Magnetic recording medium, magnetic tape cartridge, and magnetic recording / reproducing device

Abstract

To provide a magnetic recording medium with a little deterioration in electromagnetic conversion characteristics after repeated running in a high-temperature environment.SOLUTION: In a magnetic recording medium, a protrusion height difference Δ(HD-HB) is 0.7 nm or more, in which an arithmetic mean roughness Ra measured on the surface of a magnetic layer is 2.5 nm or less, a protrusion height HD, with the height of the surrounding substrate area being 0 nm, is measured by an atomic force microscope with respect to an area identified as a dark area in a first binarized image of a backscattered electron image obtained by imaging the surface of the magnetic layer with a scanning electron microscope, and a protrusion height HB, with the height of the surrounding substrate area being 0 nm, is measured by the atomic force microscope with respect to an area identified as a bright area in a second binarized image performed on the higher gradation side than the first binarized image obtained by imaging the surface of the magnetic layer with the scanning electron microscope. A magnetic tape cartridge and a magnetic recording / reproducing device include the magnetic recording medium.SELECTED DRAWING: None

Description

[Technical Field]

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

[0002] BACKGROUND ART Magnetic recording media are widely used as recording media for recording various types of data (see, for example, Patent Documents 1 and 2). [Prior art documents] [Patent documents]

[0003] [Patent Document 1] Japanese Patent Application Laid-Open No. 2014-209403 [Patent Document 2] Japanese Patent Application Laid-Open No. 2004-348897 Summary of the Invention [Problem to be solved by the invention]

[0004] Magnetic recording media are required to exhibit excellent electromagnetic conversion characteristics, and further improvements in electromagnetic conversion characteristics are desired.

[0005] Magnetic recording media typically have a magnetic layer containing ferromagnetic powder on a non-magnetic support, and the surface shape of the magnetic layer can affect the performance of the magnetic recording medium. Regarding the surface shape of the magnetic layer, the aforementioned Japanese Patent Laid-Open No. 2014-209403 (Patent Document 1) and Japanese Patent Laid-Open No. 2004-348897 (Patent Document 2) propose controlling the state of protrusions on the magnetic layer surface. In response to this, the present inventors aimed to provide a magnetic recording medium with even better electromagnetic conversion characteristics than those achieved by previously proposed methods of controlling the state of protrusions on the magnetic layer surface, specifically, a magnetic recording medium with minimal degradation in electromagnetic conversion characteristics after repeated running in a high-temperature environment (e.g., an ambient temperature of 40°C or higher, or even a severe high-temperature environment of 60°C or higher).

[0006] That is, an object of one aspect of the present invention is to provide a magnetic recording medium that exhibits little deterioration in electromagnetic conversion characteristics after repeated running in a high-temperature environment. [Means for solving the problem]

[0007] One aspect of the present invention relates to a magnetic recording medium [1] below. [1] A magnetic recording medium having a non-magnetic support and a magnetic layer containing ferromagnetic powder, The arithmetic mean roughness Ra measured on the surface of the magnetic layer (hereinafter also referred to as "magnetic layer surface Ra") is 2.5 nm or less, and a protrusion height HD measured by an atomic force microscope in a region identified as a dark region in a first binarized image of a backscattered electron image obtained by imaging the surface of the magnetic layer with a scanning electron microscope, the protrusion height HD being determined by taking the height of the surrounding base region as 0 nm; a protrusion height HB measured by an atomic force microscope in an area identified as a bright area in an image obtained by imaging the surface of the magnetic layer using a scanning electron microscope and subjected to a second binarization process performed on the higher gradation side than the first binarization process, with the height of the surrounding base area being 0 nm; and A magnetic recording medium in which the protrusion height difference Δ(HD-HB) is 0.7 nm or more.

[0008] In one embodiment, the magnetic recording medium [1] above can be the following magnetic recording medium.

[0009] [2] The magnetic recording medium according to [1], wherein the protrusion height difference Δ is 0.7 nm or more and 3.0 nm or less. [3] The magnetic recording medium according to [1] or [2], wherein the arithmetic mean roughness Ra is 0.8 nm or more and 2.5 nm or less. [4] The magnetic recording medium according to any one of [1] to [3], wherein the magnetic layer contains two or more types of non-magnetic powder. [5] The magnetic recording medium according to [4], wherein the non-magnetic powder in the magnetic layer contains alumina powder. [6] The magnetic recording medium according to [4] or [5], wherein the non-magnetic powder in the magnetic layer contains carbon black. [7] The magnetic recording medium according to any one of [1] to [6], further comprising a non-magnetic layer containing non-magnetic powder between the non-magnetic support and the magnetic layer. [8] The magnetic recording medium according to any one of [1] to [7], further comprising a backcoat layer containing nonmagnetic powder on the surface of the nonmagnetic support opposite to the surface on which the magnetic layer is formed. [9] The magnetic recording medium according to any one of [1] to [8], which is a magnetic tape.

[0010] One aspect of the present invention relates to the magnetic tape cartridge described below in

[10] .

[10] A magnetic tape cartridge including the magnetic tape described in [9].

[0011] One aspect of the present invention relates to a magnetic recording and reproducing device as described below in

[11] .

[11] A magnetic recording and reproducing device including the magnetic recording medium according to any one of [1] to [9]. [Effects of the Invention]

[0012] According to one aspect of the present invention, it is possible to provide a magnetic recording medium that exhibits minimal deterioration in electromagnetic conversion characteristics after repeated running in a high-temperature environment, and a magnetic tape cartridge and a magnetic recording / reproducing device that include such a magnetic recording medium. [Brief explanation of the drawings]

[0013] [Figure 1] 1 shows an example of the arrangement of data bands and servo bands. [Figure 2] An example of servo pattern layout for an LTO (Linear Tape-Open) Ultrium format tape is shown below. [Figure 3] FIG. 1 shows a schematic diagram of a reel tester used to run the magnetic tapes of the examples and comparative examples. DETAILED DESCRIPTION OF THE INVENTION

[0014] [Magnetic recording media] One aspect of the present invention relates to a magnetic recording medium having a non-magnetic support and a magnetic layer containing ferromagnetic powder. The arithmetic mean roughness Ra measured on the surface of the magnetic layer is 2.5 nm or less. Furthermore, the protrusion height difference Δ(HD-HB) between the protrusion height HD measured by atomic force microscope in a region identified as a dark region in a first binarized image of a backscattered electron image obtained by imaging the surface of the magnetic layer with a scanning electron microscope, where the height of the surrounding base material region is 0 nm, and the protrusion height HB measured by atomic force microscope in a region identified as a bright region in a second binarized image of a backscattered electron image obtained by imaging the surface of the magnetic layer with a scanning electron microscope, where the height of the surrounding base material region is 0 nm, is 0.7 nm or more. In "HD" and "HB," "H" is used as an abbreviation for height. In "HD," "D" is used as an abbreviation for dark region. In "HB," "B" is used as an abbreviation for bright region.

[0015] Typically, during the running of a magnetic recording medium, the magnetic layer surface and the magnetic head come into contact and slide against each other. Repeated running can cause wear of the magnetic head (hereinafter referred to as "head wear"), which can result in a deterioration of electromagnetic conversion characteristics after repeated running. After extensive research, the present inventors have newly discovered that by providing a magnetic layer having excellent surface smoothness with an arithmetic mean roughness Ra of 2.5 nm or less, with regions identified as the dark regions (specifically, protrusions) and regions identified as the bright regions (specifically, protrusions) with a protrusion height difference Δ of 0.7 nm or more, it is possible to provide a magnetic recording medium with minimal deterioration of electromagnetic conversion characteristics after repeated running in a high-temperature environment. The present inventors believe that this is because such a magnetic recording medium can suppress head wear during repeated running in a high-temperature environment. Regarding the height of the protrusions, in JP 2014-209403 A (Patent Document 1), the height of the protrusions on the surface of the magnetic layer is measured using an atomic force microscope, and the plane where the volumes of the convex and concave components are equal is defined as the reference plane, and the height of the protrusions is calculated using the height of this reference plane as 0 nm (see paragraph 0016 of JP 2014-209403 A (Patent Document 1)). Also, in JP 2004-348897 A (Patent Document 2), the height of the protrusions on the surface of the magnetic layer is measured using an atomic force microscope, and the plane where the volumes of the protrusions and concaves are equal is defined as the reference plane, and the height of the protrusions is calculated using the height of this reference plane as 0 nm (see paragraph 0025 of JP 2004-348897 A (Patent Document 2)). It is believed that the height of the protrusions is calculated in this way taking into account the presence of undulations on the surface of the magnetic layer. However, in reality, magnetic recording media typically run under tension within a magnetic recording / reproducing device, and it is believed that the waviness of the magnetic layer surface of the magnetic recording medium is stretched during the run. In particular, because magnetic layers tend to soften easily under high-temperature conditions, it is believed that the waviness of the magnetic layer surface may become even more stretched. In this regard, the protrusion heights HD and HB are determined by setting the height of the "surrounding substrate region," described in detail below, at 0 nm. The inventors believe that the protrusion heights HD and HB thus determined can serve as indicators of the protrusion state of the protrusions on the magnetic layer surface when the waviness is stretched. The inventors have discovered that by controlling the state of the protrusions on the magnetic layer surface with respect to the protrusion height (i.e., by controlling the protrusion height difference Δ), it is possible to provide a magnetic recording medium with minimal degradation in electromagnetic conversion characteristics after repeated running under high-temperature conditions. The protrusion height difference Δ and the magnetic layer surface Ra will be explained in more detail below. In this invention and this specification, the protrusion height Δ and the magnetic layer surface Ra of a magnetic recording medium are values ​​measured using a new magnetic recording medium that has not been used since it was shipped as a product.

[0016] <Protrusion height difference Δ> In the present invention and this specification, the protrusion heights HD and HB are values ​​determined on the surface of the magnetic layer by the following method. In the present invention and this specification, the "surface of the magnetic layer" is synonymous with the magnetic layer-side surface of a magnetic recording medium. The following measurements are carried out using a sample piece cut out from the magnetic recording medium to be measured. The size of the sample piece need only allow for the following measurements. The measurement environment is an ambient temperature of 25°C ± 2°C and a relative humidity of 45% ± 25%. (1) An atomic force microscope (AFM) is used in tapping mode to capture an image of a 10.0 μm × 10.0 μm area on the surface of the magnetic layer of the magnetic recording medium to be measured, obtaining an AFM image. For imaging, the sample piece is held with the magnetic layer surface facing upward, and the surface opposite the magnetic layer surface is attached to the AFM sample stage using a fixing film. Commercially available fixing films can be used as the fixing film. Examples of such fixing films include the FIXFILM series manufactured by Fujicopian. In the examples and comparative examples described below, Fujicopian's FIXFILM HGA2 was used as the fixing film. One side of the FIXFILM HGA2 is an adhesive surface, and the other side is an adsorption surface. In the examples and comparative examples described below, the adhesive side of the FIXFILM HGA2 was attached to the AFM sample stage, and the adsorption surface of the FIXFILM HGA2 was attached to the surface opposite the magnetic layer surface of the sample piece. The imaging conditions are a scanning frequency of 0.70 Hz and a resolution of 512 pixels x 512 pixels. By imaging in this manner, AFM height data is obtained for the imaging area. The AFM can be an S-image / Nanonavi manufactured by Hitachi High-Tech Science Corporation in measurement mode DFM (Dynamic Force Microscope), and the probe can be an SI-DF40 (with Al coating on the back surface) manufactured by Hitachi High-Tech Science Corporation. This AFM and probe were used in measurements for the examples and comparative examples described below, with the measurement mode set to DFM. (2) An SEM image is obtained using a scanning electron microscope (SEM) from the same region as the AFM image. A field emission scanning electron microscope (FE-SEM) is used as the scanning electron microscope. For example, a Hitachi High-Technologies Corporation FE-SEM SU8220 can be used as the FE-SEM, and this FE-SEM was used for measurements in the examples and comparative examples described below. Furthermore, no coating treatment was performed on the magnetic layer surface before capturing the SEM image. The SEM image obtained is a low-angle backscattered electron (LA-BSE) image. Hereinafter, this will be referred to simply as a "backscattered electron image." The imaging conditions were: acceleration voltage: 2 kV, emission: 10 μA, working distance: 4 mm, and magnification: 13,000x. The focus was adjusted under these imaging conditions, and an SEM image (backscattered electron image) was captured. The backscattered electron image, from which the size indicators (micron bar, cross mark, etc.) were removed, was imported into image processing software, aligned with the AFM image captured in (1) above, and then binarized. Alignment was performed on the central 8.5 μm × 8.5 μm area within the previously described 10.0 μm × 10.0 μm imaging area. For example, free software such as ImageJ can be used as image analysis software. ImageJ was used in the examples and comparative examples described below. The binarization process divided the image into bright areas (white areas) and dark areas (black areas). The following two binarization processes (first binarization process and second binarization process) were performed. The backscattered electron image captured under the above-described imaging conditions is subjected to a first binarization process as follows to create a first binarized image. The lower limit is set to 0 gradation and the upper limit to a value in the range of 75 to 90 gradations, and binarization processing is performed using these two thresholds (lower limit and upper limit). Before the binarization processing, noise component removal processing is performed using image analysis software. The noise component removal processing can be performed, for example, by the following method. For the examples and comparative examples described below, noise removal processing was performed by the following method. In the image analysis software ImageJ, select the blurring Gauss Filter to remove noise components. The first binarized image obtained in this way is used as an image for identifying dark areas, and the parts of this image that are displayed as dark areas (i.e., black areas) are identified as "dark areas." Image analysis software is used to calculate the area of ​​each of the dark areas contained in the first binarized image. The circular equivalent diameter of each dark area is calculated from the calculated area. Specifically, the circular equivalent diameter L is calculated from the calculated area A using (A / π)^(1 / 2)×2=L. Here, the operator "^" represents exponentiation. The equivalent circle diameter is calculated in 1 nm increments by rounding off to one decimal place and discarding any decimal places after the first decimal place. In addition to the above binarization process, a second binarization process is performed as follows to create a second binarized image. In the backscattered electron image captured under the above imaging conditions, the lower limit is set to a value in the range of 140 to 170 gradations, the upper limit is set to 255 gradations, and binarization processing is performed using these two thresholds (lower limit and upper limit). Before the binarization processing, noise component removal processing is performed using image analysis software. The noise component removal processing can be performed, for example, by the following method. For the examples and comparative examples described below, noise removal processing was performed by the following method. In the image analysis software ImageJ, select the blurring Gauss Filter to remove noise components. The second binarized image obtained in this way is used as an image for identifying bright areas, and the areas displayed as bright areas (i.e., white areas) in this image are identified as "bright areas." Image analysis software is used to calculate the area of ​​each bright area contained in the second binarized image. From the calculated areas, the circular equivalent diameter of each dark area is calculated. Specifically, from the calculated area A, the circular equivalent diameter L is calculated using (A / π)^(1 / 2)×2=L. (3) From the AFM height data of the regions identified as dark areas by the above-described alignment, the height from the reference plane, i.e., the height with the reference plane set to 0 nm, is calculated for each of the regions identified as dark areas. This height is calculated as the arithmetic mean of the AFM height data within each dark area. In this invention and this specification, the "reference plane" refers to a plane where the volumes of the convex and concave components in the imaging area are equal. In this way, for all areas identified as dark areas, the height is determined with the reference plane set to 0 nm. Furthermore, for all areas identified as dark areas, a "surrounding base area" is identified as follows. For each dark area, a circle is set as a reference circle, with the center of gravity of the area as its center and the equivalent diameter of the area as its diameter. A margin area and a surrounding base area are identified concentrically with the reference circle. If the radius of the reference circle is R (unit: nm), then R = L / 2. The margin area is a 50 nm-wide donut-shaped area obtained by excluding the area enclosed by the reference circle from the area enclosed by a circle with a radius of (R + 50) nm. The surrounding base area is a 100 nm-wide donut-shaped area obtained by excluding the area enclosed by the circle with a radius of (R + 50 + 100) nm from the area enclosed by the circle with a radius of (R + 50) nm. However, the shape of the area identified as a dark area is not limited to a circle, and it is possible that part of the shape of the actual dark area overlaps with the donut-shaped area identified as the surrounding base area. In such cases, the overlapping portion is excluded from the surrounding base area. The height of the peripheral substrate region thus identified, i.e., the height with the reference plane at 0 nm, is calculated from the AFM height data, and this height is calculated as the arithmetic mean of the AFM height data within each peripheral substrate region. In this way, the height of the surrounding substrate area of ​​all dark areas is determined with the reference surface being 0 nm. For all dark areas (excluding dark areas where the height of the surrounding substrate area is a negative value when the reference surface is set to 0 nm), the value obtained by subtracting the "height of the surrounding substrate area when the reference surface is set to 0 nm" from the "height when the reference surface is set to 0 nm" is calculated. The arithmetic mean of the values ​​thus calculated is the "protrusion height HD when the height of the surrounding substrate area is set to 0 nm." The above processing can be performed using image analysis software (for example, the free software ImageJ), and ImageJ was used in the examples and comparative examples described below. (4) Separately from (3) above, for all regions identified as bright regions, the height from the reference plane, i.e., the height with the reference plane at 0 nm, is calculated from the AFM height data of the regions identified as bright regions by the above-described alignment. The height is calculated as the arithmetic mean of the AFM height data within each bright region. In this way, for all areas identified as bright areas, the height is calculated with the reference plane set to 0 nm. Furthermore, for all areas identified as bright areas, the "surrounding substrate areas" are identified using the method described above for identifying the surrounding substrate areas of dark areas. More specifically, the "surrounding substrate areas" are identified as follows. For each bright area, a circle is set as a reference circle, with the center of gravity of the area as its center and the equivalent diameter of the area as its diameter. A margin area and a peripheral base area are identified concentrically with the reference circle. If the radius of the reference circle is R (unit: nm), then R = L / 2. The margin area is a 50 nm-wide donut-shaped area obtained by excluding the area enclosed by the reference circle from the area enclosed by a circle with a radius of (R + 50) nm. The peripheral base area is a 100 nm-wide donut-shaped area obtained by excluding the area enclosed by the circle with a radius of (R + 50 + 100) nm from the area enclosed by the circle with a radius of (R + 50) nm. However, the shape of the area identified as a bright area is not limited to a circle, and it is possible that part of the shape of the actual bright area overlaps with the donut-shaped area identified as the peripheral base area. In such cases, the overlapping portion is excluded from the peripheral base area. The height of the peripheral substrate region thus identified, i.e., the height with the reference plane at 0 nm, is calculated from the AFM height data, and this height is calculated as the arithmetic mean of the AFM height data within each peripheral substrate region. In this way, the height of the surrounding substrate area of ​​all bright areas is determined with the reference surface set to 0 nm. For all bright areas (excluding bright areas where the height of the surrounding base material area is a negative value when the reference surface is set to 0 nm), the value obtained by subtracting the "height of the surrounding base material area when the reference surface is set to 0 nm" from the "height when the reference surface is set to 0 nm" is calculated. The arithmetic mean of the values ​​calculated in this way is the "protrusion height HB when the height of the surrounding base material area is set to 0 nm." The above processing can be performed using image analysis software (for example, the free software ImageJ), and ImageJ was used in the examples and comparative examples described below.

[0017] The above steps (1) to (4) are performed on three different measurement regions selected at random on the surface of the magnetic layer (n=3). The arithmetic mean of the three HD values ​​obtained in this way is defined as the "protrusion height HD when the height of the surrounding substrate region is 0 nm" for the magnetic recording medium being measured. The arithmetic mean of the three HB values ​​obtained in this way is defined as the "protrusion height HB when the height of the surrounding substrate region is 0 nm" for the magnetic recording medium being measured. Then, the value (HD-HB) obtained by subtracting the "protrusion height HB when the height of the surrounding substrate region is 0 nm" for the magnetic recording medium being measured is defined as the "protrusion height difference Δ" for the magnetic recording medium being measured.

[0018] The present inventors speculate as follows about the protrusion height difference Δ. The magnetic layer of a magnetic recording medium typically contains a non-magnetic powder (hereinafter also referred to as "abrasive") for imparting abrasive properties to the magnetic layer surface and a non-magnetic powder (hereinafter also referred to as "filler") for forming appropriate protrusions on the magnetic layer surface to control friction characteristics. The inventors believe that the regions identified as dark areas by (2) above are protrusions formed on the magnetic layer surface by the filler, and that the regions identified as bright areas by (2) above are protrusions formed on the magnetic layer surface by the abrasive. It is believed that head wear is primarily caused by contact between the head and the protrusions formed by the abrasive. Furthermore, making the protrusions formed by the filler higher than the protrusions formed by the abrasive is believed to suppress head wear caused by such factors. Furthermore, for the magnetic recording medium, the protrusion height difference Δ is determined based on the height of the surrounding base region rather than the reference surface (i.e., the height of the surrounding base region is set to 0 nm). A protrusion height difference Δ of 0.7 nm or more can contribute to suppressing deterioration of electromagnetic conversion characteristics after repeated running in a high-temperature environment. Details of this point are as described above.

[0019] In the magnetic recording medium, the protrusion height difference Δ is 0.7 nm or more, preferably 0.8 nm or more, more preferably 0.9 nm or more, and even more preferably 1.0 nm or more, from the viewpoint of suppressing deterioration of electromagnetic conversion characteristics after repeated running in a high-temperature environment. Furthermore, the protrusion height difference Δ can be, for example, 3.0 nm or less. In one embodiment, a protrusion height difference Δ of 3.0 nm or less is preferred from the viewpoint of improving electromagnetic conversion characteristics at the beginning of running.

[0020] Regarding the control of the protrusion height difference Δ, for example, the value tends to increase by using a small-sized abrasive and / or a large-sized filler. The value of the protrusion height difference Δ tends to decrease by strengthening the dispersion process of the filler-containing dispersion liquid (hereinafter also referred to as "filler liquid") during the preparation of the magnetic layer-forming composition (e.g., by increasing the number of dispersion processes). Furthermore, at any stage before shipping as a product, the magnetic recording medium can be run under tension while sliding the magnetic layer surface against a sliding member. This can either remove particles of nonmagnetic powder (e.g., filler and / or abrasive) protruding from the magnetic layer surface and / or push the particles toward the interior of the magnetic layer, thereby changing one or both of the HD and HB. Any sliding member can be used as the sliding member. For example, a magnetic head can also be used as the sliding member. For example, by employing one or more of the above methods, the protrusion height difference Δ can be controlled to 0.7 nm or greater.

[0021] <Magnetic layer surface Ra> In the present invention and this specification, the arithmetic mean roughness Ra (magnetic layer surface Ra) measured on the surface of the magnetic layer is determined by the following method. An atomic force microscope (AFM) is used to measure the arithmetic mean roughness Ra. The measurement area is a 40 μm square (40 μm × 40 μm). Measurements are performed at three different measurement points selected at random on the magnetic layer surface (n=3). The arithmetic mean of the three values ​​obtained by these measurements is taken as the magnetic layer surface Ra of the magnetic recording medium being measured. The following measurement conditions can be cited as an example of AFM measurement conditions. The following measurement conditions were used for the examples and comparative examples described below. An AFM (Veeco Nanoscope 4) was used in tapping mode to measure an area of ​​40 μm × 40 μm on the surface of the magnetic layer of the magnetic recording medium. A BRUKER RTESP-300 probe was used, with a resolution of 512 pixels × 512 pixels and a scan speed of 341 seconds per screen (512 pixels × 512 pixels).

[0022] The magnetic layer surface Ra of the magnetic recording medium is 2.5 nm or less, preferably 2.4 nm or less, and more preferably 2.3 nm or less. Setting the protrusion height difference Δ required for the surface of a magnetic layer with excellent surface smoothness, having a magnetic layer surface Ra of 2.5 nm or less, to 0.7 nm or more can contribute to suppressing deterioration of electromagnetic conversion characteristics after repeated running in a high-temperature environment. Furthermore, the magnetic layer surface Ra of the magnetic recording medium can be, for example, 0.8 nm or more, 0.9 nm or more, 1.0 nm or more, 1.1 nm or more, or 1.2 nm or more, or it can be less than the values ​​exemplified here.

[0023] The surface Ra of the magnetic layer can be controlled by known methods such as adjusting the manufacturing conditions of the magnetic recording medium.

[0024] The above magnetic recording medium will now be described in more detail.

[0025] <Magnetic layer> (Ferromagnetic powder) The ferromagnetic powder contained in the magnetic layer can be one or a combination of two or more known ferromagnetic powders used in the magnetic layers of various magnetic recording media. 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 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 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 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.

[0026] Regarding the particle size of the ferromagnetic powder, the average particle volume can also be used as an index of particle size. From the viewpoint of improving recording density, the average particle volume is set to 2500 nm 3 Preferably, it is 2300 nm or less. 3 More preferably, it is 2000 nm or less. 3 More preferably, it is 1500 nm or less. 3 From the viewpoint of magnetization stability, the average particle volume of the ferromagnetic powder is more preferably 500 nm or less. 3 It is preferable that the thickness is 600 nm or more. 3 More preferably, it is 650 nm or more. 3 More preferably, it is 700 nm or more. 3 The above average particle volume is a value determined as a sphere-equivalent volume from the average particle size determined by the method described below.

[0027] Hexagonal ferrite powder A preferred example of the ferromagnetic powder is hexagonal ferrite powder. For details of the hexagonal ferrite powder, see, for example, JP 2011-225417 A, paragraphs 0012 to 0030, JP 2011-216149 A, paragraphs 0134 to 0136, JP 2012-204726 A, paragraphs 0013 to 0030, and JP 2015-127985 A, paragraphs 0029 to 0084.

[0028] 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. The main phase refers to the structure to which the highest diffraction peak belongs in the X-ray diffraction spectrum obtained by X-ray diffraction analysis. For example, if the highest diffraction peak in the X-ray diffraction spectrum obtained by X-ray diffraction analysis is assigned to the hexagonal ferrite crystal structure, it is determined that the hexagonal ferrite crystal structure has been detected as the main phase. If only a single structure is detected by X-ray diffraction analysis, this detected structure is considered to be 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 of such atoms include alkaline earth metal atoms such as strontium atoms, barium atoms, and calcium atoms, and lead atoms. In the present invention and this specification, "hexagonal strontium ferrite powder" refers to a powder in which the main divalent metal atom contained therein is strontium atom, and "hexagonal barium ferrite powder" refers to a powder in which the main divalent metal atom contained therein is barium atom. "Main divalent metal atom" refers to the divalent metal atom that is the most abundant, on an atomic % basis, among the divalent metal atoms contained in the powder. However, rare earth atoms are not included in the above divalent metal atoms. In the present invention and this specification, "rare earth atom" refers to a powder selected from the group consisting of scandium atom (Sc), yttrium atom (Y), and lanthanoid atom. The lanthanoid atom is selected from the group consisting of lanthanum atom (La), cerium atom (Ce), praseodymium atom (Pr), neodymium atom (Nd), promethium atom (Pm), samarium atom (Sm), europium atom (Eu), gadolinium atom (Gd), terbium atom (Tb), dysprosium atom (Dy), holmium atom (Ho), erbium atom (Er), thulium atom (Tm), ytterbium atom (Yb), and lutetium atom (Lu).

[0029] Hereinafter, hexagonal strontium ferrite powder, which is one form of hexagonal ferrite powder, will be described in more detail.

[0030] The activation volume of the hexagonal strontium ferrite powder is preferably 800 to 1600 nm 3 The finely divided hexagonal strontium ferrite powder exhibiting an activation volume in the above range is suitable for producing a magnetic recording medium that exhibits 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 From the viewpoint of further improving the electromagnetic conversion characteristics, the activation volume of the hexagonal strontium ferrite powder can be 1500 nm or more. 3 More preferably, it is 1400 nm or less. 3 More preferably, it is 1300 nm or less. 3 More preferably, it is 1200 nm or less. 3 It is even more preferable that the value is equal to or less than 1100 nm. 3 It is even more preferable that the activation volume of the hexagonal barium ferrite powder is equal to or less than 10 ...

[0031] "Activation volume" is a unit of magnetization reversal and is an index showing the magnetic size of a particle. The activation volume described in this invention and this specification and the anisotropy constant Ku described below are values ​​obtained by measuring the coercive force Hc using a vibrating sample magnetometer at magnetic field sweep rates of 3 minutes and 30 minutes (measurement temperature: 23°C ± 1°C) in the coercive force Hc measurement section, and then calculating from the following relational expression between Hc and activation volume V. The unit of the anisotropy constant Ku is 1 erg / cc = 1.0 x 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)]

[0032] The anisotropy constant Ku can be used as an index for reducing thermal fluctuation, in other words, improving thermal stability. The hexagonal strontium ferrite powder preferably has an anisotropy constant of 1.8×10 5 J / m 3 and more preferably 2.0×10 5 J / m 3 The Ku of the hexagonal strontium ferrite powder can be, for example, 2.5×10 5 J / m 3 However, since a higher Ku value means higher thermal stability and is therefore preferred, the Ku value is not limited to the above-mentioned values.

[0033] The hexagonal strontium ferrite powder may or may not contain rare earth atoms. When the hexagonal strontium ferrite powder contains rare earth atoms, it is preferable that the rare earth atoms are contained at a content (bulk content) of 0.5 to 5.0 atomic % relative to 100 atomic % of iron atoms. In one embodiment, the hexagonal strontium ferrite powder containing rare earth atoms can have rare earth atoms unevenly distributed in the surface layer portion. In the present invention and this specification, the term "surface layer distribution of rare earth atoms" refers to the ratio of the rare earth atom content relative to 100 atomic % of iron atoms in a solution obtained by partially dissolving a hexagonal strontium ferrite powder in an acid (hereinafter referred to as "surface layer content of rare earth atoms" or simply "surface layer content" with respect to rare earth atoms) to the ratio of the rare earth atom content relative to 100 atomic % of iron atoms in a solution obtained by completely dissolving a hexagonal strontium ferrite powder in an acid (hereinafter referred to as "bulk content of rare earth atoms" 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 of "surface rare earth atom content / bulk rare earth atom content > 1.0" is satisfied. The rare earth atom content of the hexagonal strontium ferrite powder described below is synonymous with the rare earth atom bulk content. In contrast, partial dissolution using an 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 surface rare earth atom content satisfies the ratio "surface rare earth atom content / bulk rare earth atom content > 1.0", this means that the rare earth atoms are unevenly distributed in the surface layer of the particles constituting the hexagonal strontium ferrite powder (i.e., present in greater amounts in the surface layer than in the interior). In the present invention and this specification, the surface layer refers to a partial region extending from the surface toward the interior of the particles constituting the hexagonal strontium ferrite powder.

[0034] When the 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. Having rare earth atoms in the above-mentioned bulk content range and having the rare earth atoms unevenly distributed in the surface layers of the particles constituting the hexagonal strontium ferrite powder is thought to contribute to suppressing a decrease in playback output during repeated playback. This is presumably because the hexagonal strontium ferrite powder contains rare earth atoms in the above-mentioned bulk content range and having the rare earth atoms unevenly distributed in the surface layers of the particles constituting the hexagonal strontium ferrite powder can increase the anisotropy constant Ku. The higher the anisotropy constant Ku, the more the occurrence of a phenomenon known as thermal fluctuation can be suppressed (in other words, thermal stability can be improved). By suppressing the occurrence of thermal fluctuation, the decrease in playback output during repeated playback can be suppressed. It is speculated that the uneven distribution of rare earth atoms in the surface layer of hexagonal strontium ferrite powder particles contributes to stabilizing the spin of the iron (Fe) sites in the crystal lattice in the surface layer, thereby increasing the anisotropy constant Ku. Furthermore, it is believed that using a hexagonal strontium ferrite powder having rare earth atoms unevenly distributed in the surface layer as the ferromagnetic powder for the magnetic layer also contributes to preventing the magnetic layer surface from being worn away by sliding with the magnetic head. That is, it is believed that a hexagonal strontium ferrite powder having rare earth atoms unevenly distributed in the surface layer can also contribute to improving the running durability of magnetic recording media. This is believed to be because the uneven distribution of rare earth atoms on the surfaces of the particles that make up the hexagonal strontium ferrite powder contributes to improving the interaction between the particle surfaces and the organic substances (e.g., binders and / or additives) contained in the magnetic layer, thereby improving the strength of the magnetic layer. From the viewpoint of further suppressing a decrease in reproduction output during repeated reproduction 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 %, even more preferably in the range of 1.0 to 4.5 atomic %, and even more preferably in the range of 1.5 to 4.5 atomic %.

[0035] The bulk content is the content determined by completely dissolving the hexagonal strontium ferrite powder. In the present invention and this specification, unless otherwise specified, the content of an atom refers to the bulk content determined by completely dissolving the hexagonal strontium ferrite powder. The hexagonal strontium ferrite powder containing rare earth atoms may contain only one rare earth atom or two or more rare earth atoms. When two or more rare earth atoms are contained, the bulk content is determined for the total of the two or more rare earth atoms. This also applies to other components in the present invention and this specification. That is, unless otherwise specified, a certain component may be used alone or in combination with two or more. When two or more components are used, the content or content refers to the total of the two or more components.

[0036] When the hexagonal strontium ferrite powder contains rare earth atoms, the rare earth atoms may be any one or more of rare earth atoms. From the viewpoint of further suppressing the decrease in playback output during repeated playback, 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.

[0037] In a hexagonal strontium ferrite powder having rare earth atoms unevenly distributed in the surface layer, the rare earth atoms need only be unevenly distributed in the surface layer of the particles constituting the hexagonal strontium ferrite powder, and the degree of uneven distribution is not limited. For example, for a hexagonal strontium ferrite powder having rare earth atoms unevenly distributed in the surface layer, the ratio of the surface content of rare earth atoms determined by partial dissolution under the dissolution conditions described below to the bulk content of rare earth atoms determined by complete dissolution under the dissolution conditions described below, i.e., "surface content / bulk content," is greater than 1.0 and can be 1.5 or greater. A "surface content / bulk content" ratio greater than 1.0 means that the rare earth atoms are unevenly distributed in the surface layer (i.e., present in greater amounts than in the interior) in the particles constituting the hexagonal strontium ferrite powder. Furthermore, the ratio of the surface content of rare earth atoms determined by partial dissolution under the dissolution conditions described below to the bulk content of rare earth atoms determined by complete dissolution under the dissolution conditions described below, i.e., "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 a hexagonal strontium ferrite powder having rare earth atoms unevenly distributed in the surface layers, it is sufficient that the rare earth atoms are unevenly distributed in the surface layers of the particles constituting the hexagonal strontium ferrite powder, and the above "surface content / bulk content" is not limited to the exemplified upper or lower limits.

[0038] The partial and total dissolution of hexagonal strontium ferrite powder is described below. For hexagonal strontium ferrite powder present as a powder, sample powders to be partially and completely dissolved are collected from the same powder lot. On the other hand, for hexagonal strontium ferrite powder contained in the magnetic layer of a magnetic recording medium, a portion of the hexagonal strontium ferrite powder removed from the magnetic layer is subjected to partial dissolution, and another portion is subjected to total dissolution. The hexagonal strontium ferrite powder can be removed from the magnetic layer by, for example, the method described in paragraph 0032 of JP 2015-91747 A. The term "partial dissolution" as used herein refers to dissolving the hexagonal strontium ferrite powder to such an extent that the remaining hexagonal strontium ferrite powder can be visually confirmed in the liquid at the end of dissolution. For example, partial dissolution can dissolve 10 to 20 mass% of the particles constituting the hexagonal strontium ferrite powder, with the total particles being 100 mass%. On the other hand, the term "complete dissolution" as used herein refers to dissolving the hexagonal strontium ferrite powder to such an extent that the remaining hexagonal strontium ferrite powder cannot be visually confirmed in the liquid at the end of dissolution. The partial dissolution and measurement of the surface layer content are carried out, for example, by the following method. Note that the dissolution conditions such as the amount of sample powder described below are merely examples, and any dissolution conditions that allow partial or complete dissolution can be adopted. A container (e.g., a beaker) containing 12 mg of sample powder and 10 mL of 1 mol / L hydrochloric acid is placed on a hot plate set at 70°C for 1 hour. The resulting solution is filtered through a 0.1 μm membrane filter. Elemental analysis of the filtrate is performed using an inductively coupled plasma (ICP) analyzer. This allows the surface content of rare earth atoms relative to 100 atomic percent iron atoms to be determined. If multiple types of rare earth atoms are detected by elemental analysis, the total content of all rare earth atoms is taken as the surface content. This also applies to measurements of bulk content. On the other hand, the total dissolved and bulk contents are measured, 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 placed on a hot plate set at 80°C for 3 hours. After that, the bulk content relative to 100 atomic % of iron atoms can be determined by carrying out the same procedures as for the partial dissolution and surface layer content measurements described above.

[0039] From the viewpoint of increasing the reproduction output when reproducing data recorded on a magnetic recording medium, it is desirable that the mass magnetization σs of the ferromagnetic powder contained in the magnetic recording medium is high. In this regard, hexagonal strontium ferrite powder that contains rare earth atoms but does not have the rare earth atoms unevenly distributed in the surface layer tends to have a significantly lower σs than hexagonal strontium ferrite powder that does not contain rare earth atoms. On the other hand, hexagonal strontium ferrite powder that has the rare earth atoms unevenly distributed in the surface layer is considered preferable in order to suppress such a large decrease in σs. In one embodiment, the σs of the hexagonal strontium ferrite powder is 45 A·m 2 / kg or more, and 47A·m 2 / kg or more. On the other hand, from the viewpoint of noise reduction, σs is 80A·m 2 / kg or less is preferable, and 60A·m 2 / kg or less is more preferable. 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 a value measured at a magnetic field strength of 15 kOe. 1 [kOe] = 10 6 / 4π[A / m].

[0040] Regarding the content (bulk content) of constituent atoms of the hexagonal strontium ferrite powder, the strontium atom content can be, for example, in the range of 2.0 to 15.0 atomic % relative to 100 atomic % of iron atoms. In one embodiment, the hexagonal strontium ferrite powder can contain only strontium atoms as divalent metal atoms. In another embodiment, the hexagonal strontium ferrite powder can contain one or more other divalent metal atoms in addition to strontium atoms. For example, barium atoms and / or calcium atoms can be contained. When divalent metal atoms other than strontium atoms are contained, the barium atom content and calcium atom content in the hexagonal strontium ferrite powder can each be, for example, in the range of 0.05 to 5.0 atomic % relative to 100 atomic % of iron atoms.

[0041] Known crystal structures of hexagonal ferrite include magnetoplumbite type (also called "M type"), W type, Y type, and Z type. The hexagonal strontium ferrite powder may have any of these crystal structures. The crystal structure can be confirmed by X-ray diffraction analysis. The hexagonal strontium ferrite powder may be one in which a single crystal structure or two or more types of crystal structures are detected by X-ray diffraction analysis. For example, in one embodiment, the hexagonal strontium ferrite powder may be one in which only the M-type crystal structure is detected by X-ray diffraction analysis. For example, the M-type hexagonal ferrite is AFe 12 O 19The composition formula is represented by the formula: where A represents a divalent metal atom. When the hexagonal strontium ferrite powder is of M type, A is only strontium atom (Sr). Alternatively, when A contains multiple divalent metal atoms, strontium atom (Sr) accounts for the largest proportion on an atomic % basis, as described above. The divalent metal atom content of the hexagonal strontium ferrite powder is usually determined by the type of crystalline structure of the hexagonal ferrite and is not particularly limited. The same applies to the iron atom content and oxygen atom content. The hexagonal strontium ferrite powder contains at least iron atoms, strontium atoms, and oxygen atoms, and may further contain rare earth atoms. Furthermore, the hexagonal strontium ferrite powder may or may not contain atoms other than these atoms. As an example, the hexagonal strontium ferrite powder may contain aluminum atoms (Al). The content of aluminum atoms can be, for example, 0.5 to 10.0 atomic % relative to 100 atomic % of iron atoms. From the viewpoint of further suppressing a decrease in playback output during repeated playback, the hexagonal strontium ferrite powder 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 % or less, more preferably 0 to 5.0 atomic %, relative to 100 atomic % of iron atoms, and may even be 0 atomic %. That is, in one embodiment, the hexagonal strontium ferrite powder may contain no atoms other than iron atoms, strontium atoms, oxygen atoms, and rare earth atoms. The content expressed in atomic % above is determined by converting the content (unit: mass %) of each atom obtained by completely dissolving the hexagonal strontium ferrite powder into a value expressed in atomic % using the atomic weight of each atom. Furthermore, in the present invention and this specification, "not containing" a certain atom means that the content is 0 mass % when completely dissolved and measured by an ICP analyzer. The detection limit of an ICP analyzer is usually 0.01 ppm (parts per million) or less by mass. The above term "free from" is used to mean that the substance is contained in an amount below the detection limit of the ICP analyzer.In one form, the hexagonal strontium ferrite powder can be one that does not contain bismuth atoms (Bi).

[0042] metal powder A preferred example of the ferromagnetic powder is ferromagnetic metal powder. For details of the ferromagnetic metal powder, see, for example, paragraphs 0137 to 0141 of JP-A No. 2011-216149 and paragraphs 0009 to 0023 of JP-A No. 2005-251351.

[0043] ε-iron oxide powder A preferred example of the ferromagnetic powder is ε-iron oxide powder. In the present invention and this specification, the term "ε-iron oxide powder" refers to a ferromagnetic powder in which an ε-iron oxide crystalline structure is detected as the main phase by X-ray diffraction analysis. For example, if the highest intensity diffraction peak in an X-ray diffraction spectrum obtained by X-ray diffraction analysis is assigned to the ε-iron oxide crystalline structure, it is determined that the ε-iron oxide crystalline structure is detected as the main phase. Known methods for producing ε-iron oxide powder include a method using goethite and a reverse micelle method. All of these production methods are publicly known. For a method for producing ε-iron oxide powder in which part of the Fe atoms are replaced by atoms such as Ga, Co, Ti, Al, or Rh, see, for example, J. Jpn. Soc. Powder Metallurgy, Vol. 61, Supplement, No. S1, pp. S280-S284 and J. Mater. Chem. C, 2013, 1, pp. 5200-5206. However, the method for producing the ε-iron oxide powder that can be used as the ferromagnetic powder in the magnetic layer of the magnetic recording medium is not limited to the method given here.

[0044] The activation volume of the ε-iron oxide powder is preferably 300 to 1500 nm 3 The activation volume of the ε-iron oxide powder is preferably in the range of 300 nm. The ε-iron oxide powder having an activation volume in the above range is suitable for producing a magnetic recording medium that exhibits excellent electromagnetic conversion characteristics. The activation volume of the ε-iron oxide powder is preferably in the range of 300 nm. 3or more, for example, 500 nm 3 From the viewpoint of further improving the electromagnetic conversion characteristics, the activation volume of the ε-iron oxide powder can be 1400 nm or more. 3 More preferably, it is 1300 nm or less. 3 More preferably, it is 1200 nm or less. 3 More preferably, it is 1100 nm or less. 3 It is even more preferred that:

[0045] The anisotropy constant Ku can be used as an index of the reduction in thermal fluctuation, in other words, the improvement in thermal stability. The ε-iron oxide powder is preferably 3.0×10 4 J / m 3 and more preferably 8.0×10 4 J / m 3 The Ku of the ε-iron oxide powder can be, for example, 3.0×10 5 J / m 3 However, since a higher Ku value means higher thermal stability and is therefore preferred, the Ku value is not limited to the above-mentioned values.

[0046] From the viewpoint of increasing the reproduction output when reproducing data recorded on the magnetic recording medium, it is desirable that the mass magnetization σs of the ferromagnetic powder contained in the magnetic recording medium is high. In this regard, in one embodiment, the σs of the ε-iron oxide powder is 8 A m 2 / kg or more, and 12 A m 2 On the other hand, the σs of ε-iron oxide powder can be 40 A m 2 / kg or less, and 35A·m 2 / kg or less is more preferable.

[0047] In the present invention and this specification, unless otherwise specified, the average particle size of various powders such as ferromagnetic powders is a value measured by the following method using a transmission electron microscope. The powder is photographed using a transmission electron microscope at a magnification of 100,000x, and a photograph of the particles that make up the powder is obtained by printing it on photographic paper or displaying it on a display so that the total magnification is 500,000x. From the obtained particle photograph, the target particle is selected and the particle outline is traced with a digitizer to measure the particle (primary particle) size. Primary particles are independent particles that do not aggregate. The above measurements are performed on 500 randomly selected particles. The arithmetic mean of the particle sizes of the 500 particles thus obtained is the average particle size of the powder. The transmission electron microscope may be, for example, a Hitachi H-9000 transmission electron microscope. Furthermore, particle size measurements can be performed using known image analysis software, such as Carl Zeiss KS-400 image analysis software. Unless otherwise specified, the average particle sizes shown in the examples below are values ​​measured using a Hitachi H-9000 transmission electron microscope and Carl Zeiss KS-400 image analysis software. In this invention and this specification, "powder" refers to a collection of multiple particles. For example, "ferromagnetic powder" refers to a collection of multiple ferromagnetic particles. Furthermore, a collection of multiple particles is not limited to a configuration in which the particles constituting the collection are in direct contact with each other, but also includes a configuration in which binders, additives, etc., as described below, are interposed between the particles. The term "particle" is sometimes used to refer to powder.

[0048] As a method for collecting sample powder from a magnetic recording medium for particle size measurement, for example, the method described in paragraph 0015 of JP-A No. 2011-048878 can be used.

[0049] In the present invention and this specification, unless otherwise specified, the size of particles constituting the powder (particle size) is determined by the shape of the particles observed in the particle photograph. (1) In the case of needle-shaped, spindle-shaped, columnar (where the height is greater than the maximum diameter of the base), etc., the particle size is expressed by the length of the major axis that constitutes the particle, i.e., the major axis length. (2) In the case of a plate or columnar shape (where the thickness or height is smaller than the maximum major axis of the plate surface or base), it is expressed by the maximum major axis of the plate surface or base, (3) When the particle is spherical, polyhedral, irregular, etc., and the long axis of the particle cannot be identified from its shape, it is expressed as the equivalent circle diameter, which is determined by the circle projection method.

[0050] The acicular ratio of a powder is determined by measuring the minor axis length of the particles in the above measurement, i.e., the minor axis length, and calculating the arithmetic mean of the major axis lengths (average major axis length) and the minor axis lengths (average minor axis length) obtained for the 500 particles as "average major axis length / average minor axis length." Here, unless otherwise specified, the minor axis length refers to the length of the minor axis constituting the particle in the above definition of particle size (1), and to the thickness or height in the above definition of particle size (2). In the above definition of particle size (3), since there is no distinction between the major axis and the minor axis, (average major axis length / average minor axis length) is considered to be 1 for convenience. Unless otherwise specified, when the particle shape is specific, for example, in the case of the above definition of particle size (1), the average particle size is the average major axis length, in the case of definition (2), the average particle size is the average plate diameter, and in the case of definition (3), the average particle size is the average diameter (also called the average particle diameter or average particle size).

[0051] The content (filling rate) of the ferromagnetic powder in the magnetic layer is preferably in the range of 50 to 90 mass % and more preferably in the range of 60 to 90 mass % relative to the total mass of the magnetic layer. A high filling rate of the ferromagnetic powder in the magnetic layer is preferable from the viewpoint of improving recording density.

[0052] (binder) The magnetic recording medium may be a coating-type magnetic recording medium, and the magnetic layer may contain a binder. The binder is one or more resins. Various resins commonly used as binders for coating-type magnetic recording media can be used as binders. For example, binders may be selected from polyurethane resins, polyester resins, polyamide resins, vinyl chloride resins, acrylic resins copolymerized with styrene, acrylonitrile, methyl methacrylate, etc., cellulose resins such as nitrocellulose, epoxy resins, phenoxy resins, polyvinyl acetal, polyvinyl butyral, etc., and may be used alone or in combination. Among these, polyurethane resins, acrylic resins, cellulose resins, and vinyl chloride resins are preferred. These resins may be homopolymers or copolymers. These resins may also be used as binders in the nonmagnetic layer and / or backcoat layer, which will be described later. For details of the binders described above, see paragraphs 0028 to 0031 of JP 2010-24113 A. The average molecular weight of the resin used as the binder can be, for example, 10,000 to 200,000 in weight-average molecular weight. Unless otherwise specified, the weight-average molecular weight in this invention and this specification refers to a value measured by gel permeation chromatography (GPC) under the following measurement conditions and converted into polystyrene equivalents. The weight-average molecular weight of the binder shown in the examples below is a value measured under the following measurement conditions and converted into polystyrene equivalents. The binder can be used in an amount of, for example, 1.0 to 30.0 parts by mass per 100.0 parts by mass of the ferromagnetic powder. GPC equipment: HLC-8120 (Tosoh Corporation) Column: TSK gel Multipore HXL-M (Tosoh Corporation, 7.8 mm ID (inner diameter) x 30.0 cm) Eluent: tetrahydrofuran (THF)

[0053] (hardening agent) A curing agent can also be used together with a resin usable as a binder. In one embodiment, the curing agent can be a thermosetting compound, which undergoes a curing reaction (crosslinking reaction) upon heating. In another embodiment, the curing agent can be a photocurable compound, which undergoes a curing reaction (crosslinking reaction) upon irradiation with light. As the curing reaction progresses during the magnetic layer formation process, at least a portion of the curing agent can be included in the magnetic layer in a state where it has reacted (crosslinked) with other components, such as the binder. This also applies to layers formed using compositions containing a curing agent when the composition used to form other layers contains the curing agent. A preferred curing agent is a thermosetting compound, and polyisocyanate is suitable. For details about polyisocyanates, see paragraphs 0124 to 0125 of JP 2011-216149 A. The curing agent can be used in the magnetic layer-forming composition in an amount of, for example, 0 to 80.0 parts by weight per 100.0 parts by weight of the binder, preferably 50.0 to 80.0 parts by weight from the viewpoint of improving the strength of the magnetic layer.

[0054] (additives) The magnetic layer may contain one or more additives as needed. The additives may be selected from commercially available products or prepared by known methods according to the desired properties and used in any amount. Examples of additives include the curing agents mentioned above. Examples of additives contained in the magnetic layer include nonmagnetic powders, lubricants, dispersants, dispersion aids, antifungal agents, antistatic agents, and antioxidants. For information on lubricants, see paragraphs

[0030] to

[0033] ,

[0035] , and

[0036] of JP 2016-126817 A. The nonmagnetic layer described below may contain a lubricant. For information on lubricants that may be contained in the nonmagnetic layer, see paragraphs

[0030] ,

[0031] , and

[0034] to

[0036] of JP 2016-126817 A. For information on dispersants, see paragraphs

[0061] and

[0071] of JP 2012-133837 A. A dispersant may be added to the nonmagnetic layer-forming composition. For dispersants that can be added to the composition for forming the nonmagnetic layer, reference can be made to paragraph 0061 of JP-A No. 2012-133837.

[0055] Filler Non-magnetic powders that can be included in the magnetic layer include non-magnetic powders (fillers) that form appropriate protrusions on the magnetic layer surface to control friction characteristics. Examples of fillers that can be used include non-magnetic powders with an average particle size of 20 to 200 nm. Carbon black is one type of filler. Another type of filler is colloidal particles. Inorganic colloidal particles are preferred due to their availability, with inorganic oxide colloidal particles being more preferred, and silica colloidal particles (colloidal silica) being even more preferred. In the present invention and this specification, "colloidal particles" refers to particles that can disperse without settling and form a colloidal dispersion when added at 1 g per 100 mL of at least one organic solvent, such as methyl ethyl ketone, cyclohexanone, toluene, or ethyl acetate, or a mixed solvent containing two or more of the above solvents in any desired ratio. The filler content in the magnetic layer is preferably 0.2 to 3.0 parts by weight, more preferably 0.3 to 1.0 part by weight, per 100.0 parts by weight of ferromagnetic powder.

[0056] abrasives Non-magnetic powders that can be contained in the magnetic layer include non-magnetic powders (abrasives) that impart abrasive properties to the magnetic layer surface. Abrasives are preferably non-magnetic powders with a Mohs hardness of greater than 8, and more preferably non-magnetic powders with a Mohs hardness of 9 or greater. The maximum Mohs hardness is 10. On the other hand, fillers can be non-magnetic powders with a lower Mohs hardness than the non-magnetic powder used as the abrasive, such as non-magnetic powders with a Mohs hardness of 8 or less. Abrasives can be powders of inorganic or organic substances. Abrasives can be, for example, powders of inorganic or organic oxides or carbides. Examples of carbides include boron carbide (e.g., BC) and titanium carbide (e.g., TiC). Diamond can also be used as an abrasive. In one form, the abrasive is preferably a powder of an inorganic oxide. Specific examples of inorganic oxides include alumina such as α-alumina (e.g., Al2O3), titanium oxide (e.g., TiO2), cerium oxide (e.g., CeO2), and zirconium oxide (e.g., ZrO2), with alumina being preferred. Alumina has a Mohs hardness of approximately 9. For details about alumina powder, see paragraph 0021 of JP2013-229090A. For example, nonmagnetic powder with an average particle size of 0.05 to 0.2 μm can be used as the abrasive. The abrasive content in the magnetic layer is preferably 2.0 to 10.0 parts by weight, more preferably 4.0 to 8.0 parts by weight, per 100.0 parts by weight of ferromagnetic powder. Additives to improve the dispersibility of the abrasive can also be added to the magnetic layer containing the abrasive. Examples of such additives include the dispersants described in paragraphs 0012 to 0022 of JP2013-131285A.

[0057] The magnetic layer described above can be provided directly on the surface of the non-magnetic support, or indirectly via a non-magnetic layer.

[0058] <Nonmagnetic layer> Next, the non-magnetic layer will be described. The magnetic recording medium may have a magnetic layer directly on the surface of a non-magnetic support, or may have a magnetic layer on the surface of a non-magnetic support via one or more non-magnetic layers containing non-magnetic powder.

[0059] From the viewpoint of improving the smoothness of the magnetic layer surface, it is preferable to improve the surface smoothness of the non-magnetic layer on which the magnetic layer will be formed. From this viewpoint, it is preferable to use a non-magnetic powder with a small average particle size as the non-magnetic powder contained in the non-magnetic layer. The average particle size of the non-magnetic powder is preferably in the range of 500 nm or less, more preferably 200 nm or less, even more preferably 100 nm or less, and even more preferably 50 nm or less. Furthermore, from the viewpoint of facilitating the improvement of the dispersibility of the non-magnetic powder, the average particle size of the non-magnetic powder is preferably 5 nm or more, more preferably 7 nm or more, and even more preferably 10 nm or more.

[0060] The non-magnetic powder used in the non-magnetic layer may be either an inorganic powder or an organic powder, and carbon black or the like may also be used.

[0061] For information on carbon black that can be used in the nonmagnetic layer, see, for example, paragraphs 0040 to 0041 of Japanese Patent Laid-Open Publication No. 2010-24113. Carbon black generally tends to have a large particle size distribution and poor dispersibility. As a result, nonmagnetic layers containing carbon black tend to have low surface smoothness. In one embodiment, the nonmagnetic layer adjacent to the magnetic layer preferably contains a nonmagnetic powder other than carbon black as the nonmagnetic powder, or a nonmagnetic layer containing carbon black as one of multiple types of nonmagnetic powder, with a low proportion of carbon black in the total amount of nonmagnetic powder. It is also preferable to provide multiple nonmagnetic layers, with the nonmagnetic layer closest to the magnetic layer containing a nonmagnetic powder other than carbon black as the nonmagnetic powder. For example, it is preferable to provide two non-magnetic layers between the non-magnetic support and the magnetic layer, with the non-magnetic layer on the non-magnetic support side (also referred to as the "lower non-magnetic layer") containing carbon black as the non-magnetic powder, and the non-magnetic layer on the magnetic layer side (also referred to as the "upper non-magnetic layer") containing a non-magnetic powder other than carbon black as the non-magnetic powder. Furthermore, non-magnetic layer-forming compositions containing multiple types of non-magnetic powder tend to have lower dispersibility of the non-magnetic powder than non-magnetic layer-forming compositions containing only one type of non-magnetic powder. From this perspective, it is preferable to provide multiple non-magnetic layers and reduce the number of types of non-magnetic powder contained in each non-magnetic layer. In one embodiment, it is preferable to use a dispersant to improve the dispersibility of the non-magnetic powder in non-magnetic layer-forming compositions containing multiple types of non-magnetic powder. Such dispersants will be described later.

[0062] Examples of inorganic powders include powders of metals, metal oxides, metal carbonates, metal sulfates, metal nitrides, metal carbides, and metal sulfides. These non-magnetic powders are commercially available or can be produced by known methods. For details, see paragraphs 0146 to 0150 of JP 2011-216149 A.

[0063] One form of non-magnetic powder is non-magnetic iron oxide powder. From the viewpoint of improving the surface smoothness of the non-magnetic layer on which the magnetic layer is formed, it is preferable to use non-magnetic iron oxide powder with a small particle size. From this viewpoint, it is preferable to use non-magnetic iron oxide powder with an average particle size within the range described above. In one form, α-iron oxide powder is preferable as the non-magnetic iron oxide powder. α-iron oxide is iron oxide whose main phase is the α phase.

[0064] The non-magnetic powder content (filling rate) in the non-magnetic layer is preferably in the range of 50 to 90 mass %, more preferably 60 to 90 mass %, relative to the total mass of the non-magnetic layer. When multiple non-magnetic layers are provided, it is preferable that the non-magnetic powder content in at least one non-magnetic layer be in the above range, and it is even more preferable that the non-magnetic powder content in more non-magnetic layers be in the above range.

[0065] The non-magnetic layer contains a non-magnetic powder and may also contain a binder together with the non-magnetic powder. For details of the binder, additives, etc. of the non-magnetic layer, known techniques related to non-magnetic layers can be applied. Furthermore, for example, known techniques related to magnetic layers can also be applied to the type and content of the binder and the type and content of the additives.

[0066] Additives that can be included in the nonmagnetic layer include dispersants that contribute to improving the dispersibility of nonmagnetic powders. Examples of such dispersants include fatty acids represented by the formula RCOOH (R is an alkyl or alkenyl group) (e.g., caprylic acid, capric acid, lauric acid, myristic acid, palmitic acid, stearic acid, behenic acid, oleic acid, elaidic acid, linoleic acid, linolenic acid, etc.); alkali metal salts or alkaline earth metal salts of the above fatty acids; esters of the above fatty acids; fluorine-containing compounds of the above fatty acid esters; amides of the above fatty acids; polyalkylene oxide alkyl phosphate esters; lecithin; trialkyl polyolefinoxy quaternary ammonium salts (containing alkyl groups with 1 to 5 carbon atoms and olefins such as ethylene and propylene); phenylphosphonic acid; and copper phthalocyanine. These may be used alone or in combination. The content of the dispersant is preferably 0.2 to 5.0 parts by mass per 100.0 parts by mass of the nonmagnetic powder.

[0067] Further, an example of an additive is an organic tertiary amine. For details of organic tertiary amines, see paragraphs 0011 to 0018 and 0021 of JP 2013-049832 A. The organic tertiary amine can contribute to improving the dispersibility of carbon black. For details of the formulation of a composition for enhancing the dispersibility of carbon black using an organic tertiary amine, see paragraphs 0022 to 0024 and 0027 of the same publication.

[0068] The amine is more preferably a trialkylamine. The alkyl group in the trialkylamine is preferably an alkyl group having 1 to 18 carbon atoms. The three alkyl groups in the trialkylamine may be the same or different. For details about the alkyl groups, see paragraphs 0015 to 0016 of JP 2013-049832 A. As the trialkylamine, trioctylamine is particularly preferred.

[0069] In the present invention and this specification, the term "nonmagnetic layer" also includes a substantially nonmagnetic layer that contains a small amount of ferromagnetic powder, for example as an impurity or intentionally, along with a nonmagnetic powder. Here, a substantially nonmagnetic layer refers to a layer having a remanence of 10 mT or less, a coercivity of 7.96 kA / m (100 Oe) or less, or a remanence of 10 mT or less and a coercivity of 7.96 kA / m (100 Oe) or less. It is preferable that the nonmagnetic layer have no remanence or coercivity.

[0070] <Nonmagnetic support> Next, the non-magnetic support will be described. Examples of the non-magnetic support (hereinafter also simply referred to as "support") include known biaxially stretched polyethylene terephthalate, polyethylene naphthalate, polyamide, polyamideimide, aromatic polyamide, etc. Among these, polyethylene terephthalate, polyethylene naphthalate, and polyamide are preferred. These supports may be previously subjected to corona discharge, plasma treatment, easy-adhesion treatment, heat treatment, etc.

[0071] <Backcoat layer> The magnetic recording medium may or may not have a backcoat layer containing nonmagnetic powder on the surface of the nonmagnetic support opposite the surface having the magnetic layer. The backcoat layer preferably contains either carbon black or inorganic powder, or both. The backcoat layer may contain a binder and / or additives. Known techniques related to backcoat layers can be applied to the binder and additives of the backcoat layer, and known techniques related to the formulation of magnetic and / or nonmagnetic layers can also be applied. For example, see paragraphs

[0018] to

[0020] of Japanese Patent Laid-Open No. 2006-331625 and U.S. Patent No. 7,029,774, column 4, line 65 to column 5, line 38, for information regarding the backcoat layer.

[0072] <Various thicknesses> With regard to the thickness (total thickness) of magnetic recording media, the enormous increase in the amount of information in recent years has led to a demand for increased recording capacity (higher capacity) for magnetic recording media. One way to achieve higher capacity is to reduce the thickness of the magnetic recording media (hereinafter also referred to as "thinning"), for example, by increasing the length of magnetic tape housed in one magnetic tape cartridge. For example, from the above perspective, the thickness (total thickness) of the magnetic recording media is preferably 5.6 μm or less, more preferably 5.5 μm or less, even more preferably 5.4 μm or less, even more preferably 5.3 μm or less, and even more preferably 5.2 μm or less. Furthermore, from the perspective of ease of handling, the thickness of the magnetic recording media is preferably 3.0 μm or more, and more preferably 3.5 μm or more.

[0073] For example, the thickness (total thickness) of the magnetic recording medium can be measured by the following method. Ten samples (e.g., 5 to 10 cm long) are cut from any part of the magnetic recording medium, and these samples are stacked and measured for thickness. The measured thickness is divided by 10, and the resulting value (thickness per sample) is taken as the thickness of the magnetic recording medium. The thickness measurement can be performed using a known measuring device capable of measuring thickness to the order of 0.1 μm.

[0074] The thickness of the non-magnetic support is preferably 3.0 to 5.0 μm.

[0075] The thickness of the 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., and is generally 0.01 μm to 0.15 μm. From the viewpoint of high-density recording, it is preferably 0.02 μm to 0.12 μm, and more preferably 0.03 μm to 0.1 μm. At least one magnetic layer is sufficient, and the magnetic layer may be separated into two or more layers with different magnetic properties, and known configurations related to multilayer magnetic layers can be applied. When the magnetic layer is separated into two or more layers, the thickness of the magnetic layer refers to the total thickness of these layers. This also applies to the thickness of the nonmagnetic layer in a magnetic recording medium having multiple nonmagnetic layers.

[0076] Regarding the thickness of the nonmagnetic layer, the thicker the nonmagnetic layer, the more likely it is that the nonmagnetic powder particles will be unevenly distributed during the coating and drying processes of the nonmagnetic layer-forming composition, resulting in greater differences in thickness at different locations and a rougher surface for the nonmagnetic layer. From the perspective of improving the smoothness of the magnetic layer surface, a high level of surface smoothness for the nonmagnetic layer is desirable. From this perspective, the thickness of the nonmagnetic layer is preferably 1.5 μm or less, and more preferably 1.0 μm or less. Furthermore, from the perspective of improving the uniformity of the coating of the nonmagnetic layer-forming composition, the thickness of the nonmagnetic layer is preferably 0.05 μm or more, and more preferably 0.1 μm or more.

[0077] The thickness of the backcoat layer is preferably 0.9 μm or less, and more preferably 0.1 to 0.7 μm.

[0078] The thickness of the magnetic layer and other thicknesses can be determined by the following method. After exposing a cross section of the magnetic recording medium in the thickness direction with an ion beam, the exposed cross section is observed using a scanning electron microscope. The thicknesses can be calculated as the arithmetic mean of the thicknesses measured at any two points during the cross section observation. Alternatively, the thicknesses can be calculated as the design thickness calculated from the manufacturing conditions, etc.

[0079] <Manufacturing process> (Preparation of compositions for forming each layer) The process for preparing a composition for forming a magnetic layer, nonmagnetic layer, or backcoat layer typically includes at least a kneading step, a dispersion step, and optionally, a mixing step before or after these steps. Each step may be divided into two or more stages. The components used to prepare each layer-forming composition may be added at the beginning or during any step. Solvents may include one or more of the various solvents commonly used in the manufacture of particulate magnetic recording media. For details about solvents, see, for example, paragraph 0153 of JP 2011-216149 A. Individual components may also be added in separate steps in two or more steps. For example, the binder may be added in separate steps during the kneading step, dispersion step, and a mixing step for adjusting the viscosity after dispersion. Known manufacturing techniques can be used in the various steps to produce the magnetic recording medium. For the kneading step, it is preferable to use a device with strong kneading power, such as an open kneader, continuous kneader, pressure kneader, or extruder. For details of the kneading process, see JP-A-1-106338 and JP-A-1-79274. Known dispersers can be used. In one embodiment, the dispersion of the abrasive (hereinafter also referred to as "abrasive liquid") can be prepared by dispersing it separately from the ferromagnetic powder and filler. In another embodiment, the dispersion of the filler (filler liquid) can be prepared by dispersing it separately from the ferromagnetic powder and abrasive. Filtration may be performed by a known method at any stage in preparing the composition for forming each layer. Filtration can be performed, for example, by filter filtration. The filter used for filtration may be, for example, a filter with a pore size of 0.01 to 3 μm (for example, a glass fiber filter, a polypropylene filter, etc.).

[0080] (Coating process) The magnetic layer can be formed by applying the magnetic layer-forming composition directly to the surface of the non-magnetic support, or by sequential or simultaneous multilayer coating with the non-magnetic layer-forming composition. Sequential multilayer coating is preferred from the viewpoint of improving the smoothness of the magnetic layer surface. The backcoat layer can be formed by applying the backcoat layer-forming composition to the surface of the non-magnetic support opposite to the surface carrying the non-magnetic layer and / or magnetic layer (or the surface on which the non-magnetic layer and / or magnetic layer will be subsequently formed). For details on the coating for forming each layer, see paragraph 0066 of JP 2010-231843 A.

[0081] (Other processes) After the coating step, various treatments can be performed, such as drying, magnetic layer orientation, and surface smoothing (calendering). For details of these processes, see known techniques, such as paragraphs 0052 to 0057 of JP 2010-24113 A. For example, the coated layer of the magnetic layer-forming composition can be subjected to an orientation treatment while still wet. For the orientation treatment, various known techniques, including those described in paragraph 0067 of JP 2010-231843 A, can be applied. For example, vertical orientation can be performed by known methods, such as using magnets with opposite poles facing each other. In the orientation zone, the drying rate of the coated layer can be controlled by the temperature and volume of the drying air and / or the transport speed of the non-magnetic support bearing the coated layer. The coated layer may also be pre-dried before being transported to the orientation zone. Regarding calendering, strengthening the calendering conditions tends to improve the smoothness of the magnetic layer surface. Calendering conditions include the number of calendering passes (hereinafter also referred to as "calendering passes"), calendering pressure, calendering temperature (calender roll surface temperature), calendering speed, and calender roll hardness. The more the calendering passes, the stronger the calendering. The higher the calendering pressure, calendering temperature, and calender roll hardness, the stronger the calendering, and the slower the calendering speed. For example, the calendering pressure (linear pressure) can be 200 to 500 kg / cm, and preferably 250 to 350 kg / cm. The calendering temperature (calender roll surface temperature) can be, for example, 85 to 120°C, and preferably 90 to 110°C, and the calendering speed can be, for example, 50 to 300 m / min, and preferably 50 to 200 m / min. A long magnetic tape roll can be obtained through various processes. The obtained magnetic tape roll is cut (slit) using a known cutting machine to the width of the magnetic tape to be wound into a magnetic tape cartridge, for example. The width is determined according to a standard and is usually 1 / 2 inch. 1 / 2 inch = 12.65 mm. A servo pattern is usually formed on the magnetic tape obtained by slitting, as will be described in detail later.

[0082] (Heat treatment) In one embodiment, the magnetic recording medium can be a magnetic tape manufactured through the following heat treatment: In another embodiment, the magnetic recording medium can be a magnetic tape manufactured without the following heat treatment.

[0083] The heat treatment can be carried out by winding the magnetic tape, which has been slit and cut to a width determined in accordance with a standard, around a core member and carrying out the heat treatment in the wound state.

[0084] In one embodiment, the above-mentioned heat treatment is performed with the magnetic tape wound around a core-shaped member for heat treatment (hereinafter referred to as the "heat treatment core"), and the magnetic tape after the heat treatment is wound onto a reel of a magnetic tape cartridge, thereby producing a magnetic tape cartridge with the magnetic tape wound onto a reel. The heat treatment core can be made of metal, resin, paper, or the like. The material of the heat treatment core is preferably a highly rigid material from the viewpoint of suppressing winding defects such as spalling. From this viewpoint, the heat treatment core is preferably made of metal or resin. Furthermore, as an index of rigidity, the flexural modulus of the material of the heat treatment core is preferably 0.2 GPa (gigapascal) or more, more preferably 0.3 GPa or more. On the other hand, since highly rigid materials are generally expensive, using a heat treatment core made of a material with a rigidity exceeding the rigidity required to suppress winding defects leads to increased costs. In consideration of the above, the flexural modulus of the material of the heat treatment core is preferably 250 GPa or less. The flexural modulus is a value measured in accordance with ISO (International Organization for Standardization) 178, and the flexural moduli of various materials are known. Furthermore, the heat treatment core can be a solid or hollow core-shaped member. If the core is hollow, the wall thickness is preferably 2 mm or more from the viewpoint of maintaining rigidity. The core for heat treatment may or may not have a flange.It is preferable to prepare a magnetic tape having a length equal to or greater than the length to be ultimately accommodated in a magnetic tape cartridge (hereinafter referred to as the "final product length") as the magnetic tape to be wound around the heat treatment core, and to perform heat treatment by placing this magnetic tape wound around the heat treatment core in a heat treatment environment. The length of the magnetic tape to be wound around the heat treatment core is equal to or greater than the final product length, and from the viewpoint of ease of winding onto the heat treatment core, it is preferable that it be "final product length + α". From the viewpoint of ease of winding, this α is preferably 5 m or more. The tension during winding onto the heat treatment core is preferably 0.1 N (Newton) or more. Furthermore, from the viewpoint of preventing excessive deformation, the tension during winding onto the heat treatment core is preferably 1.5 N or less, more preferably 1.0 N or less. The outer diameter of the heat treatment core is preferably 20 mm or more, more preferably 40 mm or more, from the viewpoints of ease of winding and prevention of coiling (longitudinal curl). The outer diameter of the heat treatment core is preferably 100 mm or less, more preferably 90 mm or less. The width of the heat treatment core need only be equal to or greater than the width of the magnetic tape wound around the core. After the heat treatment, when removing the magnetic tape from the heat treatment core, it is preferable to remove the magnetic tape from the heat treatment core after the magnetic tape and the heat treatment core have cooled sufficiently to prevent unintended deformation of the tape during the removal operation. The removed magnetic tape is preferably first wound onto another core (referred to as a "temporary take-up core"), and then wound from the temporary take-up core onto a reel of a magnetic tape cartridge (typically with an outer diameter of approximately 40 to 50 mm). This allows the magnetic tape to be wound onto the reel of a magnetic tape cartridge while maintaining the inner and outer positions of the magnetic tape relative to the heat treatment core during the heat treatment. For details about the temporary take-up core and the tension when winding the magnetic tape onto this core, please refer to the previous description of the heat treatment core. In a form in which the above-mentioned heat treatment is performed on a magnetic tape having a length of "final product length + α", the length of "+ α" can be cut off at any stage. For example, in one form, the magnetic tape of the final product length can be wound from the temporary winding core onto the reel of the magnetic tape cartridge, and the remaining length of "+ α" can be cut off.From the viewpoint of reducing the portion that is cut off and discarded, it is preferable that the above-mentioned α be 20 m or less.

[0085] A specific example of the heat treatment carried out in the state where the core member is wound as described above will be described below. The atmospheric temperature at which the heat treatment is performed (hereinafter referred to as "heat treatment temperature") is preferably 40° C. or higher, and more preferably 50° C. or higher. On the other hand, from the viewpoint of suppressing excessive deformation, the heat treatment temperature is preferably 75° C. or lower, more preferably 70° C. or lower, and even more preferably 65° C. or lower. The absolute humidity by weight of the atmosphere in which the heat treatment is carried out is preferably 0.1 g / kg dry air or more, more preferably 1 g / kg dry air or more. An atmosphere with an absolute humidity by weight in the above range is preferred because it can be prepared without using special equipment for reducing moisture. On the other hand, from the viewpoint of preventing condensation from forming and reducing workability, the absolute humidity by weight is preferably 70 g / kg dry air or less, more preferably 66 g / kg dry air or less. The heat treatment time is preferably 0.3 hours or more, more preferably 0.5 hours or more. Furthermore, from the viewpoint of production efficiency, the heat treatment time is preferably 48 hours or less.

[0086] (Servo pattern formation) The magnetic recording medium can be a tape-shaped magnetic recording medium (i.e., magnetic tape) or a disk-shaped magnetic recording medium (i.e., magnetic disk). In either form, the magnetic layer can have a servo pattern. "Forming a servo pattern" can also be called "recording a servo signal." Hereinafter, the formation of a servo pattern will be explained using magnetic tape as an example.

[0087] The servo pattern is usually formed along the length of the magnetic tape. Control methods that use servo signals (servo control) include timing-based servo (TBS), amplitude servo, and frequency servo.

[0088] As specified in ECMA (European Computer Manufacturers Association)-319 (June 2001), magnetic tapes conforming to the LTO (Linear Tape-Open) standard (commonly referred to as "LTO tapes") employ a timing-based servo system. In this timing-based servo system, a servo pattern is formed by a pair of non-parallel magnetic stripes (also called "servo stripes") arranged continuously along the longitudinal direction of the magnetic tape. In this invention and this specification, the term "timing-based servo pattern" refers to a servo pattern that enables head tracking in a timing-based servo system. The reason why the servo pattern is formed by a pair of non-parallel magnetic stripes as described above is to inform a servo signal reading element passing over the servo pattern of its passing position. Specifically, the spacing between the pair of magnetic stripes is formed so that it continuously changes along the width direction of the magnetic tape. By reading this spacing, the servo signal reading element can determine the relative position between the servo pattern and the servo signal reading element. This relative position information enables tracking of data tracks. For this reason, a plurality of servo tracks are usually set on the servo pattern along the width direction of the magnetic tape.

[0089] A servo band is made up of a continuous servo pattern along the length of the magnetic tape. Typically, multiple servo bands are provided on a magnetic tape. For example, there are five servo bands on an LTO tape. The area between two adjacent servo bands is the data band. A data band is made up of multiple data tracks, and each data track corresponds to one of the servo tracks.

[0090] Also, in one embodiment, as disclosed in Japanese Patent Application Laid-Open No. 2004-318983, information indicating the servo band number (also called "servo band ID (identification)" or "UDIM (Unique Data Band Identification Method) information") is embedded in each servo band. This servo band ID is recorded by shifting a specific one of a plurality of pairs of servo stripes in the servo band so that its position is displaced relative to the longitudinal direction of the magnetic tape. Specifically, the way in which a specific one of a plurality of pairs of servo stripes is shifted varies for each servo band. As a result, the recorded servo band ID is unique for each servo band, and therefore, simply by reading one servo band with a servo signal reading element, that servo band can be uniquely identified.

[0091] One method for uniquely identifying servo bands is the staggered method described in ECMA-319 (June 2001). In this staggered method, pairs of non-parallel magnetic stripes (servo stripes) are continuously arranged along the length of the magnetic tape, and are recorded so that each servo band is offset along the length of the magnetic tape. The combination of this offset between adjacent servo bands is unique across the entire magnetic tape, making it possible to uniquely identify servo bands when reading the servo pattern with two servo signal reading elements.

[0092] As specified in ECMA-319 (June 2001), each servo band also typically contains information indicating the longitudinal position of the magnetic tape (also known as "LPOS (Longitudinal Position) information"). Like UDIM information, this LPOS information is recorded by shifting the positions of a pair of servo stripes along the longitudinal direction of the magnetic tape. However, unlike UDIM information, the same signal is recorded in each servo band with this LPOS information.

[0093] It is also possible to embed information other than the UDIM information and LPOS information described above in the servo bands. In this case, the embedded information may be different for each servo band, such as UDIM information, or it may be common to all servo bands, such as LPOS information. Furthermore, methods other than those described above can be used to embed information in the servo bands. For example, a predetermined code may be recorded by thinning out a predetermined pair from a group of pairs of servo stripes.

[0094] The head for forming a servo pattern is called a servo write head. A servo write head typically has a pair of gaps corresponding to the pair of magnetic stripes, the number of which is equal to the number of servo bands. Typically, a core and a coil are connected to each pair of gaps. By supplying a current pulse to the coil, the magnetic field generated in the core can generate a leakage magnetic field in the pair of gaps. To form a servo pattern, a current pulse is input while running a magnetic tape over the servo write head, thereby transferring the magnetic pattern corresponding to the pair of gaps onto the magnetic tape, thereby forming the servo pattern. The width of each gap can be appropriately set depending on the density of the servo pattern to be formed. The width of each gap can be set, for example, to 1 μm or less, 1 to 10 μm, or 10 μm or more.

[0095] Before forming a servo pattern on a magnetic tape, the magnetic tape is usually subjected to a demagnetization (erase) process. This erase process can be performed by applying a uniform magnetic field to the magnetic tape using a direct current magnet or an alternating current magnet. There are two types of erase processes: direct current (DC) erase and alternating current (AC) erase. AC erase is performed by gradually reducing the strength of the magnetic field applied to the magnetic tape while reversing the direction of the magnetic field. DC erase, on the other hand, is performed by applying a unidirectional magnetic field to the magnetic tape. There are two other DC erase methods. The first method is horizontal DC erase, in which a unidirectional magnetic field is applied along the length of the magnetic tape. The second method is vertical DC erase, in which a unidirectional magnetic field is applied along the thickness of the magnetic tape. The erase process can be performed on the entire magnetic tape or on each servo band of the magnetic tape.

[0096] The direction of the magnetic field of the formed servo pattern is determined by the erase direction. For example, when a magnetic tape is subjected to horizontal DC erasure, the servo pattern is formed so that the direction of the magnetic field is opposite to the erase direction. This increases the output of the servo signal obtained by reading the servo pattern. As shown in Japanese Patent Application Laid-Open No. 2012-53940, when a magnetic pattern is transferred to a magnetic tape that has been vertically DC erased using the gap, the servo signal obtained by reading the formed servo pattern has a unipolar pulse shape. On the other hand, when a magnetic pattern is transferred to a magnetic tape that has been horizontally DC erased using the gap, the servo signal obtained by reading the formed servo pattern has a bipolar pulse shape.

[0097] In one embodiment, the width of the magnetic tape can be controlled by acquiring width dimensional information of the magnetic tape while it is running using a servo signal and adjusting and changing the tension applied to the magnetic tape in the longitudinal direction in accordance with the acquired dimensional information. Adjusting the tension in this manner can contribute to preventing the magnetic head for recording or reproducing data from being displaced from the target track position due to width deformation of the magnetic tape during recording or reproduction.

[0098] [Magnetic tape cartridge] In one embodiment, the magnetic recording medium may be a magnetic tape. Another aspect of the present invention relates to a magnetic tape cartridge including the magnetic tape.

[0099] The details of the magnetic tape contained in the tape cartridge are as described above.

[0100] A magnetic tape cartridge generally contains a magnetic tape wound on a reel within the cartridge body. The reel is rotatably mounted within the cartridge body. Widely used magnetic tape cartridges include single-reel magnetic tape cartridges with one reel within the cartridge body and dual-reel magnetic tape cartridges with two reels within the cartridge body. When a single-reel magnetic tape cartridge is loaded into a magnetic tape device for recording and / or reproducing data on the magnetic tape, the magnetic tape is pulled out from the magnetic tape cartridge and rewound onto a reel on the magnetic tape device. A magnetic head is located in the magnetic tape transport path from the magnetic tape cartridge to the take-up reel. The magnetic tape is fed and rewound between the reel (supply reel) on the magnetic tape cartridge and the reel (take-up reel) on the magnetic tape device. During this process, the magnetic head comes into contact with and slides against the magnetic layer surface of the magnetic tape, thereby recording and / or reproducing data. In contrast, a dual-reel magnetic tape cartridge is provided with both a supply reel and a take-up reel inside the magnetic tape cartridge.

[0101] [Magnetic recording and playback device] One aspect of the present invention relates to a magnetic recording and reproducing device including the above magnetic recording medium.

[0102] In this invention and this specification, the term "magnetic recording and reproducing device" refers to a device capable of at least one of recording data to a magnetic recording medium and reproducing data recorded on the magnetic recording medium. Such devices are generally called drives. In one embodiment, in the magnetic recording and reproducing device, data can be recorded on the magnetic recording medium and / or reproduced by contacting and sliding a magnetic head with the magnetic layer surface of the magnetic recording medium. Such a magnetic recording and reproducing device is generally called a sliding drive or a contact sliding drive. The magnetic head included in the magnetic recording and reproducing device can be a recording head capable of recording data on the magnetic recording medium or a reproducing head capable of reproducing data recorded on the magnetic recording medium. In one embodiment, the magnetic recording and reproducing device can include both a recording head and a reproducing head as separate magnetic heads. In another embodiment, the magnetic head included in the magnetic recording and reproducing device can have both a recording element and a reproducing element in a single magnetic head. The reproducing head is preferably a magnetic head (MR head) including a magnetoresistive (MR) element as a reproducing element, which can read information recorded on a magnetic recording medium with high sensitivity. Various known MR heads (e.g., GMR (Giant Magnetoresistive) heads, TMR (Tunnel Magnetoresistive) heads, etc.) can be used as the MR head. The magnetic head that records and / or reproduces data may include a servo signal reading element. Alternatively, the magnetic recording and reproducing device may include a magnetic head (servo head) equipped with a servo signal reading element, separate from the magnetic head that records and / or reproduces data. For example, the magnetic head that records and / or reproduces recorded data (hereinafter also referred to as a "recording and reproducing head") may include two servo signal reading elements, each of which can simultaneously read two adjacent servo bands sandwiching a data band.One or more data elements can be placed between the two servo signal read elements. The elements for recording data (recording elements) and the elements for reproducing data (reproducing elements) are collectively called "data elements."

[0103] When recording and / or reproducing data, 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 data track is moved by changing the servo track read by the servo signal reading element in the tape width direction. The read / write head can also record and / or read data from other data bands by using the UDIM information described above to move the servo signal read element to a specific servo band and start tracking that servo band.

[0104] FIG. 1 shows an example of the arrangement of data bands and servo bands. In FIG. 1, multiple servo bands 1 are sandwiched between guide bands 3 on the magnetic layer of a magnetic tape MT. Multiple regions 2 sandwiched between two servo bands form data bands. Servo patterns are magnetized regions formed by magnetizing specific regions of the magnetic layer with a servo write head. The regions magnetized by the servo write head (the positions where servo patterns are formed) are determined by standards. 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 FIG. 2. Specifically, in FIG. 2, a servo frame SF on servo band 1 is composed of a servo subframe 1 (SSF1) and a servo subframe 2 (SSF2). Servo subframe 1 is composed of an A burst (labeled A in FIG. 2) and a B burst (labeled B in FIG. 2). The A burst is composed of servo patterns A1 to A5, and the B burst is composed of servo patterns B1 to B5. On the other hand, servo subframe 2 is composed of a C burst (labeled C in FIG. 2) and a D burst (labeled D in FIG. 2). The C burst is composed of servo patterns C1 through C4, and the D burst is composed of servo patterns D1 through D4. These 18 servo patterns are arranged in sets of 5 and 4 in a 5, 5, 4, 4 arrangement in a subframe, and are used to identify servo frames. For illustrative purposes, FIG. 2 shows one servo frame. However, in reality, on the magnetic layer of a magnetic tape where head tracking is performed using a timing-based servo system, multiple servo frames are arranged in each servo band in the running direction. In FIG. 2, the arrow indicates the running direction. For example, an LTO Ultrium format tape typically has more than 5,000 servo frames per meter of tape length in each servo band of the magnetic layer.

[0105] In one embodiment of the magnetic recording and reproducing device, the magnetic recording medium is treated as a removable medium (so-called interchangeable medium), and for example, a magnetic tape cartridge containing a magnetic tape is inserted into and removed from the magnetic recording and reproducing device. In another embodiment, the magnetic recording medium is not treated as an interchangeable medium, and for example, the magnetic tape is wound around a reel of a magnetic recording and reproducing device equipped with a magnetic head, and the magnetic tape is housed in the magnetic recording and reproducing device. In one embodiment, in such a magnetic recording and reproducing device, the magnetic tape and the magnetic head can be housed in an enclosed space within the magnetic recording and reproducing device. In the present invention and this specification, the term "enclosed space" refers to a space in which the degree of airtightness evaluated by the immersion method (bombing method) using helium (He) specified in JIS Z 2331:2006 Helium Leak Test Method is 10 x 10 -8 Pa·m 3 / sec or less. The airtightness of an enclosed space is, for example, 5 x 10 ―9 Pa·m 3 / sec or more 10 x 10 -8 Pa·m 3 / sec or less, or may be below the above range. In one embodiment, the entire space in the housing can be the sealed space, and in another embodiment, a portion of the space in the housing can be the sealed space. The sealed space can be the internal space of a housing that covers all or part of the magnetic recording and reproducing device. The material and shape of the housing are not particularly limited, and can be, for example, the same as the material and shape of the housing of a typical magnetic recording and reproducing device. Examples of materials for the housing include metal and resin. [Example]

[0106] An embodiment of the present invention will be described below based on an example. However, the present invention is not limited to the embodiment shown in the example. The indications of "parts" and "%" below mean "parts by mass" and "% by mass" unless otherwise specified. "eq" is an equivalent, and is a unit that cannot be converted to SI units. Unless otherwise specified, the following steps and operations were carried out in an environment with a temperature of 20 to 25°C and a relative humidity of 40 to 60%.

[0107] In Table 1 below, "BaFe" represents hexagonal barium ferrite powder with an average particle size (average plate diameter) of 21 nm.

[0108] The ferromagnetic powder referred to as "metal powder" in Table 1 below was an iron-cobalt alloy ferromagnetic powder (average particle size (average major axis length) 50 nm) similar to the metal powder used in Example 1 of JP-A No. 2004-348897 (Patent Document 2) shown above.

[0109] In Table 1 below, "SrFe" represents hexagonal strontium ferrite powder prepared by the method described below, and "ε-iron oxide" represents ε-iron oxide powder prepared by the method described below. The average particle volumes of the various ferromagnetic powders described below are values ​​determined by the method described above. The various values ​​relating to the particle sizes of the various powders described below are also values ​​determined by the method described above. The anisotropy constant Ku is a value determined for each ferromagnetic powder by the method described above using a vibrating sample magnetometer (manufactured by Toei Kogyo Co., Ltd.). The mass magnetization σs is a value measured at a magnetic field strength of 15 kOe using a vibrating sample magnetometer (manufactured by Toei Kogyo Co., Ltd.).

[0110] [Method for producing ferromagnetic powder] <Method for producing hexagonal strontium ferrite powder> 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 were weighed and mixed in a mixer to obtain a raw material mixture. The resulting raw material mixture was melted in a platinum crucible at a melting temperature of 1390°C, and the melt was stirred while heating the tapping hole at the bottom of the platinum crucible, causing the melt to be poured into a rod shape at a rate of approximately 6 g / sec. The tapped liquid was rolled and quenched using a water-cooled twin roller to produce an amorphous body. 280 g of the produced amorphous body was placed in an electric furnace, heated to 635°C (crystallization temperature) at a rate of 3.5°C / min, and held at that temperature for 5 hours to precipitate (crystallize) hexagonal strontium ferrite particles. The crystallized material obtained above, containing hexagonal strontium ferrite particles, was then coarsely crushed in a mortar. 1,000 g of 1 mm zirconia beads and 800 ml of 1% acetic acid solution were added to a glass bottle and dispersed for 3 hours using a paint shaker. The resulting dispersion was then separated from the beads and placed in a stainless steel beaker. The dispersion was left to stand at 100°C for 3 hours to dissolve the glass components, after which it was precipitated in a centrifuge and washed by repeated decantation. It was then dried for 6 hours in a heating furnace at 110°C to obtain hexagonal strontium ferrite powder. The average particle volume of the hexagonal strontium ferrite powder obtained above ("SrFe" in Table 1 below) was 900 nm 3 , the anisotropy constant Ku is 2.2×10 5 J / m 3 , mass magnetization σs is 49A m 2 / kg. A 12 mg sample powder was taken from the hexagonal strontium ferrite powder obtained above, and this sample powder was partially dissolved under the dissolution conditions exemplified above. The filtrate was subjected to elemental analysis using an ICP analyzer to determine the neodymium atom content in the surface layer. Separately, 12 mg of sample powder was collected from the hexagonal strontium ferrite powder obtained above, and this sample powder was completely dissolved under the dissolution conditions exemplified above. The filtrate thus obtained was subjected to elemental analysis using an ICP analyzer to determine the bulk content of neodymium atoms. The neodymium atom content (bulk content) of the hexagonal strontium ferrite powder obtained above relative to 100 atomic percent of iron atoms was 2.9 atomic percent. The neodymium atom content in the surface layer was 8.0 atomic percent. The ratio of the surface layer content to the bulk content, "surface layer content / bulk content," was 2.8, confirming that neodymium atoms were unevenly distributed in the surface layers of the particles.

[0111] The powder obtained above was confirmed to have a hexagonal ferrite crystal structure by scanning with CuKα radiation at a voltage of 45 kV and an intensity of 40 mA and measuring the X-ray diffraction pattern under the following conditions (X-ray diffraction analysis). The powder obtained above exhibited a magnetoplumbite (M-type) hexagonal ferrite crystal structure. Furthermore, the crystalline phase detected by X-ray diffraction analysis was a single magnetoplumbite phase. PANalytical X'Pert Pro diffractometer, PIXcel detector Soller slits for incident and diffracted beams: 0.017 radians Dispersion slit fixed angle: 1 / 4 degree Mask: 10mm Anti-scatter 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

[0112] <Method for producing ε-iron oxide powder> 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 in 90 g of pure water. While stirring using a magnetic stirrer, 4.0 g of 25% aqueous ammonia was added in air at an ambient temperature of 25°C. The mixture was stirred for 2 hours at 25°C. A citric acid solution prepared by dissolving 1 g of citric acid in 9 g of pure water was added to the resulting solution and stirred for 1 hour. The precipitated powder was collected by centrifugation, washed with pure water, and dried in a heating oven at 80°C. 800g of pure water was added to the dried powder, and the powder was dispersed in water again to obtain a dispersion. The resulting 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 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 for 24 hours in a heating furnace at 80°C to obtain a precursor of the ferromagnetic powder. The obtained precursor of the ferromagnetic powder was placed in a heating furnace at an internal temperature of 1000° C. in an air atmosphere and subjected to heat treatment for 4 hours. The heat-treated ferromagnetic powder precursor was placed in a 4 mol / L aqueous solution of sodium hydroxide (NaOH), and the liquid temperature was maintained at 70°C while stirring for 24 hours, thereby removing the impurity silicate compound from the heat-treated ferromagnetic powder precursor. Thereafter, the silicate compound was removed by centrifugation, and the ferromagnetic powder was collected and washed with pure water to obtain a ferromagnetic powder. The composition of the obtained ferromagnetic powder was confirmed by inductively coupled plasma optical emission spectrometry (ICP-OES). It was found that the powder consisted of Ga, Co, and Ti substituted ε-iron oxide (ε-Ga 0.28 Co 0.05 Ti 0.05 Fe 1.62 O3). Furthermore, X-ray diffraction analysis was performed under the same conditions as those described above for the preparation of hexagonal strontium ferrite powder, and it was confirmed from the peaks in the X-ray diffraction pattern that the obtained ferromagnetic powder had a single-phase ε-phase crystal structure (ε-iron oxide crystal structure) that did not contain α-phase or γ-phase crystal structures. The average particle volume of the obtained ε-iron oxide powder (referred to as "ε-iron oxide" in Table 1 below) was 750 nm 3 , the anisotropy constant Ku is 1.2×10 5 J / m 3 , mass magnetization σs is 16A m 2 / kg.

[0113] In Table 1 below, for Examples and Comparative Examples in which only one nonmagnetic layer was formed, information relating to the nonmagnetic layer is shown in the "Lower Nonmagnetic Layer" column.

[0114] [Example 1] (1) Preparation of alumina dispersion 100.0 parts of α-alumina powder with the average particle size shown in Table 1 was mixed with 3.0 parts of 2,3-dihydroxynaphthalene (Tokyo Chemical Industry Co., Ltd.), 31.3 parts of a 32% solution of polyester polyurethane resin (UR-4800 manufactured by Toyobo Co., Ltd. (polar group amount: 80 meq / kg)) having SO3Na groups as polar groups (solvent: a mixed solvent of methyl ethyl ketone and toluene), and 570.0 parts of a mixed solution of methyl ethyl ketone and cyclohexanone in a 1:1 (mass ratio) solvent, and dispersed for 5 hours using a paint shaker in the presence of zirconia beads. After dispersion, the dispersion and the beads were separated using a mesh to obtain an alumina dispersion.

[0115] (2) Magnetic Layer Formulation (Magnetic liquid) Ferromagnetic powder (type: see Table 1) 100.0 parts Polyurethane resin 10.0 parts Toyobo UR-4800 (sulfonic acid group-containing polyester polyurethane resin) Cyclohexanone 150.0 parts Methyl ethyl ketone 150.0 parts (abrasive liquid) 6.0 parts of the alumina dispersion prepared in (1) above (filler liquid) Filler 0.5 parts Type: Carbon black (average particle size: 80 nm) Methyl ethyl ketone 1.4 parts (Other ingredients) Stearic acid 2.0 parts Stearic acid amide 0.2 parts Butyl stearate 2.0 parts Polyisocyanate (Tosoh Corporation, Coronate (registered trademark) L) 2.5 parts (Finishing additive solvent) Cyclohexanone 200.0 parts Methyl ethyl ketone 200.0 parts

[0116] (3) Nonmagnetic Layer Formulation Non-magnetic powder: α-iron oxide 100.0 parts Average particle size (average major axis length): See Table 1 Acicular ratio: 7 BET (Brunauer-Emmett-Teller) specific surface area: 52m 2 / g Carbon black 20.0 parts Average particle size: 20nm SO3Na group-containing polyurethane resin 18.0 parts Weight average molecular weight: 70,000, SO3Na group: 0.2meq / g Stearic acid 2.0 parts Stearic acid amide 0.2 parts Butyl stearate 2.0 parts Cyclohexanone 300.0 parts Methyl ethyl ketone 300.0 parts

[0117] (4) Formulation of composition for forming backcoat layer Carbon black 100.0 parts DBP (Dibutyl phthalate) oil absorption: 74cm 3 / 100g Nitrocellulose 27.0 parts Polyester polyurethane resin containing sulfonic acid groups and / or their salts 62.0 copies Polyester resin 4.0 parts Alumina powder (BET specific surface area: 17 m 2 / g) 0.6 parts Methyl ethyl ketone 600.0 parts Toluene 600.0 parts Polyisocyanate (Tosoh Corporation, Coronate (registered trademark) L) 15.0 parts

[0118] (5) Preparation of compositions for forming each layer A magnetic layer-forming composition was prepared by the following method. The magnetic liquid was prepared by dispersing the above components for 24 hours (bead dispersion) using a batch-type vertical sand mill. Zirconia beads with a bead diameter of 0.5 mm were used as the dispersion beads. The filler liquid was liquefied by mixing the components of the filler liquid in a batch-type ultrasonic disperser equipped with an agitator at a rotation speed of 1500 rpm (revolutions per minute) for 30 minutes. The liquefied filler liquid was dispersed in a horizontal bead mill disperser using zirconia beads with a bead diameter of 0.5 mm, with a bead filling rate of 80% by volume, a rotor tip peripheral speed of 10 m / s, a residence time per pass of 2 minutes, and the number of passes shown in the "Filler Liquid Dispersion Treatment" column in Table 1. The dispersed liquid was stirred in a dissolver agitator at a peripheral speed of 10 m / s for 30 minutes, and then subjected to three passes in a flow-type ultrasonic disperser at a flow rate of 3 kg / min. The prepared magnetic liquid and filler liquid were mixed with the abrasive liquid and other components (other components and finishing additive solvent) using the sand mill, and then bead dispersed for 5 minutes, followed by treatment (ultrasonic dispersion) for 0.5 minutes using a batch-type ultrasonic device (20 kHz, 300 W).The mixture was then filtered using a filter with a pore size of 0.5 μm to prepare a magnetic layer-forming composition. The nonmagnetic layer-forming composition was prepared by the following method. The above components, excluding the lubricants (stearic acid, stearic acid amide, and butyl stearate), were kneaded and diluted using an open kneader, and then dispersed using a horizontal bead mill disperser. The lubricants (stearic acid, stearic acid amide, and butyl stearate) were then added, and the mixture was stirred and mixed using a dissolver stirrer to prepare the nonmagnetic layer-forming composition. The backcoat layer-forming composition was prepared by the following method. The above components, excluding the polyisocyanate, were introduced into a dissolver mixer and stirred at a peripheral speed of 10 m / s for 30 minutes, and then dispersed using a horizontal bead mill disperser. The polyisocyanate was then added, and the mixture was stirred and mixed using the dissolver mixer to prepare the backcoat layer-forming composition.

[0119] (6) Manufacturing of magnetic tapes and magnetic tape cartridges The nonmagnetic layer-forming composition prepared in (5) above was applied and dried on the surface of a 4.1 μm-thick biaxially oriented polyethylene terephthalate support to a thickness after drying listed in Table 1, forming a nonmagnetic layer. The magnetic layer-forming composition prepared in (5) above was then applied to the nonmagnetic layer to a thickness after drying of 0.1 μm, forming a coating layer. While the magnetic layer-forming composition coating was still wet, a magnetic field with a strength of 0.3 T was applied perpendicular to the surface of the coating layer to perform a vertical orientation treatment, followed by drying to form a magnetic layer. In other words, sequential coating was used as the coating method. The backcoat layer-forming composition prepared in (5) above was then applied to the surface of the support opposite the surface on which the nonmagnetic and magnetic layers were formed, to a thickness after drying of 0.3 μm, forming a backcoat layer. Thereafter, a surface smoothing treatment (calendering treatment) was carried out using a calender roll consisting only of a metal roll at a speed of 100 m / min, a linear pressure of 300 kg / cm, and a calender temperature (surface temperature of the calender roll) of 90°C (calendering number of times: 2). The long magnetic tape was then heat-treated by storing it in a heat treatment furnace at an ambient temperature of 70°C (heat treatment time: 36 hours). After heat treatment, the tape was slit into 1 / 2-inch widths to obtain magnetic tape. Servo signals were recorded on the magnetic layer of the obtained magnetic tape using a commercially available servo writer, resulting in a magnetic tape with data bands, servo bands, and guide bands arranged in accordance with the LTO (Linear Tape-Open) Ultrium format, and with servo patterns (timing-based servo patterns) on the servo bands arranged and shaped in accordance with the LTO Ultrium format. The servo patterns thus formed conform to the specifications of JIS (Japanese Industrial Standards) X6175:2006 and Standard ECMA-319 (June 2001). The total number of servo bands was five, and the total number of data bands was four. The magnetic tape (length 970 m) after the servo pattern formation was taken up onto a core for heat treatment, and was heat treated while still wound on this core. A solid core-shaped member (outer diameter: 50 mm) made of resin with a flexural modulus of elasticity of 0.8 GPa was used as the core for heat treatment, and the tension during winding was 0.6 N. The heat treatment was carried out at a heat treatment temperature of 50°C for 5 hours. The weight absolute humidity of the atmosphere in which the heat treatment was carried out was 10 g / kg dry air. After the above heat treatment, once the magnetic tape and heat treatment core had sufficiently cooled, the magnetic tape was removed from the heat treatment core and wound onto a temporary take-up core. Thereafter, the final product length (960 m) of magnetic tape was wound from the temporary take-up core onto the reel of the magnetic tape cartridge (reel outer diameter: 44 mm). The remaining 10 m was cut off, and a leader tape in accordance with item 9 of Section 3 of Standard ECMA (European Computer Manufacturers Association)-319 (June 2001) was spliced ​​to the end of the cut-off side using commercially available splicing tape. The temporary take-up core was a solid core-shaped member made of the same material and with the same outer diameter as the heat treatment core, and the tension during winding was 0.6 N. As a result of the above, a single-reel magnetic tape cartridge was produced in which a 960 m long magnetic tape was wound on a reel.

[0120] [Example 2] Magnetic tapes and magnetic tape cartridges were prepared by the method described for Example 1, except that α-alumina powder with the average particle size shown in Table 1 was used as the abrasive and the number of passes of the filler liquid dispersion treatment was changed as shown in Table 1.

[0121] [Example 3] A magnetic tape and a magnetic tape cartridge were produced by the same method as described in Example 1, except that the non-magnetic powder for the non-magnetic layer was changed to α-iron oxide having the average particle size shown in Table 1.

[0122] [Example 4] A magnetic tape and a magnetic tape cartridge were prepared by the method described for Example 3, except that the nonmagnetic layer was formed by applying and drying the composition for forming the nonmagnetic layer so that the thickness after drying was the thickness shown in Table 1.

[0123] [Example 5] A magnetic tape and a magnetic tape cartridge were produced by the method described in Example 4, except that the ferromagnetic powders shown in the "Ferromagnetic Powder" column in Table 1 were used as the ferromagnetic powders.

[0124] [Example 6] Two non-magnetic layers were formed as described below, and a magnetic layer was formed by applying a magnetic layer-forming composition to the upper non-magnetic layer as described in Example 1, and a magnetic tape and a magnetic tape cartridge were produced by the method described in Example 1, except that the number of calendering passes was one.

[0125] <Formulation of composition for forming lower nonmagnetic layer> Carbon black (average particle size: 20 nm) 100.0 parts Trioctylamine 4.0 parts Vinyl chloride resin 12.0 parts 1.5 parts stearic acid Stearic acid amide 0.3 parts Butyl stearate 1.5 parts Cyclohexanone 200.0 parts Methyl ethyl ketone 510.0 parts

[0126] <Formulation of composition for forming upper nonmagnetic layer> Non-magnetic powder α-iron oxide 100.0 parts Average particle size (average major axis length): 30nm Average short axis length: 15nm Acicular ratio: 2.0 SO3Na group-containing polyurethane resin 18.0 parts Weight average molecular weight: 70,000, SO3Na group: 0.2meq / g Stearic acid 1.0 parts Cyclohexanone 300.0 parts Methyl ethyl ketone 300.0 parts

[0127] For each of the compositions for forming the lower nonmagnetic layer and the upper nonmagnetic layer, the components were kneaded in an open kneader for 240 minutes and then dispersed in a sand mill. The dispersion conditions for each nonmagnetic layer composition were a dispersion time of 24 hours, and zirconia beads with a diameter of 0.1 mm were used. 4.0 parts of polyisocyanate (Tosoh Coronate 3041) was added to each dispersion, which was then stirred and mixed for another 20 minutes, and then filtered using a filter with a pore size of 0.5 μm. In this manner, a lower nonmagnetic layer-forming composition and an upper nonmagnetic layer-forming composition were prepared. The lower nonmagnetic layer-forming composition was applied to one surface of a support similar to that in Example 1 so as to have a thickness after drying shown in Table 1, and then dried in an environment with an ambient temperature of 100°C to form a lower nonmagnetic layer. The upper nonmagnetic layer-forming composition was applied to the lower nonmagnetic layer so as to have a thickness after drying shown in Table 1, and then dried in an environment with an ambient temperature of 100°C to form an upper nonmagnetic layer.

[0128] [Examples 7 and 8] A magnetic tape and a magnetic tape cartridge were produced by the method described in Example 6, except that the ferromagnetic powders shown in the "Ferromagnetic Powder" column in Table 1 were used as the ferromagnetic powders.

[0129] [Comparative Example 1] A magnetic tape and a magnetic tape cartridge were prepared by the method described in Example 1, except that α-alumina powder having the average particle size shown in Table 1 was used as the abrasive.

[0130] [Example 9] A magnetic tape was produced by the method described for Comparative Example 1. Before the produced magnetic tape was housed in a magnetic tape cartridge, the entire length of the magnetic tape was subjected to the following sliding treatment (hereinafter referred to as "sliding treatment during the manufacturing process"). A recording / playback head (LTO8 head) mounted on an IBM LTO (Linear Tape-Open) 8 tape drive was used as the sliding member. The magnetic tape was run in a magnetic tape transport device under the following running conditions, causing the sliding member to come into contact with the surface of the magnetic layer and slide. The tension values ​​applied to the magnetic tape in the longitudinal direction and the magnetic tape running speed below are the settings for the magnetic tape transport device. Regarding units, "gf" stands for gram force, and 1 N (Newton) is approximately 102 gf. (Driving conditions) Magnetic tape running speed: 4 m / s Tension applied to the magnetic tape in the longitudinal direction: 100 gf Magnetic tape travel passes: 20,000 single passes Wrap angle θ: 1°

[0131] Comparative Example 2 A magnetic tape and a magnetic tape cartridge were produced by the method described for Comparative Example 1, except that the number of passes of the filler liquid dispersion treatment was changed as shown in Table 1.

[0132] Comparative Example 3 A magnetic tape was produced according to the description of Example 1 of JP 2014-209403 A (Patent Document 1). However, the ferromagnetic powder used was the same as in Example 1, as shown in Table 1; the abrasive used was the same α-alumina powder with the average particle size listed in Table 1 as in Comparative Example 1, as shown in Table 1; and the nonmagnetic powder used for the nonmagnetic layer was the same carbon black as in Example 1 and α-iron oxide with the average particle size listed in Table 1. As described in paragraph 0067 of JP 2014-209403 A (Patent Document 1), the filler contained in the magnetic layer was colloidal silica. For Comparative Example 3, the magnetic layer-forming composition was prepared according to the description in paragraph 0071 of JP 2014-209403 A (Patent Document 1), and therefore the liquid containing colloidal silica was not subjected to a dispersion treatment (dispersion treatment of the filler liquid) before mixing with other components. A servo pattern was formed on the prepared magnetic tape in the same manner as in Example 1, and then the magnetic tape was housed in a magnetic tape cartridge. In this way, a single-reel magnetic tape cartridge was produced in which a 960 m long magnetic tape was wound on a reel.

[0133] Comparative Example 4 A magnetic tape was produced using the metal powder described above, in accordance with Example 1 of JP 2004-348897 A (Patent Document 2). As described in paragraph 0050 of JP 2004-348897 A (Patent Document 2), the magnetic layer and non-magnetic layer were coated by simultaneous multilayer coating. For Comparative Example 4, the magnetic layer-forming composition was prepared in accordance with the description in paragraph 0050 of JP 2004-348897 A (Patent Document 2), and therefore the liquid containing carbon black was not subjected to a dispersion treatment (dispersion treatment of the filler liquid) before being mixed with other components. A servo pattern was formed on the prepared magnetic tape in the same manner as in Example 1, and then the magnetic tape was housed in a magnetic tape cartridge. In this way, a single-reel magnetic tape cartridge was produced in which a 960 m long magnetic tape was wound on a reel.

[0134] For each of the examples and comparative examples, two magnetic tape cartridges were produced, one of which was used to determine the protrusion height difference Δ and magnetic layer surface Ra below, and the other was used to evaluate the electromagnetic conversion characteristics before and after repeated running in a high-temperature environment as described below.

[0135] [Physical property evaluation] (1)Magnetic layer surface Ra The following conditions were used as the AFM measurement conditions, and the magnetic layer surface Ra was determined for each of the magnetic tapes of the Examples and Comparative Examples by the method described above. An AFM (Veeco Nanoscope 4) was used in tapping mode to measure an area of ​​40 μm × 40 μm on the surface of the magnetic layer of the magnetic tape. A BRUKER RTESP-300 probe was used, with a resolution of 512 pixels × 512 pixels and a scan speed of 341 seconds per screen (512 pixels × 512 pixels).

[0136] (2) Protrusion height difference Δ For each of the magnetic tapes of the examples and comparative examples, the protrusion height HD and the protrusion height HB were determined by the method described above, and the protrusion height difference Δ(HD-HB) was calculated from the determined values. For the Examples and Comparative Examples shown in Table 2 below, the reference protrusion height difference Δ was calculated as follows from the measurement results obtained by the method described above for three measurement areas on the surface of the magnetic layer. ref asked for. For each measurement area, the arithmetic mean of the height of the dark area was calculated as the arithmetic mean of all the dark areas, with the reference plane being 0 nm. The arithmetic mean of the three values ​​obtained for the three measurement areas was calculated, and this was used as the HD ref In addition, for each measurement area, the arithmetic mean of the height of the bright area, with the reference plane at 0 nm, was calculated as the arithmetic mean of all the bright areas. The arithmetic mean of the three values ​​obtained for the three measurement areas was calculated, and this was used as the HB ref It was decided. The above HD ref and H.B. ref From the reference protrusion height difference Δ ref =HD ref -HB ref The reference protrusion height difference Δ ref can be said to be the difference in protrusion height between the dark and bright areas, calculated with the reference surface at 0 nm. Here, "ref" is used as an abbreviation for "reference."

[0137] [Evaluation of electromagnetic conversion characteristics before and after repeated driving in a high-temperature environment] (1) Evaluation of electromagnetic conversion characteristics before repeated driving in a high-temperature environment The following electromagnetic characteristics were evaluated in an environment with an ambient temperature of 23°C ± 1°C and a relative humidity of 50%. Figure 3 shows a schematic diagram of the reel tester used to run the magnetic tape. For each of the examples and comparative examples, a 100-m-long tape sample was cut from an arbitrary position in the longitudinal direction of the magnetic tape removed from the magnetic tape cartridge and attached to a 1 / 2-inch reel tester equipped with a recording / playback head (LTO8 head) mounted on an IBM LTO (Linear Tape-Open) 8 tape drive, as shown in Figure 3. Specifically, one end of the tape sample was attached to one tape reel of the reel tester, and the other end was attached to the other tape reel of the reel tester, and the tape sample was attached to the reel tester. The "LTO8 head" is a magnetic head conforming to the LTO8 standard. The tape sample was run in the reel tester, and data was recorded and played back by sliding the magnetic head against the surface of the magnetic layer. The running conditions for the magnetic tape (the above-mentioned tape sample) were as follows: The tension value applied in the longitudinal direction of the magnetic tape and the running speed of the magnetic tape were the settings for the reel tester. As noted above, in terms of units, "gf" is gram force, and 1 N (Newton) is approximately 102 gf. (Driving conditions) Magnetic tape running speed: 4 m / s Tension applied to the magnetic tape in the longitudinal direction: 100 gf Magnetic tape running path: 1 single path Wrap angle θ: 1° Recording was performed at a linear recording density of 300 kfci, and the playback output was measured during playback to determine the signal-to-noise ratio (SNR) (ratio of playback output to noise). The unit kfci is the unit of linear recording density (not convertible to SI units).

[0138] (2) Repeated driving in high temperature environments The tape sample after the evaluation in (1) above was attached to a reel tester as described in (1) above in an environment of an ambient temperature of 65°C and a relative humidity of 10%, and run under the running conditions described in (1) above, causing the surface of the magnetic layer to come into contact with the magnetic head and slide.

[0139] (3) Evaluation of electromagnetic conversion characteristics after repeated driving in a high-temperature environment The electromagnetic conversion characteristics of the magnetic tape after the repeated running in (2) above were evaluated as described in (1) above, except that the magnetic tape was run 20,000 single passes in an environment with an ambient temperature of 23°C ± 1°C and a relative humidity of 50%.

[0140] (4) SNR decrease (ΔSNR) before and after repeated driving in a high-temperature environment The SNR obtained by recording and reproducing in (1) above was defined as the "SNR before repeated running," and the SNR obtained by recording and reproducing on the 20,000th single pass in (3) above was defined as the "SNR after repeated running," and ΔSNR was calculated using the following formula: If the change in SNR after repeated running from the SNR before repeated running is within 3.0 dB, it can be said that there is little deterioration in the electromagnetic conversion characteristics after repeated running in a high-temperature environment. ΔSNR = (SNR after repeated driving) - (SNR before repeated driving)

[0141] In Comparative Example 4, numerous scratches were generated in the magnetic layer during repeated running in the high-temperature environment described above in (2), and therefore it was not possible to evaluate the electromagnetic conversion characteristics described above in (3) (in Table 1, the ΔSNR column is marked "Evaluation not possible").

[0142] [Table 1]

[0143] [Table 2]

[0144] From the results shown in Table 1, it can be confirmed that the magnetic tapes of the examples are less susceptible to deterioration in electromagnetic conversion characteristics after repeated running in a high temperature environment than the magnetic tapes of the comparative examples. In addition, from the results shown in Table 2, the magnitude relationship of the protrusion height difference Δ obtained when the height of the surrounding substrate area is set to 0 nm is the same as the reference protrusion height Δ obtained when the reference surface is set to 0 nm. refIt can be seen that this does not correspond to the magnitude relationship of [Industrial Applicability]

[0145] One aspect of the present invention is useful in the field of magnetic tape for data storage.

Claims

1. A magnetic recording medium having a non-magnetic support and a magnetic layer containing ferromagnetic powder, The arithmetic mean roughness Ra measured on the surface of the magnetic layer is 2.5 nm or less, and a protrusion height HD measured by an atomic force microscope in a region identified as a dark region in a first binarized image of a backscattered electron image obtained by imaging the surface of the magnetic layer with a scanning electron microscope, the protrusion height HD being determined by taking the height of a surrounding base region as 0 nm; a protrusion height HB measured by an atomic force microscope for a region identified as a bright region in an image obtained by imaging a backscattered electron image of the surface of the magnetic layer using a scanning electron microscope and subjected to a second binarization process performed on the higher gradation side than the first binarization process, with the height of the surrounding base region being taken as 0 nm; A magnetic recording medium in which the protrusion height difference Δ, HD-HB, is 0.7 nm or more.

2. 2. The magnetic recording medium according to claim 1, wherein the protrusion height difference .DELTA. is 0.7 nm or more and 3.0 nm or less.

3. 3. The magnetic recording medium according to claim 1, wherein the arithmetic mean roughness Ra is 0.8 nm or more and 2.5 nm or less.

4. 4. The magnetic recording medium according to claim 1, wherein the magnetic layer contains two or more types of non-magnetic powder.

5. 5. The magnetic recording medium according to claim 4, wherein the non-magnetic powder of the magnetic layer includes alumina powder.

6. 6. The magnetic recording medium according to claim 4, wherein the non-magnetic powder in the magnetic layer contains carbon black.

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

8. 8. The magnetic recording medium according to claim 1, further comprising a backcoat layer containing nonmagnetic powder on the surface of said nonmagnetic support opposite to the surface on which said magnetic layer is formed.

9. 9. The magnetic recording medium according to claim 1, which is a magnetic tape.

10. A magnetic tape cartridge comprising the magnetic tape of claim 9.

11. A magnetic recording and reproducing device comprising the magnetic recording medium according to any one of claims 1 to 9.

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