Magnetic recording medium, magnetic tape cartridge, and magnetic recording reproduction device

Abstract

Provided is a magnetic recording medium comprising a non-magnetic support and a magnetic layer containing ferromagnetic powder, and further comprising a non-magnetic layer containing non-magnetic powder between the non-magnetic support and the magnetic layer. The average pore diameter of the magnetic layer and the non-magnetic layer is 30.0 nm or less, and the standard deviation of the pore diameter distribution of the magnetic layer and the non-magnetic layer is 30.0 nm or less.

Description

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

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

[0002] A magnetic recording medium is known to have a configuration that includes a magnetic layer and a non-magnetic layer (see, for example, Patent Document 1).

[0003] Japanese Patent Publication No. 2005-25905

[0004] In the embodiment described in Japanese Patent Publication No. 2005-25905 (Patent Document 1), the electromagnetic conversion characteristics of a magnetic recording medium having a magnetic layer and a non-magnetic layer are evaluated (see paragraph 0114, Table 2 and Table 3 of Japanese Patent Publication No. 2005-25905). Thus, it is desirable for magnetic recording media to have excellent electromagnetic conversion characteristics.

[0005] One aspect of the present invention aims to provide a magnetic recording medium having a magnetic layer and a non-magnetic layer, which can exhibit excellent electromagnetic conversion characteristics.

[0006] One aspect of the present invention is as follows: [1] A magnetic recording medium comprising a non-magnetic support and a magnetic layer containing ferromagnetic powder, further comprising a non-magnetic layer containing non-magnetic powder between the non-magnetic support and the magnetic layer, wherein the average pore diameter of the magnetic layer and the non-magnetic layer is 30.0 nm or less, and the standard deviation of the pore diameter distribution of the magnetic layer and the non-magnetic layer is 30.0 nm or less. [2] The magnetic recording medium according to [1], wherein the thickness of the non-magnetic layer is 1.00 μm or less. [3] The magnetic recording medium according to [1] or [2], wherein the non-magnetic powder of the non-magnetic layer contains carbon black. [4] The magnetic recording medium according to [3], wherein the non-magnetic layer contains 60.0% by mass or more of carbon black relative to the total amount of non-magnetic powder. [5] The magnetic recording medium according to [3] or [4], wherein the pH of the carbon black is 7.0 or more and 10.0 or less. [6] The specific surface area of ​​the carbon black is 280 m². ２ / g or more 500m ２The magnetic recording medium according to any one of [3] to [5], which is 0.5 g or less. [7] The magnetic recording medium according to any one of [1] to [6], further comprising a back coat layer containing non-magnetic powder on the surface side opposite to the surface side having the magnetic layer and the non-magnetic layer of the non-magnetic support. [8] The magnetic recording medium according to any one of [1] to [7], which is a magnetic tape. [9] The thickness of the non-magnetic layer is 1.00 μm or less, the non-magnetic layer contains 60.0% by mass or more of carbon black with respect to the total amount of non-magnetic powder, the pH of the carbon black is 7.0 or more and 10.0 or less, and the specific surface area of the carbon black is 280 m ２ / g or more and 500 m ２ / g or less, further comprising a back coat layer containing non-magnetic powder on the surface side opposite to the surface side having the magnetic layer and the non-magnetic layer of the non-magnetic support, and being a magnetic tape. The magnetic recording medium according to any one of [1] to [8].

[10] A magnetic tape cartridge containing the magnetic tape according to [8] or [9].

[11] A magnetic recording and reproducing apparatus containing the magnetic recording medium according to any one of [1] to [9].

[0007] According to one aspect of the present invention, a magnetic recording medium having a magnetic layer and a non-magnetic layer can be provided, which can exhibit excellent electromagnetic conversion characteristics. Also, according to one aspect of the present invention, a magnetic tape cartridge and a magnetic recording and reproducing apparatus containing such a magnetic recording medium can be provided.

[0008] [Magnetic Recording Medium] One embodiment of the present invention relates to a magnetic recording medium having a non-magnetic support and a magnetic layer containing ferromagnetic powder, and further having a non-magnetic layer containing non-magnetic powder between the non-magnetic support and the magnetic layer. The average pore diameter of the magnetic layer and the non-magnetic layer (hereinafter, also referred to as "average pore diameter (magnetic layer + non-magnetic layer)") is 30.0 nm or less, and the standard deviation of the pore diameter distribution of the magnetic layer and the non-magnetic layer (hereinafter, also referred to as "standard deviation of pore diameter distribution (magnetic layer + non-magnetic layer)") is 30.0 nm or less.

[0009] <Method for measuring average pore diameter (magnetic layer + non-magnetic layer) and standard deviation of pore diameter distribution (magnetic layer + non-magnetic layer)> In this invention and specification, the above-mentioned "average pore diameter (magnetic layer + non-magnetic layer)" and "standard deviation of pore diameter distribution (magnetic layer + non-magnetic layer)" shall be determined by the nitrogen adsorption / desorption method and the BJH (Barrett-Joyner-Halenda) method. Specifically, they shall be determined by the following method. From the magnetic recording medium to be measured, a sample area of ​​500 cm² ２Cut out a sample for measurement. For samples with a back coat layer on the surface opposite to the surface with the magnetic and non-magnetic layers of the non-magnetic support, the back coat layer is removed using a solvent. Therefore, the following measurements are performed on a sample that has a magnetic layer, a non-magnetic layer, and a non-magnetic support, and does not have a back coat layer or the back coat layer has been removed. Measure the mass of the sample. Subtract the mass of the non-magnetic support, obtained by a known method, from the measured mass to determine the masses of the magnetic and non-magnetic layers. The masses of the magnetic and non-magnetic layers thus obtained are used as the sample mass for calculating the amount of adsorption from the measurement results of the nitrogen adsorption / desorption isotherm measurement described below. After degassing at room temperature for 6 hours using a nitrogen adsorption analyzer, nitrogen adsorption / desorption isotherm measurement is performed at liquid nitrogen temperature under the following measurement conditions. For example, a BELSORP MINI X (Microtrac-Bel) can be used as the nitrogen adsorption analyzer. The above "room temperature" is in the range of 20 to 25°C. The adsorption / desorption isotherms obtained by these measurements are analyzed using the BJH method to determine the average pore size and the standard deviation of the pore size distribution. The standard deviation is σ (i.e., the positive square root of the variance). The "relative pressure" below is generally expressed as "P / P0". The measured values ​​are those obtained for a sample that also has a non-magnetic support. However, assuming that there are no pores in the non-magnetic support, the "average pore size (magnetic layer + non-magnetic layer)" and the "standard deviation of the pore size distribution (magnetic layer + non-magnetic layer)" are determined from the measurement results of the nitrogen adsorption / desorption isotherm measurement. (Measurement conditions) Measurement temperature: Liquid nitrogen temperature (i.e., 77K (-196°C)) Adsorbate to be measured: Nitrogen gas Relative pressure range: 0 to 0.999 range measured in 30 steps Equilibrium pressure tolerance range: 0.5% (Therefore, in adsorption measurements, equilibrium pressure is considered to have been reached at a pressure of [target pressure - 0.5%] or higher, and in desorption measurements, at a pressure of [target pressure + 0.5%] or lower.)

[0010] A magnetic recording medium with a high saturation magnetic flux density Bm of the magnetic layer can exhibit excellent electromagnetic conversion characteristics. In this regard, the inventor believes that making the average pore diameter (magnetic layer + non-magnetic layer) and the standard deviation of the pore diameter distribution (magnetic layer + non-magnetic layer) within the above ranges respectively contributes to increasing the saturation magnetic flux density Bm of the magnetic layer. More specifically, the inventor speculates as follows. The composition for forming the non-magnetic layer usually contains a binder. Although it is desirable from the viewpoints of improving the dispersibility of the non-magnetic powder in the non-magnetic layer and enhancing the film strength of the non-magnetic layer to adsorb the binder onto the particle surfaces of the non-magnetic powder as much as possible, it is generally considered that not all of the total amount is adsorbed. Therefore, it is speculated that free non-adsorbed binder components exist in the non-magnetic layer in a state mixed with the adsorbed binder components. If this non-adsorbed binder component of the non-magnetic layer penetrates into the magnetic layer through the pores between the non-magnetic powder and the adsorbed binder component during the manufacturing process of the magnetic recording medium, increasing the non-magnetic components of the magnetic layer, the ratio of the ferromagnetic powder in the volume of the magnetic layer will decrease. This is speculated to be the cause of the decrease in the saturation magnetic flux density Bm of the magnetic layer. In this regard, the inventor believes that fine pores with a sharp pore diameter distribution are difficult for the non-adsorbed binder component to pass through. Therefore, in a magnetic recording medium where both the average pore diameter (magnetic layer + non-magnetic layer) and the standard deviation of the pore diameter distribution (magnetic layer + non-magnetic layer) obtained by the method described above are small, it is considered that it is difficult for the non-adsorbed binder component to penetrate from the non-magnetic layer into the magnetic layer. The inventor speculates that this is the reason why the magnetic recording medium with the "average pore diameter (magnetic layer + non-magnetic layer)" and the "standard deviation of the pore diameter distribution (magnetic layer + non-magnetic layer)" within the ranges described above can exhibit excellent electromagnetic conversion characteristics. However, the present invention is not limited to the speculation described in this specification.

[0011] <Average pore size (magnetic layer + non-magnetic layer)> From the viewpoint of improving electromagnetic conversion characteristics, the average pore size (magnetic layer + non-magnetic layer) of the magnetic recording medium is 30.0 nm or less, preferably 28.0 nm or less, and more preferably 26.0 nm or less, 24.0 nm or less, 22.0 nm or less, 20.0 nm or less, 18.0 nm or less, 16.0 nm or less, 14.0 nm or less, 12.0 nm or less, 10.0 nm or less, and 8.0 nm or less, in that order. The average pore size (magnetic layer + non-magnetic layer) of the magnetic recording medium can be, for example, 1.0 nm or more, 2.0 nm or more, 3.0 nm or more, or 4.0 nm or more. From the viewpoint of improving electromagnetic conversion characteristics, the smaller the average pore size (magnetic layer + non-magnetic layer) of the magnetic recording medium, the better, so it may be below the range exemplified here.

[0012] <Standard deviation of pore size distribution (magnetic layer + non-magnetic layer)> From the viewpoint of improving electromagnetic conversion characteristics, the standard deviation of the pore size distribution (magnetic layer + non-magnetic layer) of the above magnetic recording medium is preferably 30.0 nm or less, preferably 28.0 nm or less, and more preferably 26.0 nm or less, 24.0 nm or less, 22.0 nm or less, 20.0 nm or less, 18.0 nm or less, 16.0 nm or less, 14.0 nm or less, 12.0 nm or less, and 10.0 nm or less, in that order. The average pore size (magnetic layer + non-magnetic layer) of the above magnetic recording medium can be, for example, 1.0 nm or more, 2.0 nm or more, 3.0 nm or more, 4.0 nm or more, 5.0 nm or more, or 6.0 nm or more. From the viewpoint of improving electromagnetic conversion characteristics, a smaller standard deviation of the pore size distribution (magnetic layer + non-magnetic layer) of the above magnetic recording medium is preferable, so it may be below the range exemplified here.

[0013] The magnetic recording medium described above will be explained in more detail below.

[0014] <Thickness of each layer and non-magnetic support> The magnetic recording medium has a magnetic layer and a non-magnetic layer. In one embodiment, a back coat layer containing non-magnetic powder may be further provided on the surface side of the non-magnetic support opposite to the surface side having the magnetic and non-magnetic layers. In another embodiment, the magnetic recording medium may be a magnetic recording medium without such a back coat layer. The thickness of the magnetic layer can be optimized according to the saturation magnetization amount of the magnetic head used, the head gap length, the bandwidth of the recording signal, etc. From the viewpoint of high-density recording, the thickness of the magnetic layer is preferably 100 nm or less, more preferably 10 to 100 nm, and even more preferably 20 to 90 nm. There only needs to be at least one magnetic layer, and the magnetic layer may be separated into two or more layers having different magnetic properties, and known configurations for multilayer magnetic layers can be applied. When separated into two or more layers, the thickness of the magnetic layer is the total thickness of these layers. This also applies when other layers are separated into two or more layers. From the viewpoint of increasing the capacity of the magnetic recording medium, the thickness of the non-magnetic layer is preferably 1.00 μm or less, preferably 0.80 μm or less, and more preferably 0.60 μm or less. Furthermore, from the viewpoint of ease of uniformly applying the composition for forming the non-magnetic layer, the thickness of the non-magnetic layer is preferably 0.05 μm or more, more preferably 0.07 μm or more, and even more preferably 0.10 μm or more. Generally, the non-magnetic layer can be thicker than the magnetic layer (for example, the thickness of the non-magnetic layer can be 2 or more, 3 or more, 4 or more, or 5 or more times the thickness of the magnetic layer). Therefore, generally, the pores of the non-magnetic layer can mainly influence the average pore diameter (magnetic layer + non-magnetic layer) and the standard deviation of the pore diameter distribution (magnetic layer + non-magnetic layer). The thickness of the back coat layer is preferably 0.90 μm or less, and more preferably in the range of 0.10 to 0.70 μm. The thickness of the non-magnetic support is, for example, in the range of 2.00 to 80.00 μm, preferably in the range of 2.00 to 50.00 μm, and more preferably in the range of 2.00 to 10.00 μm.

[0015] In the present invention and this specification, the thickness of the magnetic layer is a value obtained from a cross-sectional image using a scanning electron microscope (SEM). A method for determining the thickness of the magnetic layer is described below.

[0016] (1) Preparation of cross-sectional observation samples A cross-sectional observation sample is prepared by cutting it from a randomly determined position on the magnetic recording medium to be measured. The preparation of the cross-sectional observation sample involves gallium ions (Ga ＋ The process is carried out by FIB (Focused Ion Beam) processing using a gallium ion beam. A specific example of this fabrication method is as follows: (i) Cut out a sample of magnetic recording medium with dimensions of 10 mm in width and 10 mm in length using a razor. Form a protective film on the magnetic layer surface of the cut-out sample to obtain a sample with a protective film. The protective film is formed by the following method. Form a platinum (Pt) film (30 nm thick) on the magnetic layer surface of the above sample by sputtering. Sputtering of the platinum film is carried out under the following conditions: (Sputtering conditions for platinum film) Target: Pt Vacuum in the chamber of the sputtering apparatus: 7 Pa or less Current value: 15 mA Further form a carbon film with a thickness of 100 to 150 nm on the platinum film sample prepared above. Form the carbon film using a gallium ion (Ga) equipped in the FIB (Focused Ion Beam) apparatus used in (ii) below. ＋ (ii) The protective film-coated sample prepared in (i) above is treated with a CVD (Chemical Vapor Deposition) mechanism using a beam. ＋ The cross-section of the magnetic recording medium is exposed by FIB processing using a beam. The acceleration voltage in FIB processing is 30 kV and the probe current is 1300 pA. For example, the cross-sectional sample thus exposed is used for SEM observation to determine the thickness of the magnetic layer.

[0017] (2) Identification of the interface between the magnetic layer and the non-magnetic layer The prepared sample for cross-sectional observation is observed using a scanning electron microscope (SEM), and a cross-sectional image (SEM image) is taken. A field emission scanning electron microscope (FE-SEM) is used as the scanning electron microscope. For example, the Hitachi FE-SEM S4800 can be used, and this FE-SEM was used in the examples and comparative examples described later. For the same cross-sectional observation sample, SEM images are taken at randomly selected locations, except for points where (i) the imaging range does not overlap, (ii) the outermost surface on the magnetic layer side (magnetic layer surface) is included in the SEM image, and (iii) the entire thickness direction of the cross-sectional observation sample (i.e., the region from the outermost surface on the magnetic layer side to the outermost surface on the other side) is included in the SEM image, or if the entire thickness direction of the cross-sectional observation sample is not included in the SEM image, points are selected so that the imaging portion of the cross-sectional observation sample occupies 80-100% of the total image area of ​​the SEM image, based on area, to obtain a total of 10 images. The above SEM images are secondary electron images (SE (secondary electron) images) taken with an acceleration voltage of 5 kV, an imaging magnification of 50,000x, and a resolution of 960 pixels vertically × 1280 pixels horizontally. The captured SEM image is imported into WinROOF, an image processing software manufactured by Mitani Corporation, and the non-magnetic layer portion (measurement area) in the SEM image is selected. In selecting the measurement area, the width of the measurement area is the total width of the captured SEM image. The "width direction" as described in relation to the SEM image refers to the width direction of the photographed cross-sectional observation sample. The width direction of the cross-sectional observation sample is the width direction of the magnetic recording medium from which this sample was cut. The same applies to the thickness direction. Regarding the thickness direction, the interface between the magnetic layer and the non-magnetic layer is identified by the following method. The SEM image is digitized to create image brightness data in the thickness direction (consisting of three components: coordinates in the thickness direction, coordinates in the width direction, and brightness). In digitization, the SEM image is divided into 1280 parts in the width direction, processed with 8 bits of brightness to obtain 256-level data, and the image brightness of each divided coordinate point is converted to a predetermined level value.Next, using the obtained image brightness data, a brightness curve is created by plotting the average brightness in the width direction at each coordinate point in the thickness direction (i.e., the average brightness at each of the 1280 divided coordinate points) on the vertical axis and the coordinates in the thickness direction on the horizontal axis. A differential curve is created by differentiating the created brightness curve, and the coordinates of the boundary between the magnetic layer and the non-magnetic layer are identified from the peak position of the created differential curve. The position on the SEM image corresponding to the identified coordinates is defined as the interface between the magnetic layer and the non-magnetic layer. If the SEM image includes a portion of the non-magnetic support, the interface between the non-magnetic layer and the non-magnetic support can be identified. For example, in a coated magnetic recording medium, the interface between the non-magnetic layer and the non-magnetic support is more clearly recognizable than the interface between the magnetic layer and the non-magnetic layer. Therefore, the interface between the non-magnetic layer and the non-magnetic support can be identified by visually inspecting the SEM image. However, it may also be identified using a brightness curve as described above. The entire region from the interface between the identified magnetic layer and the non-magnetic layer (i.e., the magnetic layer-side surface of the non-magnetic layer) to the interface between the non-magnetic layer and the non-magnetic support (i.e., the non-magnetic support-side surface of the non-magnetic layer) can be identified as the non-magnetic layer.

[0018] (3) Measurement of the thickness of the magnetic layer Each of the 10 SEM images described above is a secondary electron image (SE image) taken with an acceleration voltage of 5 kV, a magnification of 50,000x, and a resolution of 960 pixels vertically × 1280 pixels horizontally. The interface between the magnetic layer and the non-magnetic layer is identified by the method described in (2) above. The surface of the magnetic layer is identified by visual inspection. At any one position on each SEM image, the distance in the thickness direction between the interface between the magnetic layer and the non-magnetic layer identified by the above method and the surface of the magnetic layer is determined, and the arithmetic mean of the values ​​obtained for the 10 images is taken as the thickness of the magnetic layer. The thicknesses of other layers and the non-magnetic support can also be determined by the same method. Alternatively, the thicknesses of other layers may be determined as the design thickness calculated from the manufacturing conditions.

[0019] <Non-magnetic layer> (Non-magnetic powder)The non-magnetic layer is a layer containing at least non-magnetic powder. As the non-magnetic powder contained in the non-magnetic layer, only one type of non-magnetic powder may be used, or two or more types of non-magnetic powder may be used. As the non-magnetic powder, it is preferable to use at least carbon black, and more preferably to use carbon black having a pH in the range of 7.0 to 10.0. In the present invention and this specification, the pH of non-magnetic powder such as carbon black is a value measured according to the standard test method ASTM D1512.

[0020] Carbon black generally has a graphite structure which is a condensed polycyclic aromatic structure, and the structure contains π electrons that can move freely, and the presence of π electrons leads to the manifestation of good electrical conductivity. Therefore, carbon black can exhibit the property as an electron donor, and when dispersed in water, H ＋ ions and OH － ions, among which H ＋ ions are selectively adsorbed, and the result is to leave OH － ions, resulting in a basic pH. This is due to the fact that the original graphite structure of carbon black is mainly present and it retains a large amount of π electrons. On the other hand, there are also commercially available carbon blacks that exhibit an acidic pH. In carbon black that exhibits an acidic pH, oxygen is introduced into the structure by oxidation treatment. Oxygen chemically bonds with carbon (C) to form functional groups that exhibit acidity such as hydroxy groups and carboxy groups. Due to such oxygen-carbon chemical bonds, as the property of the electron donor decreases, the pH decreases, and the pH exhibited by the carbon black becomes an acidic pH. Therefore, it can be said that in carbon black that exhibits an acidic pH, the original graphite structure is chemically altered and the condensed polycyclic aromatic structure is reduced. The inventor believes that for the above reasons, carbon black having a pH in the range of 7.0 to 10.0, which is a neutral to basic pH, has more condensed polycyclic aromatic structures than carbon black that exhibits an acidic pH. Regarding the binder that is preferably used in combination with carbon black having a pH in the range of 7.0 to 10.0 from the viewpoint of improving dispersibility, it will be described later.

[0021] Carbon black can be produced, for example, by the furnace process, in which petroleum-based or coal-based oil is injected into high-temperature gas as a raw material and incompletely combusted to obtain carbon black. Carbon black produced by the furnace process is called furnace black. In addition, the following types of carbon black are also known: lamp black, produced by the lamp black process, in which oil, pine, etc., is burned in a shallow dish; acetylene black, produced by the acetylene black process, in which acetylene gas is thermally decomposed; thermal black, produced by the thermal black process, in which other hydrocarbon gases are thermally decomposed; and channel black, produced by the channel black process, in which natural gas, aromatic oils, etc., are partially oxidized. For carbon black exhibiting a pH in the neutral to basic range of 7.0 to 10.0, for example, carbon black selected from the group consisting of furnace black, lamp black, acetylene black, and thermal black can be used. In contrast, channel black tends to exhibit an acidic pH. However, the method of producing carbon black contained in the non-magnetic layer is not particularly limited.

[0022] From the perspective of reducing the average pore size (magnetic layer + non-magnetic layer) and the standard deviation of the pore size distribution (magnetic layer + non-magnetic layer) of magnetic recording media, the specific surface area of ​​carbon black is 280 m². ２ It is preferable that it be 300m or more per gram. ２ It is more preferable that the amount is greater than or equal to 1 / g. From the viewpoint of ease of improving the dispersibility of carbon black in the non-magnetic layer, the specific surface area of ​​carbon black should be 500 m². ２ It is preferable that the amount is less than or equal to 400m ２It is more preferable that the specific surface area is less than or equal to / g. In the present invention and this specification, the specific surface area of ​​the various powders is the specific surface area obtained using the BET (Brunauer-Emmett-Teller) formula derived by Brunauer, Emmett, and Teller by the nitrogen adsorption method in accordance with JIS K 6217-7:2013. The specific surface areas of the various powders used in the examples and comparative examples described later are the specific surface areas measured for the raw material powders used in the preparation of each layer-forming composition. However, it is also possible to extract powder from a magnetic recording medium by a known method and determine the specific surface area of ​​the extracted powder. The same applies to the pH of the powder.

[0023] The proportion of carbon black in the non-magnetic powder of the non-magnetic layer is preferably 60.0% by mass or more, more preferably 70.0% by mass or more, 75.0% by mass or more, 80.0% by mass or more, 85.0% by mass or more, 90.0% by mass or more, and 95.0% by mass or more, in that order, and more preferably 100.0% by mass (i.e., the non-magnetic powder is carbon black only). Furthermore, the proportion of carbon black in the non-magnetic powder of the non-magnetic layer can be 100.0% by mass or less, less than 100.0% by mass, 99.0% by mass or less, or 98.0% by mass or less, relative to the total amount of non-magnetic powder. The content (filling rate) of non-magnetic powder in the non-magnetic layer is preferably in the range of 50 to 90% by mass, more preferably in the range of 60 to 90% by mass, relative to the total mass of the non-magnetic layer.

[0024] In addition to carbon black, inorganic or organic powders may be used as the non-magnetic powder. The average particle size of the non-magnetic powder other than carbon black is preferably in the range of 10 to 200 nm. The specific surface area of ​​the non-magnetic powder other than carbon black is, for example, 50 to 150 m². ２The pH can be in the range of / g. The pH of non-magnetic powders other than carbon black can be in the range of, for example, 7.0 to 10.0. 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 available commercially and can also be manufactured by known methods. For details, see, for example, paragraphs 0146 to 0150 of Japanese Patent Application Publication No. 2011-216149. An example of a non-magnetic powder is α-iron oxide powder (commonly called "red iron oxide"). However, if α-iron oxide powder accounts for a large portion of the non-magnetic powder in the non-magnetic layer, the average pore size (magnetic layer + non-magnetic layer) and the standard deviation of the pore size distribution (magnetic layer + non-magnetic layer) of the magnetic recording medium tend to be large. Therefore, in a non-magnetic layer containing α-iron oxide powder, the proportion of α-iron oxide powder in the non-magnetic powder of the non-magnetic layer is preferably less than 60.0% by mass, more preferably 50.0% by mass or less, in order of increasing preference between 30.0% by mass or less, 20.0% by mass or less, 10.0% by mass or less, and 5.0% by mass or less, and even more preferably 0.0% by mass (i.e., no α-iron oxide powder).

[0025] (Binder) The above magnetic recording medium may be a coated magnetic recording medium and may contain a binder in the non-magnetic layer. The binder is one or more resins. In the present invention and this specification, the resin may be a homopolymer or a copolymer. As the binder for the non-magnetic layer, for example, a resin selected from polyurethane resin, polyester resin, polyamide resin, vinyl chloride resin, acrylic resin copolymerized with styrene, acrylonitrile, methyl methacrylate, etc., cellulose resin such as nitrocellulose, epoxy resin, phenoxy resin, polyvinyl acetal, polyvinyl alkylal resin such as polyvinyl butyral, etc. may be used alone or a mixture of multiple resins may be used. The average molecular weight of the resin used as the binder for the non-magnetic layer may be, for example, 5,000 to 200,000 or 10,000 to 200,000 as a weight-average molecular weight. Unless otherwise specified, the weight-average molecular weight and number-average molecular weight in this invention and specification are values ​​obtained by converting the values ​​measured by gel permeation chromatography (GPC) under the following measurement conditions to polystyrene equivalent. The binder content can be, for example, 1.0 to 30.0 parts by mass per 100.0 parts by mass of non-magnetic powder. GPC apparatus: HLC-8120 (Tosoh Corporation) Column: TSK gel Multipore HXL-M (Tosoh Corporation, 7.8 mm ID (Inner Diameter) × 30.0 cm) Eluent: Tetrahydrofuran (THF)

[0026] Improving the dispersibility of non-magnetic powder in the non-magnetic layer can lead to a reduction in the average pore size (magnetic layer + non-magnetic layer) and the standard deviation of the pore size distribution (magnetic layer + non-magnetic layer) of the magnetic recording medium. From the viewpoint of improving dispersibility, a polyurethane resin having an aromatic cyclic structure is a preferred binder to be used in combination with carbon black exhibiting a pH in the range of 7.0 to 10.0. The inventors believe that polyurethane resins having an aromatic cyclic structure have high affinity and adsorption properties for carbon black exhibiting a pH in the range of 7.0 to 10.0, and therefore can contribute to improving the dispersibility of carbon black exhibiting the above pH range.

[0027] Examples of polyurethane resins include polyurethane resins with various skeletons such as polyether urethane, polyester urethane, polycarbonate urethane, and polyether ester urethane. A polyurethane resin having an aromatic cyclic structure can be, for example, a polyurethane resin synthesized using an isocyanate having an aromatic cyclic structure as the isocyanate compound that serves as a raw material for polyurethane. The urethane group concentration of a polyurethane resin synthesized using an isocyanate having an aromatic cyclic structure is preferably 1.0 meq / g to 5.0 meq / g, and more preferably 1.5 meq / g to 4.5 meq / g. The above polyurethane resin with a urethane group concentration of 1.0 meq / g or more, and more preferably 1.5 meq / g or more, has a sufficient aromatic ring skeleton derived from the isocyanate having an aromatic cyclic structure contained in the polyurethane chain, and exhibits good affinity and adsorption to carbon black. Furthermore, polyurethane resins with a urethane group concentration of 5.0 meq / g or less, and even 4.5 meq / g or less, exhibit high solubility in solvents commonly used for the manufacture of coated magnetic recording media, which can also contribute to improving the dispersibility of carbon black. "eq" stands for equivalent and is a unit that cannot be converted to SI units.

[0028] Polyols and isocyanate compounds used as raw materials for polyurethane resins include the long-chain diols, short-chain diols, and diisocyanate compounds listed in the "Polyurethane Resin Handbook" (edited by Keiji Iwata, 1986, Nikkan Kogyo Shimbun). The polyurethane chain may be linear, or it may have a side-chain structure and / or a branched structure.

[0029] To give polyurethane resin a side chain structure, a compound having two or more functional groups that react with the side chain structure and isocyanate in one molecule can be used. Compounds having a side chain structure and two or more functional groups that react with isocyanates in one molecule include 1,2-propylene glycol, 1,3-propylene glycol, 1,3-butylene glycol, 2,3-butylene glycol, 2,2-dimethyl-1,3-propanediol, 3-methyl-1,5-pentanediol, 2,2,4-trimethyl-1,3-pentanediol, 2-ethyl-1,3-hexanediol, 2,2-dimethyl-3-hydroxypropyl-2',2'-dimethyl-3-hydroxypropanate, 2-n-butyl-2-ethyl-1,3-propanediol, 3-ethyl-1,5-pentanediol, 3-propyl-1,5-pentanediol, 2,2-diethyl-1,3-propanediol, 2-butyl, 2-ethyl-1,3-propanediol, 3- Octyl-1,5-pentanediol, 3-phenyl-1,5-pentanediol, 2,5-dimethyl-3-sodium sulfo-2,5-hexanediol, 1,3-bis(hydroxymethyl)cyclohexane, 1,4-bis(hydroxymethyl)cyclohexane, 1,4-bis(hydroxyethyl)cyclohexane, 1,4-bis(hydroxypropyl)cyclohexane, 1,4-bis(hydroxymethoxy)cyclohexane, 1,4-bis(hydroxyethoxy)cyclohexane, 2,2-bis(4-hydroxymethoxycyclohexyl)propane, 2,2-bis(4-hydroxyethoxycyclohexyl)propane, bis(4-hydroxycyclohexyl)methane, 2,2-bis(4-hydroxycyclohexyl)propane, 3(4),8(9)-tricyclo[5.2.1.0 ２，６Examples include decanedimethanol, hydrogenated bisphenol A, ethylene oxide adducts of hydrogenated bisphenol A, and propylene oxide adducts of hydrogenated bisphenol A. Compounds having two or more functional groups that react with the side chain structure and isocyanate in one molecule include 2,2-dimethyl-1,3-propanediol, 2,2-dimethyl-3-hydroxypropyl-2',2'-dimethyl-3-hydroxypropanate, 2-n-butyl-2-ethyl-1,3-propanediol, 2,2-diethyl-1,3-propanediol, 2-butyl-2-ethyl-1,3-propanediol, and 3(4),8(9)-tricyclo[5.2.1.0 ２，６ Decandimethanol, hydrogenated bisphenol A having an alicyclic skeleton, ethylene oxide adducts of hydrogenated bisphenol A, and propylene oxide adducts of hydrogenated bisphenol A are preferred, and hydrogenated bisphenol A having an alicyclic skeleton is more preferred.

[0030] To give polyurethane resin a branched structure, a small amount of trifunctional or higher alcohol can be used in addition to the diol compound during polyurethane polymerization. Preferred trifunctional or higher alcohols include trimethylolpropane, glycerin, pentaerythritol, or their ethylene oxide adducts, propylene oxide adducts, etc.

[0031] For example, if the polyurethane resin is polyester urethane, it is preferable that it has the following skeleton.

[0032] When the total amount of the acid component and glycol component of the polyester polyol is set to 100 mol%, it is preferable that aromatic dibasic acids are present at least 20 mol%, with the remainder consisting of aliphatic and / or alicyclic dibasic acids. Furthermore, it is preferable that 50 to 100 mol% of the total diol components of the polyester polyol consist of aliphatic diol components having an alkyl group with one or more carbon atoms in its side chain. The presence of 20 mol% or more aromatic dibasic acids is preferable from the viewpoint of improving the dispersibility of carbon black and the running durability of magnetic recording media. In addition, the molar ratio of dibasic acid to glycol is not limited to 1 / 1, and it is also preferable that the glycol component is in excess. The preferred range is 1 / 1 to 1 / 2.5.

[0033] Examples of aromatic dibasic acids include terephthalic acid, isophthalic acid, orthophthalic acid, and naphthalenedicarboxylic acid. Among these, isophthalic acid and orthophthalic acid are preferred from the viewpoint of achieving both the dispersibility of carbon black and the running durability of magnetic recording media.

[0034] Aliphatic diol components having an alkyl group with one or more carbon atoms in the side chain include 1,2-propylene glycol, 1,3-butylene glycol, 2,3-butylene glycol, 2-methyl-1,3-propanediol, 2,2-dimethyl-1,3-propanediol, 3-methyl-1,5-pentanediol, 2,2-dimethyl-3-hydroxypropyl-2',2'-dimethyl-3-hydroxypropaneate, 2-butyl, 2-ethyl-1,3-propanediol, and 2,2-diethyl-1,3-propanediol. Among these, 2,2-dimethyl-1,3-propanediol, 2,2-diethyl-1,3-propanediol, 2-butyl, 2-ethyl-1,3-propanediol, and 2,2-dimethyl-3-hydroxypropyl-2',2'-dimethyl-3-hydroxypropaneate are preferred.

[0035] Other diol components used in polyester polyols include aliphatic glycols such as ethylene glycol, 1,3-propanediol, 1,4-butanediol, 1,5-pentanediol, and 1,6-hexanediol, as well as 1,3-bis(hydroxymethyl)cyclohexane, 1,4-bis(hydroxymethyl)cyclohexane, 1,4-bis(hydroxyethyl)cyclohexane, 1,4-bis(hydroxypropyl)cyclohexane, 1,4-bis(hydroxymethoxy)cyclohexane, 1,4-bis(hydroxyethoxy)cyclohexane, 2,2-bis(4-hydroxymethoxycyclohexyl)propane, 2,2-bis(4-hydroxyethoxycyclohexyl)propane, bis(4-hydroxycyclohexyl)methane, 2,2-bis(4-hydroxycyclohexyl)propane, and 3(4),8(9)-tricyclo[5.2.1.0 ２，６ Examples include alicyclic glycols such as decanedimethanol. Among these, 1,3-propanediol, 1,4-bis(hydroxymethyl)cyclohexane and 3(4),8(9)-tricyclo[5.2.1.0 ２，６ Decandimethanol is preferred.

[0036] The above-mentioned polyester polyol is preferably such that its number average molecular weight, calculated from the hydroxyl value measured by the method described in JIS standard K1557-1:2007, is 300 to 800, from the viewpoint of obtaining a polyurethane resin with high solubility in solvents commonly used for the manufacture of coated magnetic recording media.

[0037] Aromatic isocyanates used in the synthesis of polyurethane resins include 2,4-tolylene diisocyanate, 2,6-tolylene diisocyanate, p-phenylenediisocyanate, 4,4'-diphenylmethane diisocyanate, m-phenylenediisocyanate, 3,3'-dimethoxy-4,4'-biphenylenediisocyanate, 2,6-naphthalenediisocyanate, 3,3'-dimethyl-4,4'-biphenylenediisocyanate, 4,4'-diphenylenediisocyanate, 4,4'-diisocyanate diphenyl ether, 1,5-naphthalenediisocyanate, and m-xylenediisocyanate. Among these, 4,4'-diphenylmethane diisocyanate is more preferred.

[0038] For short-chain diol components used in the synthesis of polyurethane resins, refer to the previous descriptions regarding other diol components used in polyester polyols, as well as the previous descriptions regarding compounds having two or more functional groups that react with side chain structures and isocyanates in one molecule. Specific examples of short-chain diols include the short-chain diol used in the synthesis of polyurethane resin A, described later. Using short-chain diols with side chain structures in the synthesis of polyurethane resins is preferable from the viewpoint of improving the solvent solubility of the unit in which the short-chain diol and aromatic isocyanate are bonded.

[0039] From the viewpoint of improving the dispersibility of carbon black exhibiting a pH in the range of 7.0 to 10.0, it is preferable that the polyurethane resin contains a sulfonic acid metal base in its backbone. The sulfonic acid metal base has the function of adsorbing to basic sites on the surface of carbon black. From the viewpoint of improving the dispersibility of carbon black exhibiting the above pH range and improving the solvent solubility of the polyurethane resin, the sulfonic acid metal base content in the polyurethane resin is preferably in the range of 60 to 400 eq / ton. As a method for introducing the sulfonic acid metal base into the polyurethane resin, for example, there is a method of copolymerizing 5-Na sulfisophthalic acid or 5-K sulfisophthalic acid into a polyester polyol, or a method of using a diol containing a sulfonic acid metal base.

[0040] The number-average molecular weight of the polyurethane resin is preferably in the range of 5,000 to 100,000, and more preferably in the range of 10,000 to 80,000. The polyurethane resin may be synthesized either by synthesizing the raw materials in a molten state or by dissolving the raw materials in a solution. As a reaction catalyst, stannous octoate, dibutyltin dilaurate, triethylamine, etc., can be used. Furthermore, ultraviolet absorbers, hydrolysis inhibitors, antioxidants, etc., may be added before, during, or after the production of the polyurethane resin.

[0041] The content of polyurethane resin having an aromatic cyclic structure is preferably 50.0% by mass or more, in order of preference to 60.0% by mass or more, 70.0% by mass or more, 80.0% by mass or more, 90.0% by mass or more, and 95.0% by mass or more, and more preferably 100.0% by mass (i.e., the binder of the non-magnetic layer is only polyurethane resin having an aromatic cyclic structure).

[0042] As the binder for the non-magnetic layer, only one or more polyurethane resins having an aromatic cyclic structure may be used, or one or more polyurethane resins having an aromatic cyclic structure and one or more resins other than polyurethane resins having an aromatic cyclic structure may be used. If the binder for the non-magnetic layer contains vinyl chloride resin, the content of vinyl chloride resin is preferably 50.0% by mass or less, 40.0% by mass or less, 30.0% by mass or less, 20.0% by mass or less, 10.0% by mass or less, and 5.0% by mass or less, in that order, and more preferably 0.0% by mass (i.e., no vinyl chloride resin).

[0043] The non-magnetic layer may contain one or more other known additives, selected appropriately from commercially available products or manufactured by known methods, in any amount, depending on the desired properties. Further information regarding additives that may be included in the non-magnetic layer can be found in the description below.

[0044] In the present invention and this specification, the non-magnetic layer includes a substantially non-magnetic layer containing a small amount of ferromagnetic powder, for example as an impurity or intentionally, along with the non-magnetic powder. A substantially non-magnetic layer is defined as a layer having a remanent magnetic flux density of 10 mT or less, a coercivity of 100 Oe or less, or a layer having a remanent magnetic flux density of 10 mT or less and a coercivity of 100 Oe or less. The unit Oe is given by 1 [kOe] = 10⁶ / 4π [A / m]. It is preferable that the non-magnetic layer has no remanent magnetic flux density and no coercivity.

[0045] <Magnetic Layer> (Ferromagnetic Powder) As the ferromagnetic powder contained in the magnetic layer, known ferromagnetic powders used in the magnetic layers of various magnetic recording media can be used. Using a ferromagnetic powder with a small average particle size is preferable from the viewpoint of improving recording density. From this viewpoint, the average particle size of the ferromagnetic powder 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, and even more preferably 15 nm or more.

[0046] A preferred specific example of hexagonal ferrite powder ferromagnetic powder is hexagonal ferrite powder. For details on hexagonal ferrite powder, see, for example, paragraphs 0012 to 0030 of Japanese Patent Publication No. 2011-225417, paragraphs 0134 to 0136 of Japanese Patent Publication No. 2011-216149, paragraphs 0013 to 0030 of Japanese Patent Publication No. 2012-204726, and paragraphs 0029 to 0084 of Japanese Patent Publication No. 2015-127985.

[0047] 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 intensity diffraction peak in the X-ray diffraction spectrum obtained by X-ray diffraction analysis belongs. For example, if the highest intensity diffraction peak in the X-ray diffraction spectrum obtained by X-ray diffraction analysis is attributed to the hexagonal ferrite crystal structure, it shall be 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 shall be considered the main phase. The hexagonal ferrite crystal structure contains at least iron atoms, divalent metal atoms, and oxygen atoms as constituent atoms. Divalent metal atoms are metal atoms that can become divalent cations as ions, and examples include strontium atoms, barium atoms, alkaline earth metal atoms such as calcium atoms, and lead atoms. In the present invention and this specification, hexagonal strontium ferrite powder refers to powder in which the main divalent metal atom contained is strontium, and hexagonal barium ferrite powder refers to powder in which the main divalent metal atom contained is barium. The main divalent metal atom refers to the divalent metal atom that accounts for the largest proportion on an atomic percentage 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 atoms" are selected from the group consisting of scandium atoms (Sc), yttrium atoms (Y), and lanthanide atoms. Lanthanide atoms are selected from the group consisting of lanthanum (La), cerium (Ce), praseodymium (Pr), neodymium (Nd), promethium (Pm), samarium (Sm), europium (Eu), gadolinium (Gd), terbium (Tb), dysprosium (Dy), holmium (Ho), erbium (Er), thulium (Tm), ytterbium (Yb), and lutetium (Lu).

[0048] Below, we will describe hexagonal strontium ferrite powder, a form of hexagonal ferrite powder, in more detail.

[0049] The activation volume of the hexagonal strontium ferrite powder is preferably 800 to 1500 nm. ３ The activation volume is within the range described above. Finely milled hexagonal strontium ferrite powder exhibiting an activation volume within the above range is suitable for the fabrication of magnetic recording media that exhibit excellent electromagnetic conversion properties. The activation volume of the hexagonal strontium ferrite powder is preferably 800 nm. ３ That's all, for example, 850 nm ３ It can also be the above. Furthermore, from the viewpoint of further improving electromagnetic conversion characteristics, the activation volume of hexagonal strontium ferrite powder is 1400 nm. ３ The following is more preferable: 1300 nm ３ It is even more preferable that the following conditions are met. For hexagonal barium ferrite powder, it is also preferable that the activation volume be within the above range.

[0050] "Activation volume" is a unit of magnetization reversal and an indicator of the magnetic size of a particle. The activation volume and the anisotropy constant Ku described herein and below are values ​​obtained from the following relationship between Hc and activation volume V, measured using a vibrating sample type magnetometer at magnetic field sweep speeds of 3 minutes and 30 minutes in the coercivity Hc measurement section (measurement temperature: 23°C ± 1°C). Regarding the unit of the anisotropy constant Ku, 1 erg / cc = 1.0 × 10⁻⁶ －１ J / m ３ Therefore, 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)]

[0051] As an indicator of reducing thermal fluctuations, or in other words, improving thermal stability, the anisotropy constant Ku can be cited. The hexagonal strontium ferrite powder is preferably 1.8 × 10⁻⁶ ５ J / m ３It can have a Ku of the above, and more preferably 2.0 × 10 ５ J / m ３ It can have a Ku of the above. Also, the Ku of hexagonal strontium ferrite powder is, for example, 2.5 × 10 ５ J / m ３ The following values ​​are possible. However, since a higher Ku value is preferable as it indicates higher thermal stability, the values ​​are not limited to those exemplified above.

[0052] Hexagonal strontium ferrite powder may or may not contain rare earth atoms. When hexagonal strontium ferrite powder contains rare earth atoms, it is preferable that the rare earth atoms are present at a concentration of 0.5 to 5.0 atomic percent (bulk concentration) per 100 atomic percent of iron atoms. In one embodiment, hexagonal strontium ferrite powder containing rare earth atoms may have a surface layer segregation of rare earth atoms. In the present invention and this specification, "rare earth atom surface segregation" means that the rare earth atom content relative to 100% of iron atoms in a solution obtained by partially dissolving hexagonal strontium ferrite powder with acid (hereinafter referred to as "rare earth atom surface content" or simply "surface content" with respect to rare earth atoms) satisfies the ratio of rare earth atom surface content / rare earth atom bulk content > 1.0 with respect to rare earth atoms in a solution obtained by completely dissolving hexagonal strontium ferrite powder with acid (hereinafter referred to as "rare earth atom bulk content" or simply "bulk content" with respect to rare earth atoms). The rare earth atom content of hexagonal strontium ferrite powder described later is synonymous with the rare earth atom bulk content. In contrast, partial dissolution using acid dissolves the surface layer of the particles constituting the hexagonal strontium ferrite powder. Therefore, the rare earth atom content in the solution obtained by partial dissolution is the rare earth atom content in the surface layer of the particles constituting the hexagonal strontium ferrite powder. When the rare earth atom surface layer content satisfies the ratio "rare earth atom surface layer content / rare earth atom bulk content > 1.0", it means that in the particles constituting the hexagonal strontium ferrite powder, rare earth atoms are concentrated in the surface layer (i.e., there are more of them in the surface layer than in the interior). In this invention and specification, the surface layer refers to a part of the region extending from the surface to the interior of the particles constituting the hexagonal strontium ferrite powder.

[0053] When hexagonal strontium ferrite powder contains rare earth atoms, the rare earth atom content (bulk content) is preferably in the range of 0.5 to 5.0 atomic percent relative to 100 atomic percent of iron atoms. It is believed that containing rare earth atoms at the bulk content within the above range, and having the rare earth atoms unevenly distributed on the surface of the particles constituting the hexagonal strontium ferrite powder, contributes to suppressing the decrease in regeneration output during repeated regeneration. This is presumed to be because the anisotropy constant Ku can be increased by containing rare earth atoms at the bulk content within the above range, and having the rare earth atoms unevenly distributed on the surface of the particles constituting the hexagonal strontium ferrite powder. The higher the value of the anisotropy constant Ku, the more it is possible to suppress the occurrence of a phenomenon called thermal fluctuation (in other words, to improve thermal stability). By suppressing the occurrence of thermal fluctuation, the decrease in regeneration output during repeated regeneration can be suppressed. It is speculated that the uneven distribution of rare earth atoms on the surface of hexagonal strontium ferrite powder particles contributes to stabilizing the spin of iron (Fe) sites in the crystal lattice of the surface layer, thereby increasing the anisotropy constant Ku. Furthermore, it is speculated that using hexagonal strontium ferrite powder with uneven distribution of rare earth atoms on the surface as the ferromagnetic powder for the magnetic layer contributes to suppressing wear on the magnetic layer surface due to sliding with the magnetic head. In other words, it is speculated that hexagonal strontium ferrite powder with uneven distribution of rare earth atoms on the surface may also contribute to improving the running durability of magnetic recording media. This is speculated to be because the uneven distribution of rare earth atoms on the surface of the particles constituting the hexagonal strontium ferrite powder contributes to improved interaction between the particle surface and organic substances (e.g., binders and / or additives) contained in the magnetic layer, resulting in improved strength of the magnetic layer. From the viewpoint of further suppressing the decrease in regeneration output during repeated regeneration and / or further improving running durability, the rare earth atom content (bulk content) is more preferably in the range of 0.5 to 4.5 atomic percent, even more preferably in the range of 1.0 to 4.5 atomic percent, and even more preferably in the range of 1.5 to 4.5 atomic percent.

[0054] The bulk content mentioned above is the content obtained by completely dissolving the hexagonal strontium ferrite powder. In this invention and specification, unless otherwise specified, the content of atoms refers to the bulk content obtained by completely dissolving the hexagonal strontium ferrite powder. The hexagonal strontium ferrite powder containing rare earth atoms may contain only one type of rare earth atom, or it may contain two or more types of rare earth atoms. In the case of containing two or more types of rare earth atoms, the bulk content mentioned above is determined for the sum of the two or more types of rare earth atoms.

[0055] When hexagonal strontium ferrite powder contains rare earth atoms, the rare earth atoms included may be one or more of any type of rare earth atom. From the viewpoint of further suppressing the decrease in regeneration output during repeated regeneration, preferred rare earth atoms include neodymium atoms, samarium atoms, yttrium atoms, and dysprosium atoms, with neodymium atoms, samarium atoms, and yttrium atoms being more preferred, and neodymium atoms being even more preferred.

[0056] In hexagonal strontium ferrite powder having a rare-earth atom surface segregation, the rare-earth atoms only need to be segregated in the surface layer of the particles constituting the hexagonal strontium ferrite powder, and the degree of segregation is not limited. For example, in hexagonal strontium ferrite powder having a rare-earth atom surface segregation, the ratio of the rare-earth atom surface content obtained by partial dissolution under the dissolution conditions described later to the rare-earth atom bulk content obtained by total dissolution under the dissolution conditions described later, "surface content / bulk content," is greater than 1.0 and can be 1.5 or greater. A "surface content / bulk content" greater than 1.0 means that in the particles constituting the hexagonal strontium ferrite powder, the rare-earth atoms are segregated in the surface layer (i.e., there are more of them in the surface layer than in the interior). Furthermore, the ratio of the surface content of rare earth atoms obtained by partial dissolution under the dissolution conditions described later to the bulk content of rare earth atoms obtained by total dissolution under the dissolution conditions described later, "surface content / bulk content," can be, for example, 10.0 or less, 9.0 or less, 8.0 or less, 7.0 or less, 6.0 or less, 5.0 or less, or 4.0 or less. However, in hexagonal strontium ferrite powder having a rare earth atom surface distribution bias, the rare earth atoms only need to be biased towards the surface of the particles constituting the hexagonal strontium ferrite powder, and the above "surface content / bulk content" is not limited to the upper or lower limits exemplified.

[0057] The partial and total dissolution of hexagonal strontium ferrite powder is described below. For hexagonal strontium ferrite powder existing as a powder, the sample powders to be partially and completely dissolved are taken from the same lot of powder. On the other hand, for hexagonal strontium ferrite powder contained in the magnetic layer of a magnetic recording medium, a portion of the hexagonal strontium ferrite powder extracted from the magnetic layer is subjected to partial dissolution, and another portion is subjected to total dissolution. The extraction of hexagonal strontium ferrite powder from the magnetic layer can be performed, for example, by the method described in paragraph 0032 of Japanese Patent Application Publication No. 2015-91747. Partial dissolution refers to dissolving to the extent that residual hexagonal strontium ferrite powder can be visually confirmed in the liquid at the end of dissolution. For example, partial dissolution can dissolve a region of 10 to 20% by mass of the particles constituting the hexagonal strontium ferrite powder, with the total particles being 100% by mass. On the other hand, total dissolution refers to dissolution to the point where no residue of hexagonal strontium ferrite powder can be visually confirmed in the liquid at the end of dissolution. The measurement of partial dissolution and surface layer content is performed, for example, by the following method. However, the dissolution conditions such as the amount of sample powder below are examples, and any dissolution conditions that enable partial and total dissolution can be arbitrarily adopted. A container (e.g., a beaker) containing 12 mg of sample powder and 10 ml of 1 mol / L hydrochloric acid is held on a hot plate at a set temperature of 70°C for 1 hour. The resulting solution is filtered through a 0.1 μm membrane filter. Elemental analysis of the resulting filtrate is performed using an inductively coupled plasma (ICP) analyzer. In this way, the surface layer content of rare earth atoms relative to 100 atomic percent of iron atoms can 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 layer content. This also applies to the measurement of bulk content. On the other hand, the measurement of total dissolution and bulk content is performed, for example, by the following method: A container (e.g., a beaker) containing 12 mg of sample powder and 10 ml of 4 mol / L hydrochloric acid is held on a hot plate at a set temperature of 80°C for 3 hours.Subsequently, the bulk content relative to 100 atomic percent of iron atoms can be determined by performing the same procedure as described above for partial dissolution and surface layer content measurement.

[0058] From the perspective of increasing the playback output when reproducing data recorded on a magnetic recording medium, it is desirable for the mass magnetization σs of the ferromagnetic powder contained in the magnetic recording medium to be high. In this regard, hexagonal strontium ferrite powder containing rare earth atoms but lacking surface segregation of rare earth atoms tended to have a significantly lower σs compared to hexagonal strontium ferrite powder that does not contain rare earth atoms. In contrast, hexagonal strontium ferrite powder having surface segregation of rare earth atoms is considered preferable in order to suppress such a large decrease in σs. In one embodiment, the σs of hexagonal strontium ferrite powder is 45 A·m ２ It can be 47 A·m or more per kg. ２ It can also be greater than / kg. On the other hand, σs is 80 A·m from the viewpoint of noise reduction. ２ It is preferable that the value be less than or equal to 60 A·m / kg. ２ It is more preferable that it be less than or equal to / kg. σs can be measured using a known measuring device capable of measuring magnetic properties, such as a vibrating sample magnetometer. In the present invention and this specification, unless otherwise specified, the mass magnetization σs is the value measured at a magnetic field strength of 15 kOe.

[0059] Regarding the constituent atom content (bulk content) of hexagonal strontium ferrite powder, the strontium atom content can be in the range of, for example, 2.0 to 15.0 atomic percent relative to 100 atomic percent of iron atoms. In one embodiment, the hexagonal strontium ferrite powder may contain only strontium atoms as divalent metal atoms. In another embodiment, the hexagonal strontium ferrite powder may contain one or more other divalent metal atoms in addition to strontium atoms. For example, it may contain barium atoms and / or calcium atoms. When other divalent metal atoms besides strontium atoms are included, the barium atom content and calcium atom content in the hexagonal strontium ferrite powder can be in the range of, for example, 0.05 to 5.0 atomic percent relative to 100 atomic percent of iron atoms.

[0060] The known crystal structures of hexagonal ferrites include magnetoplumbite (also called "M-type"), W-type, Y-type, and Z-type. Hexagonal strontium ferrite powder may have any of these crystal structures. The crystal structure can be confirmed by X-ray diffraction analysis. Hexagonal strontium ferrite powder may show a single crystal structure or two or more crystal structures by X-ray diffraction analysis. For example, in one embodiment, hexagonal strontium ferrite powder may show only the M-type crystal structure by X-ray diffraction analysis. For example, M-type hexagonal ferrite is AFe １２ O １９It is represented by the following compositional formula: Here, A represents a divalent metal atom, and if the hexagonal strontium ferrite powder is of type M, A is either only a strontium atom (Sr), or if A contains multiple divalent metal atoms, then as described above, strontium atoms (Sr) make up the largest proportion on an atomic percentage basis. The divalent metal atom content of hexagonal strontium ferrite powder is usually determined by the type of crystal structure of the hexagonal ferrite and is not particularly limited. The same applies to the iron atom content and oxygen atom content. Hexagonal strontium ferrite powder contains at least iron atoms, strontium atoms and oxygen atoms, and may also contain rare earth atoms. Furthermore, hexagonal strontium ferrite powder may or may not contain atoms other than these atoms. As an example, hexagonal strontium ferrite powder may contain aluminum atoms (Al). The aluminum atom content can be, for example, 0.5 to 10.0 atomic percent relative to 100 atomic percent of iron atoms. From the viewpoint of further suppressing the decrease in regeneration output during repeated regeneration, 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 percent or less, more preferably in the range of 0 to 5.0 atomic percent, and may even be 0 atomic percent, relative to 100 atomic percent of iron atoms. That is, in one embodiment, the hexagonal strontium ferrite powder does not have to contain atoms other than iron atoms, strontium atoms, oxygen atoms, and rare earth atoms. The above content expressed in atomic percent is obtained by converting the content of each atom (unit: mass%) obtained by completely dissolving the hexagonal strontium ferrite powder into an atomic percent value using the atomic weight of each atom. Furthermore, in the present invention and this specification, "does not contain" for a certain atom means that the content measured by an ICP analyzer after complete dissolution is 0 mass%. The detection limit of an ICP analyzer is typically 0.01 ppm (parts per million) or less by mass. The term "does not contain" above is used to include the presence of substances in amounts below the detection limit of the ICP analyzer.Hexagonal strontium ferrite powder can, in one form, be bismuth atom-free (Bi).

[0061] A preferred specific example of a metal powder ferromagnetic powder is a ferromagnetic metal powder. For details on ferromagnetic metal powders, see, for example, paragraphs 0137 to 0141 of Japanese Patent Application Publication No. 2011-216149 and paragraphs 0009 to 0023 of Japanese Patent Application Publication No. 2005-251351.

[0062] A preferred specific example of ε-iron oxide powder ferromagnetic powder is ε-iron oxide powder. In the present invention and this specification, "ε-iron oxide powder" refers to ferromagnetic powder in which the crystal structure of ε-iron oxide is detected as the main phase by X-ray diffraction analysis. For example, if the highest intensity diffraction peak in the X-ray diffraction spectrum obtained by X-ray diffraction analysis is attributed to the crystal structure of ε-iron oxide, it shall be determined that the crystal structure of ε-iron oxide was detected as the main phase. Methods for producing ε-iron oxide powder include methods from goethite and the reverse micelle method. All of the above production methods are publicly known. Furthermore, for a method of producing ε-iron oxide powder in which a portion of Fe is substituted with substitution atoms such as Ga, Co, Ti, Al, and Rh, see, for example, J. Jpn. Soc. Powder Metallurgy Vol. 61 Supply, No. S1, pp. See S280-S284, J. Mater. Chem. C, 2013, 1, pp. 5200-5206, etc. However, the method for producing ε-iron oxide powder that can be used as ferromagnetic powder in the magnetic layer of the above magnetic recording medium is not limited to the method listed herein.

[0063] The activation volume of ε-iron oxide powder is preferably 300 to 1500 nm. ３ The activation volume is within the range described above. Micronized ε-iron oxide powder exhibiting an activation volume within this range is suitable for the fabrication of magnetic recording media that exhibit excellent electromagnetic conversion properties. The activation volume of the ε-iron oxide powder is preferably 300 nm. ３ That's all, for example, 500 nm ３It can also be the above. Furthermore, from the viewpoint of further improving electromagnetic conversion characteristics, the activation volume of ε-iron oxide powder is 1400 nm. ３ The following is more preferable: 1300 nm ３ It is even more preferable that the following conditions apply: 1200 nm ３ It is even more preferable that the following conditions be met: 1100 nm ３ The following is even more preferable.

[0064] The anisotropy constant Ku can be cited as an indicator of the reduction of thermal fluctuations, or in other words, the improvement of thermal stability. The ε-iron oxide powder is preferably 3.0 × 10 ４ J / m ３ It can have a Ku of the above, and more preferably 8.0 × 10 ４ J / m ３ It is possible to have the above Ku content. Also, the Ku content of ε-iron oxide powder is, for example, 3.0 × 10 ５ J / m ３ The following values ​​are possible. However, a higher Ku value indicates higher thermal stability, which is preferable, so the values ​​are not limited to those exemplified above.

[0065] From the perspective of increasing the playback output when reproducing data recorded on a magnetic recording medium, it is desirable for the mass magnetization σs of the ferromagnetic powder contained in the magnetic recording medium to be high. In this regard, in one embodiment, the σs of ε-iron oxide powder is 8 A·m ２ It can be 12 A·m or more per kg. ２ It can also be more than / kg. On the other hand, the σs of ε-iron oxide powder is 40 A·m from the viewpoint of noise reduction. ２ It is preferable that it be less than or equal to 35 A·m / kg. ２ It is more preferable that the amount is less than or equal to / kg.

[0066] In the present invention and this specification, unless otherwise specified, the average particle size of various powders such as ferromagnetic powder shall be the value measured using a transmission electron microscope by the following method: The powder is photographed using a transmission electron microscope at a magnification of 100,000x, and the resulting image is printed on photographic paper or displayed on a screen to obtain a photograph of the particles constituting the powder, with a total magnification of 500,000x. The target particles are selected from the obtained photographs of the particles, and their contours are traced with a digitizer to measure the size of the particles (primary particles). Primary particles refer to independent particles that do not aggregate. The above measurement is performed on 500 randomly selected particles. The arithmetic mean of the particle sizes of these 500 particles is taken as the average particle size of the powder. As the above-mentioned transmission electron microscope, for example, a Hitachi H-9000 transmission electron microscope can be used. Furthermore, the particle size measurement can be performed using known image analysis software, for example, Carl Zeiss KS-400 image analysis software. The average particle sizes shown in the Examples section below were measured using a Hitachi H-9000 transmission electron microscope and Carl Zeiss KS-400 image analysis software, unless otherwise specified. In the present invention and this specification, "powder" means an aggregate of multiple particles. For example, ferromagnetic powder means an aggregate of multiple ferromagnetic particles. Furthermore, an aggregate of multiple particles is not limited to a configuration in which the particles constituting the aggregate are in direct contact, but also includes configurations in which binders, additives, etc., described later, are interposed between the particles. The term "particle" is sometimes used to refer to powder.

[0067] 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 Japanese Patent Application Publication No. 2011-048878 can be employed.

[0068] In the present invention and this specification, unless otherwise specified, the size of the particles constituting the powder (particle size) is expressed as follows: (1) If the shape of the particles observed in the above particle photograph is needle-shaped, spindle-shaped, columnar (however, the height is greater than the longest major axis of the base), it is expressed as the length of the long axis constituting the particle, i.e., the long axis length; (2) If the shape is plate-shaped or columnar (however, the thickness or height is less than the longest major axis of the plate surface or base), it is expressed as the longest major axis of the plate surface or base; (3) If the shape is spherical, polyhedral, unspecified, etc., and the long axis constituting the particle cannot be determined from the shape, it is expressed as the equivalent diameter of a circle. The equivalent diameter of a circle refers to the diameter determined by the circular projection method.

[0069] Furthermore, the average needle-shape ratio of the powder refers to the arithmetic mean of the values ​​obtained for the 500 particles by measuring the length of the short axis of each particle in the above measurement, i.e., the short axis length, and determining the (long axis length / short axis length) value for each particle. Here, unless otherwise specified, the short axis length refers to the length of the short axis constituting the particle in the definition of particle size above (1), the thickness or height in the definition of (2), and in the case of (3), since there is no distinction between the long axis and the short axis, (long axis length / short axis length) is considered to be 1 for convenience. And unless otherwise specified, when the shape of the particle is specific, for example, in the definition of particle size above (1), the average particle size is the average long axis length, and in the definition of (2), the average particle size is the average plate diameter. In the definition of (3), the average particle size is the average diameter (also called the average particle size or average particle diameter).

[0070] The content (filling rate) of ferromagnetic powder in the magnetic layer is preferably in the range of 50 to 90% by mass, and more preferably in the range of 60 to 90% by mass, relative to the total mass of the magnetic layer. A high filling rate of ferromagnetic powder in the magnetic layer is preferable from the viewpoint of improving recording density.

[0071] (Non-magnetic powder) The above magnetic recording medium may contain one or more non-magnetic powders in the magnetic layer. Preferably, the non-magnetic powder contains at least a non-magnetic powder that contributes to forming protrusions on the surface of the magnetic layer (hereinafter referred to as "protrusion-forming agent"). It is also preferable that the magnetic layer contains a non-magnetic powder that can function as an abrasive (hereinafter referred to as "abrasive"). The protrusion-forming agent and abrasive will be described further below.

[0072] One form of the protrusion-forming agent is carbon black. The average particle size of the carbon black can be, for example, 30 to 300 nm, and preferably 40 to 200 nm. Another form of the protrusion-forming agent is colloidal particles. As colloidal particles, inorganic colloidal particles are preferred from the viewpoint of availability, inorganic oxide colloidal particles are more preferred, and silica colloidal particles (colloidal silica) are even more preferred. In the present invention and this specification, "colloidal particles" means particles that, when added at a rate of 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 mixing ratio, disperse without settling and yield a colloidal dispersion. The average particle size of the colloidal particles can be, for example, 30 to 300 nm, and preferably 40 to 200 nm.

[0073] The content of the protrusion-forming agent in the magnetic layer is preferably 0.1 to 10.0 parts by mass, and more preferably 0.1 to 5.0 parts by mass, per 100.0 parts by mass of ferromagnetic powder. In the present invention and this specification, one component may be used, or two or more components may be used. When two or more components are used, the content and content rate refer to the total content and total content rate of the two or more components.

[0074] (Abrasive) The abrasive is a component that can remove deposits that adhere to the magnetic head during operation (abrasive properties). Abrasives commonly used as abrasives for magnetic layers include alumina (Al ２ O ３), silicon carbide, boron carbide (B ４ C), TiC, chromium oxide (Cr ２ O ３ ), cerium oxide, zirconium oxide (ZrO ２ Examples of ferromagnetic powders include iron oxide and diamond, with alumina such as α-alumina, silicon carbide, and diamond powder being particularly preferred. The abrasive content of the magnetic layer is preferably 1.0 to 20.0 parts by mass, more preferably 3.0 to 15.0 parts by mass, and even more preferably 4.0 to 10.0 parts by mass, per 100.0 parts by mass of ferromagnetic powder. The average particle size of the abrasive can be, for example, 30 to 300 nm, and preferably 40 to 200 nm.

[0075] (Binder, curing agent) The magnetic recording medium described above may contain a binder in the magnetic layer. Various resins commonly used as binders for coated magnetic recording media can be used as the binder for the magnetic layer. For example, as the binder for the magnetic layer, a resin selected from polyurethane resin, polyester resin, polyamide resin, vinyl chloride resin, acrylic resin copolymerized with styrene, acrylonitrile, methyl methacrylate, etc., cellulose resin such as nitrocellulose, epoxy resin, phenoxy resin, polyvinyl acetal, polyvinyl alkylal resin such as polyvinyl butyral can be used alone or in mixture of multiple resins. Among these, polyurethane resin, acrylic resin, cellulose resin, and vinyl chloride resin are preferred. For more information on the binders, refer to paragraphs 0028 to 0031 of Japanese Patent Application Publication No. 2010-24113. The average molecular weight of the resin used as the binder for the magnetic layer can be, for example, 10,000 or more and 200,000 or less as a weight-average molecular weight. The content of the binder in the magnetic layer can be, for example, 1.0 to 30.0 parts by mass per 100.0 parts by mass of ferromagnetic powder.

[0076] Furthermore, a curing agent can be used together with a resin that can be used as a binder. In one form, the curing agent can be a thermosetting compound, which is a compound that undergoes a curing reaction (crosslinking reaction) by heating, and in another form, it can be a photocurable compound, which undergoes a curing reaction (crosslinking reaction) by light irradiation. As the curing reaction progresses during the magnetic layer formation process, at least a portion of the curing agent may 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 a composition that contains a curing agent when the composition used to form other layers contains a curing agent. Preferred curing agents are thermosetting compounds, and polyisocyanates are preferred. For details on polyisocyanates, refer to paragraphs 0124 to 0125 of Japanese Patent Application Publication No. 2011-216149. The curing agent can be used in an amount of, for example, 0 to 80.0 parts by mass, preferably 50.0 to 80.0 parts by mass from the viewpoint of improving the strength of the magnetic layer, per 100.0 parts by mass of the binder.

[0077] (Additives) The magnetic layer may contain one or more additional additives as needed. An example of an additive is the curing agent mentioned above. Other additives that can be included in the magnetic layer include lubricants, dispersants, dispersion aids, antifungal agents, antistatic agents, antioxidants, etc. Additives can be appropriately selected from commercially available products according to the desired properties, or manufactured by known methods, and used in any amount. For example, for lubricants, refer to paragraphs 0030 to 0033, 0035 and 0036 of Japanese Patent Application Publication No. 2016-126817. A lubricant may also be included in the non-magnetic layer described later. For lubricants that can be included in the non-magnetic layer, refer to paragraphs 0030 to 0031, 0034, 0035 and 0036 of Japanese Patent Application Publication No. 2016-126817. For dispersants, refer to paragraphs 0061 and 0071 of Japanese Patent Application Publication No. 2012-133837. A dispersant may be added to the composition for forming a non-magnetic layer. For dispersants that can be added to the composition for forming a non-magnetic layer, see paragraph 0061 of Japanese Patent Application Publication No. 2012-133837. An example of an additive that can be used to improve the dispersibility of an abrasive in a magnetic layer containing an abrasive is the dispersant described in paragraphs 0012 to 0022 of Japanese Patent Application Publication No. 2013-131285.

[0078] <Non-magnetic support> Next, we will describe non-magnetic support (hereinafter also simply referred to as "support"). Known non-magnetic support materials include biaxially oriented polyethylene terephthalate, polyethylene naphthalate, polyamide, polyamide-imide, and aromatic polyamide. Among these, polyethylene terephthalate, polyethylene naphthalate, and polyamide are preferred. These support materials may be subjected to corona discharge, plasma treatment, easy-adhesion treatment, heat treatment, etc. in advance.

[0079] <Backcoat Layer> The above magnetic recording medium may also have a backcoat layer containing non-magnetic powder on the surface side opposite to the surface side of the non-magnetic support having the magnetic layer and non-magnetic layer. Alternatively, the above magnetic recording medium may be a magnetic recording medium without a backcoat layer. Preferably, the backcoat layer contains either or both carbon black and inorganic powder. For details of the backcoat layer, known technology relating to backcoat layers can be applied. The backcoat layer may also contain a binder. For the binder contained in the backcoat layer and various additives that may be optionally contained, the prior descriptions and known technology relating to the formulation of the magnetic layer and / or non-magnetic layer can be optionally applied. For example, paragraphs 0018 to 0020 of Japanese Patent Application Publication No. 2006-331625 and lines 65 to 38 of column 5 of U.S. Patent No. 7,029,774 can be referenced for the backcoat layer.

[0080] <Manufacturing Process> (Preparation of Compositions for Forming Each Layer) Compositions for forming a magnetic layer, a non-magnetic layer, or a backcoat layer usually contain a solvent along with the various components described above. As the solvent, one or more of the various solvents commonly used in the manufacture of coated magnetic recording media can be used. The solvent content of each layer-forming composition is not particularly limited. For solvents, refer to paragraph 0153 of Japanese Patent Application Publication No. 2011-216149. The solid content concentration and solvent composition of each layer-forming composition may be appropriately adjusted in accordance with the handling suitability of the composition, the coating conditions, and the thickness of each layer to be formed. The process of preparing a composition for forming a magnetic layer, a non-magnetic layer, or a backcoat layer usually includes at least a kneading step, a dispersion step, and mixing steps provided before or after these steps as needed. Each individual step may be divided into two or more stages. All raw materials used in the preparation of each layer-forming composition may be added at the beginning or in the middle of any step. Alternatively, individual raw materials may be added in two or more separate steps. For example, the binder may be added in separate steps: a kneading step, a dispersion step, and a mixing step for adjusting the viscosity after dispersion. In the above manufacturing process for magnetic recording media, conventional known manufacturing techniques can be used as some of the steps. In the kneading step, it is preferable to use a kneader with strong kneading force, such as an open kneader, a continuous kneader, a pressure kneader, or an extruder. Details of the kneading step are described in Japanese Patent Publication No. 1-106338 and Japanese Patent Publication No. 1-79274. As the disperser, various known dispersers that utilize shear force, such as a bead mill, a ball mill, a sand mill, or a homomixer, can be used. Dispersion beads can preferably be used for dispersion. Examples of dispersion beads include ceramic beads and glass beads, with zirconia beads being preferred. Two or more types of beads may be used in combination. The bead diameter (particle size) and bead packing rate of the dispersion beads are not particularly limited and should be set according to the powder to be dispersed. Each layer-forming composition may be filtered by a known method before being subjected to the coating step. Filtration can be performed, for example, by filter filtration.For filtration, filters with a pore size of 0.01 to 3 μm (e.g., glass fiber filters, polypropylene filters, etc.) can be used.

[0081] (Coating process) The non-magnetic layer and the magnetic layer can be formed by sequentially or simultaneously applying the non-magnetic layer-forming composition and the magnetic layer-forming composition in multiple layers. The back coat layer can be formed by applying the back coat-forming composition to the surface of the non-magnetic support opposite to the surface having the non-magnetic layer and the magnetic layer (or to which the non-magnetic layer and / or magnetic layer are subsequently provided). For details of the coating process for each layer formation, refer to paragraph 0066 of Japanese Patent Application Publication No. 2010-231843.

[0082] (Other Processes) For other processes for manufacturing magnetic recording media, see, for example, paragraphs 0067 to 0070 of Japanese Patent Publication No. 2010-231843. For example, for orientation processing, the orientation processing can be performed on the coated layer in the orientation zone while the coated layer of the magnetic layer forming composition is wet. For orientation processing, various known techniques, including the description in paragraph 0052 of Japanese Patent Publication No. 2010-24113, can be applied. For example, vertical orientation processing can be performed by known methods such as using opposite-polarity opposing magnets. In the orientation zone, the drying rate of the coated layer can be controlled by the temperature of the drying air, the airflow rate and / or the transport speed in the orientation zone. Alternatively, the coated layer may be pre-dried before being transported to the orientation zone. As an example, the magnetic field strength in vertical orientation processing can be 0.10 to 0.80 T, or 0.10 to 0.60 T. Additionally, a calendaring treatment can be performed to improve the surface smoothness of the magnetic recording media. Regarding the conditions for the calendering process, for example, the calendering pressure (linear pressure) can be 200 to 500 kN / m, and preferably 250 to 350 kN / m. The calendering temperature (surface temperature of the calendering roll) can be 70 to 120°C, and preferably 80 to 100°C, and the calendering speed can be 50 to 300 m / min, and preferably 50 to 200 m / min.

[0083] A magnetic recording medium according to one aspect of the present invention may be a tape-shaped magnetic recording medium (magnetic tape) or a disk-shaped magnetic recording medium (magnetic disk). For example, a magnetic tape is usually housed in a magnetic tape cartridge, and the magnetic tape cartridge is mounted in a magnetic recording and playback device. A servo pattern may also be formed on the magnetic recording medium by known methods to enable head tracking in the magnetic recording and playback device. "Formation of a servo pattern" can also be referred to as "recording of a servo signal." The formation of a servo pattern will be described below using a magnetic tape as an example.

[0084] Servo patterns are typically formed along the longitudinal direction of the magnetic tape. Examples of control methods that utilize servo signals (servo control) include timing-based servo (TBS), amplitude servo, and frequency servo.

[0085] As indicated in ECMA (European Computer Manufacturers Association)-319 (June 2001), magnetic tapes conforming to the LTO (Linear Tape-Open) specification (commonly referred to as "LTO tapes") employ a timing-based servo system. In this timing-based servo system, the servo pattern is composed of multiple pairs of non-parallel magnetic stripes (also called "servo stripes") arranged continuously along the longitudinal direction of the magnetic tape. The reason the servo pattern is composed of pairs of non-parallel magnetic stripes is to inform the servo signal reading element of its position as it passes over the servo pattern. Specifically, the pair of magnetic stripes is formed such that their spacing changes continuously along the width of the magnetic tape, and the servo signal reading element can determine the relative position between the servo pattern and the servo signal reading element by reading this spacing. This relative position information enables the tracking of the data track. Therefore, multiple servo tracks are typically set up on the servo pattern, aligned with the width of the magnetic tape.

[0086] A servo band consists of servo signals that run continuously along the longitudinal direction of the magnetic tape. Typically, multiple servo bands are provided on a magnetic tape. For example, in an LTO tape, there are five. The area between two adjacent servo bands is called the data band. A data band consists of multiple data tracks, each corresponding to a specific servo track.

[0087] In one embodiment, as shown in Japanese Patent Publication No. 2004-318983, each servo band has embedded information indicating the servo band number (also called "servo band ID (identification)" or "UDIM (Unique DataBand Identification Method) information"). This servo band ID is recorded by shifting a specific pair of servo stripes within a servo band so that its position is displaced relative to the longitudinal direction of the magnetic tape. Specifically, the method of shifting a specific pair of servo stripes is changed for each servo band. As a result, the recorded servo band ID is unique for each servo band, so that a servo band can be uniquely identified by reading it with a servo signal reading element.

[0088] One method for uniquely identifying a servo band is the staggered method, as described in ECMA-319 (June 2001). In this staggered method, a group of non-parallel magnetic stripes (servo stripes) arranged continuously along the longitudinal direction of the magnetic tape are recorded, shifted along the longitudinal direction of the magnetic tape for each servo band. Since the combination of these shifts between adjacent servo bands is unique across the entire magnetic tape, it is possible to uniquely identify a servo band when reading the servo pattern with two servo signal reading elements.

[0089] Furthermore, each servo band typically has embedded information indicating its position along the longitudinal direction of the magnetic tape (also known as "LPOS (Longitudinal Position) information"), as shown in ECMA-319 (June 2001). This LPOS information, like the UDIM information, is recorded by shifting the positions of a pair of servo stripes along the longitudinal direction of the magnetic tape. However, unlike the UDIM information, the same signal is recorded for each servo band in this LPOS information.

[0090] It is also possible to embed information other than the UDIM and LPOS information described above into the servo bands. In this case, the embedded information may be different for each servo band, like the UDIM information, or it may be common to all servo bands, like the LPOS information. Furthermore, methods other than those described above can be used to embed information into the servo bands. For example, a predetermined code may be recorded by thinning out a predetermined pair from a group of servo stripes.

[0091] The servo pattern forming head is called a servo light head. The servo light head has a pair of gaps corresponding to the pair of magnetic stripes, for each servo band. Typically, a core and a coil are connected to each pair of gaps, and by supplying current pulses to the coils, the magnetic field generated in the core can create a leakage magnetic field in the pair of gaps. When forming a servo pattern, by inputting current pulses while running a magnetic tape over the servo light head, the magnetic patterns corresponding to the pair of gaps are transferred to the magnetic tape, thereby forming the servo pattern. The width of each gap can be appropriately set according to the density of the servo pattern to be formed. For example, the width of each gap can be set to 1 μm or less, 1 to 10 μm, 10 μm or more, etc.

[0092] Before forming a servo pattern on a magnetic tape, it is usually demagnetized (erased). This erasing process can be performed by applying a uniform magnetic field to the magnetic tape using a DC or AC magnet. There are two types of erasing: DC (Direct Current) erasing and AC (Alternating Current) erasing. AC erasing is performed by gradually reducing the strength of the magnetic field while reversing the direction of the magnetic field applied to the magnetic tape. On the other hand, DC erasing is performed by applying a unidirectional magnetic field to the magnetic tape. There are two further methods of DC erasing. The first method is horizontal DC erasing, which applies a unidirectional magnetic field along the longitudinal direction of the magnetic tape. The second method is vertical DC erasing, which applies a unidirectional magnetic field along the thickness direction of the magnetic tape. The erasing process may be performed on the entire magnetic tape or on each servo band of the magnetic tape.

[0093] The direction of the magnetic field of the formed servo pattern is determined according to the direction of the erase. For example, when a magnetic tape is horizontally DC erased, the servo pattern is formed such that the direction of the magnetic field is opposite to the direction of the erase. This makes it possible to increase the output of the servo signal obtained by reading the servo pattern. As shown in Japanese Patent Application Publication No. 2012-53940, when a magnetic pattern using the above gap is transferred to a vertically DC erased magnetic tape, the servo signal obtained by reading the formed servo pattern is in the shape of a single-pole pulse. On the other hand, when a magnetic pattern using the above gap is transferred to a horizontally DC erased magnetic tape, the servo signal obtained by reading the formed servo pattern is in the shape of a double-pole pulse.

[0094] Magnetic tape is typically housed in a magnetic tape cartridge, which is then mounted in a magnetic recording and playback device.

[0095] [Magnetic Tape Cartridge] One aspect of the present invention relates to a magnetic tape cartridge including the above-mentioned tape-shaped magnetic recording medium (i.e., magnetic tape).

[0096] Details of the magnetic tape included in the above magnetic tape cartridge are as described above.

[0097] In a magnetic tape cartridge, the magnetic tape is generally housed inside the cartridge body, wound onto a reel. The reel is rotatably mounted inside the cartridge body. Two types of magnetic tape cartridges are widely used: single-reel cartridges, which have one reel inside the cartridge body, and double-reel cartridges, which have two reels inside the cartridge body. When a single-reel magnetic tape cartridge is mounted in a magnetic recording / reproduction device for recording and / or reproducing data onto magnetic tape, the magnetic tape is pulled out of the cartridge and wound onto the reel on the magnetic recording / reproduction device side. A magnetic head is positioned 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 on the magnetic tape cartridge side (supply reel) and the reel on the magnetic recording / reproduction device side (take-up reel). During this time, the magnetic head and the surface of the magnetic layer of the magnetic tape come into contact and slide against each other, enabling data recording and / or reproduction. In contrast, a dual-reel magnetic tape cartridge has both a supply reel and a take-up reel housed inside the magnetic tape cartridge. The magnetic tape cartridge may be either a single-reel or dual-reel type. The magnetic tape cartridge may contain a magnetic tape according to one aspect of the present invention, and prior art may be applied for other aspects.

[0098] [Magnetic Recording and Reproducing Device] One aspect of the present invention relates to a magnetic recording and reproducing device including the above-mentioned magnetic recording medium.

[0099] In the present invention and this specification, "magnetic recording and reproduction device" means a device capable of recording data onto a magnetic recording medium and reproducing data recorded on a magnetic recording medium, at least one of the above. Such a device is generally called a drive. The magnetic recording and reproduction device may be, for example, a sliding type magnetic recording and reproduction device. A sliding type magnetic recording and reproduction device is a device in which the surface of the magnetic layer and the magnetic head come into contact and slide when recording data onto a magnetic recording medium and / or reproducing recorded data. For example, the magnetic recording and reproduction device may include a magnetic tape cartridge that can be detachably attached.

[0100] The above magnetic recording and playback device may include a magnetic head. The magnetic head may be a recording head capable of recording data onto a magnetic recording medium, or a playback head capable of playing back data recorded on the magnetic recording medium. In one embodiment, the above magnetic recording and playback device may include both a recording head and a playback head as separate magnetic heads. In another embodiment, the magnetic head included in the above magnetic recording and playback device may have a configuration in which both an element for recording data (recording element) and an element for playing back data (playback element) are provided on a single magnetic head. Hereinafter, the elements for recording data and the elements for playing back data will be collectively referred to as "data elements". As the playback head, a magnetic head (MR head) that includes a magnetoresistive (MR) element as a playback element capable of sensitively reading data recorded on a magnetic recording medium is preferred. Various known MR heads such as AMR (Anisotropic Magnetoresistive) heads, GMR (Giant Magnetoresistive) heads, and TMR (Tunnel Magnetoresistive) heads can be used as MR heads. Furthermore, the magnetic head used for recording and / or reproducing data may include a servo signal reading element. Alternatively, a magnetic head equipped with a servo signal reading element (servo head) may be included in the magnetic recording and / or reproducing device as a separate head from the magnetic head used for recording and / or reproducing data. For example, a magnetic head used for recording and / or reproducing recorded data (hereinafter also referred to as the "recording and reproducing head") may include two servo signal reading elements, and each of the two servo signal reading elements may simultaneously read two adjacent servo bands. One or more data elements may be placed between the two servo signal reading elements.

[0101] In the above-described magnetic recording and reproduction apparatus, data can be recorded on a magnetic recording medium and / or data recorded on the magnetic recording medium can be reproduced by bringing the magnetic head into contact with the magnetic layer surface of the magnetic recording medium and sliding it. The above-described magnetic recording and reproduction apparatus may include a magnetic recording medium according to one aspect of the present invention, and prior art can be applied for other aspects.

[0102] For example, when recording data and / or playing back recorded data, tracking using servo signals is performed first. That is, by making the servo signal reading element follow a predetermined servo track, the data element is controlled to pass over the target data track. The movement of the data track is achieved by changing the servo track read by the servo signal reading element in the tape width direction. The recording / playback head can also record and / or play back data on other data bands. In this case, the UDIM information described above is used to move the servo signal reading element to a predetermined servo band, and tracking for that servo band is started.

[0103] The present invention will be described below based on examples. However, the present invention is not limited to the embodiments shown in the examples. The terms "parts" and "%" used below are based on mass. Furthermore, the processes and evaluations described below were carried out in an environment with an ambient temperature of 23°C ± 1°C unless otherwise specified.

[0104] In the examples and comparative examples, the polyurethane resin A used to prepare the non-magnetic layer forming composition is a polyurethane resin having an aromatic cyclic structure synthesized by the following method.

[0105] [Synthesis of Polyurethane Resin A Having an Aromatic Cyclic Structure] Polyester polyol (a) shown in Table 2, and compound (b), which is an EO (ethylene oxide) adduct of HPN (2,2-dimethyl-1,3-propanediol mono(hydroxypivalate)) and Na sulfisophthalic acid as short-chain diols, were dissolved in a 30% cyclohexanone solution at a liquid temperature of 60°C under a nitrogen stream in a container equipped with a reflux condenser and stirrer and pre-purged with nitrogen. Next, 60 ppm (by mass) of dibutyltin dilaurate was added as a catalyst and dissolved for a further 15 minutes. Furthermore, MDI (4,4-diphenylmethane diisocyanate) was added as a diisocyanate and the reaction was heated at a liquid temperature of 90°C for 6 hours to obtain polyurethane resin A. The raw material composition ratios were as shown in Table 1.

[0106]

[0107]

[0108]

[0109]

[0110]

[0111] The number-average molecular weight of polyester polyol (a) shown in Table 2 is calculated from the hydroxyl value measured by the method described in JIS standard K1557-1:2007.

[0112]

[0113] The weight-average molecular weight of polyurethane resin A, determined by the method described above, was 67,000.

[0114] Regarding polyurethane resin A, a metal sulfonate base (SO ３ The Na content was determined from the concentration of the metallic component Na using the following formula, and was found to be 70 eq / ton. In the formula below, "23" is the atomic weight of Na. The Na concentration (unit: ppm (mass basis)) was determined by atomic absorption spectrometry after carbonizing 0.1 g of the sample and dissolving it in acid. Metallic sulfonic acid (SO ３ Na) content (eq / ton) = Na concentration (ppm) / 23

[0115] Polyurethane resin A was analyzed in chloroform-d solvent using a Varian Gemini-200 nuclear magnetic resonance analyzer (NMR). １ The polyurethane composition was determined by performing 1H-NMR analysis and analyzing the integral ratio. １ The urethane group concentration was determined from the molar percentage of isocyanate obtained by 1H-NMR analysis using the following formula. The determined urethane group concentration was 3.5 meq / g. Urethane group concentration (meq / g) = 10 ３ (x {2 x molar percentage of isocyanate x (1 / 100)} / unit molecular weight of polyurethane)

[0116] [Example 1] <Formulation of composition for forming a magnetic layer> (Magnetic liquid) Hexagonal barium ferrite powder: 100.0 parts (Average particle size: 17 nm, Activation volume: 1300 nm) ３ ) Oleic acid: 1.5 parts Polyvinyl chloride resin: 10.0 parts (Kaneka Corporation MR-104) Polyurethane resin: 4.0 parts (Toyobo Corporation UR-4800 (sulfonic acid group-containing polyester polyurethane resin)) Methyl ethyl ketone: 300.0 parts Cyclohexanone: 200.0 parts (Abrasive liquid) Alumina powder (α-alumina with average particle size of 110 nm): 9.0 parts Polyvinyl chloride resin: 0.7 parts (Kaneka Corporation MR-110) Cyclohexanone: 20.0 parts (Protrusion-forming agent liquid) Carbon black (average particle size: 80 nm): 0.5 parts Cyclohexanone: 4.0 parts (Other components) Stearic acid: 1.0 part, Stearamide: 0.3 parts, Butyl stearate: 1.5 parts, Methyl ethyl ketone: 110.0 parts, Cyclohexanone: 110.0 parts, Polyisocyanate (Coronate L, manufactured by Tosoh Corporation): 2.5 parts

[0117] <Formulation for Non-Magnetic Layer Forming Composition> Carbon black: See Table 3 (Specific surface area: See Table 3, pH: See Table 3) Trioctylamine: 4.0 parts Polyurethane resin A: 30.0 parts Methyl ethyl ketone: 510.0 parts Cyclohexanone: 200.0 parts Stearic acid: 1.5 parts Stearamide: 0.3 parts Butyl stearate: 1.5 parts

[0118] <Formulation for backcoat layer formation composition> Carbon black: 100.0 parts (average particle size: 40 nm, DBP (Dibutyl phosphate) oil absorption: 74 cm²) ３ ( / 100g) Copper phthalocyanine: 3.0 parts Nitrocellulose: 25.0 parts Polyurethane resin: 60.0 parts (UR-8401 (sulfonic acid group-containing polyester polyurethane resin) manufactured by Toyobo Co., Ltd.) Polyester resin: 4.0 parts (Byron 500 manufactured by Toyobo Co., Ltd.) Alumina powder (specific surface area 17 m²) ２ α-alumina (per g): 1.0 part Polyisocyanate: 15.0 parts (Colonate L, manufactured by Tosoh Corporation) Methyl ethyl ketone: 600.0 parts Toluene: 600.0 parts

[0119] <Preparation of Compositions for Forming Each Layer> The magnetic layer-forming compositions were prepared as follows: After kneading and diluting the components of the magnetic liquid using an open kneader, zirconia (ZrO) with a particle size of 0.1 mm was dispersed using a horizontal bead mill disperser. ２ Using Zr beads (hereinafter referred to as "Zr beads"), a dispersion treatment was performed for 30 passes with a bead filling rate of 80 vol% and a rotor tip peripheral speed of 10 m / sec, with a residence time of 2 minutes per pass. For the abrasive liquid, a mixture of the components of the abrasive liquid described above (alumina powder, vinyl chloride resin, and cyclohexanone) was prepared, and this mixture was placed in a horizontal bead mill disperser together with Zr beads with a particle size of 0.3 mm, and the bead volume / (abrasive liquid volume + bead volume) was adjusted to 80 vol%. The bead mill dispersion treatment was performed for 120 minutes, and the treated liquid was removed and subjected to ultrasonic dispersion filtration using a flow-type ultrasonic dispersion filtration device. The magnetic liquid, abrasive liquid, protrusion-forming agent liquid, and other components were introduced into a dissolver stirrer and stirred at a peripheral speed of 10 m / sec for 30 minutes. Then, it was treated in 3 passes with a flow-type ultrasonic disperser at a flow rate of 7.5 kg / min, and then filtered through a 1 μm filter to prepare a composition for forming a magnetic layer.

[0120] A composition for forming a non-magnetic layer was prepared as follows. The above components, excluding the lubricants (stearic acid, stearic acid amide, and butyl stearate), were kneaded and diluted in an open kneader, and then dispersed in a horizontal bead mill disperser. Subsequently, the lubricants (stearic acid, stearic acid amide, and butyl stearate) and methyl ethyl ketone for adjusting the coating thickness were added, and the mixture was stirred and mixed in a dissolver stirrer to prepare a composition for forming a non-magnetic layer. In Example 1, and in Examples 2 to 4 and Comparative Examples 1 to 3 described later, when preparing the composition for forming a non-magnetic layer, methyl ethyl ketone for adjusting the coating thickness was used in an amount ranging from 70.0 to 510.0 parts by mass per 100.0 parts by mass of non-magnetic powder used for preparing the composition for forming a non-magnetic layer.

[0121] A composition for forming a backcoat layer was prepared as follows: The above components, excluding polyisocyanate, were introduced into a dissolver stirrer and stirred at a peripheral speed of 10 m / s for 30 minutes, after which dispersion was carried out using a horizontal bead mill disperser. Subsequently, polyisocyanate was added and stirred and mixed using a dissolver stirrer to produce a composition for forming a backcoat layer.

[0122] <Machining of Magnetic Tape> A non-magnetic layer was formed by coating one surface of a 6.00 μm thick biaxially oriented polyethylene naphthalate support with a non-magnetic layer composition so that the thickness after drying would be as shown in Table 3, and then drying it. A coated layer was then formed on the formed non-magnetic layer by coating it with the magnetic layer composition prepared above so that the thickness after drying would be 40 nm. While the coated layer of the magnetic layer composition was still wet (not yet dry), a vertical orientation treatment was performed by applying a magnetic field of magnetic field strength of 0.15 T perpendicular to the surface of the coated layer. After that, the coated layer was dried to form a magnetic layer. Then, a back coat layer composition was coated on the surface of the support opposite to the surface where the non-magnetic and magnetic layers were formed, so that the thickness after drying would be 0.50 μm, and then dried. After that, calendering was performed using a calendering machine consisting only of metal rolls at a speed of 100 m / min, a linear pressure of 294 kN / m, and a calendering temperature of 100°C. Subsequently, the material was heat-treated for 36 hours in an ambient temperature of 70°C. After heat treatment, the material was slit into 1 / 2-inch wide strips (1 inch = 0.0254 meters) to obtain magnetic tape.

[0123] [Examples 2-4, Comparative Examples 1-3] Magnetic tapes were prepared using the method described for Example 1, except that the items shown in Table 3 were changed as shown in Table 3. The thickness of the non-magnetic layer was adjusted by the amount of methyl ethyl ketone used for adjusting the coating thickness when preparing the composition for forming the non-magnetic layer.

[0124] [Comparative Example 4] A magnetic tape was prepared by the method described for Example 1, except that the items shown in Table 3 were changed as shown in Table 3, and the content of polyurethane resin A in the non-magnetic layer forming composition was changed from 30.0 parts to 75.0 parts. The thickness of the non-magnetic layer was adjusted by the amount of methyl ethyl ketone used for adjusting the coating thickness when preparing the non-magnetic layer forming composition.

[0125] [Evaluation Method] <Average pore diameter (magnetic layer + non-magnetic layer), standard deviation of pore diameter distribution (magnetic layer + non-magnetic layer)> For each magnetic tape in the examples and comparative examples, the "average pore diameter (magnetic layer + non-magnetic layer)" and the "standard deviation of pore diameter distribution (magnetic layer + non-magnetic layer)" were determined using the method described above. A BELSORP MINI X (Microtrac-Bel) was used as the nitrogen adsorption measuring instrument.

[0126] <Various Thicknesses> Samples for cross-sectional observation were prepared using the method described above as a specific example. The thickness of the magnetic layer was determined using the method described above with respect to the prepared cross-sectional observation samples. The thickness of the non-magnetic layer was also determined using the same method as described above for measuring the thickness of the magnetic layer. A Hitachi FE-SEM S4800 was used as the field emission scanning electron microscope (FE-SEM) for SEM observation. The thickness of the magnetic layer of each magnetic tape in Examples 1 to 4 and Comparative Examples 1 to 4 was 40 nm, and the thickness of the non-magnetic layer was as shown in Table 3.

[0127] <Magnetic Layer Bm> For each magnetic tape in the examples and comparative examples, the magnetization amount Φm [G・μm] was determined by sweeping the magnetic tape with an external magnetic field at a maximum external magnetic field of 15 kOe and a scan speed of 60 Oe / sec using a vibrating sample type magnetometer (manufactured by Toyo Kogyo) at a measurement temperature of 23°C ± 1°C. The magnetic layer Bm was calculated as the value obtained by dividing Φm [G・μm] by the magnetic layer thickness (unit: μm) determined above. The magnetic layer Bm is preferably 1500 G (Gauss) or higher, and more preferably 1600 G or higher, 1700 G or higher, and 1800 G or higher in that order. A higher magnetic layer Bm is preferable from the viewpoint of improving electromagnetic conversion characteristics. In one embodiment, the magnetic layer Bm can be, for example, 3000 G or less, 2500 G or less, or 2200 G or less.

[0128] <Electromagnetic Conversion Characteristics (SNR: Signal-to-Noise Ratio)> The SNR was measured for each magnetic tape in the examples and comparative examples using a 1 / 2-inch reel tester with a fixed head. The magnetic head / magnetic tape relative speed was set to 6 m / sec. Recording was performed using a MIG (Metal-In-Gap) head (gap length 0.15 μm, track width 1.0 μm), and the recording current was set to the optimal recording current for each magnetic tape. A GMR head with an element thickness of 15 nm, a shielding gap of 0.1 μm, and a lead width of 0.5 μm was used as the playback head. A signal with a linear recording density (270 kfci) was recorded, and the playback signal was measured using a spectrum analyzer manufactured by Shibasoku Corporation. The ratio of the carrier signal output to the integrated noise across the entire spectral band was defined as the SNR, and it was calculated as a relative value with Comparative Example 1 as the reference (0.0 dB). The unit kfci is a unit of linear recording density (not convertible to the SI unit system). The signal used was from a portion of the magnetic tape after it had started running and the signal was sufficiently stable.

[0129] The results are shown in Table 4.

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[0131]

[0132] The results shown in Table 4 confirm that the magnetic tapes of Examples 1 to 4 exhibit excellent electromagnetic conversion characteristics.

[0133] The present invention is useful in the field of various magnetic recording media, such as magnetic tapes for data storage.

Claims

1. A magnetic recording medium comprising a non-magnetic support and a magnetic layer containing ferromagnetic powder, further comprising a non-magnetic layer containing non-magnetic powder between the non-magnetic support and the magnetic layer, wherein the average pore diameter of the magnetic layer and the non-magnetic layer is 30.0 nm or less, and the standard deviation of the pore diameter distribution of the magnetic layer and the non-magnetic layer is 30.0 nm or less.

2. The magnetic recording medium according to claim 1, wherein the thickness of the non-magnetic layer is 1.00 μm or less.

3. The magnetic recording medium according to claim 1, wherein the non-magnetic powder of the non-magnetic layer contains carbon black.

4. The magnetic recording medium according to claim 3, wherein the non-magnetic layer contains 60.0% by mass or more of carbon black relative to the total amount of non-magnetic powder.

5. The magnetic recording medium according to claim 3, wherein the pH of the carbon black is 7.0 or higher and 10.0 or lower.

6. The specific surface area of ​​the carbon black is 280 m². ２ / g or more 500m ２ The magnetic recording medium according to claim 5, wherein the value is less than or equal to / g.

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

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

9. The thickness of the non-magnetic layer is 1.00 μm or less, the non-magnetic layer contains 60.0% by mass or more of carbon black relative to the total amount of non-magnetic powder, the pH of the carbon black is 7.0 or more and 10.0 or less, and the specific surface area of ​​the carbon black is 280 m². ２ / g or more 500m ２ The magnetic recording medium according to claim 1, wherein the amount is less than or equal to / g, and the non-magnetic support further has a back coat layer containing non-magnetic powder on the surface side opposite to the surface side having the magnetic layer and the non-magnetic layer, and is a magnetic tape.

10. A magnetic tape cartridge comprising the magnetic tape according to claim 8 or 9.

11. A magnetic recording and reproducing apparatus including a magnetic recording medium according to any one of claims 1 to 9.

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