JPH11185242A - Magnetic recording medium - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a magnetic recording medium which is less in self degaussing loss in recording and thickness loss in reproducing, is high in output even in high-frequency recording and is excellent in durability. SOLUTION: This magnetic recording medium is constituted by forming a nonmagnetic ground surface layer contg. nonmagnetic powder on a flexible nonmagnetic base and laminating a magnetic layer contg. ferromagnetic powder, inorg. fillers and binder thereon. In such a case, the film thickness (duc) of the nonmagnetic ground surface layer is larger than the film thickness (dmg) of the magnetic layer. The Mohs hardness of the inorg. fillers is >=8 and the average grain size (d) thereof satisfies the equation dmag<d<dmag+0.5 duc. In addition, the average roughness (Rz) of the magnetic layer surface is <=300 nm.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a magnetic recording medium having a high output at a high frequency suitable for high-density recording, and having excellent durability and weather resistance.

[0002]

2. Description of the Related Art In recent years, the density of magnetic recording media has been increasing, and the recording wavelength has been shortened. In response to this demand for higher density,
A magnetic recording medium using a metal thin film for the magnetic layer has been studied, but a coating type magnetic recording medium is superior in terms of productivity, durability, corrosiveness and the like. For this reason, studies are being made to improve the electromagnetic conversion characteristics of the coating type magnetic recording medium. In recording at a high frequency, the problems of self-demagnetization loss at the time of recording and thickness loss at the time of reproduction are increasing. In order to avoid this problem, the coating thickness is reduced.
However, simply reducing the thickness of the magnetic layer deteriorates durability and surface properties. For this reason, magnetic recording media having a multilayer structure in which a lower layer containing a non-magnetic powder is provided below a layer containing a magnetic powder have been proposed (for example, JP-A-62-159338 and JP-A-62-193338). -1542
No. 25).

However, the conventional technology cannot satisfy both output and durability sufficient for high-density recording. JP-B-6-73176 discloses that an inorganic filler having a Mohs hardness of 5 or more whose surface is coated with an oxide is blended in a magnetic layer so that the proportion of the filler increases near the surface. It has been proposed to improve durability. However, when performing multi-layer coating in a wet state, if the coating thickness is reduced to a certain degree or more, the inorganic filler in the magnetic layer will settle down to the lower layer, and the interface will be roughened, electromagnetic conversion characteristics will be deteriorated, and the roughened interface will affect the surface And satisfactory surface properties cannot be obtained. Furthermore, when controlling the proportion of the inorganic filler present in the magnetic layer and on the surface, if the amount of the inorganic filler is insufficient, the durability is deteriorated in any case.

[0004]

SUMMARY OF THE INVENTION The present invention provides a magnetic recording medium which has a small self-demagnetization loss during recording and a small thickness loss during reproduction, has a high output even in high-frequency recording, and has excellent durability. Things.

[0005]

Means for Solving the Problems The inventors of the present invention have conducted intensive studies in order to solve the above problems, and as a result, have found that the nonmagnetic powder added to the magnetic layer has a Mohs hardness of 8 or more and the average particle size thereof is It has been found that by specifying the film thickness and the thickness of the nonmagnetic underlayer, satisfactory durability can be obtained without impairing the output during reproduction and the S / N at the time of high-density recording. That is, the present invention provides a magnetic layer containing a ferromagnetic powder, an inorganic filler and a binder formed on a non-magnetic underlayer containing a non-magnetic powder on a flexible non-magnetic support. In a magnetic recording medium on which
The thickness of the nonmagnetic underlayer (duc) is the thickness of the magnetic layer (dmag)
The inorganic filler has a Mohs hardness of 8 or more, an average particle diameter (d) satisfying the following formula (1), and an average roughness (Rz) of the magnetic layer surface of 300 nm or less. Magnetic recording media. dmag <d <dmag + 0.5duc (1)

[0006]

DESCRIPTION OF THE PREFERRED EMBODIMENTS In the magnetic recording medium of the present invention, a non-magnetic underlayer is formed on a flexible non-magnetic support, and a magnetic layer is laminated thereon. The non-magnetic support used for the magnetic recording medium of the present invention is not particularly limited, and polyesters such as polyethylene terephthalate and polyethylene naphthalate, polypropylene, polyolefins such as polyethylene, cellulose derivatives such as cellulose acetate, polycarbonate, polyamide, and polyimide. Various plastics and the like can be used. Among them, polyesters such as polyethylene terephthalate and polyethylene naphthalate are preferred from the viewpoints of excellent mechanical properties, heat resistance, electrical properties, chemical resistance and the like. However, these polyester films are poor in adhesion to the magnetic layer and the underlayer because of the crystal orientation. Therefore, these supports may be subjected to a treatment with a surface modifier such as an aqueous alkali solution, an aqueous amine solution, trichloroacetic acid, or phenols, and then used in the production of a magnetic recording medium. The support has a thickness of 4 to 100 μm, for example, 4 to 10 μm for a tape, and 30 to 100 μm for a magnetic disk.
100 μm.

The non-magnetic underlayer according to the present invention contains a non-magnetic powder and a binder. Non-magnetic powders include carbon black, titanium oxide, α-iron oxide, α-alumina, silicon carbonate, chromium oxide, cerium oxide, goethite, corundum, silicon nitride, silicon dioxide, tin oxide, magnesium oxide, zirconium oxide, calcium carbonate, Examples thereof include calcium sulfate, barium sulfate, and molybdenum disulfide. The shape may be any of a needle shape, a spherical shape, a polyhedral shape, and a plate shape. The particle size is preferably smaller than the average particle size of the ferromagnetic powder contained in the magnetic layer, and particularly preferably 10 to 200 nm. Among these, carbon black is preferred because it carries a lubricant, and when carbon black is used in combination with other materials, the carbon black content is preferably 80% or more. More preferably, the nonmagnetic powder is only carbon black.

As the binder, it is preferable to use a resin having excellent adhesion to a support and abrasion resistance, a glass transition point of -100 to 150 ° C. and a number average molecular weight of about 1,000 to 150,000. Examples of the resin used include polyurethane resins, polyester resins, cellulose acetate butyrate, cellulose derivatives such as cellulose diacetate and nitrocellulose, vinyl chloride-vinyl acetate copolymers, vinyl chloride-vinylidene chloride copolymers, and chlorides. Vinyl chloride resins such as vinyl-acrylic copolymers, various synthetic rubbers such as styrene-butadiene copolymers, epoxy resins, phenoxy resins, and the like. These may be used alone or as a mixture of two or more. be able to. The content of the binder resin with respect to the nonmagnetic powder in the nonmagnetic underlayer is 2%.
It is preferably used in an amount of from 100 to 100% by weight, especially from 10 to 80% by weight.

The nonmagnetic underlayer may further contain a crosslinking agent, a lubricant, a dispersant, and the like, and the amount thereof is not particularly limited as long as the present invention is not hindered. Known techniques for the magnetic layer can also be applied to the above. The weight ratio (P / B) of the nonmagnetic powder (P) to the binder (B) in the nonmagnetic underlayer is preferably 1.0 or more, and more preferably 1.4 to 4.0. The weight ratio (P / B) is 1.
If the value is less than 0, the magnetic layer is applied after the application and drying of the nonmagnetic underlayer, in particular, in the case of a so-called sequential coating step, the dried nonmagnetic underlayer is melted by the magnetic coating agent at the time of applying the magnetic layer, and the magnetic layer becomes non-magnetic. If the interface of the magnetic underlayer is roughened and the magnetic layer is thinner, the surface of the magnetic layer may be roughened.

In the present invention, the thickness of the nonmagnetic underlayer is formed to be larger than the thickness of the magnetic layer. Preferably, it is 1.1 times or more and 10 times or less of the magnetic layer. More preferably, it is 0.4 μm or more and 2.5 μm or less. If the thickness of the underlayer is less than or equal to the thickness of the magnetic layer, the amount of lubricant retained in the magnetic layer increases, and the wear resistance of the magnetic layer may deteriorate. By forming the non-magnetic underlayer thicker than the magnetic layer, the lubricant is mainly held by the non-magnetic underlayer, so that the magnetic layer is not excessively retained, and the polishing resistance of the magnetic layer is improved. Sufficient lubricant can be added without deterioration.

[0011] A magnetic layer is formed on the non-magnetic underlayer.
As the ferromagnetic powder used for the magnetic layer, magnetic recording
Any of those known for media can be used.
For example, γ-Fe_TwoO_Three, Fe_ThreeO_Four, Between these
Oxide, Co-containing γ-Fe _TwoO_Three, Co-containing Fe_ThreeO
_FourNeedle-like iron oxide powder, barium ferrite, straw
Hexagonal plate-like ferrite powder such as n-ferrite
Powder, Fe, Co, Ni, their alloys or these
From other metals or alloys containing small amounts of non-metallic atoms
Metal powder.

Of these, metal magnetic powders are particularly preferred.
To achieve high density magnetic recording, the specific surface area must be 35 m ²
/ G or more is preferred. If it is less than 35 m ² / g, sufficient surface properties cannot be obtained when a magnetic recording medium is used, and it may not be suitable for high-density recording. Also, since the filling ratio of the magnetic powder in the magnetic layer is not sufficiently increased, a high output cannot be obtained. Conversely, if the specific surface area exceeds 65 m ² / g, the dispersibility in the magnetic paint may decrease, or the durability of the magnetic recording medium may decrease. The average particle size of the ferromagnetic powder is 0.5 μm or less,
Among them, those having a diameter of 0.15 μm or less are preferable. The amount of the ferromagnetic powder to be used in the magnetic layer is preferably 50 to 90% by weight, particularly preferably 60 to 70% by weight. If the content is less than 50% by weight, a high coercive force cannot be obtained due to a low filling rate of the ferromagnetic powder in the magnetic layer, and it is difficult to obtain a high-density magnetic recording medium. , And the durability of the magnetic recording medium may be reduced.

The magnetic layer is usually formed by applying a magnetic coating material in which a ferromagnetic powder and a non-magnetic powder are dispersed in a binder onto a support. If necessary, dispersants, lubricants,
An antistatic agent may be dispersed in the binder. As the binder, those having excellent adhesion to the support and abrasion resistance are appropriately used. For example, polyurethane resins, polyester resins, cellulose acetate butyrate, cellulose diacetate, cellulose derivatives such as nitrocellulose,
Various synthetic rubbers such as a vinyl chloride-vinyl acetate copolymer, a vinyl chloride-vinylidene chloride copolymer, a vinyl chloride-acrylic copolymer, an epoxy resin, a phenoxy resin, and the like. These may be used alone or as a mixture of two or more at any ratio. The binder is preferably used in an amount of usually 2 to 50% by weight, particularly 5 to 25% by weight in the magnetic layer.

Further, by including a polyisocyanate compound having a plurality of isocyanate groups in the magnetic layer, a three-dimensional network structure can be formed in the magnetic layer, and its mechanical strength can be improved. Examples of such a low molecular weight polyisocyanate compound include a trimethylolpropane adduct of tolylene diisocyanate. Such a low molecular weight polyisocyanate compound is preferably used in a proportion of 10 to 50% by weight based on the binder. In the present invention, the thickness of the magnetic layer is arbitrary. However, at the time of high density recording, it is preferably 1.0 μm or less, and
m or less is preferable. However, the thickness of the magnetic layer is 0.05 μm
If it is less than 0.05 μm, the durability of the magnetic recording medium may be reduced.

In the present invention, the inorganic filler to be added to the magnetic layer is alumina, silicon carbide, chromium oxide or the like having a Mohs hardness of 8 or more. Among them, alumina is particularly preferred. Further, an appropriate range of the average particle diameter (d) of the inorganic filler is determined by the following formula (1) from the thickness of the magnetic layer (dmag) and the thickness of the nonmagnetic underlayer (duc). dmag <d <dmag +0.5 duc (1) Among them, the average particle diameter of the inorganic filler is 0.2 μm.
It is preferably at least m and at most 2.0 μm.

If the average particle size of the inorganic filler exceeds the upper limit of the formula (1), the amount of exposure to the surface of the magnetic layer becomes too large, the surface roughness, especially Rz, is deteriorated, and the reproduction output and S / N are reduced. In addition, the inorganic filler may be lost while the head is running, resulting in an increase in errors and deterioration in durability. When the average particle size of the inorganic filler is less than the lower limit of the formula (1), the amount of exposure to the surface of the magnetic layer is reduced, and the durability may be deteriorated. Further, the average particle size of the inorganic filler is 0.1.
If it is less than 2 μm, the inorganic filler may sink into the magnetic layer due to dispersion. If it is more than 2.0 μm, the dispersion of the ferromagnetic powder, which is a fine powder, is hindered. N may decrease. The amount of the inorganic filler used is 1 to 3 in the magnetic layer.
It is preferably in the range of 0% by weight.

In the present invention, the area ratio of the inorganic filler exposed on the surface of the magnetic layer is 0.2% or more and 3.0% or less. The presence area ratio of the inorganic filler exposed in the present invention indicates the ratio of the area occupied by the inorganic filler to the surface of the magnetic layer, which is detected in a SEM photograph described later. If the area ratio of the inorganic filler is less than 0.2%, the durability is remarkably deteriorated, and the cleaning performance of the head is deteriorated, so that the head is clogged, and the electromagnetic conversion characteristic is deteriorated during long-time sliding. I do. In addition, if the area ratio of the inorganic filler in the magnetic layer is higher than 0.2% in the magnetic layer, the interface is disturbed by the precipitated inorganic filler.

If the area ratio of the inorganic filler exceeds 3.0%, the durability of the head is remarkably deteriorated, and the ratio of the inorganic filler detached from the surface is high. The problems of lowering the production yield and increasing the error occurrence rate occur due to the damage of the substrate. The inorganic filler presence area ratio is obtained by calculating the area occupied by the inorganic filler per unit area from an SEM photograph taken from a direction substantially perpendicular to the surface of the magnetic layer. For example, it can be easily calculated by using a computer and performing image processing on a SEM photograph with a difference in contrast. The surface of the magnetic layer means a surface (= projected surface from a direction perpendicular to the magnetic layer) when the surface is flat, ignoring irregularities.

Further, a lubricant, a dispersant, an antistatic agent and the like may be added to the magnetic paint as required. Here, as the lubricant, various lubricants such as aliphatic, fluorine, silicone, and hydrocarbon can be used. Examples of the aliphatic lubricants include fatty acids such as oleic acid, lauric acid, myristic acid, palmitic acid, stearic acid, and behenic acid; salts thereof with metals such as magnesium, aluminum, sodium, and calcium; and butyl esters of the fatty acids. And octyl esters and fatty acid esters such as glyceride, and amides, linoleic acid amides and caproic acid amides of the above-mentioned fatty acids. Also, fatty acid alcohols such as lauryl alcohol, stearyl alcohol, myristyl alcohol, palmityl alcohol, and oleyl alcohol; fluorine-based lubricants such as perfluoroalkyl polyethers and perfluoroalkyl carboxylic acids; and silicone oils such as silicone oils and modified silicone oils Examples of the lubricant include a hydrocarbon-based lubricant such as a lubricant, paraffin, squalane, and wax. Further, solid lubricants such as molybdenum disulfide and tungsten disulfide, phosphates, and the like can also be used.

The amount of the fatty acid ester-based lubricant used is such that the content in the magnetic layer is usually in the range of 0.1 to 15% by weight, preferably 1 to 15% by weight. The amount of lubricant is 0.
If it is less than 1% by weight, the durability is insufficient. Also,
If it exceeds 15% by weight, the head may be stained with a lubricant. The amount of fatty acid is 0.1 to 10% by weight, more preferably 1 to 5% by weight. When the amount of the fatty acid is small, the running property tends to decrease, and when the amount exceeds 10% by weight, the durability and the output decrease easily.

Examples of the dispersant include fatty acids having 10 to 18 carbon atoms, such as capric acid, lauric acid, myristic acid, oleic acid, and linoleic acid, and metal soaps containing these alkali metal salts or alkaline earth metal salts, lecithin and the like. Is used. The amount of the dispersant used is usually from 0 to 5% by weight in the magnetic layer. Examples of the solvent used in kneading, dispersing, and applying the magnetic paint include ketones such as methyl ethyl ketone, methyl isobutyl ketone, and cyclohexane; alcohols such as methanol, ethanol, propanol, and isopropyl alcohol; methyl acetate; and ethyl acetate. Conventionally known solvents such as esters such as butyl acetate, ethers such as diethyl ether and tetrahydrofuran, aromatic hydrocarbons such as benzene, toluene and xylene, and aliphatic hydrocarbons such as hexane are exemplified.

As the antistatic agent, carbon black,
Conductive metal and its compounds, oxides, natural surfactants such as saponin, alkylene oxide-based, glycerin-based nonionic surfactants, higher alkylamines,
Quaternary ammonium salts, cationic surfactants such as pyridinium salts and other heterocyclic salts, anionic surfactants containing an acidic group such as a carboxylic acid group, a sulfonic acid group, a phosphoric acid group, a sulfate group, a phosphate group, amino acids, Amphoteric surfactants such as aminosulfonic acids, sulfuric acid or phosphoric acid esters of amino alcohol, and the like are used. Incidentally, these surfactants can be used alone or in combination.

Among them, acetylene black, black for color, furnace black, thermal black and the like can be used as the carbon black used in the present invention. As a specific example, BLACKP manufactured by Cabot Corporation
EARLS 2000, 1000, 900, 800, MOGUL-L, VULCAN XC-7
2, RAVEN 8800, 8000, 700 made by Columbian Carbon
0, Mitsubishi Chemical # 3750B, # 3750, # 3250B, # 3250, # 95
0, # 850B, # 650B, # 45, # 40, # 5, MA-77, MA-7 and the like. These carbon blacks, alone,
Alternatively, a plurality of them can be used in combination. Also,
It may be treated with a carbon black surface dispersant or the like, or a part thereof may be graphitized.

As the conductive metal and its compound and oxide, tin oxide, indium tin oxide and the like can be used. The amount of the antistatic agent used is usually in the range of 0.1 to 10% by weight in the magnetic layer. These are used as antistatic agents, but sometimes their purpose is to improve lubricity, for example. In the production of the magnetic recording medium of the present invention, the constituent materials of each layer are kneaded / dispersed with a solvent or the like by a conventional technique to form a coating agent, which may be coated by appropriately selecting a coating method. The means are optional as long as they are satisfied.

The non-magnetic underlayer and the magnetic layer may be manufactured by either a sequential coating method or a simultaneous coating method. However, the sequential coating method is preferable because the interface between the two layers can be made uniform. It is preferable that a magnetic underlayer is applied and dried, and then a magnetic layer is applied. In this case, a magnetic layer (a single layer or a plurality of layers) may be formed on the nonmagnetic underlayer without performing a calendering process after the application of the nonmagnetic underlayer, and the upper magnetic layer may be applied, dried, and then calendered. By doing so, a magnetic recording medium having good electromagnetic conversion characteristics and durability can be obtained. In particular, there is an effect that a lubricant is supported and supplied to the surface of the magnetic layer in small amounts. Preferably, calendering is performed after coating and drying of the magnetic layer. More preferably, the processing conditions are a line thickness of 300 kg / cm or more and a processing temperature of 6 kg / cm.
The temperature is set to 0 ° C. or higher. The calendering treatment can improve the surface smoothness of the magnetic layer.

[0026]

According to the present invention, the magnetic layer is calendered by making the nonmagnetic underlayer thicker than the magnetic layer and making the average particle size of the inorganic filler larger than the magnetic layer and within a specific range. The surface state and the dispersed structure of the inorganic filler form a preferable state as a magnetic recording medium, and a magnetic recording medium with high output and excellent durability can be obtained.

[0027]

EXAMPLES The present invention will be described in more detail with reference to the following Examples, which should not be construed as limiting the scope of the invention. In the examples, “parts” indicates “parts by weight”. The output in the electromagnetic conversion characteristics was measured at various recording densities by using a MIG head with a spin stand, changing the recording frequency at a rotation speed of 2945 rpm, and measuring position R of 40 mm. Durability: 25 ° C, 50% humidity with the above spin stand
In an atmosphere of RH, the number of rotations is 2945 rpm, and the measurement position R
After running for 120 hours at 40 mm, the output was measured and the output before running was evaluated as 100.

Example 1 After kneading the following material compositions, 10 parts by weight of a polyisocyanate (AD30 manufactured by Mitsubishi Chemical Corporation) was added to a dispersion dispersed by a sand mill.
The solution was filtered using a filter having an average pore size of 3 μm. The lower layer is applied to a 62 μm-thick polyethylene terephthalate film so as to have a dry film thickness of 0.5 μm, and the obtained coating solution is dried for several minutes. Coating agent for non-magnetic underlayer Carbon black (BET = 25 m ² / g, DBP = 72 ml / 100 g, average particle size 75 nm) 100 parts Polyester polyurethane resin 40 parts Tridecyl stearate 10 parts Oleic acid 2 parts Methyl ethyl ketone 300 parts Cyclohexanone 100 Department

Coating agent for magnetic layer Metal magnetic powder (Fe / Co = 85/15, σs = 150 emu / g, Hc = 1850 Oe, BET = 40m ² / g, particle size = 0.135 μm) 100 parts Polymer 15 parts Polyester polyurethane resin 4 parts α-alumina (Average particle size: 0.5 μm, Mohs hardness: 9) 15 parts Carbon black 8 parts Tridecyl stearate 9 parts Oleic acid 1 part Methyl ethyl ketone 280 parts Cyclohexanone 120 parts

In order to form a magnetic layer, each of the above-mentioned material compositions was kneaded, and then dispersed by a sand mill to prepare a coating agent. 5 parts by weight of a polyisocyanate (AD30 manufactured by Mitsubishi Chemical Corporation) was added to this dispersion, and the mixture was filtered using a filter having an average pore diameter of 1 μm. The obtained coating liquid was applied on the underlayer so that the dry thickness of the magnetic layer became 0.4 μm. After this, a calender (temperature: 70 ° C,
(Linear pressure: 300 kg / cm) treatment, punched out into a 3.5-inch diameter disk, placed horizontally, allowed to stand at 50 ° C. for 48 hours to prepare a magnetic recording medium, and evaluated.

Example 2 The procedure of Example 1 was repeated, except that the thickness of the nonmagnetic underlayer was 1.5 μm and the particle size of the inorganic filler used in the magnetic layer was 0.8 μm. A magnetic medium was manufactured. Comparative Example 1: The same method as in Example 1 was performed except that the particle diameter of the inorganic filler used in the magnetic layer in Example 1 was 0.3 μm. Comparative Example 2: The same procedure as in Example 1 was carried out except that the particle diameter of the inorganic filler used in the magnetic layer in Example 1 was 0.8 μm.

Comparative Example 3 The procedure of Example 2 was repeated, except that the temperature in the calendering step was 40 ° C. Comparative Example 4: The same method as in Example 1 was performed except that the thickness of the underlayer was changed to 0.3 μm. Comparative Example 5: The same method as in Example 1 was performed except that the inorganic filler used in the magnetic layer in Example 1 was silicon oxide having an average particle diameter of 0.5 μm (Mohs hardness: 7).

Example 3 The ferromagnetic metal powder used in the magnetic layer in Example 1 had a particle size of 0.06 μm (Fe / Co
= 80/20, σs = 133 emu / g, Hc = 195
0 Oe, BET = 64 m ² / g, particle size = 0.060 μ
m) was carried out in the same manner as in Example 1 except for m). Example 4: The nonmagnetic powder used in the nonmagnetic underlayer in Example 1 was Fe ₂ O ₃ (BET = 0.30 μm).
The same method as in Example 1 was performed except that the particle size was 62 m ² / g and the particle size was 0.30 μm. Example 5: Except that the amount of the urethane resin dispersed in the nonmagnetic underlayer in Example 1 was 190 parts by weight,
Performed in the same manner as in Example 1.

Example 6 Example 1 was the same as Example 1 except that the linear pressure in the calendering step was 200 kg / cm.
Performed in the same manner as Example 7: In Example 1, the particle size of the inorganic filler used in the magnetic layer was 0.3 μm, and the thickness of the magnetic layer was 0.2 μm.
m, the thickness of the underlayer was set to 1.0 μm, and the dispersion time of the magnetic paint was tripled. Example 8: The inorganic filler used in the magnetic layer in Example 1 was changed to silicon carbide having a particle size of 0.5 μm (Mohs hardness:
Except for 9), the procedure was the same as in Example 1.

Example 9: In Example 1, the inorganic filler used in the magnetic layer was alumina having a particle size of 2.5 μm, the thickness of the magnetic layer was 2.0 μm, and the thickness of the underlayer was 3.0 μm.
The procedure was performed in the same manner as in Example 1 except for the above. Example 10: The ferromagnetic metal powder used in the magnetic layer in Example 1 had a particle size of 0.17 μm (Fe / Co = 90/1).
0, σs = 134 emu / g, Hc = 1680 Oe,
BET = 57 m ² / g, particle size = 170 μm), except that the same method as in Example 1 was used. Example 11: The ferromagnetic powder used in the magnetic layer in Example 1 had a particle size of 0.035 μm (σs = 56 emu / g,
Hc = 1990 Oe, BET = 45 m ² / g, particle size =
Except that it was a hexagonal barium ferrite having a thickness of 0.035 μm and a plate ratio of 3), the same method as in Example 1 was used. Example 12: In Example 1, the thickness of the non-magnetic layer was 1.
Except for 5 μm, the same method as in Example 1 was used. Tables 1 and 2 show the conditions and results of these experiments.

[0036]

[Table 1]

[0037]

[Table 2] Table 2 表面 Surface abrasive presence rate Durability Output%%% ─────── ─────────────────── Example 1 1.0 95 100 Example 2 1.7 101 97 Comparative Example 1 0.4 37 93 Comparative Example 2 2.0 76 42 Comparative Example 3 2.3 86 37 Comparative Example 4 1.3 41 64 Comparative Example 5 1.1 38 96 Example 3 2.0 92 83 Example 4 2.5 88 78 Example 5 2.5 84 75 Example 6 2.5 79 76 Example 7 0.1 72 94 Example 8 1.2 76 97 Example 9 2.3 96 74 Example 10 0.8 99 81 Example 11 1.6 92 55 Example 12 0.8 72 98 ──────────────────────────

The output is a relative value when the output of Example 1 is set to 100, and the display durability is 120 using a spin stand.
Output after running for hours is displayed as output before running as 100

Claims (7)

[Claims] 1. A non-magnetic underlayer containing a non-magnetic powder is formed on a flexible non-magnetic support, and a magnetic layer containing a ferromagnetic powder, an inorganic filler and a binder is formed thereon. In the laminated magnetic recording medium, the thickness (d
uc) is greater than the thickness (dmag) of the magnetic layer, the inorganic filler has a Mohs hardness of 8 or more, the average particle diameter (d) satisfies the following formula (1), and the average roughness ( Rz)
Is 300 nm or less. dmag <d <dmag + 0.5duc (1) 2. The magnetic recording medium according to claim 1, wherein the average particle size of the nonmagnetic powder contained in the nonmagnetic underlayer is smaller than the average particle size of the ferromagnetic powder contained in the magnetic layer. 3. The magnetic recording medium according to claim 1, wherein the nonmagnetic powder contained in the nonmagnetic underlayer is carbon black. 4. The weight ratio (P / B) of the nonmagnetic powder (P) and the binder (B) constituting the nonmagnetic underlayer is 1.0 or more. Magnetic recording medium. 5. The magnetic recording medium according to claim 1, wherein the area ratio of the inorganic filler exposed on the surface of the magnetic layer is from 0.2% to 3.0% of the surface of the magnetic layer. 6. The magnetic recording medium according to claim 1, wherein the average particle diameter of the ferromagnetic powder contained in the magnetic layer is 0.15 μm or less. 7. The magnetic recording medium according to claim 1, wherein the ferromagnetic powder contained in the magnetic layer is a ferromagnetic metal powder.