JP2005216349A - Magnetic recording medium - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a magnetic recording medium wherein noise generated by a head media interface is improved and ≥3 G bite/inch<SP>2</SP>recording density can be attained. <P>SOLUTION: The magnetic recording medium wherein a substantially non magnetic lower layer is formed on a substrate and a magnetic layer formed by dispersing ferromagnetic powder in a binder is formed thereon is characterized in that the surface of the magnetic layer has ≤10.0 pieces/900 μm<SP>2</SP>protrusions having ≥30 nm height and ≤2.0 pieces /900 μm<SP>2</SP>recesses having ≥30 nm depth measured by using an AFM (Atomic Force Microscope). <P>COPYRIGHT: (C)2005,JPO&NCIPI

Description

  The present invention relates to a coating type high recording density magnetic recording medium. More particularly, the present invention relates to a magnetic recording medium suitable for a recording / reproducing system employing an MR head, which has a substantially non-magnetic lower layer and a magnetic layer containing ferromagnetic powder in this order.

In the field of flexible magnetic disks, 2 MB MF-2HD floppy disks using Co-modified iron oxide have come to be standard installed in personal computers. However, with today's rapidly increasing data capacity, the capacity is not sufficient, and it is desired to increase the capacity of flexible magnetic disks.
As a large capacity flexible magnetic disk, 10MB MF-2TD, 21MB MF-2SD, Zip100, Zup250, or 4MB as a large capacity disk using a ferromagnetic metal powder with excellent high density recording characteristics. In particular, in the field of optical disc media, DVD-R has reached 3 Gbit / inch ² and has not been sufficient in terms of capacity and performance.

On the other hand, a magnetic head (induction type magnetic head) whose operation principle is electromagnetic induction is widely used in recording and reproducing systems for magnetic recording media. However, there is a limit in using the induction type magnetic head in a higher density recording / reproducing area. That is, in order to obtain a large reproduction output, it is necessary to increase the number of coil turns of the reproduction head. However, when the number of coil turns is increased, the inductance increases and the resistance at high frequency increases, resulting in a problem that the reproduction output decreases.
In view of such circumstances, a recording / reproducing MR head based on MR (magnetoresistance) has recently been developed and put into practical use in a hard disk device.
A magnetic disk using hexagonal ferrite has low noise but low sensitivity and has not yet achieved sufficient performance. However, when an MR head is used for a magnetic disk using low noise hexagonal ferrite, the sensitivity is low. It has been found that the recording density characteristics are remarkably improved.
However, in a flexible magnetic disk in which the head and the magnetic recording medium are easily in contact with each other, a lot of noise that seems to be related to the head media interface is generated, and desired characteristics are not obtained.

Some attempts have been made to improve these. In Patent Document 1, the thermal asperity noise generated specifically in the MR head is improved by setting the number of protrusions having a height of 100 nm or more to 10 pieces / 900 cm ² or less.
According to Patent Document 2, thermal asperity noise can be improved by setting the number of protrusions having a height of 30 nm or more to 100 pieces / 900 μm ² or less at a recording density of 0.5 to 2 Gbit / inch ² .
In Patent Document 3, the number of dents having a depth of 1/3 or more of the minimum recording bit length is set to 100 pieces / 10000 μm ² or less, and the average surface roughness of the center plane is set to 6 nm or less, so that noise caused by an increase in spacing loss is obtained. It can be improved.
However, in Patent Document 1, thermal asperity is evaluated at a linear recording density of 100 kfci and a track width of 1.9 μm. This recording density is 1.3 Gbit / inch ² at most, which is the 3 Gbit targeted by the present application. It was found that sufficient effects could not be obtained at ² inches / inch or more.
Patent Document 2 states that it is difficult to obtain the effect of the present invention at a density exceeding 2 Gbit / inch ² .
In the example of Patent Document 3, a high S / N ratio is obtained by suppressing the number of recesses having a depth of 33 nm to 92 nm to a predetermined number according to the bit length. However, as a result of studies by the present inventors, it has been found that although the SN ratio is high, thermal asperity is often generated, and sufficient characteristics cannot be obtained at 3 Gbit / inch ² or more.

JP 2002-367140 A JP-A-10-302243 JP 2003-115104 A

SUMMARY OF THE INVENTION An object of the present invention is to provide a magnetic recording medium, particularly a flexible magnetic disk using a hexagonal ferrite magnetic body, which can improve noise generated in connection with a head media interface and achieve a recording density of 3 gigabytes / inch ² or more. It is in.

The problems of the present invention can be solved by the following means.
1) In a magnetic recording medium in which a substantially non-magnetic lower layer on a support and a magnetic layer formed by dispersing ferromagnetic powder in a binder are formed on the support, the surface of the magnetic layer is AFM (interatomic A magnetic recording medium characterized in that protrusions with a height of 30 nm or more measured by a force microscope) are 10.0 pieces / 900 μm ² or less, and recesses with a depth of 30 nm or more are 2.0 pieces / 900 μm ² or less.
2) The magnetic recording medium as described in 1) above, wherein the ferromagnetic powder is a hexagonal ferrite powder.
3) The magnetic recording medium as described in 1) or 2) above, wherein the magnetic recording medium is a magnetic disk.
4) The magnetic recording medium according to any one of 1) to 3) above, wherein the number of protrusions of 10 nm or more is 100 to 400/900 μm ² .

  The present invention can provide a magnetic recording medium that can significantly reduce thermal asperity noise generated by contact between the head and the magnetic recording medium in a magnetic recording system employing an MR head.

In the present invention, the height of the protrusion on the surface of the magnetic layer and the depth of the dent are the average surface of the magnetic layer surface roughness when the three-dimensional surface roughness is measured using Nanoscope III manufactured by Digital Instruments Inc. It is the distance from the surface where the volume of the unevenness is equal) to the top of the convex part and the distance to the deepest part of the concave part. The measurement is performed within a range of 30 μm × 30 μm (900 μm ² ), and 10 arbitrary locations of the magnetic recording medium are measured, and the average value is obtained.

The reason why the magnetic recording medium of the present invention exhibits excellent characteristics in reproduction with an MR head is not clear, but is considered as follows. Conventionally, it has been known that the magnetic resistance of the MR head changes due to the heat generated by the collision of the surface protrusions with the MR head, resulting in thermal asperity noise, but at a high density of 3 Gbit / inch ² or more, It may be necessary to reduce smaller protrusions.
On the other hand, it was found that this was not sufficient, and the recesses caused thermal asperity. This is thought to be because the air flow between the head and the surface of the magnetic recording medium changes due to the presence of the concave portion, and noise of the opposite polarity to the noise generated by the collision with the convex portion occurs due to the cooling of the head. . Thus, it has been found that the thermal asperity cannot be reduced only by reducing only the convex portions and by reducing only the concave portions. Furthermore, in the present application, it has been found that the thermal asperity is further improved by setting the projections of 10 nm or more to 100 to 400 pieces / 900 μm ^2, and the reason is considered as follows.
Even if there are many protrusions of 10 nm or more on the surface, the height is sufficiently low so that the collision energy with the head is small and thermal asperity is unlikely. On the other hand, due to the presence of such minute projections, the spacing between the head and the magnetic recording medium is maintained at a certain value, and even if a higher projection is encountered, the relative height is lowered and the impact is reduced. It can be relieved.
When the number of minute protrusions is small, the spacing between the head and the magnetic recording medium decreases. However, when encountering a high convex part, not only does thermal energy increase due to collision, the thermal asperity increases, but the convex part is shaved or magnetic. Subsequent thermal asperity is likely to occur due to layer surface scraping.

The thermal asperity noise aimed at improvement in the present invention is generated by the interaction between the magnetic recording medium and the head. The flying height of the head changes depending on the structure of the head / slider and the relative speed. Specifically, not only the flying height changes depending on the rail width of the slider and the number, width, and depth of slits provided in the rail, but also the flying height changes when the relative speed is large. When the relative speed is high, the flying height generally increases and the collision frequency with the protrusion decreases, but the energy when the protrusion collides increases. Therefore, the frequency of thermal asperity changes depending on both the head flying height and the relative speed. The magnetic recording medium of the present invention is used in the range of 1 m / s to 15 m / s relative to the head and the head flying height in the range of 10 nm to 100 nm. However, if the requirements of the present invention are satisfied, this range is practical. Thermal asperity can be improved to the extent that there is no problem.
That is, in the present invention, the number of protrusions having a height of 30 nm or more measured by AFM on the surface of the magnetic layer is 10.0 / 900 μm ² or less, preferably 5/900 μm ² or less, ideally 0, and deep. The number of recesses of 30 nm or more is 2.0 pieces / 900 μm ² or less, ideally 0 pieces.

There are the following methods for controlling the height and number of protrusions on the surface of the magnetic layer of the magnetic recording medium of the present invention.
(1) Nonmagnetic powders such as abrasives and carbon black added to the magnetic layer have a sharp particle size distribution, specifically, a coefficient of variation (100 × standard deviation / average particle size) of the particle size distribution. 30% or less, preferably 10% or less is used. When the coefficient of variation is large, coarse particles form large surface protrusions, which is not preferable.
(2) A nonmagnetic powder having an average particle size of 60 to 140%, preferably 80 to 120%, is used with respect to the magnetic layer thickness. When the average particle size is larger than 140% of the magnetic layer thickness, large surface protrusions are formed, and when the average particle size is smaller than 60%, the functions of the non-magnetic powder (reduction of frictional force, polishing action, etc.) are undesirably reduced.
(3) In order to reduce protrusions caused by undispersed magnetic materials, a highly dispersed binder is used, and sufficient dispersion is performed using a highly dispersed medium such as Zr beads.
(4) If the support has many protrusions due to filler aggregates, it will appear as protrusions in the magnetic layer. In particular, in order to reduce the number of protrusions of 30 nm or more, it is preferable that the number of protrusions of 100 nm or more of the support is 10.0 / 900 μm ² or less.
(5) The coating method is more non-magnetic than the simultaneous multi-layer method in which a magnetic layer is formed on a lower layer (hereinafter, also simply referred to as “lower layer” or “nonmagnetic layer”) in a wet state. It is easier to control the protrusions by using the sequential multi-layer method in which the magnetic underlayer is dried and then the magnetic layer is formed thereon. In the case of simultaneous multi-layering, part of the non-magnetic powder in the magnetic layer tends to be buried in the wet lower layer, whereas in the successive multi-layering, the lower layer is solidified, so the non-magnetic powder is contained in the magnetic layer. It is because it is easy to arrange uniformly.
(6) The magnetic recording medium is preferably subjected to a surface cutting treatment with a polishing tape. In particular, when a diamond or alumina polishing tape is used, protrusions of 30 nm or more can be easily removed.

There are the following methods for controlling the depth and number of recesses on the surface of the magnetic layer of the magnetic recording medium of the present invention.
(1) If a minute foreign matter is mixed between webs in the manufacturing process and stored in a roll state, a copy is generated between adjacent webs, resulting in a dent. The web is preferably handled in a class 100 or less clean process.
(2) It is preferable that the elastic modulus of the magnetic layer is 9.8 GPa (1000 Kg / mm ² ) or more because the influence of image transfer between adjacent webs is reduced. Specifically, it is preferable to use a binder having a high glass transition point (Tg), a functional group such as an OH group, and a high reactivity with the curing agent.
(3) The thickness variation of the support is preferably 3% or less, more preferably 1% or less. When the thickness variation of the support becomes larger than 3%, a high pressure is locally generated between the webs while the web is wound, and a dent due to the projection of the projection is likely to occur.
(4) The variation in the thickness of the coating layer is preferably 10% or less, more preferably 3% or less. When the variation in the thickness of the coating layer is greater than 10%, a high pressure is locally generated between the webs while the web is wound, and a dent due to projection of the projection is likely to occur.
(5) It is preferable that the tension at the time of winding the web is lowered within a range where no turbulence is generated, so that the transfer between the webs hardly occurs.
The method for controlling the protrusions and the recesses is not limited to the above.

Next, the layer structure of the magnetic recording medium of the present invention will be described.
[Magnetic layer]
<Ferromagnetic powder>
The ferromagnetic powder used in the magnetic layer is not particularly limited, but ferromagnetic metal powder or hexagonal ferrite powder is preferable, and hexagonal ferrite powder is particularly preferable.

  The ferromagnetic metal powder is not particularly limited as long as it has α-Fe as a main component (including an alloy), but these ferromagnetic powders include Al, Si, S, Ca, Ti, V, Cr, Cu, Y, Mo, Rh, Pd, Ag, Sn, Ba, Ta, W, Au, Bi, La, Ce, Pr, Nd, P, Co, Mn, Zn, Ni, Sr, It may contain atoms such as B. It is preferable that at least one of Al, Si, Ca, Y, Ba, La, Nd, Co, Ni, and B is included in addition to α-Fe, and it is particularly preferable that Co, Al, Y, and Nd are included. More specifically, it is preferable that Co is contained in an amount of 10 to 50 atom%, Al is contained in an amount of 2 to 20 atom%, and Y and Nd are contained in an amount of 3 to 20 atom%.

The ferromagnetic metal powder used in the present invention uses a magnetic material excellent in high output, high dispersibility, and orientation in order to maximize the suitability of the high density region. That is, a ferromagnetic metal powder that is very fine particles and can achieve high output, in particular, has an average major axis length of 30 to 65 nm, a crystallite size of 80 to 140 mm, further contains Co, and contains Al as a sintering inhibitor. High output and high durability can be achieved by the ferromagnetic metal powder containing Y and Y compound. In addition, the powder size distribution must be excellent, the long axis length variation coefficient (100 × standard deviation of long axis length / average long axis length) is 0 to 30%, and the average needle ratio is 3.5 to Preferably, the coercive force is 7.5 to 223 kA / m, the saturation magnetization is 85 to 125 A · m ² / kg, and the specific surface area (S _BET ) by the BET method is 45 to 120 m ² / g. Can be obtained by JP-A-9-22522, JP-A-9-106535, JP-A-6-340426, JP-A-11-1001303 and combinations thereof.
In order to achieve high-density recording, the ferromagnetic powder preferably has a high coercive force, and preferably 143 to 223 kA / m, depending on the performance of the recording head used. As the coercive force increases, signal overwriting becomes a problem. Since the coercive force of the ferromagnetic metal powder is mainly derived from shape anisotropy, it is preferable that the coefficient of variation in shape is small.

  Examples of the hexagonal ferrite powder include barium ferrite, strontium ferrite, lead ferrite, calcium ferrite, and substitution products thereof such as Co. Specific examples include magnetoplumbite type barium ferrite and strontium ferrite, magnetoplumbite type ferrite whose particle surface is coated with spinel, and magnetoplumbite type barium ferrite and strontium ferrite partially containing a spinel phase. . Other than the predetermined atoms, Al, Si, S, Sc, Ti, V, Cr, Cu, Y, Mo, Rh, Pd, Ag, Sn, Sb, Te, Ba, Ta, W, Re, Au, Hg, It may contain atoms such as Pb, Bi, La, Ce, Pr, Nd, P, Co, Mn, Zn, Ni, Sr, B, Ge, and Nb. In general, an element added with an element such as Co—Zn, Co—Ti, Co—Ti—Zr, Co—Ti—Zn, Ni—Ti—Zn, Nb—Zn—Co, Sb—Zn—Co, Nb—Zn, etc. Can be used. Some raw materials and manufacturing methods contain specific impurities.

The particle size is an arithmetic average of hexagonal plate diameters (hereinafter referred to as average plate diameter) of 10 to 55 nm, preferably 15 to 35 nm, and particularly preferably 18 to 30 nm. When the average plate diameter exceeds 55 nm, noise increases, and when it is less than 10 nm, stable magnetization cannot be expected due to thermal fluctuation. The average plate ratio [arithmetic average of (plate diameter / plate thickness)] is preferably 3-7. Preferably it is 3-5. When the average plate ratio is small, the filling property in the magnetic layer is preferably high, but sufficient orientation is difficult to obtain. If the plate ratio is 7 or less, noise will not increase due to stacking between particles.
The specific surface area (S _BET ) of the particles in the above particle size range by the BET method is 40 to 100 m ² / g. The specific surface area is generally signified as an arithmetic calculation value from the particle plate diameter and plate thickness. The distribution of the particle plate diameter and plate thickness is generally preferably as narrow as possible. Digitization can be compared by randomly measuring 500 particles from a particle TEM photograph. In many cases, the distribution is not a normal distribution, but when calculated and expressed as a standard deviation with respect to the average size, σ / average size = 0.1 to 2.0. In order to sharpen the particle size distribution, the particle generation reaction system is made as uniform as possible, and the generated particles are subjected to a distribution improvement process. For example, a method of selectively dissolving ultrafine particles in an acid solution is also known.
The hexagonal ferrite particles can be prepared in the range of a coercive force Hc measured with a magnetic material of 80 kA / m (1000 Oe) to 319 kA / m (4000 Oe). A higher coercive force Hc is advantageous for high-density recording, but is limited by the capacity of the recording head. In the present invention, the coercive force Hc is about 80 kA / m (1000 Oe) to 319 kA / m (4000 Oe), preferably 143 to 239 kA / m (1800 to 3000 Oe). The coercive force Hc can be controlled by the particle size (plate diameter / plate thickness), the type and amount of the contained element, the substitution site of the element, the particle generation reaction conditions, and the like. The saturation magnetization σs is 40 to 70 A · m ² / kg. The saturation magnetization σs is preferably higher, but tends to be smaller as the particles become finer. In order to improve the saturation magnetization σs, it is well known to combine spinel ferrite with magnetoplumbite ferrite and to select the type and amount of elements contained. It is also possible to use W-type hexagonal ferrite.

When dispersing hexagonal ferrite, the surface of the magnetic particles is also treated with a substance suitable for the dispersion medium and polymer. As the surface treatment material, an inorganic compound or an organic compound is used. Typical examples of the main compound include compounds such as Si, Al, and P, various silane coupling agents, and various titanium coupling agents. The amount is 0.1 to 10% with respect to the magnetic substance. The pH of the magnetic material is also important for dispersion. Usually, about 4 to 12 has optimum values depending on the dispersion medium and polymer, but about 6 to 11 is selected from the chemical stability and storage stability of the magnetic recording medium. Water contained in the magnetic material also affects the dispersion. Although there is an optimum value depending on the dispersion medium and the polymer, 0.01 to 2.0% is usually selected.
The hexagonal ferrite can be manufactured by (1) mixing a metal oxide replacing barium oxide, iron oxide, iron and boron oxide as a glass forming substance so as to have a desired ferrite composition, and then melting and quenching. Glass crystallization method to obtain barium ferrite crystal powder by making it amorphous and then reheating treatment, (2) neutralizing barium ferrite composition metal salt solution with alkali, Hydrothermal reaction method to obtain barium ferrite crystal powder by liquid phase heating at 100 ° C or higher after removal, washing, drying and grinding, (3) neutralizing barium ferrite composition metal salt solution with alkali, by-product There is a coprecipitation method in which barium ferrite crystal powder is obtained by drying, treating at 1100 ° C. or less, and pulverizing, but the production method is not limited.

<Binder>
The binder used in the magnetic layer of the present invention is not particularly limited as long as it is usually used, and any of thermoplastic resins, thermosetting resins, reactive resins, radiation curable resins, and mixtures thereof. Can also be used.
The thermoplastic resin has a glass transition temperature of −100 to 150 ° C., a number average molecular weight of 1,000 to 200,000, preferably 10,000 to 100,000, and a degree of polymerization of about 50 to 1,000. In order to satisfy the requirements of the present invention for the depression on the surface of the magnetic layer, the glass transition point is preferably 0 ° C. or higher, more preferably 50 ° C. or higher. The elongation at break is preferably 100 to 2000%, the break stress is preferably 0.49 to 98 MPa (0.05 to 10 kg / mm ² ), and the yield point is preferably 0.49 to 98 MPa (0.05 to 10 kg / mm ² ).

Specific examples of such thermoplastic resins include, for example, vinyl chloride, vinyl acetate, vinyl alcohol, maleic acid, acrylic acid, acrylic ester, vinylidene chloride, acrylonitrile, methacrylic acid, methacrylic ester, styrene, butadiene, ethylene, There are polymers or copolymers containing vinyl butyral, vinyl acetal, vinyl ether and the like as structural units, polyurethane resins, and various rubber resins. Thermosetting resins or reactive resins include phenolic resins, epoxy resins, polyurethane curable resins, urea resins, melamine resins, alkyd resins, acrylic reactive resins, formaldehyde resins, silicone resins, epoxy-polyamide resins, polyesters. Examples thereof include a mixture of resin and isocyanate prepolymer, a mixture of polyester polyol and polyisocyanate, a mixture of polyurethane and polyisocyanate, and the like. These resins are described in detail in “Plastic Handbook” published by Asakura Shoten. It is also possible to use a known electron beam curable resin for each layer. These examples and their production methods are described in detail in JP-A No. 62-256219. The above resins can be used alone or in combination, but are preferably selected from vinyl chloride resin, vinyl chloride vinyl acetate copolymer, vinyl chloride vinyl acetate vinyl alcohol copolymer, vinyl chloride vinyl acetate vinyl maleic anhydride copolymer. A combination of at least one kind and a polyurethane resin, or a combination of these with a polyisocyanate may be mentioned.
As the structure of the polyurethane resin, known structures such as polyester polyurethane, polyether polyurethane, polyether polyester polyurethane, polycarbonate polyurethane, polyester polycarbonate polyurethane, and polycaprolactone polyurethane can be used. For all binding agents indicated herein, if necessary in order to obtain more excellent dispersibility and _{durability, -COOM, -SO 3 M, -OSO} 3 M, -P = O (OM) 2, - O—P═O (OM) ₂ (wherein M is a hydrogen atom or an alkali metal), —OH, —NR ₂ , —N ⁺ R ₃ (R is a hydrocarbon group), epoxy group, —SH, —CN It is preferable to use one in which at least one polar group selected from the above is introduced by copolymerization or addition reaction. The amount of such a polar group is 10 ⁻¹ to 10 ⁻⁸ mol / g, preferably 10 ⁻² to 10 ⁻⁶ mol / g.

  Specific examples of these binders used in the present invention include VAGH, VYHH, VMCH, VAGF, VAGD, VROH, VYES, VYNC, VMCC, XYHL, XYSG, PKHH, PKHJ, PKHC, Union Carbide, PKFE, Nissin Chemical Industry MPR-TA, MPR-TA5, MPR-TAL, MPR-TSN, MPR-TMF, MPR-TS, MPR-TM, MPR-TAO, Denki Kagaku 1000W, DX80, DX81, DX82, DX83, 100FD, Nippon Zeon's MR-104, MR-105, MR110, MR100, MR555, 400X-110A, Nippon Polyurethane's Nipporan N2301, N2302, N2304, Dainippon Ink, Pandex T-5105, T-R3080, T- 201, Vernock D-400, D-210-80, Crisbon 6109, 7209, Byron UR8200, UR8300, UR-8700, RV530, RV280, Toyobo Co., Ltd. Daiferamin 4020, 5020, 5100, 5300, 9020 9022, 7020, MX5004 manufactured by Mitsubishi Kasei Co., Ltd., Sanprene SP-150 manufactured by Sanyo Kasei Co., Ltd., Saran F310, F210 manufactured by Asahi Kasei Co., Ltd. and the like.

The binder used in the magnetic layer of the present invention is used in the range of 5 to 50 parts by mass, preferably in the range of 10 to 30 parts by mass with respect to 100 parts by mass of the ferromagnetic powder. In the case of using a vinyl chloride resin, 5 to 30 parts by mass, in the case of using a polyurethane resin combination, 2 to 20 parts by mass, polyisocyanate is preferably used in a combination of 2 to 20 parts by mass. When head corrosion occurs due to a small amount of dechlorination, it is possible to use only polyurethane or only polyurethane and isocyanate.
The magnetic recording medium of the present invention comprises two or more layers, a lower layer and a magnetic layer. Therefore, the amount of the binder, the amount of vinyl chloride resin, polyurethane resin, polyisocyanate or other resin in the binder, the molecular weight of each resin forming the magnetic layer, the polar group amount, or the resin described above Of course, it is possible to change the physical characteristics of each layer between the non-magnetic layer and each magnetic layer as needed, and rather, it should be optimized for each layer, and a known technique relating to a multilayer magnetic layer can be applied. For example, when changing the binder amount in each layer, it is effective to increase the binder amount of the magnetic layer in order to reduce scratches on the surface of the magnetic layer, and in order to improve the head touch to the head, the nonmagnetic layer The amount of binder can be increased to give flexibility.

Polyisocyanates that can be used in the present invention include tolylene diisocyanate, 4,4′-diphenylmethane diisocyanate, hexamethylene diisocyanate, xylylene diisocyanate, naphthylene-1,5-diisocyanate, o-toluidine diisocyanate, isophorone diisocyanate, triphenyl. Examples thereof include isocyanates such as methane triisocyanate, products of these isocyanates and polyalcohols, and polyisocyanates formed by condensation of isocyanates.
Commercially available product names of these isocyanates include Coronate L, Coronate HL, Coronate 2030, Coronate 2031, Millionate MR, Millionate MTL, Takenate Pharmaceuticals Takenate D-102, Takenate D-110N, Takenate D-200, Takenate D-202, Sumitomo Bayer's Death Module L, Death Module IL, Death Module N, Death Module HL, etc. Each layer can be used in the above combination.

<Abrasive>
Further, the magnetic layer can contain an abrasive in order to increase the mechanical strength of the magnetic layer and prevent clogging of the magnetic head. As the abrasive used in the magnetic layer of the present invention, known ones can be used, but diamond particles or alumina particles are preferred. As diamond, natural diamond is expensive, and artificial diamond is usually used. Diamond production methods include a method in which graphite and iron, Co, Ni, etc. are used for production under high temperature and high pressure, a method for static synthesis in which graphite or furan resin carbon is reacted under high temperature and pressure, and a dynamic synthesis method. There is a gas phase synthesis method. The present invention does not choose a method for producing diamond. Industrially, diamond used for cutting and polishing can be used secondarily by using impurities that have been washed by differentiating impurities. As a method of classifying diamond particles, there is a method of using a special mesh filter using centrifugal force from a dispersion. As the alumina particles, α-alumina, β-alumina or the like having an α conversion rate of 90% or more is used.
Examples of abrasives other than diamond or alumina that can be used in the present invention include silicon carbide, chromium oxide, cerium oxide, α-iron oxide, corundum, silicon nitride, silicon carbide, titanium carbide, titanium oxide, silicon dioxide, and nitride. Known materials having a Mohs hardness of 6 or more, such as boron, are mainly used. Those having different particle sizes can be combined as required.

The average particle size of the abrasive used in the present invention is 0.03 to 1.0 μm, preferably 0.05 to 0.5 μm, and more preferably 0.07 to 0.2 μm. A compounding quantity is 0.1 to 30 weight% with respect to ferromagnetic powder, Preferably it is 0.5 to 5 weight%. In order for the protrusions on the surface of the magnetic layer to satisfy the requirements of the present invention, the particle size distribution of the abrasive is preferably narrow, and the standard deviation of the distribution is 30% or less, preferably 20% or less, more preferably 10 % Or less. The tap density is preferably 0.3 to 2 g / cc, the water content is preferably 0.1 to 5% by weight, the pH is 2 to 11, and the specific surface area is preferably 1 to 30 m 2 / g. The shape of the abrasive used in the present invention may be any of a needle shape, a spherical shape, and a dice shape, but those having a corner in a part of the shape are preferable because of high polishing properties.
As the abrasive, specifically, AKP-12, AKP-15, AKP-20, AKP-30, AKP-50, HIT-20, HIT-30, HIT-55, HIT-60, manufactured by Sumitomo Chemical Co., Ltd. HIT-70, HIT-80, HIT-100, Reynolds, ERC-DBM, HP-DBM, HPS-DBM, Fujimi Abrasives, WA10000, Uemura Kogyo, UB20, Nippon Chemical Industries, G-5, Chromex U2, Chromex U1, Toda Kogyo Co., Ltd., TF100, TF140, Ibiden Co., Beta Random Ultra Fine, Showa Mining Co., Ltd., B-3, Tomei Dia Co., Ltd. MD-150, MD-100 , Etc.
These abrasives can be added to the lower layer as needed. By adding to the lower layer, the surface shape can be controlled, and the protruding state of the abrasive can be controlled. The particle size and amount of the abrasive used in combination with or in the lower layer of these magnetic layers should of course be set to optimum values.

<Other additives>
If necessary, the magnetic layer of the present invention may contain a dispersant such as a surfactant, a lubricant such as a higher fatty acid, a fatty acid ester, or silicone oil, and various other additives such as an antistatic agent and a plasticizer. Also good.
In particular, in the present invention, the magnetic layer preferably contains at least a fatty acid and a fatty acid ester, and the fatty acid residues of the fatty acid and the fatty acid ester are preferably the same. Examples of the fatty acid include monobasic fatty acids having 10 to 24 carbon atoms (which may contain an unsaturated bond or may be branched), and examples of fatty acid esters include monobasic fatty acids having 10 to 24 carbon atoms ( Any one of monovalent, divalent, trivalent, tetravalent, pentavalent, and hexavalent alcohols having 2 to 12 carbon atoms (unsaturated bond). Or a fatty acid ester of a monoalkyl ether of an alkylene oxide polymer. Additives other than these fatty acids and fatty acid esters include molybdenum disulfide, tungsten disulfide, graphite, boron nitride, fluorinated graphite, silicone oil, silicone with polar groups, fatty acid-modified silicone, fluorine-containing silicone, fluorine-containing alcohol, fluorine Containing ester, polyolefin, polyglycol, alkyl phosphate ester and alkali metal salt thereof, alkyl sulfate ester and alkali metal salt thereof, polyphenyl ether, phenylphosphonic acid, α-naphthyl phosphoric acid, phenyl phosphoric acid, diphenyl phosphoric acid, p-ethylbenzenephosphonic acid, Phenylphosphinic acid, aminoquinones, various silane coupling agents, titanium coupling agents, fluorine-containing alkyl sulfates and alkali metal salts thereof, monobasic fatty acids having 10 to 24 carbon atoms ( Metal salts (including Li, Na, K, Cu, etc.) or monovalent, divalent, trivalent, tetravalent, pentavalent carbon atoms having 12 to 22 carbon atoms, which may contain a saturated bond or may be branched. , Hexahydric alcohols (which may contain an unsaturated bond or may be branched), alkoxy alcohols having 12 to 22 carbon atoms, fatty acid amides having 8 to 22 carbon atoms, aliphatic amines having 8 to 22 carbon atoms Etc. can be used.

  Specific examples of these fatty acids include capric acid, caprylic acid, lauric acid, myristic acid, palmitic acid, stearic acid, behenic acid, oleic acid, elaidic acid, linoleic acid, linolenic acid, and isostearic acid. For fatty acid esters, butyl stearate, octyl stearate, amyl stearate, isooctyl stearate, butyl myristate, octyl myristate, butoxyethyl stearate, butoxydiethyl stearate, 2-ethylhexyl stearate, 2-octyldodecyl palmi Tate, 2-hexyldecyl palmitate, isohexadecyl stearate, oleyl oleate, dodecyl stearate, tridecyl stearate, oleyl erucate, neopentyl glycol didecanoate, ethylene glycol dioleyl, oleyl alcohol for alcohols, Examples include stearyl alcohol and lauryl alcohol. In addition, nonionic surfactants such as alkylene oxide, glycerin, glycidol, alkylphenol ethylene oxide adducts, cyclic amines, ester amides, quaternary ammonium salts, hydantoin derivatives, heterocyclics, phosphonium or sulfoniums, etc. Cationic surfactants, anionic surfactants containing acidic groups such as carboxylic acid, sulfonic acid, phosphoric acid, sulfate ester group, phosphate ester group, amino acids, aminosulfonic acids, amino alcohol sulfuric acid or phosphate esters, alkyl bedda An amphoteric surfactant such as in-type can also be used. These surfactants are described in detail in “Surfactant Handbook” (published by Sangyo Tosho Co., Ltd.). These lubricants, antistatic agents, and the like are not necessarily 100% pure, and may contain impurities such as isomers, unreacted materials, side reaction products, decomposed products, and oxides in addition to the main components. These impurities are preferably 30% or less, more preferably 10% or less.

The lubricants and surfactants used in the present invention have different physical actions, and the type, amount and combination ratio of lubricants that produce a synergistic effect are optimally determined according to the purpose. It should be. For example, leaching to the surface is controlled by using fatty acids with different melting points in the lower layer and the magnetic layer, leaching to the surface is controlled by using esters having different boiling points, melting points, and polarities, and the amount of surfactant. It may be possible to improve the stability of the coating by adjusting the amount of oil, or to increase the lubricating effect by increasing the amount of lubricant added in the intermediate layer.Of course, it is not limited to the examples shown here. Absent. Generally, the total amount of the lubricant is 0.1 to 50 parts by mass with respect to 100 parts by mass of the ferromagnetic powder, and 2 to 25 parts by mass is preferably selected.
All or part of the additives used in the present invention may be added in any step of the method for producing the coating material for the magnetic layer. For example, when mixing with a magnetic material before the kneading step, adding at a kneading step with a magnetic material, a binder and a solvent, adding at a dispersing step, adding after dispersing, adding just before coating, etc. . Further, after applying the magnetic layer according to the purpose, the purpose may be achieved by applying a part or all of the additive by simultaneous or sequential application. Further, depending on the purpose, a lubricant can be applied to the surface of the magnetic layer after calendering or after completion of the slit.

  The coating for the magnetic layer is prepared by adding an organic solvent to the above components. There is no restriction | limiting in particular in the organic solvent to be used, A well-known thing can be used. For example, ketones such as acetone, methyl ethyl ketone, methyl isobutyl ketone, diisobutyl ketone, cyclohexanone, isophorone, tetrahydrofuran, etc., alcohols such as methanol, ethanol, propanol, butanol, isobutyl alcohol, isopropyl alcohol, methylcyclohexanol, Esters such as methyl acetate, butyl acetate, isobutyl acetate, isopropyl acetate, ethyl lactate, glycol acetate, glycol ethers such as glycol dimethyl ether, glycol monoethyl ether, dioxane, fragrances such as benzene, toluene, xylene, cresol, chlorobenzene Group hydrocarbons, methylene chloride, ethylene chloride, carbon tetrachloride, chloroform, ethylene chlorohydrin, di Chlorinated hydrocarbons such as Rorubenzen, N, N- dimethylformamide, may be used hexane.

  These organic solvents are not necessarily 100% pure, and may contain impurities such as isomers, unreacted materials, side reaction products, decomposition products, oxides, and moisture in addition to the main components. These impurities are preferably 30% or less, more preferably 10% or less. The organic solvent used in the present invention is preferably the same in the magnetic layer and the nonmagnetic layer. The amount added may be changed. It is important to improve the coating stability by using a solvent with high surface tension for the non-magnetic layer. is there. In order to improve the dispersibility of both layers, it is preferable that the polarity is somewhat strong, and it is preferable that 50% or more of a solvent having a dielectric constant of 15 or more is included in the solvent composition of both layers. Moreover, it is preferable that the dissolution parameters of the organic solvent used for both layers are 8-11.

[Underlayer]
The lower layer in the magnetic recording medium of the present invention is substantially nonmagnetic. In the present specification, “substantially non-magnetic lower layer” means that it may have magnetism to the extent that it does not participate in recording, and is synonymous with the non-magnetic layer.
The lower layer preferably contains at least carbon black and a binder made of a radiation curable resin or a thermosetting resin.
<Carbon black>
When carbon black is included in the lower layer, the lubricant can be retained, and the amount of lubricant on the surface of the magnetic layer can be easily adjusted to a desired range. In particular, when the thickness of the magnetic layer is as thin as 0.3 μm or less, it is difficult to contain a sufficient amount of lubricant with the magnetic layer alone, and therefore, the lower layer carbon black is important. In addition, the lower carbon black has the effect of lowering the surface electrical resistance of the magnetic layer, and also has the effect of preventing electrostatic breakdown of the MR head.
Examples of the carbon black that can be used for the lower layer include rubber furnaces, rubber thermals, color blacks, and acetylene blacks. By using these carbon blacks in combination, the following characteristics can be optimized and desired effects can be obtained.
The specific surface area of the underlying carbon black is 100 to 500 m ² / g, preferably from 150 to 400 m ² / g. The DBP oil absorption is 20 to 400 ml / 100 g, preferably 30 to 400 ml / 100 g. The average particle diameter of carbon black is 5 to 80 nm, preferably 10 to 50 nm, and more preferably 10 to 40 nm. The pH of the carbon black is preferably 2 to 10, the water content is 0.1 to 10%, and the tap density is 0.1 to 1 g / ml.

  Specific examples of carbon black used in the present invention include BLACKPEARLS 2000, 1300, 1000, 900, 800, 880, 700, VULCAN XC-72, manufactured by Cabot Corporation, # 3050B, # 3150B, # 3150B manufactured by Mitsubishi Kasei Corporation. 3250B, # 3750B, # 3950B, # 950, # 650B, # 970B, # 850B, MA-600, MA-230, # 4000, # 4010, CONDUCTEX SC manufactured by Colombian Carbon Co., Raven 8800, 8000, 7000, 5750, 5250, 3500, 2100, 2000, 1800, 1500, 1255, 1250, Ketjen Black EC manufactured by Akzo, and the like. Carbon black may be surface-treated with a dispersant or the like, or may be used after being grafted with a resin, or may be obtained by graphitizing a part of the surface. Moreover, before adding carbon black to a coating material, you may disperse | distribute with a binder beforehand. These carbon blacks can be used in a range not exceeding 50% by mass relative to the following inorganic powder and not exceeding 40% of the total mass of the nonmagnetic layer. These carbon blacks can be used alone or in combination. The carbon black that can be used in the present invention can be referred to, for example, “Carbon Black Handbook” (edited by Carbon Black Association).

<Inorganic powder>
Various inorganic powders other than carbon black can be used for the lower layer. The inorganic powder that can be used for the lower layer can be selected from inorganic compounds such as metal oxides, metal carbonates, metal sulfates, metal nitrides, metal carbides, and metal sulfides. Examples of inorganic compounds include α-alumina, β-alumina, γ-alumina, θ-alumina, silicon carbide, chromium oxide, cerium oxide, α-iron oxide, hematite, goethite, corundum, and nitridation with an α conversion rate of 90% or more. Silicon, titanium carbide, titanium oxide, silicon dioxide, tin oxide, magnesium oxide, tungsten oxide, zirconium oxide, boron nitride, zinc oxide, calcium carbonate, calcium sulfate, barium sulfate, molybdenum disulfide, etc. are used alone or in combination. The Particularly preferred are titanium dioxide, zinc oxide, iron oxide and barium sulfate because of their small particle size distribution and many means for imparting functions, and more preferred are titanium dioxide and α-iron oxide. Of these, spherical ultrafine iron oxide is preferably used. By using spherical ultrafine iron oxide, high dispersibility can be obtained, and the particle filling rate in the lower layer can be increased. Further, the surface property of the nonmagnetic layer itself is improved, and consequently the surface property of the magnetic layer is improved, and the electromagnetic conversion characteristics can be improved.

The particle size of the inorganic powder is preferably 0.005 to 2 μm, but if necessary, inorganic powders having different particle sizes can be combined, or even a single inorganic powder can have the same effect by widening the particle size distribution. . Particularly preferred is a particle size of the inorganic powder of 0.01 to 0.2 μm. In particular, when the inorganic powder is a granular metal oxide, the average particle size is preferably 0.08 μm or less, and when the inorganic powder is an acicular metal oxide, the average major axis length is preferably 0.3 μm or less, and 0.2 μm or less. Is more preferable. The tap density is 0.05 to 2 g / ml, preferably 0.2 to 1.5 g / ml. The moisture content of the inorganic powder is 0.1 to 5% by mass, preferably 0.2 to 3% by mass, and more preferably 0.3 to 1.5% by mass. The pH of the inorganic powder is 2 to 11, and the pH is particularly preferably between 5.5 and 10. The specific surface area of the inorganic powder is 1 to 100 m ² / g, preferably 5 to 80 m ² / g, more preferably 10 to 70 m ² / g. The crystallite size of the inorganic powder is preferably 0.004 to 1 μm, and more preferably 0.04 to 0.1 μm. The oil absorption using DBP (dibutyl phthalate) is 5 to 100 ml / 100 g, preferably 10 to 80 ml / 100 g, and more preferably 20 to 60 ml / 100 g. The specific gravity is 1 to 12, preferably 3 to 6. The shape may be any of a needle shape, a spherical shape, a polyhedron shape, and a plate shape. The Mohs hardness is preferably 4 or more and 10 or less. The SA (stearic acid) adsorption amount of the inorganic powder is 1 to 20 μmol / m ² , preferably ² to 15 μmol / m ² , more preferably 3 to 8 μmol / m ² . The pH is preferably between 3-6. The surface of these inorganic powders is preferably surface-treated and Al ₂ O ₃ , SiO ₂ , TiO ₂ , ZrO ₂ , SnO ₂ , Sb ₂ O ₃ , ZnO, and Y ₂ O ₃ are present. Particularly preferred for dispersibility are Al ₂ O ₃ , SiO ₂ , TiO ₂ , and ZrO ₂ , but more preferred are Al ₂ O ₃ , SiO ₂ , and ZrO ₂ . These may be used in combination or may be used alone. Further, a surface-treated layer co-precipitated according to the purpose may be used, or a method of first treating with alumina and then treating the surface layer with silica, or vice versa. The surface treatment layer may be a porous layer depending on the purpose, but it is generally preferable that the surface treatment layer is homogeneous and dense.

  Specific examples of the inorganic powder used in the lower layer of the present invention include Showa Denko Nanotite, Sumitomo Chemical HIT-100, ZA-G1, Toda Kogyo α-Hematite DPN-250, DPN-250BX, DPN-245. , DPN-270BX, DPN-500BX, DBN-SA1, DBN-SA3, Ishihara Sangyo Titanium oxide TTO-51B, TTO-55A, TTO-55B, TTO-55C, TTO-55S, TTO-55D, SN-100, α-Hematite E270, E271, E300, E303, Titanium Industry Titanium Oxide STT-4D, STT-30D, STT-30, STT-65C, α-Hematite α-40, Teica MT-100S, MT-100T, MT-150W MT-500B, MT-600B, MT-100F, MT-500H , FINEX-25, BF-1, BF-10, BF-20, ST-M, Dowa Mining DEFIC-Y, DEFIC-R, Nippon Aerosil AS2BM, TiO2P25, Ube Industries 100A, 500A Can be used. Particularly preferred inorganic powders are titanium dioxide and α-iron oxide.

[Support]
The material used as the support of the magnetic recording medium of the present invention, preferably the non-magnetic support, is not particularly limited, and can be selected from various flexible materials and various rigid materials according to the purpose. For example, flexible materials include polyesters such as polyethylene terephthalate and polyethylene naphthalate, polyolefins such as polypropylene, cellulose triacetate, polyamide (including aromatic polyamide such as aramid), polyimide, polyamideimide, polysulfone, polycarbonate And various resins such as polybenzoxazole. Moreover, in order to change the surface roughness of a magnetic surface and a base surface as needed, the laminated type support body as shown in Unexamined-Japanese-Patent No. 3-224127 can also be used. These supports may be subjected in advance to corona discharge treatment, plasma treatment, easy adhesion treatment, heat treatment, dust removal treatment and the like.

The roughness of the surface of the support can be freely controlled by the size and amount of filler added to the support as necessary. Examples of these fillers include oxides and carbonates of Ca, Si, Ti, Al, and the like, as well as fine organic resin powders such as acrylic, and SiO ₂ and Al ₂ O ₃ are preferable. In order to achieve the object of the present invention, it is preferable to reduce coarse protrusions due to aggregation of these fillers by sufficiently filtering. In order to reduce the number of protrusions of 30 nm or more and to provide a predetermined number of protrusions of 10 nm or more, it is preferable to use a filler having a particle size of 200 nm or less, and more preferable to use a filler of 100 nm or less.
The surface roughness of the support is 0.5 nm or more and 20 nm or less, and more preferably 1 nm or more and 10 nm or less in terms of the center plane average surface roughness Ra.
The F-5 value of the support is preferably 49 to 490 MPa (5 to 50 kg / mm ² ), and the heat shrinkage rate of the support at 100 ° C. for 30 minutes is preferably 3% or less, more preferably 1.5. %, And the heat shrinkage at 80 ° C. for 30 minutes is preferably 1% or less, more preferably 0.5% or less. The breaking strength is preferably 49 to 980 MPa (5 to 100 kg / mm ² ), and the elastic modulus is preferably 0.98 to 19.6 GPa (100 to 2000 Kg / mm ² ). The temperature expansion coefficient is 10 ⁻⁴ to 10 ⁻⁸ / ° C., preferably 10 ⁻⁵ to 10 ⁻⁶ / ° C. The humidity expansion coefficient is 10 ⁻⁴ / RH% or less, preferably 10 ⁻⁵ / RH% or less. These thermal characteristics, dimensional characteristics, and mechanical strength characteristics are preferably substantially equal with a difference within 10% in each in-plane direction of the support. The thickness of the support is 10 to 150 μm, preferably 20 to 80 μm. The thickness variation of the support is preferably 3% or less, and more preferably 1% or less. When the thickness variation of the support becomes larger than 3%, a high pressure is locally generated between the webs while the web is wound, and a dent due to reflection is likely to occur on the protrusion.

[Layer structure]
The thickness of the magnetic layer in the present invention is more preferably 0.02 to 0.30 μm, and further preferably 0.04 to 0.20 μm. If the magnetic layer exceeds 0.30 μm, the resolution and overwrite characteristics deteriorate, and if it is thinner than 0.02 μm, it is difficult to form a uniform film. The thickness variation of the magnetic layer is preferably 10% or less, more preferably 3% or less.
Further, the magnetic layer may be separated into two or more layers having different magnetic characteristics, and a configuration related to a known multilayer magnetic layer can be applied. When two or more magnetic layers are provided, the thickness of the magnetic layer is the sum of them.
The thickness of the lower layer in the present invention is usually from 0.1 to 2.5 μm, preferably from 0.3 to 2.3 μm. When the thickness of the lower layer is in the range of 0.1 to 2.5 μm, the influence of the surface property of the support can be suppressed, and the lower layer surface can be smoothed. In addition, since the surface state of the lower layer becomes good, the surface of the magnetic layer can be smoothed and the electromagnetic conversion characteristics can be improved.
The thickness variation of the coating layer composed of the lower layer and the magnetic layer is preferably 10% or less, more preferably 3% or less. When the variation in the thickness of the coating layer is greater than 10%, a high pressure is locally generated between the webs while the web is wound, and a dent due to reflection is likely to occur on the protrusion.
The fluctuation of the interface between the magnetic layer and the lower layer is preferably 10% or less, more preferably 3% or less.
In the magnetic recording medium of the present invention, an undercoat layer may be provided in order to improve adhesion between the support and the lower layer or the magnetic layer. The thickness of the undercoat layer is 0.01 to 0.5 μm, preferably 0.02 to 0.5 μm. The magnetic recording medium of the present invention may be a double-sided magnetic recording medium having a lower layer (nonmagnetic layer) and a magnetic layer on both sides of a support, or may be provided only on one side.

[Production method]
The magnetic recording medium of the present invention can be produced by preparing a coating material for a lower layer and a coating material for a magnetic layer, respectively, using the above materials, and applying them in this order on the support.
Each of the coating materials for the lower layer and the magnetic layer is produced by performing a mixing step, a viscosity adjusting step, and a filtration step at least before and after the kneading step, the dispersing step, and these steps. Each process may be divided into two or more stages.
For kneading and dispersing the paint, a conventionally known production technique can be used for some or all of the steps. In the kneading step, it is preferable to use a material having a strong kneading force such as a continuous kneader or a pressure kneader. When a continuous kneader or a pressure kneader is used, it is kneaded with all or part of a ferromagnetic powder or nonmagnetic inorganic powder and a binder. The slurry temperature during kneading is preferably 50 ° C to 100 ° C.
The dispersion medium used for dispersion of the paint is not particularly limited, but it is desirable to use a dispersion medium having a high specific gravity, and ceramic media such as zirconia and titania are preferable. In order to satisfy the requirements of the present invention, the average particle size of the dispersion medium is preferably 0.5 mm or less, and more preferably 0.3 mm or less.
As the coating means, any of gravure coating, reverse coating, extrusion nozzle and the like may be used, but a method using a die nozzle coater is preferable.
After coating, while the coating layer is in a wet state, the orientation treatment is usually performed using a known method, and in the case of a magnetic tape, longitudinal orientation is common.
In the case of a magnetic disk, a sufficiently isotropic orientation may be obtained even without non-orientation without using an orientation device. However, it is known that cobalt magnets are alternately arranged obliquely and an alternating magnetic field is applied by a solenoid. It is preferable to use a random orientation apparatus. The isotropic orientation is generally preferably in-plane two-dimensional random, but it can also be made three-dimensional random with a vertical component. Further, circumferential orientation may be used using spin coating.
After coating the magnetic layer in this way, the coating film subjected to orientation treatment is usually dried by known drying and evaporation means such as hot air, far-infrared rays, an electric heater, a vacuum device, etc. provided inside the drying furnace.・ Fixed. The drying temperature may be appropriately selected in the range from room temperature to about 300 ° C. depending on the heat resistance, solvent type, concentration, etc. of the support, and a temperature gradient may be provided in the drying furnace. Furthermore, general air or an inert gas may be used as the gas atmosphere in the drying furnace.

After the magnetic layer is dried, a calendar process is performed as a surface smoothing process as necessary. As the calendering roll, a combination of a heat-resistant plastic roll such as epoxy, polyester, nylon, polyimide, polyamide, and polyimideamide and a metal roll (a combination of 3 to 7 stages) may be used. Moreover, it can also process with metal rolls. The treatment temperature is preferably 80 ° C. or higher, more preferably 85 ° C. or higher. The linear pressure is preferably 196 kN / m (200 kg / cm) or more, more preferably 245 kN / m (250 kg / cm) or more, and the treatment speed is in the range of 20 m / min to 900 m / min.
When applying a magnetic recording medium having a multi-layer structure in the present invention, a known simultaneous multi-layer coating system or a known sequential multi-layer coating system can be used. To achieve the object of the present invention, a sequential multi-layer coating system is used. It is preferable to use it.

[Physical properties]
The SFD and SFDr of the magnetic layer of the magnetic recording medium according to the present invention are preferably 0.7 or less. The squareness ratio is 0.5 to 0.7 for two-dimensional random, preferably 0.55 to 0.65, preferably 0.4 to 0.6 for three-dimensional random, and vertical for vertical alignment. It is 0.6 or more in the direction, preferably 0.7 or more, and 0.7 or more, preferably 0.8 or more when demagnetizing field correction is performed. The orientation ratio is preferably 85% or more for both two-dimensional random and three-dimensional random.
The friction coefficient with respect to the head of the magnetic recording medium of the present invention is 0.5 or less, preferably 0.3 or less in the temperature range of −10 ° C. to 40 ° C. and humidity 0% to 95%, and the surface resistivity is preferably the magnetic surface 10. ⁴ to 10 ¹² Ω / sq, and the charging position is preferably within −500V to + 500V. The elastic modulus at 0.5% elongation of the magnetic layer is preferably 0.98 to 19.6 GPa (100 to 2000 kg / mm ² ) in each in-plane direction, and the breaking strength is preferably 0.098 to 0.686 GPa (10 ˜70 kg / mm ² ), the elastic modulus of the magnetic recording medium is preferably 0.98 to 14.7 GPa (100 to 1500 kg / mm ² ) in each in-plane direction, and the residual elongation is preferably 0.5% or less, 100 ° C. The thermal shrinkage at any of the following temperatures is preferably 1% or less, more preferably 0.5% or less, and most preferably 0.1% or less. The glass transition temperature of the magnetic layer (the maximum point of the loss modulus of dynamic viscoelasticity measured at 110 Hz) is preferably 50 to 120 ° C, and that of the nonmagnetic layer is preferably 0 to 100 ° C. The loss elastic modulus is preferably in the range of 1 × 10 ³ to 8 × 10 ⁴ N / cm ² (1 × 10 ⁸ to 8 × 10 ⁹ dyne / cm ² ), and the loss tangent is 0.2 or less. Is preferred. If the loss tangent is too large, adhesion failure is likely to occur. It is preferable that these thermal characteristics and mechanical characteristics are within 10% in each in-plane direction of the magnetic recording medium and are substantially equal.
The residual solvent contained in the magnetic layer is preferably 100 mg / m ² or less, more preferably 10 mg / m ² or less. The porosity of the coating layer is preferably 30% by volume or less, and more preferably 20% by volume or less for both the lower layer and the magnetic layer. The porosity is preferably small in order to achieve high output, but it may be better to ensure a certain value depending on the purpose.

EXAMPLES Hereinafter, although an Example demonstrates this invention further more concretely, this invention is not limited to these Examples. In the examples, “part” means part by mass unless otherwise specified.
<Creation of paint>
Coating for magnetic layer 100 parts of barium ferrite magnetic powder Surface treatment: 6% by mass of Al ₂ O ₃ is present with respect to the whole particle,
Coercive force Hc: 199 kA / m (3000 Oe)
Average plate diameter: 0.021 μm
Average plate ratio: 3
Saturation magnetization σs: 50 A · m ² / kg (emu / g)
Polyurethane resin 10 parts UR8200 (manufactured by Toyobo)
Abrasive 3 parts Diamond [Average particle size: 100 nm (variation coefficient: 20%)]
Carbon black [average particle size: 60 nm] 5 parts butyl stearate 4 parts hexadecyl stearate 4 parts stearic acid 2 parts methyl ethyl ketone 125 parts cyclohexanone 125 parts

Lower layer coating material Inorganic powder: 75 parts of acicular α-Fe ₂ O ₃ Average major axis length: 0.08 μm, Average acicular ratio: 5
Carbon black 20 parts Conductex SC-U (manufactured by Colombian Carbon)
Polyurethane (glass transition point (Tg): 70 ° C.) 12 parts phenylphosphonic acid 4 parts butyl stearate 3 parts hexadecyl stearate 3 parts stearic acid 3 parts methyl ethyl ketone / cyclohexanone (8/2 mixed solvent) 250 parts

(Production of Sample A-2 (Example))
Each component of the magnetic layer coating material and the lower layer coating material was kneaded with a kneader, and then dispersed with a sand mill using Zr beads having a diameter of 0.3 mm for 12 hours. To the obtained dispersion, 2.5 parts of polyisocyanate is added to the lower layer coating liquid and 3 parts to the magnetic layer coating liquid, and 40 parts of cyclohexanone is added to each, and a filter having an average pore diameter of 1 μm is used. Then, coating solutions for the lower layer and the magnetic layer were prepared. The obtained coating solution for the lower layer was applied to a thickness of 1.4 μm and dried, and then the thickness of the magnetic layer on it was 50 μm, and the center surface average surface roughness was 50 μm. Was successively stacked on polyethylene naphthalate having a thickness of 2 nm and a thickness variation of 2%. A 7-stage calendar composed of only metal rolls is processed at a speed of 200 m / min at a temperature of 85 ° C., then punched into a 3.5 mm diameter disk, and surface-treated using an alumina polishing tape. Disc sample A-2 was obtained.

(Production of Samples A-1, 3 to 10 (Examples) and Samples B-1 to 8 (Comparative Examples))
The sample was prepared by changing the elements as follows based on Sample A-2.
Samples prepared in the same manner as A-2 except that abrasives (diamond) having a coefficient of variation in particle size distribution of 40%, 30%, 10%, and 5% were used. B-1, A-1, A- 3, A-4.

  A sample prepared in the same manner as A-2 except that carbon black having an average particle diameter of 20 nm was used instead of carbon black contained in the magnetic layer coating material was designated as A-5.

  Samples prepared in the same manner as A-2 except that the thickness of the magnetic layer was 0.05 μm and 0.15 μm were designated as B-2 and A-6.

  A sample prepared in the same manner as A-2 was used as B-3 except that a simultaneous multi-layer coating method in which the lower layer was applied and immediately after that the upper layer was applied while the lower layer was still wet was used.

  A sample prepared in the same manner as A-2 except that Zr beads having a diameter of 1 mm were used was designated as B-4.

  Samples prepared in the same manner as A-2 except that polyurethanes having glass transition points of 0 ° C. and 100 ° C. were used instead of polyurethane contained in the lower layer coating material were designated as B-5 and A-7.

  Samples prepared in the same manner as A-2 except that a support having a thickness variation of 0.5%, 3.5%, and 5% were used were designated as A-8, B-6, and B-7.

  Samples prepared in the same manner as A-2 except that diamond polishing tape and chromium oxide polishing tape were used instead of alumina polishing tape for the surface treatment were designated as A-9 and B-8.

A sample prepared in the same manner as A-2 except that a diamond having a coefficient of variation of 5%, a polyurethane having a glass transition point of 100 ° C., a support having a thickness of 0.5%, and a diamond polishing tape was used for the surface treatment. It was set to 10.
The characteristics of these samples are shown in Table 1.

1) Method of measuring protrusions and dents on the surface of the magnetic layer by AFM The height of the protrusions and the depth of the dents on the surface of the magnetic layer are determined by measuring the three-dimensional surface roughness using Nanoscope III manufactured by Digital Instruments Inc. This is the distance from the average surface (the surface where the volume of the unevenness in the measurement surface is equal) to the apex of the convex part and the distance to the deepest part of the concave part. The measurement was performed within a range of 30 μm × 30 μm (900 μm ² ), and 10 arbitrary locations on the magnetic disk were measured to obtain an average value.

2) Thermal asperity measurement method Using a composite GMR head, a signal with a wavelength of 2 μm is recorded on a disk at a relative speed of 9 m / s, and an average signal amplitude is obtained with a digital oscilloscope. Next, after performing DC erasure, reproduction is performed with a GMR head, and the number of pulse signals having an intensity of 1/2 or more of the previously obtained average signal amplitude is measured for 10 tracks, and the pulse signal per track is obtained. The number of initial thermal asperities was determined. The pulse-like signal is the sum of the polarities generated on the positive and negative sides.
Next, the head was run in an environment of 23 ° C. and 50% in a random seek mode for 100 hours and then replayed again with a GMR head. Similarly, the thermal asperity was measured after running.

3) Measuring method of S / N ratio (S / N) Using a spinstand and a composite GMR head with a track width of 1.0 μm, a 180 kfci signal at 32 MHz is recorded and reproduced with a track width of 1.2 μm, and the signal output is Measurement was further integrated with noise in the range of 0 to 64 MHz, and the ratio was defined as the SN ratio.

From Table 1, the magnetic disk according to the present invention having 10.0 / 900 μm ² or less protrusions of 30 nm or more and 2.0 / 900 μm ² or less recesses of 30 nm or more has less noise due to thermal asperity. I understand that. Further, it can be seen that a magnetic disk having 100 to 400 protrusions of 10 nm or more / 900 μm ² has a small increase in thermal asperity after running as well as the initial thermal asperity.
The signal obtained by measuring the SN ratio corresponds to a surface recording density of 3.8 Gbit / inch ^{2. Even} at this surface recording density, the magnetic disk of the present invention has an SN ratio of 24 dB, which is a practically sufficient value. Understand.

Claims (4)

In a magnetic recording medium in which a substantially non-magnetic lower layer on a support and a magnetic layer in which a ferromagnetic powder is dispersed in a binder are formed thereon, the surface of the magnetic layer is measured with an AFM (atomic force microscope). The number of protrusions having a height of 30 nm or more measured in (1) is 10.0 / 900 μm ² or less, and the number of recesses having a depth of 30 nm or more is 2.0 / 900 μm ² or less.   The magnetic recording medium according to claim 1, wherein the ferromagnetic powder is a hexagonal ferrite powder.   The magnetic recording medium according to claim 1, wherein the magnetic recording medium is a magnetic disk. The magnetic recording medium according to claim 1, wherein the number of protrusions of 10 nm or more is 100 to 400/900 μm ² .