, 1.00 or less. The vertical aspect ratio of the magnetic recording medium can be 1.00 or less, and can be, for example, 0.95 or less, 0.90 or less, 0.85 or less, or 0.80 or less. A larger value for the vertical aspect ratio of the magnetic recording medium is preferable from the viewpoint of further improving electromagnetic conversion characteristics. The vertical aspect ratio of the magnetic recording medium can be controlled by known methods such as performing vertical orientation processing.

[0145] In the present invention and this specification, "vertical angular ratio" refers to the angular ratio measured in the vertical direction of the magnetic recording medium. With respect to the angular ratio, "vertical direction" refers to the direction perpendicular to the surface of the first magnetic layer, and can also be referred to as the thickness direction. In the present invention and this specification, the vertical angular ratio is determined by the following method. A sample piece of a size suitable for introduction into a vibrating magnetometer is cut from the magnetic recording medium to be measured. Using a vibrating magnetometer, a magnetic field is applied to this sample piece perpendicular to the sample piece (in the direction perpendicular to the surface of the first magnetic layer) at a maximum applied magnetic field of 3979 kA / m, a measurement temperature of 296 K, and a magnetic field sweep speed of 8.3 kA / m / sec, and the magnetization intensity of the sample piece with respect to the applied magnetic field is measured. The measured magnetization intensity is obtained as a value after demagnetization correction and after subtracting the magnetization of the sample probe of the vibrating magnetometer as background noise. When the magnetization intensity at the maximum applied magnetic field is Ms and the magnetization intensity at zero applied magnetic field is Mr, the squareness ratio SQ is calculated as SQ = Mr / Ms. The measurement temperature refers to the temperature of the sample piece, and the temperature of the sample piece can be set to the measurement temperature by setting the ambient temperature around the sample piece to the measurement temperature, thereby achieving thermal equilibrium.

[0146] [Magnetic tape cartridge] One aspect of the present invention relates to a magnetic tape cartridge including the tape-shaped magnetic recording medium (i.e., magnetic tape).

[0147] Details of the magnetic tape included in the above magnetic tape cartridge are as described above.

[0148] In a magnetic tape cartridge, the magnetic tape is generally housed inside the cartridge body, wound onto a reel. The reel is rotatably mounted inside the cartridge body. Two types of magnetic tape cartridges are widely used: single-reel cartridges, which have one reel inside the cartridge body, and double-reel cartridges, which have two reels inside the cartridge body. When a single-reel magnetic tape cartridge is mounted in a magnetic tape device for recording and / or playing back data onto magnetic tape, the magnetic tape is pulled out of the cartridge and wound onto the reel on the magnetic tape device. A magnetic head is positioned along the magnetic tape transport path from the magnetic tape cartridge to the take-up reel. The magnetic tape is fed and wound between the reel on the magnetic tape cartridge side (supply reel) and the reel on the magnetic tape device side (take-up reel). During this process, the magnetic head and the magnetic layer surface of the magnetic tape come into contact and slide against each other, enabling data recording and / or playback. In contrast, a dual-reel magnetic tape cartridge has both a supply reel and a take-up reel housed inside the magnetic tape cartridge. The magnetic tape cartridge may be either a single-reel or dual-reel type. The magnetic tape cartridge may include a magnetic recording medium (magnetic tape) according to one aspect of the present invention, and prior art may be applied for other aspects.

[0149] [Magnetic recording and playback device] One aspect of the present invention relates to a magnetic recording and playback apparatus including the magnetic recording medium described above. In the magnetic recording and playback apparatus, recording data onto the magnetic recording medium and / or playback of data recorded on the magnetic recording medium can be performed, for example, by bringing the magnetic layer surface of the magnetic recording medium into contact with a magnetic head and sliding it. For example, the magnetic recording and playback apparatus may detachably include a magnetic tape cartridge according to one aspect of the present invention.

[0150] <Magnetic head> In the present invention and this specification, "magnetic recording and reproduction device" means a device capable of recording data onto a magnetic recording medium and reproducing data recorded on a magnetic recording medium. Such a device is generally called a drive. The magnetic head included in the magnetic recording and reproduction device may be a recording head capable of recording data onto a magnetic recording medium, or a reproduction head capable of reproducing data recorded on a magnetic recording medium. In one embodiment, the magnetic recording and reproduction device may include both a recording head and a reproduction head as separate magnetic heads. In another embodiment, the magnetic head included in the magnetic recording and reproduction device may have a configuration in which both a recording element and a reproduction element are provided on a single magnetic head. The recording head can be a magnetic head for vertical recording or a magnetic head for in-plane recording. For magnetic heads for vertical recording and magnetic heads for in-plane recording, known technologies related to those heads can be applied. The magnetic recording medium can exhibit excellent electromagnetic conversion characteristics when reproducing data recorded using the vertical recording method. Therefore, it is preferable that the recording head included in the magnetic recording and reproduction device is a magnetic head for vertical recording. As the playback head, a magnetic head (MR head) that includes a magnetoresistive (MR) element as a playback element capable of sensitively reading information recorded on a magnetic recording medium is preferred. Various known MR heads (e.g., GMR (Giant Magnetoresistive) head, TMR (Tunnel Magnetoresistive) head, etc.) can be used as the MR head. In addition, the magnetic head that records and / or plays back data may include a servo pattern reading element. Alternatively, a magnetic head (servo head) equipped with a servo pattern reading element may be included in the magnetic recording and / or playback device as a separate head from the magnetic head that records and / or plays back data. For example, a magnetic head that records and / or plays back recorded data (hereinafter also referred to as the "recording and playback head") may include two servo signal reading elements, and each of the two servo signal reading elements can simultaneously read two adjacent servo bands separated by a data band. One or more data elements can be placed between the two servo signal reading elements. Elements for recording data (recording elements) and elements for reproducing data (reproduction elements) are collectively referred to as "data elements."

[0151] When recording data and / or playing back recorded data, head tracking using servo signals can be performed first. That is, by making the servo signal reading element follow a predetermined servo track, the data element can be controlled to pass over the target data track. The movement of the data track is achieved by changing the servo track read by the servo signal reading element in the tape width direction. Furthermore, the recording / playback head can also record and / or play back data on other data bands. In this case, the servo signal reading element can be moved to a predetermined servo band using the UDIM information described earlier, and tracking for that servo band can be started.

[0152] Figure 1 shows an example of the arrangement of data bands and servo bands in a magnetic tape. In Figure 1, multiple servo bands 1 are arranged on the first magnetic layer of the magnetic tape MT, sandwiched between guide bands 3. Multiple regions 2 sandwiched between two servo bands are the data bands. A servo pattern is a magnetized region that can be formed by magnetizing a specific region of the first magnetic layer with a servo light head. The region magnetized by the servo light head (the position where the servo pattern is formed) is defined by the standard. For example, in the industry standard LTO Ultrium format tape, multiple servo patterns inclined with respect to the tape width direction are formed on the servo bands during magnetic tape manufacturing, as shown in Figure 2. More specifically, in Figure 2, the servo frame SF on the servo band 1 consists of a servo subframe 1 (SSF1) and a servo subframe 2 (SSF2). The servo subframe 1 consists of an A-burst (indicated as A in Figure 2) and a B-burst (indicated as B in Figure 2). The A-burst consists of servo patterns A1 to A5, and the B-burst consists of servo patterns B1 to B5. On the other hand, servo subframe 2 consists of C-bursts (indicated as C in Figure 2) and D-bursts (indicated as D in Figure 2). C-bursts consist of servo patterns C1 to C4, and D-bursts consist of servo patterns D1 to D4. These 18 servo patterns are arranged in sets of 5 and 4 on subframes in a 5, 5, 4, 4 sequence, and are used to identify the servo frames. Figure 2 shows one servo frame for illustrative purposes. However, in reality, in the magnetic layer of a magnetic tape where timing-based servo head tracking is performed, multiple servo frames are arranged in the direction of travel in each servo band. In Figure 2, the arrows indicate the direction of travel. For example, LTO Ultrium format tape typically has more than 5000 servo frames per meter of tape length in each servo band of the magnetic layer. [Examples]

[0153] The present invention will be described below based on examples. However, the present invention is not limited to the embodiments shown in the examples. Unless otherwise specified, the terms "parts" and "%" below refer to "parts by mass" and "% by mass," respectively. Furthermore, unless otherwise specified, the processes and evaluations described below were carried out in an environment with an ambient temperature of 23°C ± 1°C. Furthermore, "eq" below refers to the equivalent, a unit that cannot be converted to the SI unit system.

[0154] [Preparation of ferromagnetic powder for the second magnetic layer] <Preparation of ferromagnetic powder A> The raw materials shown in Table 1 were weighed out in the proportions (based on mol%) shown in Table 1 and mixed in a mixer to obtain a raw material mixture. The obtained raw material mixture was melted in a platinum crucible at a melting temperature of 1480°C. While stirring the molten material, the outlet at the bottom of the platinum crucible was heated, and the molten material was dispensed in a rod shape at a rate of approximately 3 g / second. The dispensed material was rolled and rapidly cooled using water-cooled twin rolls to obtain an amorphous material. 280g of the obtained amorphous material was placed in an electric furnace, and the furnace temperature was raised to the crystallization temperature shown in Table 1. This temperature was maintained for 5 hours to precipitate (crystallize) ferromagnetic powder particles. Next, the crystalline material containing the precipitated particles was coarsely ground in a mortar. 1000g of 1mm diameter zirconia beads and 800ml of 1% acetic acid were added to the glass bottle containing the coarsely ground material, and the mixture was dispersed in a paint shaker for 3 hours. The dispersion was then separated from the beads and placed in a stainless steel beaker. The dispersion was allowed to stand at a liquid temperature of 80°C for 3 hours to dissolve the glass components (CaB2O4), then it was precipitated using a centrifuge and washed by repeated decantation. Finally, it was dried in a dryer at an internal atmosphere temperature of 110°C for 6 hours to obtain ferromagnetic powder.

[0155] <Preparation of ferromagnetic powders B-E, X, and Y> Each ferromagnetic powder was obtained by the method described for the preparation of ferromagnetic powder A, except that the composition of the raw material mixture and the crystallization temperature were changed as shown in Table 1.

[0156] The ferromagnetic powders A-E, X, and Y obtained above were confirmed to be spinel ferrites by scanning with CuKα rays at a voltage of 45kV and intensity of 40mA, and measuring the X-ray diffraction patterns under the following conditions (X-ray diffraction analysis). Each of the ferromagnetic powders obtained above exhibited the crystalline structure of spinel ferrite, and the crystalline phase detected by X-ray diffraction analysis was a single phase of spinel ferrite. PANalytical X'Pert Pro diffractometer, PIXcel detector Soller slit for incident and diffracted beams: 0.017 radians Fixed angle of dispersion slit: 1 / 4 degree Mask: 10mm Scatter prevention slit: 1 / 4 degree Measurement mode: Continuous Measurement time per step: 3 seconds Measurement speed: 0.017 degrees per second Measurement step: 0.05 degrees

[0157] <Measurement of average particle size of ferromagnetic powder> The average particle size of each of the ferromagnetic powders A-E, X, and Y obtained above was determined using the method described earlier. A Hitachi H-9000 transmission electron microscope was used, and Carl Zeiss KS-400 image analysis software was used for image analysis.

[0158] <Measurement of coercivity (Hc) of ferromagnetic powder> The coercivity Hc of each of the ferromagnetic powders A-E, X, and Y obtained above was determined using the method described earlier. A Vibrating Sample Magnetometer (VSM) manufactured by Toei Kogyo Co., Ltd. was used as the vibrating sample magnetometer.

[0159] The results are shown in Table 1.

[0160] [Table 1]

[0161] [Preparation of abrasive solution] <Preparation of Abrasive Solution A> To 100.0 parts of the abrasive (alumina powder) shown in Table 2, 31.3 parts of a 32% solution of 2,3-dihydroxynaphthalene (manufactured by Tokyo Chemical Industry Co., Ltd.) in the amount shown in Table 2, a polyester polyurethane resin having SO3Na groups as polar groups (Toyobo Co., Ltd. UR-4800 (amount of polar groups: 80 meq / kg)) (solvent: mixed solvent of methyl ethyl ketone and toluene) was mixed, and 570.0 parts of a mixture of methyl ethyl ketone and cyclohexanone in a 1:1 (mass ratio) solvent was mixed, and the mixture was dispersed in the presence of zirconia beads (bead diameter: 0.1 mm) using a paint shaker for the time (bead dispersion time) shown in Table 2. After dispersion, the dispersion was separated from the beads using a mesh, and the resulting dispersion was subjected to centrifugation. The centrifugation was performed using a Hitachi Koki CS150GXL centrifuge (with a Hitachi Koki S100AT6 rotor) at the rotation speed (rpm; rotations per minute) and time (centrifugation time) shown in Table 2. This centrifugation process resulted in the deposition of relatively large particles and the dispersion of relatively small particles into the supernatant. Subsequently, the supernatant liquid was collected by decantation. This collected liquid is called "Abrasive Solution A".

[0162] <Preparation of abrasive solutions B and C> Except for the changes made to various items as shown in Table 2, polishing solutions B and C were prepared using the same method as described for the preparation of polishing solution A.

[0163] [Table 2]

[0164] [Preparation of filler solution] <Preparation of filler solution D> To 100.0 parts of the filler (carbon black) shown in Table 3, 570.0 parts of polyethyleneimine, stearic acid, and a 1:1 mass ratio mixture of methyl ethyl ketone and cyclohexanone as solvent were mixed and dispersed in the presence of zirconia beads (bead diameter: 0.1 mm) using a paint shaker for the time (bead dispersion time) shown in Table 3. After dispersion, the dispersion was separated from the beads using a mesh, and the resulting dispersion was subjected to centrifugation. The centrifugation was performed using a Hitachi Koki CS150GXL centrifuge (with a Hitachi Koki S100AT6 rotor) at the rotation speed (rpm; rotations per minute) and time (centrifugation time) shown in Table 3. This centrifugation process resulted in the deposition of relatively large particles and the dispersion of relatively small particles into the supernatant. Subsequently, the supernatant liquid was collected by decantation. This collected liquid is called "filler liquid D".

[0165] The polyethyleneimine mentioned above is a commercially available product manufactured by Nippon Shokubai Co., Ltd. (number average molecular weight 600).

[0166] <Preparation of filler solutions E-G> Aside from the changes made to various items as shown in Table 3, filler solutions E to G were prepared using the method described for the preparation of filler solution D.

[0167] [Table 3]

[0168] [Example 1] <Preparation of the first magnetic layer forming composition> (Magnetic liquid) Ferromagnetic powder: 100.0 parts Hexagonal barium ferrite powder with an average particle size (average plate diameter) of 21 nm (labeled "BaFe" in Table 4). Oleic acid: 2.0 parts Vinyl chloride copolymer (MR-104, manufactured by Nippon Zeon): 10.0 parts SO3Na group-containing polyurethane resin: 4.0 parts (Weight average molecular weight 70000, SO3Na group: 0.07meq / g) Amine polymer (DISPERBYK-102, manufactured by BIC Chemie): 6.0 parts Methyl ethyl ketone: 150.0 parts Cyclohexanone: 150.0 parts (Abrasive solution) Use the abrasive solution shown in Table 4, ensuring that the amount of abrasive in the solution is as shown in Table 4. (Filler liquid) Use the filler solution shown in Table 4, ensuring that the amount of filler in the filler solution is as shown in Table 4. (Other ingredients) Stearic acid: 3.0 parts Stearic acid amide: 0.3 parts Butyl stearate: 6.0 parts Methyl ethyl ketone: 110.0 parts Cyclohexanone: 110.0 parts Polyisocyanate (Tosoh Corporation's Coronate® L): 3.0 parts

[0169] (Preparation method) The various components of the above magnetic liquid are processed using a batch-type vertical sand mill to produce zirconia beads with a diameter of 0.5 mm (first dispersion bead, density 6.0 g / cm³). 3 Dispersion A was prepared by dispersing the ferromagnetic powder for 24 hours using (first step), and then filtering it using a filter with a pore size of 0.5 μm. The amount of zirconia beads used was 10 times the mass of the ferromagnetic powder. Subsequently, dispersion A is processed using a batch-type vertical sand mill to produce diamond beads with a diameter of 500 nm (second dispersion bead, density 3.5 g / cm³). 3 The mixture was dispersed for 1 hour using (second stage), and a dispersion (dispersion B) was prepared by separating the diamond beads using a centrifuge. The amount of diamond beads used was 10 times the mass of the ferromagnetic powder. The dispersion liquid B obtained above, the abrasive liquid, the filler liquid, and the other components described above were introduced into a dissolver stirrer and stirred at a peripheral speed of 10 m / sec for 360 minutes. Thereafter, after subjecting the mixture to ultrasonic dispersion treatment at a flow rate of 7.5 kg / min for 60 minutes using a flow-type ultrasonic disperser, it was filtered three times through a filter with a pore size of 0.3 μm to prepare a composition for forming a first magnetic layer.

[0170] <Preparation of the composition for forming a second magnetic layer> The various components of the following composition for forming a second magnetic layer were dispersed for 24 hours using zirconia beads with a bead diameter of 0.1 mm by a batch-type vertical sand mill, and then filtered using a filter having a pore size of 0.5 μm to prepare a composition for forming a second magnetic layer.

[0171] Ferromagnetic powder (see Table 4): 100.0 parts SO3Na group-containing polyurethane resin: 18.0 parts (weight average molecular weight 70000, SO3Na group content 0.2 meq / g) Stearic acid: 1.0 part Cyclohexanone: 300.0 parts Methyl ethyl ketone: 300.0 parts

[0172] <Preparation of the composition for forming a non-magnetic layer> The various components of the following composition for forming a non-magnetic layer were dispersed for 24 hours using zirconia beads with a bead diameter of 0.1 mm by a batch-type vertical sand mill, and then filtered using a filter having a pore size of 0.5 μm to prepare a composition for forming a non-magnetic layer.

[0173] Non-magnetic inorganic powder α-iron oxide: 100.0 parts (average particle size 10 nm, BET specific surface area 75 m 2 / g) Carbon black: 25.0 parts (average particle size 20 nm) SO3Na group-containing polyurethane resin: 18.0 parts (weight average molecular weight 70000, SO3Na group content 0.2 meq / g) Stearic acid: 1.0 part Cyclohexanone: 300.0 parts Methyl ethyl ketone: 300.0 parts

[0174] <Preparation of composition for forming backcoat layer> Of the various components of the backcoat layer forming composition described below, all components except for the lubricants (stearic acid and butyl stearate), polyisocyanate, and 200.0 parts of cyclohexanone were kneaded and diluted using an open kneader. Then, using 1 mm diameter zirconia beads, the mixture was subjected to a dispersion process using a horizontal bead mill disperser with a bead filling rate of 80% by volume, a rotor tip peripheral speed of 10 m / sec, and a residence time of 2 minutes per pass, for 12 passes. After that, the remaining components were added and stirred with a dissolver stirrer, and the resulting dispersion was filtered using a filter with a pore size of 1 μm to prepare the backcoat layer forming composition.

[0175] Non-magnetic inorganic powder α-iron oxide: 80.0 parts (Average particle size 0.15 μm, BET specific surface area 52 m²) 2 / g) Carbon Black: 20.0 parts (Average particle size: 20 nm) Vinyl chloride copolymer: 13.0 parts Sulfonate base-containing polyurethane resin: 6.0 parts Phenylephosphonic acid: 3.0 parts Cyclohexanone: 155.0 parts Methyl ethyl ketone: 155.0 parts Stearic acid: 3.0 parts Butyl stearate: 3.0 parts Polyisocyanate: 5.0 parts Cyclohexanone: 200.0 parts

[0176] <Method for manufacturing magnetic tape and magnetic tape cartridges> A non-magnetic layer was formed on the surface of a polyethylene naphthalate support with a thickness of 4.1 μm by applying and drying the non-magnetic layer-forming composition prepared above, such that the thickness after drying was 0.7 μm. Next, the second magnetic layer-forming composition prepared above was applied to the non-magnetic layer and dried to form a second magnetic layer, with a drying thickness of 0.1 μm. Next, the first magnetic layer-forming composition prepared above was applied to the second magnetic layer to form a coating layer, with a drying thickness of 0.1 μm. Subsequently, while the coating layer of the first magnetic layer-forming composition was still wet, a magnetic field with a magnetic field strength of 0.3T was applied perpendicularly to the surface of the coating layer to perform a vertical orientation treatment, and then it was dried to form the first magnetic layer. Subsequently, the backcoat layer-forming composition prepared above was applied and dried to the surface of the support opposite to the surface on which the non-magnetic layer, the second magnetic layer, and the first magnetic layer were formed, so that the thickness after drying was 0.3 μm, thereby forming a backcoat layer. Subsequently, a surface smoothing treatment (calendering treatment) was performed using a calendering roll composed solely of metal rolls, at a speed of 100 m / min, a linear pressure of 300 kg / cm, and a calendering temperature (surface temperature of the calendering roll) of 90°C. Subsequently, after heat treatment at an ambient temperature of 70°C for 36 hours, the long magnetic tape roll was slit into 1 / 2-inch width strips to obtain magnetic tape. By recording servo signals on the first magnetic layer of the obtained magnetic tape using a commercially available servo writer, a magnetic tape was obtained having data bands, servo bands, and guide bands arranged according to the LTO (Linear Tape-Open) Ultrium format, and having a servo pattern (timing-based servo pattern) on the servo bands arranged and shaped according to the LTO Ultrium format. The servo pattern thus formed conforms to the descriptions in JIS (Japanese Industrial Standards) X6175:2006 and Standard ECMA-319 (June 2001). The obtained magnetic tape (tape length: 960m) was housed in a single-reel magnetic tape cartridge. The above process was repeated to fabricate two magnetic tape cartridges. One magnetic tape cartridge was used for the measurement of the following number distributions A and B, and the other magnetic tape cartridge was used for the evaluation of the following electromagnetic conversion characteristics.

[0177] That a compound containing an ammonium salt structure of an alkyl ester anion represented by Formula 1, which is formed by polyethyleneimine and stearic acid, is contained in the first magnetic layer of the magnetic tape can be confirmed by the following method. A sample is cut out from the magnetic tape, and X-ray photoelectron spectroscopy is performed using an ESCA apparatus on the surface of the magnetic layer (measurement region: 300 μm × 700 μm). Specifically, wide scan measurement is performed by the ESCA apparatus under the following measurement conditions. In the measurement results, peaks are confirmed at the positions of the binding energy of the ester anion and the binding energy of the ammonium cation. Apparatus: AXIS-ULTRA manufactured by Shimadzu Corporation Excitation X-ray source: Monochromatic Al-Kα ray Scan range: 0 - 1200 eV Pass energy: 160 eV [[ID=I5]] Energy resolution: 1 eV / step Capture time: 100 ms / step Integration times: 5 [[ID=2I]] Also, a 3-cm long sample piece is cut out from the magnetic tape, and ATR-FT-IR (Attenuated total reflection-fourier transform-infrared spectrometer) measurement (reflection method) is performed on the surface of the first magnetic layer. In the measurement results, the wave numbers (1540 cm - or 1430 cm -1 or) corresponding to the absorption of COO -1 , and the wave numbers (2400 cm -1 ) corresponding to the absorption of the ammonium cation show absorption.

[0178] [Examples 2 to 28, Comparative Examples 1 to 25] A magnetic tape cartridge was produced in the same manner as described in Example 1, except that the items shown in Table 4 were changed as described in Table 4. For the comparative example described as "without second magnetic layer" in Table 4, the second magnetic layer was not formed, and the first magnetic layer was formed on the non-magnetic layer.

[0179] In Table 4, "SrFe1" indicates hexagonal strontium ferrite powder produced as follows. Weighed 1707 g of SrCO3, 687 g of H3BO3, 1120 g of Fe2O3, 45 g of Al(OH)3, 24 g of BaCO3, 13 g of CaCO3, and 235 g of Nd2O3, and mixed them in a mixer to obtain a raw material mixture. The obtained raw material mixture was melted in a platinum crucible at a melting temperature of 1390 °C, and while stirring the melt, the outlet provided at the bottom of the platinum crucible was heated, and the melt was discharged in a rod shape at about 6 g / second. The discharged liquid was rolled and rapidly cooled with a water-cooled double roller to produce an amorphous body. Charged 280 g of the produced amorphous body into an electric furnace, heated it to 635 °C (crystallization temperature) at a heating rate of 3.5 °C / min, and held it at the same temperature for 5 hours to precipitate (crystallize) hexagonal strontium ferrite particles. Next, the crystallized product obtained above containing hexagonal strontium ferrite particles was coarsely pulverized in a mortar, 1000 g of zirconia beads with a particle size of 1 mm and 800 mL of an acetic acid aqueous solution with a concentration of 1% were added to a glass bottle, and dispersion treatment was performed with a paint shaker for 3 hours. Then, the obtained dispersion was separated from the beads and placed in a stainless steel beaker. After the dispersion was allowed to stand at a liquid temperature of 100 °C for 3 hours for dissolution treatment of the glass component, it was precipitated with a centrifuge and decantation was repeated for washing, and dried in a heating furnace at a furnace temperature of 110 °C for 6 hours to obtain hexagonal strontium ferrite powder. The average particle size of the hexagonal strontium ferrite powder obtained above was 18 nm, and the activation volume was 902 nm 3 , and the anisotropy constant Ku was 2.2×10 5 J / m 3 , and the mass magnetization σs was 49 A·m 2 / kg. A sample powder of 12 mg was taken from the hexagonal strontium ferrite powder obtained above, and the elemental analysis of the filtrate obtained by partially dissolving this sample powder under the dissolution conditions exemplified earlier was performed using an ICP analyzer to determine the surface layer content of neodymium atoms. Separately, 12 mg of sample powder was taken from the hexagonal strontium ferrite powder obtained above, and the elemental analysis of the filtrate obtained by completely dissolving this sample powder under the dissolution conditions exemplified earlier was performed using an ICP analyzer to determine the bulk content of neodymium atoms. The neodymium atom content (bulk content) relative to 100 atomic percent of iron atoms in the hexagonal strontium ferrite powder obtained above was 2.9 atomic percent. Furthermore, the neodymium atom content in the surface layer was 8.0 atomic percent. The ratio of surface layer content to bulk content, "surface layer content / bulk content," was 2.8, confirming that neodymium atoms were concentrated in the surface layer of the particles.

[0180] The hexagonal ferrite crystal structure of the powder obtained above was confirmed by scanning with CuKα rays at a voltage of 45kV and intensity of 40mA, and measuring the X-ray diffraction pattern under the following conditions (X-ray diffraction analysis). The powder obtained above showed a magnetoplanbite-type (M-type) hexagonal ferrite crystal structure. Furthermore, the crystalline phase detected by X-ray diffraction analysis was a single phase of the magnetoplanbite type. PANalytical X'Pert Pro diffractometer, PIXcel detector Soller slit for incident and diffracted beams: 0.017 radians Fixed angle of dispersion slit: 1 / 4 degree Mask: 10mm Scatter prevention slit: 1 / 4 degree Measurement mode: Continuous Measurement time per step: 3 seconds Measurement speed: 0.017 degrees per second Measurement step: 0.05 degrees

[0181] In Table 4, "SrFe2" refers to hexagonal strontium ferrite powder prepared as follows. 1725g of SrCO3, 666g of H3BO3, 1332g of Fe2O3, 52g of Al(OH)3, 34g of CaCO3, and 141g of BaCO3 were weighed out and mixed in a mixer to obtain a raw material mixture. The obtained raw material mixture was melted in a platinum crucible at a melting point of 1380°C. While stirring the molten material, the outlet at the bottom of the platinum crucible was heated, and the molten material was dispensed in a rod shape at approximately 6 g / second. The dispensed material was rolled and rapidly cooled using water-cooled twin rolls to produce an amorphous body. 280g of the obtained amorphous material was placed in an electric furnace, heated to 645°C (crystallization temperature), and held at the same temperature for 5 hours to precipitate (crystallize) hexagonal strontium ferrite particles. Next, the crystalline material obtained above, containing hexagonal strontium ferrite particles, was coarsely ground in a mortar. 1000g of 1mm particle size zirconia beads and 800mL of 1% aqueous acetic acid solution were added to a glass bottle, and the mixture was dispersed in a paint shaker for 3 hours. After that, the resulting dispersion was separated from the beads and placed in a stainless steel beaker. The dispersion was allowed to stand at a liquid temperature of 100°C for 3 hours to dissolve the glass components, then precipitated using a centrifuge, and washed by repeated decantation. Finally, it was dried in a heating furnace at a furnace temperature of 110°C for 6 hours to obtain hexagonal strontium ferrite powder. The average particle size of the obtained hexagonal strontium ferrite powder was 19 nm, and the activation volume was 1102 nm. 3 The anisotropy constant Ku is 2.0 × 10⁻⁶. 5 J / m 3 , mass magnetization σs is 50A m 2 It was / kg.

[0182] In Table 4, "ε-iron oxide" refers to ε-iron oxide powder prepared as follows. In 90 g of pure water, 8.3 g of iron(III) nitrate nonahydrate, 1.3 g of gallium(III) nitrate octahydrate, 190 mg of cobalt(II) nitrate hexahydrate, 150 mg of titanium(IV) sulfate, and 1.5 g of polyvinylpyrrolidone (PVP) were dissolved. While stirring with a magnetic stirrer in an air atmosphere at an ambient temperature of 25°C, 4.0 g of a 25% aqueous ammonia solution was added, and the mixture was stirred for 2 hours at the same ambient temperature of 25°C. To the resulting solution, an aqueous citric acid solution obtained by dissolving 1 g of citric acid in 9 g of pure water was added, and the mixture was stirred for 1 hour. After stirring, the precipitated powder was collected by centrifugation, washed with pure water, and dried in a heating furnace at an ambient temperature of 80°C. 800g of pure water was added to the dried powder, and the powder was dispersed in the water again to obtain a dispersion. The obtained dispersion was heated to 50°C, and 40g of a 25% aqueous ammonia solution was added dropwise while stirring. After stirring for 1 hour while maintaining the temperature at 50°C, 14mL of tetraethoxysilane (TEOS) was added dropwise, and the mixture was stirred for 24 hours. 50g of ammonium sulfate was added to the resulting reaction solution, and the precipitated powder was collected by centrifugation, washed with pure water, and dried in a heating furnace at a temperature of 80°C for 24 hours to obtain a precursor of ferromagnetic powder. The obtained ferromagnetic powder precursor was placed in a heating furnace at a temperature of 1000°C under an atmospheric environment and subjected to heat treatment for 4 hours. A heat-treated ferromagnetic powder precursor was added to a 4 mol / L sodium hydroxide (NaOH) aqueous solution, and the solution was stirred for 24 hours while maintaining the temperature at 70°C to remove silicate compounds, which are impurities, from the heat-treated ferromagnetic powder precursor. Subsequently, the ferromagnetic powder, from which the silicate compounds were removed by centrifugation, was collected and washed with pure water to obtain ferromagnetic powder. The composition of the obtained ferromagnetic powder was confirmed by inductively coupled plasma emission spectroscopy (ICP-OES), revealing that it was a Ga, Co, and Ti-substituted ε-iron oxide (ε-Ga 0.28 Co 0.05 Ti 0.05 Fe 1.62It was O3). Furthermore, X-ray diffraction analysis was performed under the same conditions as previously described for hexagonal strontium ferrite powder SrFe1, and from the peaks of the X-ray diffraction pattern, it was confirmed that the obtained ferromagnetic powder has a single-phase crystal structure of the ε phase (crystal structure of ε-iron oxide) that does not contain the crystal structures of the α phase and γ phase. The average particle size of the obtained ε-iron oxide powder was 12 nm, and the activation volume was 746 nm. 3 The anisotropy constant Ku is 1.2 × 10⁻⁶. 5 J / m 3 , mass magnetization σs is 16A m 2 It was / kg.

[0183] The activation volume and anisotropy constant Ku of the hexagonal strontium ferrite powder and ε-iron oxide powder described above were obtained for each ferromagnetic powder using a vibrating sample magnetometer (manufactured by Toei Kogyo Co., Ltd.) and the method described above. Furthermore, the mass magnetization σs was measured using a vibrating sample type magnetometer (manufactured by Toei Kogyo Co., Ltd.) at a magnetic field strength of 1194 kA / m (15 kOe).

[0184] [Measurement of distribution A and distribution B] For the magnetic tape contained in the magnetic tape cartridge, the number distributions A and B on the surface of the first magnetic layer were determined using a scanning electron microscope (FE-SEM) (FE-SEM S4800) manufactured by Hitachi, Ltd., by the following method.

[0185] (Number distribution A) A scanning electron microscope (FE-SEM) is used to capture a secondary electron image of the surface of the first magnetic layer of the magnetic tape to be measured. The imaging conditions are as follows: acceleration voltage of 5kV, working distance of 5mm, and magnification of 10,000x. During imaging, an unimaged area of ​​the surface of the first magnetic layer is selected, and the focus is adjusted under the above imaging conditions to capture a secondary electron image. The parts that indicate size, etc. (micron bars, cross marks, etc.) are removed from the captured image, and a secondary electron image with a resolution of 960 pixels × 1280 pixels is obtained. The above procedure is performed 100 times at different locations on the surface of the first magnetic layer of the magnetic tape to be measured. The resulting secondary electron image is imported into image processing software (the free software ImageJ), and binarization is performed using the following procedure. The thresholds for binarizing the secondary electron image obtained above are set to a lower limit of 210 gradations and an upper limit of 255 gradations. Binarization is performed using these two thresholds. After binarization, the noise reduction process, Despeckle, is selected in the image analysis software (ImageJ, a free software) to remove noise components. For the resulting binarized image, the number of bright areas (i.e., white areas) and the area of ​​each bright area are determined using image analysis software (the free software ImageJ). From the area A of the bright areas obtained here, the equivalent circular diameter L of each bright area is calculated using the formula (A / π)^(1 / 2)×2=L. The above steps are performed on the binarized images (100 images) obtained above. Based on the above, we can determine the distribution A of the number of items.

[0186] (Number distribution B) A scanning electron microscope (FE-SEM) is used to capture a secondary electron image of the surface of the first magnetic layer of the magnetic tape being measured. The imaging conditions are as follows: acceleration voltage of 2kV, working distance of 5mm, and magnification of 10,000x. During imaging, an unimaged area of ​​the surface of the first magnetic layer is selected, and the focus is adjusted under the above imaging conditions to capture a secondary electron image. The parts that indicate size, etc. (micron bars, cross marks, etc.) are removed from the captured image, and a secondary electron image with a resolution of 960 pixels × 1280 pixels is obtained. The above procedure is performed 100 times at different locations on the surface of the first magnetic layer of the magnetic tape to be measured. The resulting secondary electron image is imported into image processing software (the free software ImageJ), and binarization is performed using the following procedure. The thresholds for binarizing the secondary electron image obtained above are set to a lower limit of 0 gradations and an upper limit of 75 gradations. Binarization is performed using these two thresholds. After binarization, the noise reduction process, Despeckle, is selected in the image analysis software (ImageJ, a free software) to remove noise components. In the resulting binarized image, the number of dark regions (i.e., black areas) and the area of ​​each dark region are determined using image analysis software (the free software ImageJ). From the area A of the dark regions obtained here, the equivalent circular diameter L of each dark region is calculated using the formula (A / π)^(1 / 2)×2=L. The above steps are performed on the binarized images (100 images) obtained above. This allows us to determine the count distribution B.

[0187] Sample pieces were cut from the magnetic tapes contained in each of the above-mentioned magnetic tape cartridges to measure the vertical angular ratio. The vertical angular ratio of these sample pieces was determined using a Tamagawa Seisakusho TM-TRVSM5050-SMSL vibrating sample magnetometer by the method described above. As a result, the vertical angular ratio of the magnetic tapes taken from each of the magnetic tape cartridges in Examples 1 to 28 and Comparative Examples 1 to 25 was all between 0.60 and 1.00.

[0188] [Evaluation of electromagnetic conversion characteristics (vertical recording method)] For each magnetic tape in a magnetic tape cartridge, recording was performed using the vertical recording method described in paragraph 0102 of Japanese Patent No. 6531764 (Patent Document 1), and the recorded data was reproduced to determine the Signal-to-Noise Ratio (SNR). The SNR was determined as a relative value with the value of Comparative Example 1 as the reference (0 dB).

[0189] The results are shown in Table 4 (Tables 4-1 to 4-2).

[0190] [Table 4-1]

[0191] [Table 4-2]

[0192] The results shown in Table 4 confirm that the coated magnetic recording media of Examples 1 to 28 exhibited excellent electromagnetic conversion characteristics (high SNR) when reproducing data recorded using the vertical recording method. [Industrial applicability]

[0193] One aspect of the present invention is useful in various magnetic recording applications where further improvement in recording density is desired.

Claims

1. It comprises a non-magnetic support and a first magnetic layer containing ferromagnetic powder and a binder, A second magnetic layer is further provided between the non-magnetic support and the first magnetic layer, comprising a ferromagnetic powder having a coercivity Hc of 50 Oe or less and an average particle size of 50 nm or less, and a binder. The number distribution A of the equivalent circle diameters of multiple bright areas in the binarized secondary electron image obtained by imaging the surface of the first magnetic layer with a scanning electron microscope at an accelerating voltage of 5 kV is as follows (1) to (3): (1) There are 10,000 to 30,000 bright areas with an equivalent circular diameter of 1 nm to 50 nm. (2) There are 7,000 to 25,000 bright areas with an equivalent circular diameter of 51 nm to 100 nm. (3) There are 1,000 to 3,000 bright areas with an equivalent circular diameter of 101 nm or more. Satisfying the conditions, and The number distribution B of the equivalent circle diameters of multiple dark regions in the binarized image of the secondary electron image obtained by imaging the surface of the first magnetic layer with a scanning electron microscope at an accelerating voltage of 2 kV is as follows (4) to (6): (4) There are 200 to 50,000 dark regions with an equivalent diameter of 1 nm to 50 nm. (5) There are 200 to 25,000 dark regions with an equivalent diameter of 51 nm to 100 nm. (6) There are 0 to 2000 dark regions with an equivalent diameter of 101 nm or more. A magnetic recording medium that satisfies the following conditions.

2. The magnetic recording medium according to claim 1, wherein the vertical aspect ratio of the magnetic recording medium is 0.60 or greater.

3. The magnetic recording medium according to claim 1, wherein the coercivity Hc of the ferromagnetic powder contained in the second magnetic layer is 10 Oe or more and 50 Oe or less.

4. The magnetic recording medium according to claim 1, wherein the average particle size of the ferromagnetic powder contained in the second magnetic layer is 5 nm or more and 50 nm or less.

5. The magnetic recording medium according to claim 1, wherein the ferromagnetic powder contained in the second magnetic layer is spinel ferrite powder.

6. The magnetic recording medium according to claim 1, wherein the ferromagnetic powder contained in the first magnetic layer is hexagonal barium ferrite powder.

7. The magnetic recording medium according to claim 1, wherein the ferromagnetic powder contained in the first magnetic layer is hexagonal strontium ferrite powder.

8. The magnetic recording medium according to claim 1, wherein the ferromagnetic powder contained in the first magnetic layer is ε-iron oxide powder.

9. The magnetic recording medium according to claim 1, further comprising a non-magnetic layer containing non-magnetic powder and a binder between the non-magnetic support and the second magnetic layer.

10. The magnetic recording medium according to claim 1, further comprising a back coat layer containing non-magnetic powder and a binder on the surface side of the non-magnetic support opposite to the surface side having the first magnetic layer and the second magnetic layer.

11. The vertical aspect ratio of the magnetic recording medium is 0.60 or more. The coercivity Hc of the ferromagnetic powder contained in the second magnetic layer is 10 Oe or more and 50 Oe or less. The average particle size of the ferromagnetic powder contained in the second magnetic layer is between 5 nm and 50 nm. The ferromagnetic powder contained in the second magnetic layer is spinel ferrite powder. The ferromagnetic powder contained in the first magnetic layer is a ferromagnetic powder selected from the group consisting of hexagonal barium ferrite powder, hexagonal strontium ferrite powder, and ε-iron oxide powder. Between the non-magnetic support and the second magnetic layer, there is further a non-magnetic layer containing non-magnetic powder and a binder, and The magnetic recording medium according to claim 1, further comprising a back coat layer containing non-magnetic powder and a binder on the surface side of the non-magnetic support opposite to the surface side having the first magnetic layer and the second magnetic layer.

12. The magnetic recording medium according to claim 1, wherein it is a magnetic tape.

13. The magnetic recording medium according to claim 11, wherein it is a magnetic tape.

14. A magnetic tape cartridge comprising a magnetic recording medium according to claim 12 or 13.

15. A magnetic recording and regeneration apparatus including a magnetic recording medium according to any one of claims 1 to 13.

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