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

Abstract

Provided is a magnetic recording medium comprising a polymer having a structural unit represented by formula (1) and a structural unit represented by formula (2) in a part on the magnetic layer side of a non-magnetic support. The polymer contains, relative to 100 mol% of all structural units in the polymer, above 50 mol% of the structural unit represented by formula (1), and in formula (1), R11 and R12 each independently represent a hydrogen atom or an alkyl group; R13 represents a hydrogen atom or a monovalent substituent; L11 represents a single bond or a divalent linking group; and A11 represents a monovalent hydrocarbon group; and in formula (2), R21 and R22 each independently represent a hydrogen atom or an alkyl group; R23 represents a hydrogen atom or a monovalent substituent; L21 represents a single bond or a divalent linking group; and A21 represents a monovalent group selected from the group consisting of an acidic group and a basic group.

Description

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

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

[0002] Magnetic recording media are generally produced by forming a magnetic layer containing ferromagnetic powder on a non-magnetic support (see, for example, Patent Document 1).

[0003] WO98 / 35345

[0004] Recording data on a magnetic recording medium and reproducing the recorded data are usually performed by running the magnetic recording medium in a magnetic recording and reproducing device and bringing the magnetic layer surface of the magnetic recording medium into contact with a magnetic head and causing them to slide. From the viewpoint of running stability, it is desirable that the coefficient of friction during sliding between the magnetic layer surface and the magnetic head is low even when the magnetic recording medium is repeatedly run.

[0005] An object of one aspect of the present invention is to provide a magnetic recording medium that can exhibit a low coefficient of friction after repeated running.

[0006] The present inventors have conducted extensive research to provide a magnetic recording medium that can exhibit a low coefficient of friction even after repeated running, and as a result, have newly discovered that a magnetic recording medium containing the following polymer in a portion of a non-magnetic support on the magnetic layer side (details will be described later) can exhibit a low coefficient of friction even after repeated running.

[0007] That is, one aspect of the present invention is as follows: [1] A magnetic recording medium having a non-magnetic support and a magnetic layer containing ferromagnetic powder, wherein a polymer (hereinafter also simply referred to as "polymer") having a structural unit represented by formula (1) and a structural unit represented by formula (2) is contained in a portion of the non-magnetic support facing the magnetic layer, and the polymer contains more than 50 mol % of the structural unit represented by formula (1) relative to 100 mol % of all structural units contained in the polymer, In formula (1), R １１ and R １２ each independently represents a hydrogen atom or an alkyl group, R １３ represents a hydrogen atom or a monovalent substituent; L １１represents a single bond or a divalent linking group; A １１ represents a monovalent hydrocarbon group, and in formula (2), R ２１ and R ２２ each independently represents a hydrogen atom or an alkyl group, R ２３ represents a hydrogen atom or a monovalent substituent; L ２１ represents a single bond or a divalent linking group; A ２１ represents a monovalent group selected from the group consisting of an acidic group and a basic group. １１ [3] In formula (2), A is a monovalent hydrocarbon group having 8 to 50 carbon atoms. ２１ [4] The magnetic recording medium according to [1] or [2], wherein the monovalent group represented by the formula (1) is selected from the group consisting of a carboxy group, a sulfonic acid group, a phosphoric acid group, a phosphonic acid group, and an amino group. １１ and L in formula (2) ２１ each independently represents a single bond, an ester group, an alkylene group, or a divalent group consisting of a combination of one or more ester groups and one or more alkylene groups. [5] The magnetic recording medium according to any one of [1] to [4], wherein the polymer has a number-average molecular weight of 2,000 to 100,000. [6] The magnetic recording medium according to any one of [1] to [5], further comprising one or more fatty acid compounds selected from the group consisting of fatty acids, fatty acid esters, and fatty acid amides, on the magnetic layer side of the non-magnetic support. [7] The magnetic recording medium according to any one of [1] to [6], further comprising a non-magnetic layer containing non-magnetic powder between the non-magnetic support and the magnetic layer. [8] The magnetic recording medium according to any one of [1] to [7], further comprising a backcoat layer containing non-magnetic powder on the surface of the non-magnetic support opposite to the surface having the magnetic layer. [9] The magnetic recording medium according to any one of [1] to [8], wherein the magnetic recording medium is a magnetic tape.

[10] In formula (1), A １１ The number of carbon atoms of the monovalent hydrocarbon group represented by the formula (2) is 8 to 50, ２１ is selected from the group consisting of a carboxy group, a sulfonic acid group, a phosphoric acid group, and an amino group, and L in formula (1)１１ and L in formula (2) ２１ each independently represent a single bond, or a divalent group consisting of an ester group, an alkylene group, or a combination of one or more ester groups and one or more alkylene groups, the number average molecular weight of the polymer is 2,000 or more and 100,000 or less, the non-magnetic support further comprises one or more fatty acid compounds selected from the group consisting of fatty acids, fatty acid esters, and fatty acid amides, on a portion of the non-magnetic support facing the magnetic layer, the non-magnetic support further comprises a non-magnetic layer containing a non-magnetic powder between the non-magnetic support and the magnetic layer, and the non-magnetic support further comprises a backcoat layer containing a non-magnetic powder on the surface opposite to the surface having the magnetic layer, and

[0008] According to one aspect of the present invention, it is possible to provide a magnetic recording medium that exhibits a low coefficient of friction after repeated running, and a magnetic tape cartridge and a magnetic recording / reproducing device that include the magnetic recording medium.

[0009] [Magnetic Recording Medium] One aspect of the present invention relates to a magnetic recording medium having a non-magnetic support and a magnetic layer containing ferromagnetic powder. The magnetic recording medium contains the polymer in a portion of the non-magnetic support facing the magnetic layer.

[0010] In the present invention and this specification, "the portion on the non-magnetic support facing the magnetic layer" refers to the magnetic layer in a magnetic recording medium having a magnetic layer directly on the non-magnetic support, and refers to the magnetic layer and / or non-magnetic layer in a magnetic recording medium having a non-magnetic layer, which will be described later, between the non-magnetic support and the magnetic layer. "The portion on the non-magnetic support facing the magnetic layer" is also simply referred to as "the portion on the magnetic layer side." The presence of a certain component on the surface facing the magnetic layer of a magnetic recording medium is also included in the component being included in the portion facing the magnetic layer. In the present invention and this specification, "surface of the magnetic layer" is synonymous with the surface facing the magnetic layer of a magnetic recording medium.

[0011] The magnetic recording medium contains the polymer in the magnetic layer side portion. It is believed that the polymer can function as a lubricant for the magnetic recording medium. The inventors believe that the polymer's ability to function as a lubricant for the magnetic recording medium, thereby imparting lubricity to the magnetic layer surface, contributes to the magnetic recording medium being able to exhibit a low coefficient of friction after repeated running. Specifically, the polymer has a structural unit represented by formula (1) and a structural unit represented by formula (2), and contains more than 50 mol % of the structural unit represented by formula (1) relative to 100 mol % of all structural units contained in this polymer. The polymer contains A of the structural unit represented by formula (2), ２１ It is presumed that the A moiety of the structural unit represented by formula (1) in the polymer can function as an adsorptive functional group that adsorbs to the particle surface of the ferromagnetic powder. １１ It is speculated that the orientation of the polymers may contribute to reducing friction. For example, it is speculated that at least a portion of the polymer contained in the magnetic layer may be present on the surface of the magnetic layer. Furthermore, the inventors speculate that the polymer contained within the magnetic layer may migrate to the surface of the magnetic layer during sliding with the magnetic head, thereby being present on the surface of the magnetic layer. The polymer may also be contained in the non-magnetic layer described below, and it is speculated that the polymer contained in the non-magnetic layer may migrate to the magnetic layer and then migrate to the surface of the magnetic layer to be present on the surface of the magnetic layer. Furthermore, the inventors believe that the inclusion of the polymer in the magnetic layer-side portion of the magnetic recording medium may also contribute to reducing changes in the surface shape of the magnetic layer before and after repeated running. Minimizing changes in the surface shape of the magnetic layer before and after repeated running is desirable from the perspective of preventing a decrease in the performance of the magnetic recording medium after repeated running. However, the above is merely speculation and does not limit the present invention. The present invention is also not limited to other speculations described in this specification.

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

[0013] <Polymer> The polymer has a structural unit represented by formula (1) and a structural unit represented by formula (2), and contains more than 50 mol% of the structural unit represented by formula (1) relative to 100 mol% of all structural units contained in the polymer.

[0014] The polymer may contain structural units represented by formula (1) having the same structure or different structures. １１ , R １２ , R １３ , L １１ and A １１ may be the same or different in the structural units represented by formula (1). The polymer may contain structural units represented by formula (2) of the same structure or different structures. ２１ , R ２２ , R ２３ , L ２１ and A ２１ may be the same or different in multiple structural units represented by formula (2). Both ends of each structural unit bond to another structural unit or to an end group of the polymer.

[0015] The structural unit represented by formula (1) and the structural unit represented by formula (2) will be described below in order. In the present invention and this specification, unless otherwise specified, the groups described may be unsubstituted or may have a substituent. When a group has a substituent, examples of the substituent include an alkyl group (e.g., an alkyl group having 1 to 6 carbon atoms), a hydroxy group, an alkoxy group (e.g., an alkoxy group having 1 to 6 carbon atoms), a cyano group, an amino group, a nitro group, an acyl group, and a carboxy group. Furthermore, with respect to a group having a substituent, the "number of carbon atoms" refers to the number of carbon atoms in the portion excluding the substituent. Furthermore, in the present invention and this specification, unless otherwise specified, the terms "alkyl group," "alkylene group," and "hydrocarbon group" each encompass linear, branched, and cyclic forms of each group.

[0016] (Constituent unit represented by formula (1))

[0017]

[0018] In formula (1), R １１ and R１２ each independently represents a hydrogen atom or an alkyl group, R １３ represents a hydrogen atom or a monovalent substituent.

[0019] R １１ and R １２ In one embodiment, both of R represent a hydrogen atom, and in another embodiment, one of R represents a hydrogen atom and the other represents an alkyl group. １１ and R １２ represent the same or different alkyl groups.

[0020] R １１ and / or R １２ Examples of the alkyl group that can be represented by the formula (I) include a linear alkyl group having from 1 to 18 carbon atoms, a branched alkyl group having from 3 to 18 carbon atoms, and a cyclic alkyl group having from 3 to 18 carbon atoms. Specific examples of the alkyl group include a methyl group, an ethyl group, a propyl group, a butyl group, an isopropyl group, a tert(tertiary)-butyl group, and a cyclohexyl group.

[0021] In formula (1), R １３ represents a hydrogen atom or a monovalent substituent. １３ Examples of the monovalent substituent that can be represented by the formula include alkyl groups. For such alkyl groups, R １１ and / or R １２ Reference can be made to the above description of alkyl groups that can be represented by:

[0022] In one embodiment, in formula (1), R １１ and R １２ represents a hydrogen atom, R １３ represents a hydrogen atom or a methyl group. １１ and R １２ represents a hydrogen atom, R １３ When R represents a hydrogen atom, the structural unit represented by formula (1) can be a structural unit derived from an acrylate ester. １１ and R １２ represents a hydrogen atom, R １３ When represents a methyl group, the structural unit represented by formula (1) can be a structural unit derived from a methacrylic acid ester.

[0023] In formula (1), L １１ represents a single bond or a divalent linking group. Examples of the divalent linking group include a divalent group represented by an ester group (-C(=O)O-), a carbonyl group (-C(=O)-), an oxygen atom (-O-), a sulfur atom (-S-), -NH-, an alkylene group, an arylene group, an aryleneoxy group (-Ar-O-: Ar represents an arylene group, the same applies hereinafter), and an arylene alkylene group (-Ar-R-: R represents an alkylene group), and a divalent group formed by combining two or more of the above divalent linking groups. In one embodiment, L １１ The divalent linking group that can be represented by can be an ester group, an alkylene group, or a divalent group consisting of a combination of one or more (e.g., one or two) ester groups and one or more (e.g., one or two) alkylene groups. The arylene group and the arylene group represented by Ar can preferably be an arylene group having 6 to 20 carbon atoms. The alkylene group and the alkylene group represented by R can preferably be a linear or branched alkylene group having 1 to 10 carbon atoms.

[0024] In formula (1), A １１ represents a monovalent hydrocarbon group. １１ The monovalent hydrocarbon group represented by the formula (I) can be a linear hydrocarbon group, a branched hydrocarbon group, or a cyclic hydrocarbon group. The cyclic hydrocarbon group is preferably a monocyclic hydrocarbon group, and preferably does not have a cyclic structure corresponding to a condensed ring structure and / or a bridged ring structure. １１ The monovalent hydrocarbon group represented by the formula (I) can be a saturated hydrocarbon group or an unsaturated hydrocarbon group, and is preferably a saturated hydrocarbon group.

[0025] A １１ From the viewpoint of reducing friction, the number of carbon atoms in the monovalent hydrocarbon group represented by A is preferably 8 or more, more preferably 12 or more, and even more preferably 14 or more, and 15 or more in that order. １１ The number of carbon atoms in the monovalent hydrocarbon group represented by the formula (I) is preferably 50 or less, more preferably 28 or less, and even more preferably 24 or less, and further more preferably 22 or less. １１The monovalent hydrocarbon group represented by the formula (I) is preferably a linear hydrocarbon group or a branched hydrocarbon group, and more preferably a linear alkyl group or a branched alkyl group. When the alkyl group is linear, in addition to the low surface energy effect brought about by hydrophobicity, the alkyl groups tend to aggregate with each other and be oriented in the interface direction. １１ The monovalent hydrocarbon group represented by the formula (I) is more preferably a linear alkyl group.

[0026] From the viewpoint of reducing friction, the content of the structural unit represented by formula (1) in the polymer is more than 50 mol%, preferably 55 mol% or more, more preferably 60 mol% or more, 65 mol% or more, 70 mol% or more, 75 mol% or more in this order.On the other hand, since the polymer contains at least the structural unit represented by formula (2) together with the structural unit represented by formula (1), the content of the structural unit represented by formula (1) in the polymer is less than 100 mol% with respect to the total structural unit 100 mol% contained in the polymer, and can be, for example, 99 mol% or less, 95 mol% or less, 90 mol% or less or 85 mol% or less.

[0027] The contents of various structural units in a polymer can be measured by known methods, or can be calculated from the molar ratio of the polymerization components (commoners) used in synthesizing the polymer. For example, for a polymer synthesized using two polymerization components (components A and B) in a molar ratio of component A:component B = 80:20, the content of structural units derived from component A can be calculated to be 80 mol % and the content of structural units derived from component B can be calculated to be 20 mol %, relative to 100 mol % of all structural units constituting the polymer.

[0028] Specific examples of polymerization components for introducing the structural unit represented by formula (1) into a polymer include the following polymerization components, although the present invention is not limited to these examples.

[0029]

[0030]

[0031]

[0032]

[0033] (Constituent unit represented by formula (2))

[0034]

[0035] In formula (2), R ２１ and R ２２ each independently represents a hydrogen atom or an alkyl group, R ２３ represents a hydrogen atom or a monovalent substituent.

[0036] R in formula (2) ２１ For details, see R in formula (1). １１ The above description of R in formula (2) ２２ For details, see R in formula (1). １２ The above description of R in formula (2) ２３ For details, see R in formula (1). １３ Reference can be made to the above descriptions regarding the respective

[0037] In formula (2), L ２１ represents a single bond or a divalent linking group. ２１ For details, see L in formula (1). １１ See the above description regarding

[0038] In formula (2), A ２１ represents a monovalent group selected from the group consisting of acidic groups and basic groups. ２１ It is presumed that the monovalent group represented by the formula (I) can function as an adsorptive functional group that adsorbs to the particle surface of the ferromagnetic powder.

[0039] In the present invention and this specification, the term "acidic group" refers to a group that reacts with H in water or a solvent containing water (hereinafter referred to as "aqueous solvent"). ＋ A refers to a group that can release and dissociate into an anion. ２１ When represents an acidic group, such an acidic group is not particularly limited as long as it is a group that falls within the above definition. ２１ Examples of the acidic group that can be represented by the formula include a carboxy group (—COOH), a sulfonic acid group (—SO ３ H), phosphate group (-OP(=O)(OH) ２ ), a phosphonic acid group (-P(=O)(OH)２ ), a boronic acid group (-B(OH) ２ From the viewpoint of further improving the adsorptivity, the acidic group is preferably a carboxyl group or a phosphate group.

[0040] In the present invention and the present specification, a "basic group" refers to a group that can be dissociated into a cation in water or an aqueous solvent or can be dissociated into a H ＋ A refers to a group that accepts a cation. ２１ When A represents a basic group, the basic group is not particularly limited as long as it is a group that satisfies the above definition. ２１ Examples of the basic group that can be represented by the formula (I) include an amino group, a guanidyl group, an amidyl group, an imidazolyl group, etc., and from the viewpoint of further improving adsorption, an amino group is preferred. The amino group may be an unsubstituted amino group or a substituted amino group, more specifically, a primary, secondary, or tertiary amino group, and a tertiary amino group is preferred. Furthermore, the substituent substituting the hydrogen atom in the substituted amino group may be, for example, a linear, branched, or cyclic alkyl group having from 1 to 10 carbon atoms.

[0041] From the viewpoint of further improving the adsorption ability, A ２１ more preferably represents a monovalent group selected from a carboxy group, a sulfonic acid group, a phosphate group, and an amino group (preferably a tertiary amino group), even more preferably represents a monovalent group selected from a carboxy group, a phosphate group, and an amino group (preferably a tertiary amino group), and even more preferably represents a carboxy group or a tertiary amino group.

[0042] The content of the structural unit represented by formula (2) in the polymer is more than 0 mol% relative to 100 mol% of all structural units contained in the polymer, and from the viewpoint of further reducing friction by further improving the adsorption of the polymer to the particle surface of the ferromagnetic powder, it is preferably 1 mol% or more, and more preferably 3 mol% or more, 5 mol% or more, 10 mol% or more, 15 mol% or more, and 20 mol% or more in this order. On the other hand, since the polymer contains more than 50 mol% of the structural unit represented by formula (1) relative to 100 mol% of all structural units, the content of the structural unit represented by formula (2) in the polymer is less than 50 mol% relative to 100 mol% of all structural units contained in the polymer, and can also be 45 mol% or less.

[0043] Specific examples of polymerization components for introducing the structural unit represented by formula (2) into a polymer include the following polymerization components, although the present invention is not limited to these examples.

[0044] Examples of polymerization components containing a carboxy group include unsaturated carboxylic acids such as (meth)acrylic acid, carboxyethyl (meth)acrylate, crotonic acid, itaconic acid, maleic acid, fumaric acid, citraconic acid, p-vinylbenzoic acid, m-vinylbenzoic acid, and o-vinylbenzoic acid, as well as 2-methacryloyloxymethylsuccinic acid, 1-heptenoic acid, 1-hexenoic acid, 1-pentenoic acid, and 1-butenoic acid. In the present invention and this specification, the term "(meth)acrylic acid" encompasses both acrylic acid and methacrylic acid.

[0045] Examples of the polymerization component having a sulfonic acid group include p-styrenesulfonic acid, 2-acrylamido-2-methylpropanesulfonic acid, 3-sulfopropyl(meth)acrylate, and bis-(3-sulfopropyl)-itaconate.

[0046] Examples of the polymerization component having a phosphoric acid group include vinylphosphonic acid, 3-methacryloxypropylphosphonic acid, 3-acryloxypropylphosphonic acid, vinyl phosphate, and 2-(methacryloyloxy)ethyl phosphate.

[0047] Examples of the polymerization component having an amino group, including a tertiary amino group, include N,N-dimethylaminoethyl (meth)acrylate, N,N-dimethylaminopropyl (meth)acrylate, N,N-diethylaminoethyl (meth)acrylate, N,N-diethylaminopropyl (meth)acrylate, N,N-dimethylaminoethyl (meth)acrylamide, N,N-dimethylaminopropyl (meth)acrylamide, N,N-diethylaminoethyl (meth)acrylamide, N,N-diethylaminopropyl (meth)acrylamide, vinylpyrrolidone, 2-vinylpyridine, 4-vinylpyridine, 2-methyl-6-vinylpyridine, and 5-ethyl-2-vinylpyridine. Examples of the polymerization component having a secondary amino group include 2-(t-butylamino)ethyl (meth)acrylate.

[0048] Some specific examples of structures of polymerization components for introducing the constitutional unit represented by formula (2) into a polymer are shown below: In the following, "n" represents the number of repeating units, and is, for example, 1 or more.

[0049]

[0050]

[0051] In one embodiment, the structural units contained in the polymer may be only structural units represented by formula (1) and structural units represented by formula (2), and in another embodiment, one or more structural units not corresponding to formula (1) and formula (2) may be included. The total content of the structural units represented by formula (1) and the structural units represented by formula (2) relative to 100 mol% of all structural units contained in the polymer may be, for example, 90 mol% or more and 100 mol% or less, or 95 mol% or more and 100 mol% or less.

[0052] The polymer is a copolymer because it has a structural unit represented by formula (1) and a structural unit represented by formula (2). A "structural unit" is also generally referred to as a "repeating unit." In the polymer, multiple structural units represented by formula (1) may be present consecutively or discontinuously. As for the structural unit represented by formula (2), multiple structural units represented by formula (2) may be present consecutively or discontinuously. In other words, the copolymerization type of the polymer is not particularly limited, and can be any copolymerization type (for example, random copolymerization, alternating copolymerization, block copolymerization, or graft copolymerization).

[0053] (Number Average Molecular Weight) The larger the number average molecular weight of the polymer, the greater the number of A １１ As described above, the density of hydrocarbon groups represented by A tends to be high. １１ It is presumed that the hydrocarbon group represented by the formula (I) can contribute to reducing friction. From the viewpoint of increasing the density of the hydrocarbon group, the number average molecular weight of the polymer is preferably 2,000 or more, and more preferably 3,000 or more, 4,000 or more, 5,000 or more, 6,000 or more, 7,000 or more, and 8,000 or more in that order. Furthermore, from the viewpoint of the adsorption efficiency of the polymer to adsorption sites such as the particle surfaces of ferromagnetic powder, the number average molecular weight of the polymer is preferably 100,000 or less, and more preferably 90,000 or less, 80,000 or less, 70,000 or less, 60,000 or less, 50,000 or less, 40,000 or less, 30,000 or less, and 20,000 or less in that order.

[0054] In the present invention and this specification, the average molecular weight (number average molecular weight and weight average molecular weight) refers to a value measured by gel permeation chromatography (GPC) and calculated in terms of standard polystyrene.

[0055] (Synthesis Method) The polymer can be synthesized by a known method. For the synthesis method, for example, known techniques for synthesizing vinyl polymers can be referenced. Compounds used as polymerization components for synthesizing the polymer can be obtained as commercial products or can be synthesized by a known method.

[0056] (Content) For example, the magnetic layer or magnetic layer-forming composition may contain 0.1 parts by mass or more of the polymer per 100.0 parts by mass of ferromagnetic powder, preferably 0.5 parts by mass or more. Furthermore, the content of the polymer in the magnetic layer or magnetic layer-forming composition may be, for example, 50.0 parts by mass or less, 40.0 parts by mass or less, 30.0 parts by mass or less, 20.0 parts by mass or less, 15.0 parts by mass or less, or 10.0 parts by mass or less per 100 parts by mass of ferromagnetic powder. The polymer contained in the magnetic layer or magnetic layer-forming composition may be one type or two or more types. When two or more types are contained, the content is the total content of the two or more types. When the non-magnetic layer and non-magnetic layer-forming composition contain the polymer, the content of the polymer in the non-magnetic layer and non-magnetic layer-forming composition can be the same as described above, with the ferromagnetic powder being replaced with non-magnetic powder.

[0057] Specific examples of the polymer include polymers P-1 to P-11 containing the following structural units, however, the present invention is not limited to these examples.

[0058]

[0059]

[0060]

[0061] The magnetic layer of the magnetic recording medium will be described in more detail below. <Magnetic Layer> (Ferromagnetic Powder) The magnetic layer contains a ferromagnetic powder. The ferromagnetic powder contained in the magnetic layer can be one or a combination of two or more ferromagnetic powders known for use in the magnetic layers of various magnetic recording media. Using a ferromagnetic powder with a small average particle size is preferable from the viewpoint of improving recording density. From this viewpoint, the average particle size of the ferromagnetic powder is preferably 50 nm or less, more preferably 45 nm or less, even more preferably 40 nm or less, even more preferably 35 nm or less, even more preferably 30 nm or less, even more preferably 25 nm or less, and even more preferably 20 nm or less. On the other hand, from the viewpoint of magnetization stability, the average particle size of the ferromagnetic powder is preferably 5 nm or more, more preferably 8 nm or more, even more preferably 10 nm or more, even more preferably 15 nm or more, and even more preferably 20 nm or more.

[0062] Hexagonal Ferrite Powder A preferred specific example of the ferromagnetic powder is hexagonal ferrite powder. For details of the hexagonal ferrite powder, see, for example, JP 2011-225417 A, paragraphs 0012 to 0030, JP 2011-216149 A, paragraphs 0134 to 0136, JP 2012-204726 A, paragraphs 0013 to 0030, and JP 2015-127985 A, paragraphs 0029 to 0084.

[0063] 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 most intense diffraction peak belongs in the X-ray diffraction spectrum obtained by X-ray diffraction analysis. For example, if the most intense diffraction peak in the X-ray diffraction spectrum obtained by X-ray diffraction analysis is assigned to the hexagonal ferrite crystal structure, it is determined that the hexagonal ferrite crystal structure has been detected as the main phase. If only a single structure is detected by X-ray diffraction analysis, this detected structure is considered to be the main phase. The hexagonal ferrite crystal structure contains at least iron atoms, divalent metal atoms, and oxygen atoms as constituent atoms. Divalent metal atoms are metal atoms that can become divalent cations as ions, and examples thereof include alkaline earth metal atoms such as strontium atoms, barium atoms, and calcium atoms, and lead atoms. In the present invention and this specification, the term "hexagonal strontium ferrite powder" refers to a powder in which the main divalent metal atom contained therein is a strontium atom, and the term "hexagonal barium ferrite powder" refers to a powder in which the main divalent metal atom contained therein is a barium atom. The term "main divalent metal atom" refers to the divalent metal atom that is the most abundant, on an atomic percentage basis, among the divalent metal atoms contained in the powder. However, the above divalent metal atoms do not include rare earth atoms. In the present invention and this specification, the "rare earth atom" is selected from the group consisting of scandium (Sc), yttrium (Y), and lanthanoid atoms. The lanthanoid atom is selected from the group consisting of lanthanum atom (La), cerium atom (Ce), praseodymium atom (Pr), neodymium atom (Nd), promethium atom (Pm), samarium atom (Sm), europium atom (Eu), gadolinium atom (Gd), terbium atom (Tb), dysprosium atom (Dy), holmium atom (Ho), erbium atom (Er), thulium atom (Tm), ytterbium atom (Yb), and lutetium atom (Lu).

[0064] Hereinafter, hexagonal strontium ferrite powder, which is one form of hexagonal ferrite powder, will be described in more detail.

[0065] The activation volume of the hexagonal strontium ferrite powder is preferably 800 to 1500 nm ３ The finely divided hexagonal strontium ferrite powder exhibiting an activation volume in the above range is suitable for producing a magnetic recording medium that exhibits excellent electromagnetic conversion characteristics. The activation volume of the hexagonal strontium ferrite powder is preferably 800 nm ３ or more, for example, 850 nm ３ From the viewpoint of further improving the electromagnetic conversion characteristics, the activation volume of the hexagonal strontium ferrite powder can be 1400 nm or more. ３ More preferably, it is 1300 nm or less. ３ More preferably, it is 1200 nm or less. ３ More preferably, it is 1100 nm or less. ３ It is even more preferred that:

[0066] "Activation volume" is a unit of magnetization reversal and is an index showing the magnetic size of a particle. The activation volume described in this invention and this specification and the anisotropy constant Ku described below are values ​​obtained by measuring the coercive force Hc using a vibrating sample magnetometer at magnetic field sweep rates of 3 minutes and 30 minutes (measurement temperature: 23°C ± 1°C) in the coercive force Hc measurement section, and by using the following relational expression between Hc and activation volume V. Note that the unit of the anisotropy constant Ku is 1 erg / cc = 1.0 x 10 －１ J / m ３ 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)]

[0067] The anisotropy constant Ku can be used as an index of the reduction in thermal fluctuation, in other words, the improvement in thermal stability. The hexagonal strontium ferrite powder preferably has an anisotropy constant of 1.8×10 ５ J / m ３and more preferably 2.0 × 10 ５ J / m ３ The Ku of the hexagonal strontium ferrite powder can be, for example, 2.5×10 ５ J / m ３ However, since a higher Ku value means higher thermal stability and is therefore preferred, the Ku value is not limited to the above-mentioned values.

[0068] The hexagonal strontium ferrite powder may or may not contain rare earth atoms. When the hexagonal strontium ferrite powder contains rare earth atoms, it is preferable that the rare earth atoms be contained at a content (bulk content) of 0.5 to 5.0 atomic % relative to 100 atomic % of iron atoms. In one embodiment, the hexagonal strontium ferrite powder containing rare earth atoms can have rare earth atoms unevenly distributed in the surface layer portion. In the present invention and this specification, "uneven distribution of rare earth atoms in the surface layer" means that the rare earth atom content relative to 100 atomic % of iron atoms in a solution obtained by partially dissolving a hexagonal strontium ferrite powder in an acid (hereinafter referred to as "surface layer content of rare earth atoms" or simply "surface layer content" with respect to rare earth atoms) satisfies the ratio of the rare earth atom content relative to 100 atomic % of iron atoms in a solution obtained by completely dissolving a hexagonal strontium ferrite powder in an acid (hereinafter referred to as "bulk content of rare earth atoms" or simply "bulk content" with respect to rare earth atoms) > 1.0. The rare earth atom content of the hexagonal strontium ferrite powder described below is synonymous with the bulk content of rare earth atoms. In contrast, partial dissolution using an acid dissolves the surface layer of the particles constituting the hexagonal strontium ferrite powder, and 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. The rare earth atom surface layer content satisfying the ratio "rare earth atom surface layer content / rare earth atom bulk content > 1.0" means that rare earth atoms are unevenly distributed in the surface layer of the particles constituting the hexagonal strontium ferrite powder (i.e., more present in the surface layer than in the interior). In the present invention and this specification, the term "surface layer" refers to a partial region extending from the surface toward the interior of the particles constituting the hexagonal strontium ferrite powder.

[0069] When the hexagonal strontium ferrite powder contains rare earth atoms, the rare earth atom content (bulk content) is preferably in the range of 0.5 to 5.0 atomic percent relative to 100 atomic percent of iron atoms. The presence of rare earth atoms at a bulk content within the above range and the uneven distribution of rare earth atoms in the surface layer of the particles constituting the hexagonal strontium ferrite powder is believed to contribute to suppressing a decrease in playback output during repeated playback. This is presumably because the hexagonal strontium ferrite powder contains rare earth atoms at a bulk content within the above range and the uneven distribution of rare earth atoms in the surface layer of the particles constituting the hexagonal strontium ferrite powder can increase the anisotropy constant Ku. The higher the anisotropy constant Ku, the more the occurrence of a phenomenon known as thermal fluctuation can be suppressed (in other words, thermal stability can be improved). By suppressing the occurrence of thermal fluctuation, the decrease in playback output during repeated playback can be suppressed. It is speculated that the uneven distribution of rare earth atoms in the particle surface layer of hexagonal strontium ferrite powder contributes to stabilizing the spin of the iron (Fe) site in the crystal lattice of the surface layer, thereby increasing the anisotropy constant Ku.In addition, it is speculated that using hexagonal strontium ferrite powder with uneven distribution of rare earth atoms in the surface layer as the ferromagnetic powder of the magnetic layer also contributes to suppressing the abrasion of 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 in the surface layer can also contribute to improving the running durability of magnetic recording media.It is speculated that this is because the uneven distribution of rare earth atoms on the surface of the particles constituting the hexagonal strontium ferrite powder contributes to improving the interaction between the particle surface and the organic substance (e.g., binder and / or additive) contained in the magnetic layer, resulting in an improvement in the strength of the magnetic layer. From the viewpoint of further suppressing the decrease in reproduction output during repeated reproduction and / or further improving running durability, the rare earth atom content (bulk content) is more preferably in the range of 0.5 to 4.5 atomic %, even more preferably in the range of 1.0 to 4.5 atomic %, and even more preferably in the range of 1.5 to 4.5 atomic %.

[0070] The bulk content is the content determined by completely dissolving the hexagonal strontium ferrite powder. In the present invention and this specification, unless otherwise specified, the content of an atom refers to the bulk content determined by completely dissolving the hexagonal strontium ferrite powder. The hexagonal strontium ferrite powder containing rare earth atoms may contain only one rare earth atom, or may contain two or more rare earth atoms. When two or more rare earth atoms are contained, the bulk content is determined for the total of the two or more rare earth atoms. This also applies to other components in the present invention and this specification. That is, unless otherwise specified, a certain component may be used alone or in combination with two or more. When two or more components are used, the content or content refers to the total of the two or more components.

[0071] When the hexagonal strontium ferrite powder contains a rare earth atom, the rare earth atom may be any one or more of rare earth atoms. From the viewpoint of further suppressing a decrease in the reproduction output during repeated reproduction, preferred rare earth atoms include neodymium, samarium, yttrium, and dysprosium atoms, with neodymium, samarium, and yttrium atoms being more preferred, and neodymium atoms being even more preferred.

[0072] In a hexagonal strontium ferrite powder having rare earth atoms unevenly distributed in the surface layer, the rare earth atoms need only be unevenly distributed in the surface layer of the particles constituting the hexagonal strontium ferrite powder, and the degree of uneven distribution is not limited. For example, for a hexagonal strontium ferrite powder having rare earth atoms unevenly distributed in the surface layer, the ratio of the surface content of rare earth atoms determined by partial dissolution under the dissolution conditions described below to the bulk content of rare earth atoms determined by complete dissolution under the dissolution conditions described below, i.e., "surface content / bulk content," is greater than 1.0 and can be 1.5 or greater. A "surface content / bulk content" greater than 1.0 means that rare earth atoms are unevenly distributed in the surface layer (i.e., present in greater amounts than in the interior) in the particles constituting the hexagonal strontium ferrite powder. Furthermore, the ratio of the surface content of rare earth atoms determined by partial dissolution under the dissolution conditions described later to the bulk content of rare earth atoms determined by complete dissolution under the dissolution conditions described later, i.e., "surface content / bulk content," can be, for example, 10.0 or less, 9.0 or less, 8.0 or less, 7.0 or less, 6.0 or less, 5.0 or less, or 4.0 or less. However, in a hexagonal strontium ferrite powder having rare earth atoms unevenly distributed in the surface layer, it is sufficient that the rare earth atoms are unevenly distributed in the surface layer of the particles constituting the hexagonal strontium ferrite powder, and the above "surface content / bulk content" is not limited to the exemplified upper or lower limits.

[0073] The partial and total dissolution of hexagonal strontium ferrite powder is described below. For hexagonal strontium ferrite powder present as a powder, sample powders to be partially and completely dissolved are collected from the same powder lot. On the other hand, for hexagonal strontium ferrite powder contained in the magnetic layer of a magnetic recording medium, a portion of the hexagonal strontium ferrite powder removed from the magnetic layer is subjected to partial dissolution, and another portion is subjected to total dissolution. Removal of the hexagonal strontium ferrite powder from the magnetic layer can be performed, for example, by the method described in paragraph 0032 of JP 2015-91747 A. The partial dissolution refers to dissolving the hexagonal strontium ferrite powder to such an 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 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, the term "total dissolution" refers to dissolution to the point where no residual hexagonal strontium ferrite powder is visually detected in the solution at the end of dissolution. The partial dissolution and surface layer content measurement are performed, for example, by the following method. However, the dissolution conditions, such as the amount of sample powder, described below are merely examples, and any dissolution conditions that allow partial or complete dissolution can be adopted. A container (e.g., a beaker) containing 12 mg of sample powder and 10 mL of 1 mol / L hydrochloric acid is placed on a hot plate set at 70°C for 1 hour. The resulting solution is filtered through a 0.1 μm membrane filter. Elemental analysis of the filtrate obtained in this manner is performed using an inductively coupled plasma (ICP) analyzer. In this way, the surface layer content of rare earth atoms relative to 100 atomic % 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. The same applies to the measurement of the bulk content. Meanwhile, the total dissolution and bulk content measurements are carried out, for example, by the following method: A container (e.g., a beaker) containing 12 mg of sample powder and 10 mL of 4 mol / L hydrochloric acid is placed on a hot plate set at 80° C. for 3 hours.Thereafter, the same procedures as in the partial dissolution and measurement of the surface layer content are carried out, and the bulk content relative to 100 atomic % of iron atoms can be determined.

[0074] From the viewpoint of increasing the reproduction output when reproducing data recorded on a magnetic recording medium, it is desirable that the mass magnetization σs of the ferromagnetic powder contained in the magnetic recording medium is high. In this regard, hexagonal strontium ferrite powder containing rare earth atoms but not having uneven distribution of rare earth atoms in the surface layer has been shown to have a tendency to have a significantly lower σs than hexagonal strontium ferrite powder not containing rare earth atoms. In contrast, hexagonal strontium ferrite powder having uneven distribution of rare earth atoms in the surface layer is considered preferable in order to suppress such a large decrease in σs. In one embodiment, the σs of the hexagonal strontium ferrite powder is 45 A m ２ / kg or more, and ２ On the other hand, from the viewpoint of noise reduction, σs is 80 A m ２ / kg or less, and ２ / kg or less is more preferable. σs can be measured using a known measuring device capable of measuring magnetic properties, such as a vibrating sample magnetometer. In the present invention and this specification, unless otherwise specified, the mass magnetization σs is a value measured at a magnetic field strength of 1194 kA / m (15 kOe).

[0075] Regarding the content (bulk content) of the constituent atoms of the hexagonal strontium ferrite powder, the strontium atom content can be, for example, in the range of 2.0 to 15.0 atomic % relative to 100 atomic % of iron atoms. In one embodiment, the hexagonal strontium ferrite powder can contain only strontium atoms as divalent metal atoms. In another embodiment, the hexagonal strontium ferrite powder can contain one or more other divalent metal atoms in addition to strontium atoms. For example, barium atoms and / or calcium atoms can be contained. When divalent metal atoms other than strontium atoms are contained, the barium atom content and calcium atom content in the hexagonal strontium ferrite powder can each be, for example, in the range of 0.05 to 5.0 atomic % relative to 100 atomic % of iron atoms.

[0076] Known crystal structures of hexagonal ferrite include magnetoplumbite type (also called "M type"), W type, Y type, and Z type. The hexagonal strontium ferrite powder may have any of the crystal structures. The crystal structure can be confirmed by X-ray diffraction analysis. The hexagonal strontium ferrite powder may be one in which a single crystal structure or two or more types of crystal structures are detected by X-ray diffraction analysis. For example, in one embodiment, the hexagonal strontium ferrite powder may be one in which only the M-type crystal structure is detected by X-ray diffraction analysis. For example, the M-type hexagonal ferrite is AFe １２ O １９The composition is represented by the following formula: Here, A represents a divalent metal atom, and when the hexagonal strontium ferrite powder is M-type, A is only strontium atom (Sr), or when multiple divalent metal atoms are contained as A, strontium atom (Sr) accounts for the largest proportion on an atomic % basis as described above. The divalent metal atom content of the hexagonal strontium ferrite powder is usually determined by the type of crystalline structure of the hexagonal ferrite and is not particularly limited. The same applies to the iron atom content and the oxygen atom content. The hexagonal strontium ferrite powder contains at least iron atoms, strontium atoms, and oxygen atoms, and may further contain rare earth atoms. Furthermore, the hexagonal strontium ferrite powder may or may not contain atoms other than these atoms. As an example, the hexagonal strontium ferrite powder may contain aluminum atoms (Al). The content of aluminum atoms can be, for example, 0.5 to 10.0 atomic % relative to 100 atomic % of iron atoms. From the viewpoint of further suppressing a decrease in playback output during repeated playback, the hexagonal strontium ferrite powder contains iron atoms, strontium atoms, oxygen atoms, and rare earth atoms, and the content of atoms other than these atoms is preferably 10.0 atomic % or less, more preferably in the range of 0 to 5.0 atomic %, relative to 100 atomic % of iron atoms, and may even be 0 atomic %. That is, in one embodiment, the hexagonal strontium ferrite powder does not need to contain atoms other than iron atoms, strontium atoms, oxygen atoms, and rare earth atoms. The content expressed in atomic % above is determined by converting the content (unit: mass %) of each atom obtained by completely dissolving the hexagonal strontium ferrite powder into a value expressed in atomic % using the atomic weight of each atom. Furthermore, in the present invention and this specification, "not containing" a certain atom means that the content measured by completely dissolving the powder and using an ICP analyzer is 0 mass %. The detection limit of an ICP analyzer is usually 0.01 ppm (parts per million) or less by mass. The above term "free from" is used to mean that the amount is below the detection limit of the ICP analyzer.In one form, the hexagonal strontium ferrite powder can be one that does not contain bismuth atoms (Bi).

[0077] Metal Powder A preferred specific example of the ferromagnetic powder is ferromagnetic metal powder. For details of the ferromagnetic metal powder, see, for example, paragraphs

[0137] to

[0141] of JP 2011-216149 A and paragraphs

[0009] to

[0023] of JP 2005-251351 A.

[0078] ε-Iron Oxide Powder A preferred example of the ferromagnetic powder is ε-iron oxide powder. In the present invention and this specification, "ε-iron oxide powder" refers to a ferromagnetic powder in which an ε-iron oxide crystal structure is detected as the main phase by X-ray diffraction analysis. For example, if the highest intensity diffraction peak in an X-ray diffraction spectrum obtained by X-ray diffraction analysis is assigned to the ε-iron oxide crystal structure, it is determined that the ε-iron oxide crystal structure has been detected as the main phase. Known methods for producing ε-iron oxide powder include a method of producing it from goethite and a reverse micelle method. All of these production methods are publicly known. Furthermore, a method for producing ε-iron oxide powder in which part of the Fe is substituted with a substitution atom such as Ga, Co, Ti, Al, or Rh is described, for example, in J. Jpn. Soc. Powder Metallurgy, Vol. 61, Supplement, No. S1, pp. 111-115. 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 magnetic recording medium is not limited to the methods mentioned here.

[0079] The activation volume of the ε-iron oxide powder is preferably 300 to 1500 nm ３ The finely divided ε-iron oxide powder exhibiting an activation volume in the above range is suitable for producing a magnetic recording medium that exhibits excellent electromagnetic conversion characteristics. The activation volume of the ε-iron oxide powder is preferably 300 nm ３ or more, for example, 500 nm ３From the viewpoint of further improving the electromagnetic conversion characteristics, the activation volume of the ε-iron oxide powder can be 1400 nm or more. ３ More preferably, it is 1300 nm or less. ３ More preferably, it is 1200 nm or less. ３ More preferably, it is 1100 nm or less. ３ It is even more preferred that:

[0080] The anisotropy constant Ku can be used as an index of the reduction in thermal fluctuation, in other words, the improvement in thermal stability. The ε-iron oxide powder preferably has an anisotropy constant Ku of 3.0×10 ４ J / m ３ and more preferably 8.0 × 10 ４ J / m ３ The Ku of the ε-iron oxide powder can be, for example, 3.0×10 ５ J / m ３ However, since a higher Ku value means higher thermal stability and is therefore preferred, it is not limited to the above-mentioned values.

[0081] From the viewpoint of increasing the reproduction output when reproducing data recorded on the magnetic recording medium, it is desirable that the mass magnetization σs of the ferromagnetic powder contained in the magnetic recording medium is high. In this regard, in one embodiment, the σs of the ε-iron oxide powder is 8 A m ２ / kg or more, and ２ On the other hand, the σs of the ε-iron oxide powder can be 40 A m ２ / kg or less, and 35 A m ２ / kg or less is more preferable.

[0082] Unless otherwise specified, in this invention and this specification, the average particle size of various powders, such as ferromagnetic powders, is a 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 photograph is printed on photographic paper or displayed on a display so that the total magnification is 500,000x, thereby obtaining a photograph of the particles constituting the powder. From the obtained particle photograph, a target particle is selected, and the particle outline is traced with a digitizer to measure the particle (primary particle) size. Primary particles refer to independent particles without agglomeration. The above measurement is performed on 500 randomly selected particles. The arithmetic mean of the particle sizes of the 500 particles thus obtained is taken as the average particle size of the powder. As the transmission electron microscope, for example, a Hitachi transmission electron microscope model H-9000 can be used. Furthermore, particle size measurement can be performed using known image analysis software, such as Carl Zeiss image analysis software KS-400. Unless otherwise specified, the average particle size shown in the examples below is a value measured using a Hitachi H-9000 transmission electron microscope and Carl Zeiss KS-400 image analysis software. In this invention and this specification, powder refers to an aggregate of multiple particles. For example, ferromagnetic powder refers to an aggregate of multiple ferromagnetic particles. Furthermore, an aggregate of multiple particles is not limited to a form in which the particles constituting the aggregate are in direct contact with each other, but also includes a form in which a binder, additive, etc., described below, is interposed between the particles. The term "particle" is sometimes used to refer to powder.

[0083] As a method for collecting sample powder from a magnetic recording medium for particle size measurement, for example, the method described in paragraph 0015 of JP-A-2011-048878 can be used.

[0084] In the present invention and this specification, unless otherwise specified, the size of particles constituting a powder (particle size) is expressed as the length of the major axis constituting the particle, i.e., the major axis length, when the shape of the particle observed in the particle photograph is: (1) needle-like, spindle-like, columnar (however, the height is greater than the maximum major axis of the base), etc. (2) plate-like or columnar (however, the thickness or height is smaller than the maximum major axis of the plate surface or base), it is expressed as the maximum major axis of the plate surface or base, (3) spherical, polyhedral, unspecified shape, etc., and when the major axis constituting the particle cannot be identified from the shape, it is expressed as the circle-equivalent diameter. The circle-equivalent diameter is determined by the circle projection method.

[0085] The average acicular ratio of a powder refers to the arithmetic average of the minor axis length of the particles measured in the above measurement, i.e., the minor axis length, the value of (major axis length / minor axis length) for each particle, and the values ​​obtained for the 500 particles. Here, unless otherwise specified, the minor axis length refers to the length of the minor axis constituting the particle in the above particle size definition (1), and the thickness or height in the above particle size definition (2). In the above particle size definition (3), since there is no distinction between the major axis and the minor axis, the (major axis length / minor axis length) is considered to be 1 for convenience. Unless otherwise specified, when the particle shape is specific, for example, in the above particle size definition (1), the average particle size is the average major axis length, and in the above definition (2), the average particle size is the average plate diameter. In the above definition (3), the average particle size is the average diameter (also called the average particle diameter or average particle size).

[0086] The content (filling rate) of the 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, based on the total mass of the magnetic layer. The magnetic layer contains ferromagnetic powder and may contain a binder, and may optionally contain one or more additional additives. A high filling rate of the ferromagnetic powder in the magnetic layer is preferable from the perspective of improving recording density.

[0087] (Binder, Curing Agent) The magnetic recording medium may be a coating-type magnetic recording medium, and the magnetic layer may contain a binder. The binder is one or more resins. Various resins commonly used as binders for coating-type magnetic recording media can be used as binders. For example, binders may be selected from polyurethane resins, polyester resins, polyamide resins, vinyl chloride resins, acrylic resins copolymerized with styrene, acrylonitrile, methyl methacrylate, etc., cellulose resins such as nitrocellulose, epoxy resins, phenoxy resins, polyvinyl acetal, polyvinyl butyral, etc., and may be used alone or in combination. Among these, polyurethane resins, acrylic resins, cellulose resins, and vinyl chloride resins are preferred. These resins may be homopolymers or copolymers. These resins can also be used as binders in the nonmagnetic layer and / or backcoat layer, which will be described later. For more information on the binders, see paragraphs 0028 to 0031 of JP 2010-24113 A. 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 the ferromagnetic powder. The average molecular weight of the resin used as the binder can be, for example, 10,000 to 200,000 in weight average molecular weight. Unless otherwise specified, the weight average molecular weight described in the Examples section below is a value measured by gel permeation chromatography (GPC) under the following measurement conditions and calculated in terms of standard polystyrene. The binder can be used in an amount of, for example, 1.0 to 30.0 parts by mass per 100.0 parts by mass of the ferromagnetic powder. GPC apparatus: HLC-8120 (manufactured by Tosoh Corporation) Column: TSK gel Multipore HXL-M (manufactured by Tosoh Corporation, 7.8 mm ID (Inner Diameter) × 30.0 cm) Eluent: tetrahydrofuran (THF)

[0088] A curing agent can also be used in conjunction with a resin usable as a binder. In one form, the curing agent can be a thermosetting compound, which undergoes a curing reaction (crosslinking reaction) upon heating. In another form, the curing agent can be a photocurable compound, which undergoes a curing reaction (crosslinking reaction) upon irradiation with light. As the curing reaction progresses during the magnetic layer formation process, at least a portion of the curing agent can be included in the magnetic layer in a state where it has reacted (crosslinked) with other components, such as the binder. This also applies to layers formed using compositions containing a curing agent when the composition used to form other layers contains a curing agent. A preferred curing agent is a thermosetting compound, with polyisocyanate being preferred. For details on polyisocyanates, see paragraphs 0124-0125 of JP 2011-216149 A. The content of the curing agent in the magnetic layer-forming composition can be, for example, 0 to 80.0 parts by weight per 100.0 parts by weight of the binder. From the perspective of improving the strength of the magnetic layer, the content can be 50.0 to 80.0 parts by weight.

[0089] (Additives) The magnetic layer may contain one or more additives as needed. Examples of additives include the curing agents mentioned above. Examples of additives contained in the magnetic layer include non-magnetic powders, lubricants, dispersants, dispersion aids, antifungal agents, antistatic agents, antioxidants, etc. For dispersants, see paragraphs 0061 and 0071 of JP 2012-133837 A. A dispersant may also be added to the non-magnetic layer-forming composition. For dispersants that can be added to the non-magnetic layer-forming composition, see paragraph 0061 of JP 2012-133837 A. Examples of non-magnetic powders that can be contained in the magnetic layer include non-magnetic powders that can function as abrasives, and non-magnetic powders that can function as protrusion-forming agents that form moderately protruding protrusions on the magnetic layer surface. Examples of abrasives include alumina (Al), a substance commonly used as an abrasive for magnetic layers. ２ O ３ ), silicon carbide, boron carbide (B ４ C), TiC, chromium oxide (Cr ２ O ３ ), cerium oxide, zirconium oxide (ZrO ２Examples of suitable abrasives include powders of ferromagnetic powder, iron oxide, and diamond, with alumina such as α-alumina, silicon carbide, and diamond powder being preferred. The abrasive content in 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 is, for example, in the range of 30 to 300 nm, preferably 50 to 200 nm. Examples of protrusion-forming agents include carbon black and colloidal particles. The protrusion-forming agent content in the magnetic layer is preferably 0.1 to 10.0 parts by mass, more preferably 0.1 to 5.0 parts by mass, and even more preferably 0.5 to 5.0 parts by mass, per 100.0 parts by mass of ferromagnetic powder. The average particle size of the colloidal particles is, for example, in the range of 90 to 200 nm, and even more preferably 100 to 150 nm. The average particle size of the carbon black is preferably in the range of 5 to 200 nm, more preferably in the range of 10 to 150 nm. Furthermore, known additives such as various polymers described in paragraphs 0030 to 0080 of JP-A-2016-051493 can also be used as the additive.

[0090] In one embodiment, the magnetic recording medium may contain one or more fatty acid compounds selected from the group consisting of fatty acids, fatty acid esters, and fatty acid amides in the portion of the non-magnetic support facing the magnetic layer. The fatty acid compounds may function as lubricants. The portion of the magnetic layer facing the non-magnetic support may contain only one or more fatty acid compounds selected from the group consisting of fatty acids, fatty acid esters, and fatty acid amides. The fatty acid may also contain only one or more fatty acids. This also applies to fatty acid esters and fatty acid amides. Examples of fatty acids include lauric acid, myristic acid, palmitic acid, stearic acid, oleic acid, linoleic acid, linolenic acid, behenic acid, erucic acid, and elaidic acid. Stearic acid, myristic acid, and palmitic acid are preferred, with stearic acid being more preferred. The fatty acid may be contained in the magnetic layer in the form of a salt, such as a metal salt. Examples of fatty acid esters include esters of the various fatty acids listed above. Specific examples include butyl myristate, butyl palmitate, butyl stearate, neopentyl glycol dioleate, sorbitan monostearate, sorbitan distearate, sorbitan tristearate, oleyl oleate, isocetyl stearate, isotridecyl stearate, octyl stearate, isooctyl stearate, amyl stearate, and butoxyethyl stearate. Fatty acid amides include amides of the various fatty acids listed above. Specific examples include lauric acid amide, myristic acid amide, palmitic acid amide, and stearic acid amide. Regarding fatty acid and fatty acid derivatives (amides, esters, etc.), it is preferable that the fatty acid-derived portion of the fatty acid derivative has the same or similar structure as the fatty acid used in combination. For example, when stearic acid is used as the fatty acid, it is preferable to use stearic acid amide and / or stearic acid ester in combination.In one embodiment, a magnetic recording medium containing one or more fatty acid compounds selected from the group consisting of fatty acids, fatty acid esters, and fatty acid amides in the magnetic layer can be produced by forming a magnetic layer using a magnetic layer-forming composition containing one or more of the above fatty acid compounds. In another embodiment, a magnetic recording medium containing one or more of the above fatty acid compounds in the magnetic layer can be produced by forming a nonmagnetic layer using a nonmagnetic layer-forming composition containing one or more of the above fatty acid compounds. In another embodiment, a magnetic recording medium containing one or more of the above fatty acid compounds in the magnetic layer can be produced by forming a nonmagnetic layer using a nonmagnetic layer-forming composition containing one or more of the above fatty acid compounds, and then forming a magnetic layer using a magnetic layer-forming composition containing one or more of the above fatty acid compounds. The nonmagnetic layer can retain and supply components that can function as lubricants, such as fatty acids, fatty acid esters, and fatty acid amides, to the magnetic layer. Lubricants such as fatty acids, fatty acid esters, and fatty acid amides contained in the nonmagnetic layer can migrate to the magnetic layer and remain in the magnetic layer. The content of fatty acid in the magnetic layer or magnetic layer-forming composition is, for example, 0 to 3.0 parts by mass, preferably 0.5 to 3.0 parts by mass, per 100.0 parts by mass of ferromagnetic powder. The content of fatty acid ester in the magnetic layer or magnetic layer-forming composition is, for example, 0 to 10.0 parts by mass, preferably 0.5 to 7.0 parts by mass, per 100.0 parts by mass of ferromagnetic powder. The content of fatty acid amide in the magnetic layer or magnetic layer-forming composition is, for example, 0 to 1.0 part by mass, preferably 0.1 to 1.0 part by mass, per 100.0 parts by mass of ferromagnetic powder. The above description of the content of fatty acid, fatty acid ester, and fatty acid amide in the non-magnetic layer or non-magnetic layer-forming composition can be applied by replacing the above ferromagnetic powder with non-magnetic powder.With regard to the mixing ratio of the polymer and the fatty acid compound in the magnetic layer or magnetic layer-forming composition, the total amount (by mass) of the polymer and the fatty acid compound is taken as 100 mass%, and the ratio of the fatty acid compound can be 10 mass% or more, 15 mass% or more, 20 mass% or more, 25 mass% or more, 30 mass% or more, or 40 mass% or more, and can be 90 mass% or less, 80 mass% or less, 70 mass% or less, or 60 mass% or less. The above points also apply to the mixing ratio of the polymer and the fatty acid compound in the non-magnetic layer or non-magnetic layer-forming composition.

[0091] The magnetic layer described above can be provided directly on the surface of the non-magnetic support, or indirectly via a non-magnetic layer.

[0092] <Nonmagnetic Layer> Next, the nonmagnetic layer will be described. The magnetic recording medium may have a magnetic layer directly on the surface of a nonmagnetic support, or may have a magnetic layer on the surface of a nonmagnetic support via a nonmagnetic layer containing nonmagnetic powder. The nonmagnetic powder used in the nonmagnetic layer may be inorganic or organic. Carbon black, etc., may also be used. Examples of inorganic powders include powders of metals, metal oxides, metal carbonates, metal sulfates, metal nitrides, metal carbides, and metal sulfides. These nonmagnetic powders are commercially available or can be produced by known methods. For details, see paragraphs

[0146] to

[0150] of JP 2011-216149 A. For information on carbon black that can be used in the nonmagnetic layer, see paragraphs

[0040] to

[0041] of JP 2010-24113 A. The content (filling rate) of the nonmagnetic powder in the nonmagnetic layer is preferably in the range of 50 to 90% by mass, more preferably 60 to 90% by mass, based on the total mass of the nonmagnetic layer.

[0093] The non-magnetic layer may be a layer containing a non-magnetic powder and a binder, and may further contain one or more additives. For details of the binder, additives, etc., of the non-magnetic layer, known techniques related to non-magnetic layers can be applied. Furthermore, for example, known techniques related to magnetic layers can also be applied to determine the type and content of the binder, the type and content of the additives, etc.

[0094] In the present invention and this specification, the term "non-magnetic layer" also includes a substantially non-magnetic layer that contains a small amount of ferromagnetic powder, for example as an impurity or intentionally, along with a non-magnetic powder. Here, a substantially non-magnetic layer refers to a layer whose residual magnetic flux density is 10 mT or less, whose coercive force is 100 Oe or less, or whose residual magnetic flux density is 10 mT or less and whose coercive force is 100 Oe or less. 1 [kOe] = 10 ６ / 4π [A / m]. It is preferable that the non-magnetic layer has no residual magnetic flux density and no coercive force.

[0095] <Non-magnetic Support> Next, the non-magnetic support (hereinafter also simply referred to as "support") will be described. Examples of non-magnetic supports include known biaxially stretched polyethylene terephthalate, polyethylene naphthalate, polyamide, polyamideimide, aromatic polyamide, etc. Among these, polyethylene terephthalate, polyethylene naphthalate, and polyamide are preferred. These supports may be previously subjected to corona discharge, plasma treatment, easy-adhesion treatment, heat treatment, etc.

[0096] <Backcoat Layer> The magnetic recording medium may also have a backcoat layer containing a nonmagnetic powder on the surface of the nonmagnetic support opposite the surface having the magnetic layer. The backcoat layer preferably contains either carbon black or an inorganic powder, or both. The backcoat layer may be a layer containing a nonmagnetic powder and a binder, and may further contain one or more additives. The binder and various optional additives of the backcoat layer can be determined by known techniques related to backcoat layers, and known techniques related to the formulation of magnetic and / or nonmagnetic layers can also be applied. For example, see paragraphs

[0018] to

[0020] of JP 2006-331625 A and U.S. Pat. No. 7,029,774, column 4, line 65 to column 5, line 38, for information on backcoat layers.

[0097] <Various Thicknesses> The thickness of the non-magnetic support is, for example, 3.0 to 80.0 μm, preferably 3.0 to 20.0 μm, more preferably 3.0 to 10.0 μm, and even more preferably 3.0 to 6.0 μm.

[0098] The thickness of the magnetic layer can be optimized depending on the saturation magnetization of the magnetic head used, the head gap length, the bandwidth of the recording signal, etc. From the viewpoint of high-density recording, the thickness of the magnetic layer is preferably 10 nm to 150 nm, more preferably 20 nm to 120 nm, and even more preferably 30 nm to 100 nm. The magnetic layer must be at least one layer, and may be separated into two or more layers with different magnetic properties, and known configurations related to multilayer magnetic layers can be applied. When the magnetic layer is separated into two or more layers, the thickness of the magnetic layer refers to the total thickness of these layers.

[0099] The thickness of the non-magnetic layer is, for example, 0.1 to 3.0 μm, preferably 0.1 to 2.0 μm, and more preferably 0.1 to 1.5 μm.

[0100] The thickness of the backcoat layer is preferably 0.9 μm or less, and more preferably in the range of 0.1 to 0.7 μm.

[0101] The thickness of each layer of the magnetic recording medium and the non-magnetic support can be determined by known film thickness measurement methods. For example, a cross section of the magnetic recording medium in the thickness direction is exposed using a known method such as an ion beam or a microtome, and then the exposed cross section is observed using a scanning electron microscope or a transmission electron microscope. Various thicknesses can be determined as the thickness determined at any one location in the cross section observation, or as the arithmetic mean of thicknesses determined at two or more randomly selected locations (e.g., two locations). Alternatively, the thickness of each layer may be determined as a design thickness calculated from the manufacturing conditions.

[0102] <Method for Manufacturing Magnetic Recording Media> The process for preparing compositions for forming the magnetic layer, as well as the optional non-magnetic layer and backcoat layer, typically includes at least a kneading step, a dispersion step, and mixing steps optionally performed before or after these steps. Each step may be divided into two or more stages. The components used to prepare each layer-forming composition may be added at the beginning or during any step. Individual raw materials may also be added in separate steps over two or more steps. Known techniques can be used to prepare each layer-forming composition. For the kneading step, it is preferable to use a kneader with strong kneading power, such as an open kneader, continuous kneader, pressure kneader, or extruder. Details of these kneading processes are described in JP-A-1-106338 and JP-A-1-79274. Furthermore, to disperse each layer-forming composition, one or more types of dispersion beads selected from the group consisting of glass beads and other dispersion beads can be used as a dispersion medium. Suitable dispersion beads include zirconia beads, titania beads, and steel beads, which are dispersion beads with high specific gravity. The particle size (bead diameter) and packing rate of these dispersion beads can be optimized before use. Known dispersers can be used. Each layer-forming composition may be filtered by a known method before being subjected to the coating step. Filtration can be carried out, for example, by filter filtration. Filters used for filtration include those with a pore size of 0.01 to 3 μm (e.g., glass fiber filters, polypropylene filters, etc.).

[0103] The magnetic layer can be formed by applying the magnetic layer-forming composition directly onto the surface of the non-magnetic support, or by sequentially or simultaneously applying the magnetic layer-forming composition and the non-magnetic layer-forming composition in a multilayer. The backcoat layer can be formed by applying the backcoat layer-forming composition onto the surface of the non-magnetic support opposite to the surface on which the magnetic layer is provided (or will be provided later).

[0104] After the coating step, various treatments such as drying, magnetic layer orientation, and surface smoothing (calendering) can be performed. Known techniques can be applied to the coating step and various treatments; see, for example, paragraphs 0051 to 0057 of JP 2010-24113 A. For example, vertical orientation can be performed as the orientation treatment. Vertical orientation can be performed by known methods, such as using magnets with opposite poles facing each other. In the orientation zone, the drying rate of the coating layer can be controlled by the temperature and volume of the drying air and / or the transport speed of the magnetic recording medium in the orientation zone. The coating layer may also be pre-dried before being transported to the orientation zone.

[0105] On the magnetic recording medium manufactured as described above, servo patterns can be formed by known methods to enable tracking control of a magnetic head in a magnetic recording / reproducing device, control of the running speed of the magnetic recording medium, etc. "Formation of servo patterns" can also be referred to as "recording of servo signals." The magnetic recording medium may be a tape-shaped magnetic recording medium (magnetic tape) or a disk-shaped magnetic recording medium (magnetic disk). The formation of servo patterns will be described below using magnetic tape as an example.

[0106] The servo patterns are usually formed along the longitudinal direction of the magnetic tape. Control methods using servo signals (servo control) include timing-based servo (TBS), amplitude servo, and frequency servo.

[0107] As specified in ECMA (European Computer Manufacturers Association)-319, magnetic tapes conforming to the LTO (Linear Tape-Open) standard (commonly referred to as "LTO tapes") employ a timing-based servo system. In this timing-based servo system, a servo pattern is formed by a pair of non-parallel magnetic stripes (also referred to as "servo stripes") arranged continuously in the longitudinal direction of the magnetic tape. As described above, the reason why the servo pattern is formed by a pair of non-parallel magnetic stripes is to inform a servo signal reading element passing over the servo pattern of its passing position. Specifically, the pair of magnetic stripes is formed so that the spacing between them changes continuously along the width direction of the magnetic tape, and by reading this spacing, the servo signal reading element can determine the relative position between the servo pattern and the servo signal reading element. This relative position information enables tracking of data tracks. For this reason, a plurality of servo tracks are usually set on the servo pattern along the width direction of the magnetic tape.

[0108] A servo band is made up of a series of servo signals that run longitudinally on the magnetic tape. A magnetic tape typically has multiple servo bands. For example, an LTO tape has five servo bands. The area between two adjacent servo bands is called a data band. A data band is made up of multiple data tracks, each corresponding to a servo track.

[0109] Also, in one embodiment, as disclosed in Japanese Patent Laid-Open No. 2004-318983, information indicating the servo band number (also referred to as "servo band ID (identification)" or "UDIM (Unique Data Band Identification Method) information") is embedded in each servo band. This servo band ID is recorded by shifting a specific one of a plurality of pairs of servo stripes in the servo band so that its position is displaced relatively in the longitudinal direction of the magnetic tape. Specifically, the way in which a specific one of a plurality of pairs of servo stripes is shifted is changed for each servo band. As a result, the recorded servo band ID is unique for each servo band, and therefore, simply by reading one servo band with a servo signal reading element, that servo band can be uniquely identified.

[0110] One method for uniquely identifying servo bands is to use the staggered method as specified in ECMA-319. In this staggered method, a group of pairs of non-parallel magnetic stripes (servo stripes) arranged continuously in the longitudinal direction of the magnetic tape are recorded so that each servo band is shifted in the longitudinal direction of the magnetic tape. Since the combination of this shift between adjacent servo bands is unique across the entire magnetic tape, it is possible to uniquely identify a servo band when reading the servo pattern with two servo signal reading elements.

[0111] Furthermore, as specified in ECMA-319, information indicating the longitudinal position of the magnetic tape (also called "LPOS (Longitudinal Position) information") is usually embedded in each servo band. Like UDIM information, this LPOS information is also recorded by shifting the positions of a pair of servo stripes in the longitudinal direction of the magnetic tape. However, unlike UDIM information, the same signal is recorded in each servo band for this LPOS information.

[0112] It is also possible to embed information other than the above-mentioned UDIM information and LPOS information in the servo bands. In this case, the embedded information may be different for each servo band, such as UDIM information, or may be common to all servo bands, such as LPOS information. Furthermore, methods other than those described above can also be used to embed information in the servo bands. For example, a predetermined code may be recorded by thinning out a predetermined pair from a group of pairs of servo stripes.

[0113] The servo pattern forming head is called a servo write head. The servo write head has pairs of gaps corresponding to the pairs of magnetic stripes, the number of which is equal to the number of servo bands. Typically, a core and a coil are connected to each pair of gaps, and by supplying a current pulse to the coil, the magnetic field generated in the core can generate a leakage magnetic field in the pair of gaps. When forming a servo pattern, a current pulse is input while running a magnetic tape over the servo write head, thereby transferring the magnetic pattern corresponding to the pair of gaps to the magnetic tape, thereby forming the servo pattern. The width of each gap can be appropriately set depending on the density of the servo pattern to be formed. The width of each gap can be set, for example, to 1 μm or less, 1 to 10 μm, or 10 μm or more.

[0114] Before forming a servo pattern on a magnetic tape, the magnetic tape is usually subjected to a demagnetization (erase) process. This erase process can be performed by applying a uniform magnetic field to the magnetic tape using a direct current magnet or an alternating current magnet. Erase processes include DC (direct current) erase and AC (alternating current) erase. AC erase 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 erase is performed by applying a unidirectional magnetic field to the magnetic tape. There are two other DC erase methods. The first method is horizontal DC erase, in which a unidirectional magnetic field is applied along the longitudinal direction of the magnetic tape. The second method is vertical DC erase, in which a unidirectional magnetic field is applied along the thickness direction of the magnetic tape. The erase process may be performed on the entire magnetic tape or on each servo band of the magnetic tape.

[0115] The direction of the magnetic field of the formed servo pattern is determined according to the direction of erasure. For example, when horizontal DC erasure is performed on a magnetic tape, the servo pattern is formed so that the direction of the magnetic field is opposite to the direction of erasure. This makes it possible to increase the output of the servo signal obtained by reading the servo pattern. As shown in Japanese Patent Application Laid-Open No. 2012-53940, when a magnetic pattern is transferred using the above-mentioned gap to a magnetic tape that has been vertically DC erased, the servo signal obtained by reading the formed servo pattern has a unipolar pulse shape. On the other hand, when a magnetic pattern is transferred using the above-mentioned gap to a magnetic tape that has been horizontally DC erased, the servo signal obtained by reading the formed servo pattern has a bipolar pulse shape.

[0116] [Magnetic Tape Cartridge] One aspect of the present invention relates to a magnetic tape cartridge including the above-described magnetic recording medium in tape form (i.e., magnetic tape).

[0117] The details of the magnetic tape contained in the magnetic tape cartridge are as described above.

[0118] A magnetic tape cartridge generally contains a magnetic tape wound on a reel within the cartridge body. The reel is rotatably mounted within the cartridge body. Widely used magnetic tape cartridges include single-reel magnetic tape cartridges with one reel within the cartridge body and dual-reel magnetic tape cartridges with two reels within the cartridge body. When a single-reel magnetic tape cartridge is loaded into a magnetic recording and playback device for recording and / or playback of data on the magnetic tape, the magnetic tape is pulled out from the magnetic tape cartridge and wound onto a reel on the magnetic recording and playback device. A magnetic head is disposed along the magnetic tape transport path from the magnetic tape cartridge to the take-up reel. The magnetic tape is fed and wound between the reel (supply reel) on the magnetic tape cartridge side and the reel (take-up reel) on the magnetic recording and playback device side. During this process, the magnetic head comes into contact with and slides against the surface of the magnetic layer of the magnetic tape, thereby recording and / or playback of data. In contrast, a dual-reel magnetic tape cartridge has both a supply reel and a take-up reel inside the magnetic tape cartridge. The magnetic tape cartridge may be either a single-reel or dual-reel magnetic tape cartridge. The magnetic tape cartridge may include the magnetic tape according to one aspect of the present invention, and other known technologies may be applied.

[0119] [Magnetic Recording and Reproducing Apparatus] One aspect of the present invention relates to a magnetic recording and reproducing apparatus including the above-described magnetic recording medium.

[0120] In this invention and this specification, the term "magnetic recording and reproducing device" refers to a device capable of at least one of recording data to a magnetic recording medium and reproducing data recorded on the magnetic recording medium. Such devices are generally called drives. The magnetic recording and reproducing device may be, for example, a sliding-type magnetic recording and reproducing device. A sliding-type magnetic recording and reproducing device is a device in which a magnetic head contacts and slides against the surface of the magnetic layer of the magnetic recording medium when recording data on the magnetic recording medium and / or reproducing recorded data. For example, the magnetic recording and reproducing device may include the magnetic tape cartridge in a detachable manner.

[0121] The magnetic recording and reproducing device may include a magnetic head. The magnetic head may be a recording head capable of recording data on a magnetic recording medium, or a reproducing head capable of reproducing data recorded on the magnetic recording medium. In one embodiment, the magnetic recording and reproducing device may include both a recording head and a reproducing head as separate magnetic heads. In another embodiment, the magnetic head included in the magnetic recording and reproducing device may have a configuration in which both an element for recording data (a recording element) and an element for reproducing data (a reproducing element) are provided in a single magnetic head. Hereinafter, the element for recording data and the element for reproducing data are collectively referred to as a "data element." The reproducing head is preferably a magnetic head (MR head) that includes a magnetoresistive (MR) element as a reproducing element, capable of sensitively reading data recorded on a magnetic recording medium. As the MR head, various known MR heads such as an AMR (Anisotropic Magnetoresistive) head, a GMR (Giant Magnetoresistive) head, and a TMR (Tunnel Magnetoresistive) head can be used. Furthermore, the magnetic head that records and / or reproduces data may include a servo signal read element. Alternatively, the magnetic recording and reproducing device may include a magnetic head (servo head) equipped with a servo signal read element, separate from the magnetic head that records and / or reproduces data. For example, the magnetic head that records and / or reproduces recorded data (hereinafter also referred to as a "recording and reproducing head") may include two servo signal read elements, each of which can simultaneously read two adjacent servo bands. One or more data elements may be disposed between the two servo signal read elements.

[0122] In the magnetic recording and reproducing device, recording of data on the magnetic recording medium and / or reproducing of data recorded on the magnetic recording medium can be performed, for example, by contacting and sliding a magnetic head against the surface of the magnetic recording medium on the magnetic layer side. The magnetic recording and reproducing device may include the magnetic recording medium according to one aspect of the present invention, and publicly known techniques may be applied for other aspects.

[0123] For example, when recording data and / or reproducing recorded data, tracking using a servo signal is first performed. That is, by making the servo signal reading element follow a predetermined servo track, the data element is controlled so as to pass over the target data track. The data track is moved by changing the servo track read by the servo signal reading element in the tape width direction. The recording / reproducing head can also record and / or reproduce data on other data bands. In this case, the servo signal reading element is moved to a predetermined servo band using the UDIM information described above, and tracking on that servo band is started.

[0124] An embodiment of the present invention will be described below based on an example. However, the present invention is not limited to the embodiment shown in the example. The expressions "parts" and "%" used below mean "parts by mass" and "% by mass" unless otherwise specified. "eq" stands for equivalent, and is a unit that cannot be converted to SI units. Furthermore, the following various steps and operations were carried out in an environment with a temperature of 20 to 25°C and a relative humidity of 40 to 60%, unless otherwise specified.

[0125] [Ferromagnetic Powder] In the examples and comparative examples, the following hexagonal strontium ferrite was used as the ferromagnetic powder.

[0126] SrCO ３ 1707g, H ３ BO ３ 687g, Fe ２ O ３ 1120 g of Al(OH) ３ 45g, BaCO ３ 24 g, CaCO ３13g, and Nd ２ O ３ 235 g of the above was weighed and mixed in a mixer to obtain a raw material mixture. The resulting raw material mixture was melted in a platinum crucible at a melting temperature of 1390 ° C., and the melt was stirred while heating the tapping port at the bottom of the platinum crucible, and the melt was poured into a rod-like shape at approximately 6 g / sec. The tapped liquid was rolled and quenched with a water-cooled twin roller to produce an amorphous body. 280 g of the produced amorphous body was charged into an electric furnace, heated to 635 ° C (crystallization temperature) at a heating rate of 3.5 ° C / min, and held at the same temperature for 5 hours to precipitate (crystallize) hexagonal strontium ferrite particles. Next, the crystallized product obtained above containing hexagonal strontium ferrite particles was coarsely crushed in a mortar, and 1000 g of zirconia beads with a particle size of 1 mm and 800 mL of a 1% acetic acid aqueous solution were added to a glass bottle containing the crushed product, and the mixture was dispersed for 3 hours using a paint shaker. The resulting dispersion was then separated from the beads and placed in a stainless steel beaker. The dispersion was left to stand for 3 hours at a liquid temperature of 100°C to dissolve the glass component, and then precipitated in a centrifuge, washed by repeated decantation, and dried for 6 hours in a heating furnace at a furnace temperature of 110°C to obtain hexagonal strontium ferrite powder. The hexagonal strontium ferrite powder obtained above had an average particle size of 18 nm and an activation volume of 902 nm. ３ , the anisotropy constant Ku is 2.2 × 10 ５ J / m ３ , mass magnetization σs is 49A・m ２ / kg. 12 mg of sample powder was collected from the hexagonal strontium ferrite powder obtained above, and this sample powder was partially dissolved under the dissolution conditions exemplified above. The obtained filtrate was subjected to elemental analysis using an ICP analyzer to determine the surface content of neodymium atoms. Separately, 12 mg of sample powder was collected from the hexagonal strontium ferrite powder obtained above. This sample powder was completely dissolved under the dissolution conditions exemplified above. The obtained filtrate was subjected to elemental analysis using an ICP analyzer to determine the bulk content of neodymium atoms. The content of neodymium atoms (bulk content) relative to 100 atomic % of iron atoms in the hexagonal strontium ferrite powder obtained above was 2.9 atomic %. The surface content of neodymium atoms was 8.0 atomic %. The ratio of the surface content to the bulk content, "surface content / bulk content," was 2.8, confirming that neodymium atoms were unevenly distributed in the surface layers of the particles.

[0127] The powder obtained above was confirmed to have a hexagonal ferrite crystal structure by scanning with CuKα radiation at a voltage of 45 kV and an intensity of 40 mA and measuring the X-ray diffraction pattern under the following conditions (X-ray diffraction analysis). The powder obtained above exhibited a magnetoplumbite-type (M-type) hexagonal ferrite crystal structure. The crystalline phase detected by X-ray diffraction analysis was a single magnetoplumbite-type phase. PANalytical X'Pert Pro diffractometer, PIXcel detector. Soller slit for incident beam and diffracted beam: 0.017 radians. Fixed angle of dispersion slit: 1 / 4 degree. Mask: 10 mm. Anti-scatter slit: 1 / 4 degree. Measurement mode: continuous. Measurement time per step: 3 seconds. Measurement speed: 0.017 degrees per second. Measurement step: 0.05 degrees.

[0128] The activation volume and anisotropy constant Ku of the hexagonal strontium ferrite powder were determined for each ferromagnetic powder by the method described above using a vibrating sample magnetometer (manufactured by Toei Industry Co., Ltd.). The mass magnetization σs was measured using the vibrating sample magnetometer (manufactured by Toei Industry Co., Ltd.) at a magnetic field strength of 1194 kA / m (15 kOe).

[0129] Synthesis Example 1 29.0 g of cyclohexanone was placed in a 300 mL three-neck flask equipped with a stirrer, a condenser, a nitrogen inlet tube, and a thermometer, and the internal temperature was raised to 80°C and purged with nitrogen. A solution containing 27.0 g of polymerization component H-1 (dodecyl methacrylate (Fujifilm Wako Pure Chemical Industries, Ltd.)), 3.0 g of polymerization component D-1 (methacrylic acid), 1.6 g of dimethyl 2,2'-azobis(isobutyrate) (Fujifilm Wako Pure Chemical Industries, Ltd.), and 29.0 g of cyclohexanone was added dropwise over 3 hours. A mixed solution of 0.81 g of dimethyl 2,2'-azobis(isobutyrate) and 1.0 g of cyclohexanone was added, and the mixture was stirred at an internal temperature of 90°C for 3 hours. 20 g of cyclohexanone was then added to obtain polymer P-1. Polymer P-1 is a copolymer having the structural units shown above.

[0130] [Synthesis Examples 2 to 17, Comparative Synthesis Examples 1 and 2] Copolymers (Synthesis Examples 2 to 17, Comparative Synthesis Example 2) or homopolymers (Comparative Synthesis Example 1) shown in Table 1 were obtained by the method described for Synthesis Example 1, except that the blending ratio of the polymerization components and / or the types of the polymerization components were changed as shown in Table 1 below. Copolymers P-2 to P-11 shown in Table 1 each have the structural units shown above.

[0131] In Table 1, polymerization component (1) is a component for introducing the structural unit represented by formula (1) into a polymer. In Synthesis Examples 1 to 17, polymerization component (2) is a polymerization component for introducing the structural unit represented by formula (2) into a polymer. "D-30" listed in the column for polymerization component (2) in Comparative Synthesis Example 2 is a polymerization component having the following structure. The "-SO" terminal functional group of D-30 ３ "Na" does not correspond to either an acidic group or a basic group.

[0132]

[0133] The polymers obtained in each of Synthesis Examples 1 to 17, Comparative Synthesis Examples 1 and 2 were subjected to GPC measurement under the following measurement conditions, and the number average molecular weight was determined in terms of standard polystyrene. Measuring instrument: HLC-8220GPC (manufactured by Tosoh Corporation) Column: TSKgel Super HZ 2000 / TSKgel Super HZ 4000 / TSKgel Super HZ-H (manufactured by Tosoh Corporation) Eluent: tetrahydrofuran (THF) Flow rate: 0.35 mL / min Column temperature: 40°C Detector: differential refractive index (RI) detector

[0134]

[0135] Example 1 Preparation of Magnetic Layer-Forming Composition The following components were kneaded in an open kneader and then dispersed using a sand mill. Ferromagnetic powder: 100.0 parts Additive A: 10.0 parts Polyurethane resin: (Vylon (registered trademark) UR4800, manufactured by Toyobo Co., Ltd., functional group: SO ３ Na, functional group concentration: 70 eq / ton): 4.0 parts Vinyl chloride resin (Kaneka Corporation MR104): 10.0 parts Methyl ethyl ketone: 150.0 parts Cyclohexanone: 150.0 parts α-Al ２ O ３ (average particle size: 100 nm): 6.0 parts Carbon black (average particle size: 20 nm): 0.7 parts

[0136] The additive A is a polymer synthesized by the method described in paragraphs 0115 to 0123 of JP-A-2016-051493.

[0137] The following components were added to the dispersion obtained above, stirred, ultrasonicated, and filtered using a filter with a pore size of 1 μm to prepare a composition for forming a magnetic layer: Polymer (see Table 2): see Table 2 Stearic acid: see Table 2 Butyl stearate: 0.5 parts Stearic acid amide: 0.3 parts Methyl ethyl ketone: 110.0 parts Cyclohexanone: 110.0 parts Polyisocyanate compound (Coronate 3041, manufactured by Tosoh Corporation): 3.0 parts

[0138] <Preparation of composition for forming non-magnetic layer> The following components were kneaded in an open kneader and then dispersed using a sand mill. The resulting dispersion was filtered using a filter with a pore size of 1 μm to prepare a composition for forming a non-magnetic layer. Carbon black: 100.0 parts DBP (Dibutyl phthalate) oil absorption: 100 mL / 100 g pH: 8 BET (Brunauer-Emmett-Teller) specific surface area: 250 m ２ / g Volatile content: 1.5% Polyurethane resin (Vylon UR4800 manufactured by Toyobo Co., Ltd., functional group: SO ３ Na, functional group concentration: 70 eq / ton): 20.0 parts vinyl chloride resin (functional group: OSO ３ K, functional group concentration: 70 eq / ton): 30.0 parts Trioctylamine: 4.0 parts Cyclohexanone: 140.0 parts Methyl ethyl ketone: 170.0 parts Stearic acid amide: 0.3 parts Toluene: 3.0 parts Polyisocyanate compound (Tosoh Corporation, Coronate 3041): 5.0 parts

[0139] <Preparation of composition for forming backcoat layer> The following components were pre-mixed in a roll mill and then dispersed in a sand mill. The resulting dispersion was mixed with 4.0 parts of polyester resin (Vylon 500 manufactured by Toyobo Co., Ltd.), 14.0 parts of polyisocyanate compound (Coronate 3041 manufactured by Tosoh Corporation), and α-Al ２ O ３ 5.0 parts of (Sumitomo Chemical Co., Ltd.) were added, stirred, and then filtered to prepare a composition for forming a backcoat layer. Carbon black (average particle size: 40 nm): 85.0 parts Carbon black (average particle size: 100 nm): 3.0 parts Nitrocellulose: 28.0 parts Polyurethane resin: 58.0 parts Copper phthalocyanine dispersant: 2.5 parts Nipolan 2301 (Tosoh Corporation): 0.5 parts Methyl isobutyl ketone: 0.3 parts Methyl ethyl ketone: 860.0 parts Toluene: 240.0 parts

[0140] <Preparation of Magnetic Recording Media> Corona discharge treatment was performed on both surfaces of a 5.0 μm-thick biaxially oriented polyethylene naphthalate support. The nonmagnetic layer-forming composition described above was applied to one surface of the polyethylene naphthalate support so that the nonmagnetic layer would have a dry thickness of 1.0 μm. Immediately thereafter, the magnetic layer-forming composition was simultaneously multi-layer coated on top of the nonmagnetic layer so that the dry thickness of the magnetic layer would be 100 nm. While both layers were still wet, they were subjected to a vertical alignment treatment using a cobalt magnet with a magnetic force of 0.5 T (Tesla) and a solenoid with a magnetic force of 0.4 T, followed by a drying treatment. The backcoat layer-forming composition described above was then applied to the other surface of the polyethylene naphthalate support so that the dry thickness of the backcoat layer would be 0.5 μm. The backcoat layer was then calendered at a speed of 80 m / min using a seven-stage calender consisting of metal rolls at a calender roll surface temperature of 100° C. The coated film was then slit into ½-inch (1 inch = 0.0254 meters) widths to prepare magnetic tapes.

[0141] Examples 2 to 19, Comparative Examples 1 and 2 Magnetic tapes were produced by the method described for Example 1, except that the items shown in Table 2 below were changed as shown in Table 2.

[0142] [Evaluation Method] <Contact Angle> Using the magnetic layer-forming compositions prepared for producing the magnetic tapes of the Examples and Comparative Examples, the contact angle was measured by the following method.

[0143] (Preparation of Sample for Contact Angle Measurement) A 5 cm long support was cut from the original polyethylene naphthalate support described above. A magnetic layer-forming composition was applied to one surface of this support by spin coating and dried to form a magnetic layer. The spin coating conditions were a rotation speed of 1000 rpm (revolutions per minute) and a rotation time of 60 seconds. The contact angle of the magnetic layer surface of the contact angle measurement sample (hereinafter also simply referred to as "sample") thus formed was determined using the following method. The contact angle with water on the magnetic layer surface was measured using a contact angle measurement device (DropMaster 700 contact angle measurement device manufactured by Kyowa Interface Science Co., Ltd.) using the following method. The contact angle measurement was performed in a measurement environment with an ambient temperature of 20°C and a relative humidity of 25%. The contact angle measurement sample was placed on a glass slide so that the support surface was in contact with the glass slide surface. 2.0 μL of the measurement liquid (water) was dropped onto the sample surface (magnetic layer surface). After visually confirming that the dropped liquid had formed a stable droplet, the droplet image was analyzed using the contact angle analysis software FAMAS, which accompanies the contact angle measurement instrument, and the contact angle between the sample and the droplet was measured. The contact angle was calculated using the θ / 2 method. Measurement points were six randomly selected locations on the magnetic layer surface, and the contact angle was measured at each of the six locations. The arithmetic mean of the six measurements thus obtained was used as the contact angle of the sample being measured. Method: Droplet method (θ / 2 method) Droplet recognition: Automatic Droplet recognition line (distance from needle tip): 50 dots Algorithm: Automatic Image mode: Frame Threshold level: Automatic The measured contact angles were ranked according to the following evaluation criteria. The inventors believe that the higher the contact angle value measured in this way, the better the lubricating properties of the lubricant component contained in the magnetic layer. (Evaluation criteria) A: Contact angle is 95° or more B: Contact angle is 90° or more and less than 95° C: Contact angle is less than 90°

[0144] <Frictional Properties> The frictional properties of each magnetic tape in the Examples and Comparative Examples were evaluated by the following method. In an environment controlled at an ambient temperature of 13°C and a relative humidity of 80%, a magnetic head removed from an IBM LTO (registered trademark) G7 (Linear Tape-Open Generation 7) drive was attached to a tape running system, and a 20 m magnetic tape was repeatedly run at 4.0 m / s (10,000 running cycles) while being fed from a feed roll and wound around a take-up roll under a tension of 0.6 N. During the first and 10,000th running cycles, the frictional force acting on the magnetic head during each run was measured using a strain gauge, and the friction coefficient μ value was calculated from the measured frictional force. The friction coefficient after repeated running was evaluated from the measured μ value according to the following evaluation criteria. An evaluation result of A or B is preferable, with A being most preferable. (Evaluation criteria) A: μ value less than 0.08 B: μ value 0.08 or more and 0.12 or less C: μ value more than 0.12

[0145] <5 nm protrusion reduction rate (evaluation of changes in the surface shape of the magnetic layer before and after repeated running)> For each magnetic tape in the examples and comparative examples, the number of protrusions 5 nm or more in height on the magnetic layer surface was determined using the following method for the unrun magnetic tape and the magnetic tape after repeated running in the above environment. The number of protrusions 5 nm or more in height was determined by measurement using an atomic force microscope (AFM). Specifically, in a planar image of the magnetic layer surface obtained by AFM, the plane where the volumes of the convex and concave components in the measurement area are equal is defined as the reference plane, and the number of protrusions 5 nm or more in height from this reference plane is determined. Note that among the protrusions 5 nm or more in height present in the measurement area, there may be some protrusions that are partially within the measurement area and other portions outside the measurement area. When determining the number of protrusions, the number of protrusions is measured including such protrusions. The measurement area for the AFM measurement is a 5 μm square (5 μm × 5 μm) area on the magnetic layer surface. Measurements are performed in three different measurement areas on the magnetic layer surface (n = 3). The number of protrusions 5 nm or more in height is determined as the arithmetic mean of the three values ​​obtained by this measurement. The following measurement conditions are used for the AFM measurement. An AFM (Nanoscope 4 manufactured by Veeco) in tapping mode is used to measure a 5 μm square (5 μm x 5 μm) area on the surface of the magnetic layer of the magnetic tape. A BRUKER RTESP-300 is used as the probe, with a resolution of 512 pixels x 512 pixels and a scan speed of 341 seconds per screen (512 pixels x 512 pixels). The change in the surface shape of the magnetic layer before and after repeated running was evaluated using the protrusion reduction rate calculated by the following formula and the following evaluation criteria. It can be determined that the smaller the protrusion reduction rate, the more suppressed the change in the surface shape of the magnetic layer before and after repeated running. Protrusion reduction rate (%) = [(Number of protrusions found on unrun magnetic tape) - (Number of protrusions found on magnetic tape after the above repeated runs) / (Number of protrusions found on unrun magnetic tape)] x 100 (Evaluation criteria) A: Protrusion reduction rate is 40% or less B: Protrusion reduction rate is more than 40% but less than 70% C: Protrusion reduction rate is 70% or more

[0146] The results are shown in Table 2.

[0147]

[0148] The results shown in Table 2 confirm that the magnetic tapes of Examples 1 to 19 have low coefficients of friction after repeated running. Furthermore, the results shown in Table 2 also confirm that the magnetic tapes of Examples 1 to 19 show little change in the surface shape of the magnetic layer before and after repeated running.

[0149] One aspect of the present invention is useful in the technical field of magnetic recording media for high-density recording.

Claims

1. A magnetic recording medium having a non-magnetic support and a magnetic layer containing ferromagnetic powder, wherein the portion of the non-magnetic support facing the magnetic layer contains a polymer having a structural unit represented by formula (1) and a structural unit represented by formula (2), and the polymer contains more than 50 mol % of the structural unit represented by formula (1) relative to 100 mol % of all structural units contained in the polymer. In formula (1), R １１ and R １２ each independently represents a hydrogen atom or an alkyl group, R １３ represents a hydrogen atom or a monovalent substituent; L １１ represents a single bond or a divalent linking group; A １１ represents a monovalent hydrocarbon group, and in formula (2), R ２１ and R ２２ each independently represents a hydrogen atom or an alkyl group, R ２３ represents a hydrogen atom or a monovalent substituent; L ２１ represents a single bond or a divalent linking group; A ２１ represents a monovalent group selected from the group consisting of acidic groups and basic groups.

2. In formula (1), A １１ 2. The magnetic recording medium according to claim 1, wherein the monovalent hydrocarbon group represented by the formula: has 8 to 50 carbon atoms.

3. In formula (2), A ２１ 2. The magnetic recording medium according to claim 1, wherein the monovalent group represented by the formula: is selected from the group consisting of a carboxy group, a sulfonic acid group, a phosphoric acid group, a phosphonic acid group, and an amino group.

4. L in formula (1) １１ and L in formula (2) ２１ and each independently represents a single bond, an ester group, an alkylene group, or a divalent group consisting of a combination of one or more ester groups and one or more alkylene groups.

5. The magnetic recording medium according to claim 1, wherein the number average molecular weight of said polymer is 2,000 or more and 100,000 or less.

6. The magnetic recording medium according to claim 1, further comprising at least one fatty acid compound selected from the group consisting of fatty acids, fatty acid esters, and fatty acid amides in the portion of the non-magnetic support facing the magnetic layer.

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

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

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

10. In formula (1), A １１ The number of carbon atoms of the monovalent hydrocarbon group represented by the formula (2) is 8 to 50, ２１ is selected from the group consisting of a carboxy group, a sulfonic acid group, a phosphoric acid group, and an amino group, and L in formula (1) １１ and L in formula (2) ２１ each independently represent a single bond, or a divalent group consisting of an ester group, an alkylene group, or a combination of one or more ester groups and one or more alkylene groups; the number average molecular weight of the polymer is 2,000 or more and 100,000 or less; the non-magnetic support further comprises one or more fatty acid compounds selected from the group consisting of fatty acids, fatty acid esters, and fatty acid amides on a portion of the non-magnetic support facing the magnetic layer; the non-magnetic support further comprises a non-magnetic layer containing non-magnetic powder between the non-magnetic support and the magnetic layer; and the non-magnetic support further comprises a backcoat layer containing non-magnetic powder on a surface of the non-magnetic support opposite to the surface having the magnetic layer.

11. A magnetic tape cartridge containing the magnetic tape of claim 9 or 10.

12. A magnetic recording and reproducing device comprising the magnetic recording medium according to any one of claims 1 to 10.

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