JP2000200412A - Magnetic recording medium - Google Patents

Abstract

PROBLEM TO BE SOLVED: To obtain a high density magnetic recording medium with high track density and high line recording density in which servo signals are recorded and which has excellent electromagnetic conversion characteristics, high output, high resolution, high S/N ratio and excellent travelling durability. SOLUTION: In a magnetic recording medium in which servo signals are recorded, at least one base coating layer and a magnetic layer are formed in this order on a nonmagnetic supporting body. The magnetic layer has <=0.20 μm thickness, and the magnetic powder in the magnetic layer is ferromagnetic iron alloy powder containing Co and rare earth elements and having <=0.15 μm average major axial length, with the average major axial length L and the min. recording wavelength λ having the relation of L/λ<=1/3. The magnetic layer contains carbon black having a larger particle size than the thickness of the magnetic layer.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a high-density magnetic recording medium provided with a servo signal, and more particularly to such a medium having excellent electromagnetic conversion characteristics and running durability.

[0002]

2. Description of the Related Art Magnetic recording media have various uses such as audio tapes, video tapes, computer tapes, and disks, but magnetic recording media typified by digital systems are increasing year by year. For a magnetic disk having a higher density, for example, a magnetic disk having a capacity of 100 MB or more has been proposed, which has a significantly increased capacity from a conventional capacity of 1 MB using oxide magnetic powder.

In a drive using such a high-density magnetic disk, the disk rotational speed is increased from 300 min ^-1 to 7 in order to record and reproduce a large amount of data in a short time.
Speed up to 20 min ^-1 or 3,000 min
^It is necessary to perform high-speed rotation of ^-1 or more. In the future, in order to further increase the density and capacity, development is under way to increase the rotation speed. Further, in order to ensure stable recording / reproducing characteristics and reliability during high-speed rotation, development of an optimal contact mechanism of a head with a medium has been promoted.

As a magnetic recording medium corresponding to such a high density, means for improving the magnetic properties of the ferromagnetic powder, improving the dispersibility of the ferromagnetic powder, and reducing the spacing loss between the medium and the head. Means for reducing the size are becoming necessary.

In order to improve the magnetic characteristics of the ferromagnetic powder, it is desirable to increase the degree of magnetization remaining in the magnetic recording medium to increase the output. Therefore, as the magnetic powder, conventional oxide magnetic powder or cobalt-containing oxide is used. Instead of iron magnetic powder, ferromagnetic iron-based alloy powder is becoming mainstream, with a coercive force of 119.4 kA / m
The above ferromagnetic iron-based alloy powder has been proposed. For example,
JP-A-5-234064, JP-A-6-25702
And JP-A-6-139553.

As means for improving the dispersibility of the ferromagnetic powder, a binder having a polar functional group such as a sulfonic acid group, a phosphoric acid group or an alkali metal salt thereof, or a low molecular weight compound together with the binder is used. Means have been proposed, such as using the above dispersant in combination, continuously performing the kneading and dispersing step of the magnetic paint, and adding a lubricant after dispersion. For example, Japanese Patent Application Laid-Open Nos. 62-23226 and 2-1
No. 01624, JP-A-3-216812, JP-A-3-17827, JP-A-4-47586, JP-A-8-235566, and the like.

Further, as means for reducing the spacing loss between the medium and the head, in addition to the above-mentioned means for increasing the dispersibility of the magnetic powder, a smoothing treatment of the magnetic layer under high temperature and high pressure conditions in the calendar step is performed. Means have been proposed in which a nonmagnetic undercoat layer is provided below the magnetic layer to suppress the influence of the surface properties of the nonmagnetic support on the surface of the magnetic layer. For example, it is disclosed in JP-B-64-1297, JP-B-7-60504, and JP-A-4-19815.

[0008]

However, the conventional magnetic recording medium manufactured by the above-described means cannot satisfy the electromagnetic conversion characteristics and running durability in a system corresponding to a capacity of 100 MB or more. In a high-density magnetic disk, in order to increase both the track density and the linear recording density, only about a fraction of the output can be obtained with the saturation magnetization and coercive force of the conventional magnetic powder, and an extremely short recording wavelength is used. Therefore, the effects of self-demagnetization loss during recording and reproduction and the thickness loss due to the thickness of the magnetic layer, which have not been a problem in the past, become large, and sufficient resolution cannot be obtained.

On the other hand, for example, Japanese Patent Laid-Open No.
No. 9061 and the like, coercive force of 79.6 as magnetic powder
Using ferromagnetic iron-based alloy powder of kA / m or more,
In order to reduce space loss and thickness loss due to surface properties, a non-magnetic layer is provided between the non-magnetic support and the magnetic layer,
It has been proposed to reduce the thickness of the magnetic layer to 0.5 μm or less. However, in a high-density magnetic disk having a track width of 10 μm or less and a shortest recording wavelength of 1.0 μm or less,
It is necessary to further increase the saturation magnetization and coercive force of the magnetic powder, and to further reduce the above space loss and thickness loss.

Moreover, in such a high-density disk,
Since the track width becomes narrower with an increase in the track density, a data error due to a decrease in the S / N ratio due to off-tracking of the magnetic head is likely to occur. Therefore, the data track position detection is performed separately from the data track. A servo track having a servo signal is required to be provided on the magnetic layer. Therefore, in order to improve track density,
It is becoming indispensable to suppress off-tracking by the signal. However, even with a medium in which such a servo signal is provided, if the data track width is extremely narrow as 10 μm or less as described above, the problem that the data error is amplified has become apparent. . That is, even if the absolute value of the deviation amount of the magnetic head from the data track due to the off-track is the same, the data track width becomes narrower, and the ratio of the minimum on-track width to the data track width is reduced. S / N by off-track
There is a problem that the ratio of decrease is large and induces data error.

It is also possible for the device to correct such data errors due to off-track, and such a correction circuit is actually used. However, on the medium side as well, it is necessary to improve the off-track margin by improving the S / N ratio and maintain the required minimum S / N ratio. This is the same for tape-shaped media, and is a recent helical scan type digital magnetic tape.
In the tape, the data track and the servo track are provided in tandem with the same width diagonally to the tape running direction, and these tracks are read by the same magnetic head. Errors easily occur. In particular, as the data track width is reduced in order to improve the recording density, and the relative speed of the tape-magnetic head tends to increase, the load on the medium side is further increased.

On the other hand, with respect to running durability, a high-density magnetic disk has a problem that running durability, which is not a problem at the conventional low speed rotation, is extremely poor at high speed rotation. In recent years, in particular, in order to improve the reliability of magnetic disks, it is necessary to satisfy seek durability in which the magnetic head is moved between several tracks while keeping the magnetic head in contact. The magnetic layer using
Compared to a magnetic layer using an oxide magnetic powder, sliding scratches are more likely to occur during high-speed rotation, which causes a reduction in output and dropout, and lowers the reliability of the disk.

In particular, when a servo signal is provided, the magnetic head follows the data track based on information from the servo signal while detecting the position of the servo track. At that time, since the position of the servo track changes due to the change of the medium caused by heat and humidity, and the movement of the running system caused by friction and the motor, the magnetic head must always follow this change. It comes into sliding contact with the magnetic layer in a vibrating state. In addition, since the vibration has an angle almost perpendicular to the traveling direction of the magnetic head, a stress in the lateral direction other than the traveling direction is also applied to the magnetic layer, so that a sliding scratch is likely to occur.

Furthermore, with the spread of personal computers, the number of drives built into personal computers is increasing, and media are used in various environments. Accordingly, the cycle environment from low temperature and low humidity to high temperature and high humidity is used. In the durability and reliability tests under the following conditions, it is necessary that the output does not deteriorate and the dropout does not occur, so that further running durability and reliability under such severe conditions are desired. .

In view of the above circumstances, the present invention provides a high-density magnetic recording medium provided with a servo signal, particularly, a medium having a high track density and a high linear recording density, having excellent electromagnetic conversion characteristics, high output, It is an object of the present invention to provide a motor having high resolution and a high S / N ratio and having excellent running durability.

[0016]

Means for Solving the Problems The inventors of the present invention have conducted intensive studies on the above objects and as a result, have found that a high-density magnetic recording medium provided with a servo signal requires a non-magnetic support and a magnetic layer. At least one undercoat layer is provided therebetween, and a ferromagnetic iron-based alloy powder having a specific elemental composition and an average major axis length is used as the magnetic powder contained in the magnetic layer. When the particle size of the carbon black is specified, it is possible to obtain the above-described medium having excellent electromagnetic conversion characteristics, high output, high resolution, high S / N ratio, and excellent running durability. We have found and completed the present invention.

That is, according to the present invention, in a magnetic recording medium provided with a servo signal, at least one undercoat layer and a magnetic layer are formed in this order on a nonmagnetic support, and the thickness of the magnetic layer is zero. 20 μm or less, the magnetic powder contained in the magnetic layer is a ferromagnetic iron-based alloy powder containing Co and a rare earth element, the average major axis length L of which is 0.15 μm or less, the average major axis length L and the shortest recording wavelength and λ have a relationship of L / λ ≦ 1/3, and a magnetic layer having a particle size larger than the thickness of the magnetic layer in the magnetic layer.
The present invention relates to a magnetic recording medium, such as a magnetic disk or a magnetic tape, which is characterized by containing a bomb black.

In the present invention, the magnetic recording medium provided with a servo signal is defined as a magnetic recording medium provided before the servo signal records a data signal, and a magnetic recording medium provided before the servo signal records a data signal. It includes both a magnetic recording medium provided at the same time.

[0019]

DESCRIPTION OF THE PREFERRED EMBODIMENTS In the present invention, a high-density magnetic recording medium provided with a servo signal, particularly a medium having a high track density and a high linear recording density, is used to improve electromagnetic conversion characteristics and durability. As the magnetic powder to be contained in the magnetic layer, among the ferromagnetic iron-based alloy powder,
And a ferromagnetic iron-based alloy powder containing a rare earth element and having an average major axis length L of 0.15 μm or less and an average major axis length L and the shortest recording wavelength λ of L / λ ≦ 1/3. Those that have a relationship are used.

In the present invention, in order to obtain high resolution, the thickness of the magnetic layer is set to 0.20 μm or less. In this case, the filling amount in the magnetic layer is reduced with the conventional magnetic powder having a large particle diameter. Magnetic properties deteriorate, and high output cannot be obtained. Therefore,
The average major axis length L is 0.15 μm or less, preferably 0.10 μm
The use of ultrafine ferromagnetic iron-based alloy powder having a particle size of not more than μm (usually up to 0.01 μm) improves the filling property of the magnetic powder in the magnetic layer, and provides high remanence. C
When o is contained, the magnetic properties of the magnetic powder itself, such as the saturation magnetization and the coercive force, are improved, and even in high-density recording where the thickness of the magnetic layer is 0.20 μm or less and the shortest recording wavelength is 1.0 μm or less,
High output can be achieved.

The average major axis length L is obtained by actually measuring the particle size of a photograph taken with a scanning electron microscope (SEM) and obtaining the average value of 100 particles. For the same reason as described above, the ferromagnetic iron-based alloy powder used in the present invention preferably has a BET specific surface area of 35 m ² / g or more, more preferably 40 m ² / g or more, and more preferably 50 m ² / g or more. Most preferably, it is ² / g or more.

On the other hand, in a magnetic recording medium provided with a servo signal, even if a predetermined S / N ratio is obtained during on-track, a data error due to off-track is prevented.
It is necessary to improve the S / N ratio of the medium. Therefore,
Even when a medium having the above-mentioned magnetic powder and merely improving the magnetic properties of the magnetic layer to increase the output is used, when the noise is also high, the relative S / N ratio decreases, and a sufficient S / N ratio is obtained during off-track. N ratio cannot be obtained. Thus, the present inventors have studied the reduction in noise, and as a result, have found that the average major axis length L is equal to 0.
A ferromagnetic iron alloy powder having a length of 15 μm or less, wherein the average long axis length L and the shortest recording wavelength λ are in a relationship of L / λ ≦ １ ／,
And by using a magnetic powder containing a rare earth element,
We have found that we can solve the above problem.

The noise generated from the medium is attributable to particulate noise caused by magnetic powder. It is considered that the particle noise is caused by unevenness of the size and the particle size of the particles. Therefore, first, it is necessary to reduce the size of the particles. According to the study of the present inventors, the average long axis length L of the magnetic powder and the shortest recording wavelength λ are L / λ ≦ 1. When the relationship is / 3, the number of magnetic particles in a certain area contributing to the head output can be increased, the variation in the amount of magnetization in the certain area due to the difference in the data recording location can be prevented, and the size of the particles can be reduced. It has been found that noise can be reduced.

In addition, according to such fine magnetic powder, noise caused by the size of the particle can be reduced, but particle noise caused by the non-uniformity of the particle size cannot be reduced. In other words, if the size of the magnetic powder varies, the magnetic flux generated from the magnetic particles existing in a certain area contributing to the head output varies even if the data is in an unrecorded state, and the head output is reduced. 0, that is, the sum of the magnetic flux is hardly zero. As a result, an output difference occurs due to a difference in data recording location, which causes an increase in noise. This is because the noise based on the output fluctuation is also superimposed when the signal is recorded, so that the noise further increases and the relative S
/ N ratio is reduced.

The inventors of the present invention have studied the particle noise caused by the non-uniformity of the particle size. As a result, the ferromagnetic iron-based alloy powder of fine particles of 1/3 or less with respect to the shortest recording wavelength, together with Co, It has been found that when a rare earth element is included, the particle size distribution of the magnetic powder becomes uniform, the particle noise due to the non-uniform particle size can be reduced, and the S / N ratio is greatly improved. Further, it has been found that when such a rare earth element is contained, good results can also be obtained in improving the wear resistance of the magnetic powder.

In the present invention, the ferromagnetic iron-based alloy powder contains
In order to contain Co, the gasite powder is calcined into magnetite powder, which is ion-exchanged for divalent iron ions and cobalt ions in a cobalt ion-containing aqueous solution,
Heat reduction method, heat reduction method of cobalt-containing acicular gausite powder obtained from aqueous alkali suspension of iron salt and cobalt salt, coprecipitation obtained from iron salt and cobalt salt added to oxalic acid aqueous solution Method of reducing the substance, a method of heating and reducing iron oxide powder having cobalt deposited on the surface,
A method of adding a reducing agent to a solution containing an iron salt and a cobalt salt,
A method of evaporating a metal in an inert gas and colliding it with gas molecules to obtain an alloy magnetic powder, and reducing it to a metal while flowing a vapor of iron or cobalt chloride in a mixed gas of hydrogen, nitrogen, and argon There are methods. Above all, solid solution with high cobalt content is possible and excellent corrosion resistance
It is preferable to use the above method in combination.

In order to include a rare earth element together with Co in the ferromagnetic iron-based alloy powder, in each of the above-mentioned Co-containing methods, a rare earth element may be contained simultaneously with Co.
A rare earth element may be contained after Co is contained. Among them, a method of co-precipitating a rare earth element simultaneously with cobalt during the production of a gate site, and a method of suspending a raw iron oxide powder containing Co in a rare earth compound aqueous solution before heat reduction are preferred. Rare earth elements include Nd, Y, La, C
There are e, Pr, Sm, Gd, Yb, Tb and the like, and among them, Y, La and Ce are particularly preferable.

In a ferromagnetic iron-based alloy powder containing Co and a rare earth element, the higher the amount of cobalt, the higher the saturation magnetization and the higher the coercive force can be achieved. However, since the excess becomes oxide, the above characteristics cannot be achieved. Therefore, the amount of cobalt is preferably such that the weight ratio of Co / Fe is in the range of 0.2 to 0.5, and more preferably in the range of 0.2 to 0.4.

The larger the amount of the rare earth element, the more uniform the particle size distribution of the magnetic powder, the lower the particle noise, and the higher the S / S ratio.
N ratio can be achieved and the adhesion between the magnetic powder and the binder can be increased, preventing damage to the magnetic powder during the coating kneading and dispersing process. Although it is possible to prevent the powder from falling off, if it is too large, the Co content decreases, and the saturation magnetization of the magnetic powder decreases. Therefore, the amount of rare earth element is expressed as
e is preferably in the range of 0.01 to 0.1, more preferably 0.1 to 0.1.
The range of 02 to 0.07 is more preferable.

In such a ferromagnetic iron-based alloy powder containing Co and a rare earth element, other elements such as
Transition metals such as Zn, Sn, Ni, Mn, Ti, Cr, and Cu can also be added. However, if an alkali metal, particularly Ca, is present in the ferromagnetic iron-based alloy powder, it reacts with the fatty acid in the magnetic layer to form a fatty acid salt on the surface of the magnetic layer. It is preferable to avoid the contamination.

The ferromagnetic iron-based alloy powder containing Co and the rare earth element is coated with an inorganic oxide to prevent sintering during heat reduction and to improve dispersibility in a magnetic paint. Is desirable. Examples of the inorganic oxide include aluminum oxide and silicon oxide. Aluminum oxide is particularly preferable because it has excellent hardness and can improve the wear resistance of the ferromagnetic iron alloy powder.

In order to carry out the coating, a method is used in which water is applied to an alcohol solution of aluminum, silicon or the like in advance on the raw iron oxide powder, and these compounds are adhered to the surface of the particles by hydrolysis. . The amount of coating is 0.1% by weight to Fe in order to prevent sintering and improve dispersibility.
It is preferably at least 001, and if it is too large, the saturation magnetization of the magnetic powder is reduced. That is, when the particle surface is coated with aluminum oxide, it is preferable that the aluminum oxide be contained at a ratio of the Al / Fe weight ratio of 0.001 to 0.07.

Further, such a ferromagnetic iron-based alloy powder containing Co and a rare earth element is included in a magnetic paint because magnetic saturation is likely to occur due to high saturation magnetization and the particle surface becomes very active. It acts as a catalyst which causes the modification of the solvent and the modification of the isocyanate component in the crosslinking agent used as the binder. Therefore, the ferromagnetic iron-based alloy powder preferably has a pH of less than 10, especially less than 8. When the pH is less than 10, the formation of the above-mentioned denatured product in the magnetic paint can be suppressed, and a coating film that can withstand sliding during high-speed rotation when forming the magnetic layer can be obtained.

The coercive force of such a ferromagnetic iron-based alloy powder containing Co and a rare-earth element is 135.
3 to 278.5 kA / m, especially 159.2 to 222.
It is preferably 8 kA / m. The saturation magnetization is 15.1 to 25.1 μWb in order to obtain a good reproduction output at a high track density and to maintain the corrosion resistance of the magnetic powder.
/ G, particularly preferably 16.3 to 20.1 μWb / g. As the squareness ratio, σr / σs is preferably 0.46 or more, particularly 0.48 or more, and more preferably 0.49 or more.

In the present invention, the magnetic properties of the magnetic layer using the ferromagnetic iron-based alloy powder containing Co and the rare earth element are as follows: the coercive force is 135.3 to 278.5 kA /
m, especially 159.2 to 222.8 kA / m. Further, in order to make the magnetic layer highly fillable with the ferromagnetic iron-based alloy powder and to provide a high-output magnetic recording medium, the magnetic layer has a residual magnetization of 2.513 nWb or more, especially 3.1.
It is preferable that the thickness be 42 nWb or more and 7.540 nWb or less. Such a magnetic layer having high remanence can be achieved by using the above-described ferromagnetic iron-based alloy powder containing Co and a rare earth element, and by the means described below for improving the filling property of the magnetic powder.

The magnetic properties of the magnetic layer and the magnetic properties of the ferromagnetic iron-based alloy powder containing Co and the rare-earth element were all measured by using a sample vibrating magnetometer at room temperature and an external magnetic field of 795.8.
It refers to a measured value in kA / m, and the coercive force and residual magnetization of the magnetic layer are values after correction when a sample having a diameter of 8 mm and a magnetic layer surface having 20 surfaces bonded together is measured.

In the present invention, the use of the ferromagnetic iron-based alloy powder containing Co and the rare earth element improves the magnetic properties of the magnetic layer, and provides a high output even at the shortest recording wavelength of 1.0 μm or less. And noise generated by the magnetic powder itself is reduced, and a high S / N ratio can be obtained. On the other hand, when the thickness of the magnetic layer is large at a high linear recording density of the shortest recording wavelength of 1.0 μm or less, the long-wavelength output increases with the thickness, but the short-wavelength output changes little due to the thickness loss. do not do. For this reason, when the short wavelength output is relatively lower than the long wavelength output, the resolution is reduced, and improvement only by the effect of reducing the self-demagnetization loss by increasing the coercive force is not sufficient.

In order to solve such a problem of resolution,
As a result of study, at a high linear recording density of 1.0 μm or less in the shortest recording wavelength, when the thickness of the magnetic layer is set to 0.20 μm or less, which is 1/5 or less of the conventional magnetic disk, a high-resolution magnetic I found that I could get a disk. That is, in the present invention, the ferromagnetic iron-based alloy powder containing Co and the rare earth element is used, and the thickness of the magnetic layer is set to 0.20 μm or less, so that the magnetic layer has a high residual magnetic flux density, The magnetic force can be achieved, the self-demagnetization loss at a high track density can be reduced, and the thickness loss can be reduced even at a high linear recording density with a minimum recording wavelength of 1.0 μm or less. Can be achieved.

In the present invention, the thickness of the magnetic layer needs to be 0.20 μm or less from the above viewpoint.
If the thickness is too small, it becomes difficult to maintain the uniformity of the thickness of the coating film when the magnetic layer is formed, and the amount of magnetic powder that can be filled in the magnetic layer decreases, and the magnetic properties of the magnetic layer deteriorate. For this reason, the thickness of the magnetic layer is desirably 0.03 μm or more, preferably 0.05 to 0.20 μm.

On the other hand, a high-density magnetic disk requires high-speed data transfer in order to record and reproduce a large amount of data in a short time.
00min at least twice ^{-1 720min -1} or 3,000Min ^-1 or more, head-contact and near-contact type high speed rotation of the drive is utilized. However, the magnetic disk using the ferromagnetic iron-based alloy powder containing Co and the rare-earth element described above has a higher magnetic powder than that of a conventional magnetic disk using a ferromagnetic powder when traveling under a normal temperature and normal humidity environment. There is an improvement in the temporary error due to the falling of the magnetic layer, but the magnetic layer is easily damaged when running in a cycle environment from low temperature and low humidity to high temperature and high humidity, dropout and output fluctuation easily occur, and sufficient running durability I can not get the nature.

To solve this problem, the present inventors
Initially, increase the amount of abrasives such as high-hardness alumina and carbon black that have been used to improve durability by sliding contact with magnetic heads, etc., to improve the polishing ability of the magnetic layer. However, it was found that the expected effect was not obtained, and that the output, the resolution, and the S / N ratio were all reduced. In this case, it was observed that what appeared to be carbon black drops was attached to the magnetic head in the early stage of traveling. This is because, if the carbon black having a small particle size is used in view of the above-mentioned effects and conductivity, when the carbon black is present on the surface of the magnetic layer, the volume buried in the magnetic layer is small. ,
It is easily excavated by sliding contact with a magnetic head, etc., and easily falls off.In other words, a state in which a small grain carbon black is buried in the magnetic layer is the same as a state in which small gravel exists in the cement. This is presumably because carbon black easily emerges on the surface of a thin magnetic layer having a thickness of 0.20 μm or less, and the above phenomenon is likely to occur.

Further, in the present invention, since the active fine magnetic ferromagnetic iron alloy powder containing Co and the rare earth element is used as the magnetic powder, it is necessary to adsorb a large amount of the binder to the active magnetic powder when the paint is dispersed. Therefore, the amount of binder for dispersing the carbon black is considered to be insufficient.As a result, the fixation of the carbon black in the magnetic layer is weakened, so that the carbon black is more easily separated from the magnetic layer. It seems to be. In a medium provided with a servo signal as in the present invention, the magnetic head slidably contacts the magnetic layer while oscillating.
Since a stress is applied in the lateral direction, it is considered that the above-mentioned dropping is further facilitated.

Based on these findings, the present inventors have
As a result of further study, it was found that at least one undercoat layer was provided between the nonmagnetic support and the magnetic layer, and that the carbon black contained in the magnetic layer had a large particle size exceeding the thickness of the magnetic layer. By using this, even in a high-density magnetic recording medium such as a high-density magnetic disk on which a servo signal is provided, and when the thickness of the magnetic layer is made as thin as 0.20 μm or less, running at high speed rotation is possible. It has been found that durability can be greatly improved.

That is, since the magnetic recording medium of the present invention is used in a drive rotating at high speed, by providing an undercoat layer below the magnetic layer, the load from the magnetic head during sliding can be dispersed, and It is possible to compensate for the decrease in the lubricant due to the reduced thickness of the layer. Further, by providing the undercoat layer, the surface properties of the magnetic layer during calendaring can be improved, which can contribute to an improvement in output. The thickness of the undercoat layer that exerts such an effect is 0.5 to 5 μm.
m, particularly preferably 1 to 3 μm.

Both the thickness of the undercoat layer and the thickness of the magnetic layer were determined by measuring 10 cross-sectional photographs of the cross-section of the coating film with a transmission electron microscope (50,000 times) at 1 cm intervals. It is obtained from the average value of five cross sections.

Also, by using the carbon black having the large particle size, the amount of carbon black particles embedded in the magnetic layer can be increased, and even if a small amount of the binder is present on the surface of the magnetic layer in a firmly fixed state. In addition to being able to prevent falling off during sliding contact, and by appropriately projecting from the surface of the magnetic layer, sliding contact between the adhesive magnetic powder and the magnetic head can be suppressed,
Even during high-speed rotation, the friction at the time of sliding contact with the magnetic head is reduced,
Stable running durability can be obtained. Further, since the carbon black having such a large particle size is present on the surface of the magnetic layer, the function as a head support at the time of contact with the magnetic head can be sufficiently exhibited. Load can be greatly reduced, and even when used in combination with carbon black with a small particle size, the carbon black with a large particle size becomes a barrier for sliding contact between the carbon black with a small particle size and a magnetic head. It is thought that the running durability can be significantly improved.

In addition, when the carbon black having such a large particle size is contained, from the viewpoint of the prior art, the carbon black exists between the magnetic head and the magnetic layer, resulting in large spacing. However, it has been found for the first time in the present invention that the output can be suppressed to a slight decrease. The reason for this is not necessarily clear, but when the carbon black protruding from the magnetic layer comes into sliding contact with the magnetic head,
It is considered that the carbon black is deformed by the head pressure, thereby reducing the amount of spacing.

In the carbon black having a large particle diameter used in the present invention, if the particle diameter is too large, the surface smoothness of the coating film is impaired, and the output and the S / N ratio are reduced.
Greater than the thickness of the magnetic layer and 3.0 times the thickness of the magnetic layer.
The ratio is preferably not more than 2 times, particularly preferably 1.1 to 2.0 times. Commercially available products include "SEVACARB MTCI" manufactured by Columbian Carbon and "Termax Powder N-991" manufactured by Kancarb.
And the like. The amount used is generally preferably 0.5 to 5 parts by weight, particularly preferably 1 to 3 parts by weight, based on 100 parts by weight of the ferromagnetic iron-based alloy powder containing Co and the rare earth element.

In the present invention, carbon black having a large bulk specific gravity and a small particle diameter of 0.01 to 0.03 μm can be used in combination for imparting conductivity. As a commercial product, Cabot
"BLACK PEARLS 800", "Mo
gul-L "," VULCAN XC-72 "," Re
gal 660R "," Raven 1255 "and" Conductex S "manufactured by Columbian Carbon Co., Ltd.
C "and the like. The amount used is large particle size carbon black /
The weight ratio of small particle size carbon black is 10 / 90-80 /
It is preferable to set the ratio to 20, especially 20/80 to 50/50. As a result, pores in the magnetic layer can be left to some extent, the lubricant is smoothly supplied to the surface of the magnetic layer, friction with the magnetic head, and contact with disk jacket components such as nonwoven fabric. Friction can be reduced, and running durability can be further improved.

In the present invention, at least one lubricant selected from fatty acids and fatty acid esters is contained in the components extracted with n-hexane from the magnetic layer and the undercoat layer. ~250kg / m ³
When, the running durability is further improved. That is, if the above extraction amount is 10 kg / m ³ or more, the sufficient lubricant is present in the magnetic layer and the undercoat layer to help the lubricant shortage on the surface of the magnetic layer, and the carbon black having a large particle size is formed. In addition to the presence of, the friction on the surface of the magnetic layer, which causes running durability, can be reduced and a cushioning effect can be brought about. Is performed, the leaching of the lubricant into the surface of the magnetic layer becomes easy, and good durability is obtained. Further, when the extraction amount is 250 kg / m ³ or less, it is possible to avoid the possibility that the filling property of the magnetic powder is deteriorated and the electromagnetic conversion characteristics are reduced, and the coating film of the magnetic layer is weakened.

The lubricant extracted with n-hexane is preferably a mixed system of a fatty acid and a fatty acid ester, and in particular, a fatty acid having 10 or more carbon atoms and a fatty acid ester having a melting point of 35 ° C. or less, as described later. And the extraction weight ratio of both components, that is, the ratio of fatty acid having 10 or more carbon atoms / fatty acid ester having a melting point of 35 ° C. or less is 0.3.
/99.7 to 10/90, especially 0.5 / 99.5 to 6
/ 94 is preferred. By having such a lubricant composition, it is possible to reduce the friction coefficient and improve the running durability while maintaining the oil film strength on the surface of the magnetic layer.

The extraction amount of the lubricant extracted by n-hexane means that 600 cm ^{2 of} a magnetic recording medium was cut out and immersed in 1 liter of n-hexane for 10 minutes, and the operation was repeated twice at 60 ° C. 5 minutes after extraction, the weight of the sample after extraction is measured within 30 seconds, and the value obtained from the difference in weight before and after immersion is divided by the total volume of the magnetic layer and the undercoat layer. Further, the composition ratio of the extracted lubricant refers to a value obtained by measuring the temperature from 150 ° C. to 200 ° C. at a rate of temperature increase of 10 ° C./min by gas chromatography using a calibration curve method.

In the present invention, when the thickness of the magnetic layer is set to 0.20 μm or less, the steel ball of the magnetic layer is used in order to improve the self-polishing ability for improving the running durability at the time of high-speed rotation. Wear volume of 0.5 × 10 ^{−4 to} 5.0 × 10 ⁻⁴ mm ³ ,
It is particularly preferable to set the range of 1.0 × 10 ^{−4 to} 3.0 × 10 ⁻⁴ mm ³ . By setting such a range, deterioration of the output and S / N ratio during long running can be suppressed, and wear of the magnetic head and clogging of the head can be reduced.

The above-mentioned steel ball wear volume is measured by rotating and sliding test while moving the position of the sliding surface, and is measured by reciprocating sliding of the same portion in the related art. However, it reflects the properties of the surface of the magnetic layer that is in sliding contact with the magnetic head at high speed. In other words, in the conventional reciprocating sliding test, since the same portion is slid only for a short distance, the internal polishing component existing on the surface of the magnetic layer and not contributing to durability is taken into consideration. It is considered that the actual durability at the time of sliding is influenced by factors such as the cohesive force of the coating film, the abrasive, and the friction reducing component only in the vicinity of the magnetic layer surface, not the entire polishing component as described above. From this viewpoint, it has been found that the steel ball wear volume of the present invention can accurately reflect the surface properties of the magnetic layer when the magnetic layer surface is in sliding contact with the magnetic head at high speed in relation to durability.

That is, in the measurement of the steel ball wear volume of the present invention, as shown in FIG. 1, the upper carriage of the drive 1 is removed and the steel ball 3 [made of high carbon chromium bearing steel; 6.35mm diameter (JISB-150
1)] is fixed, and a metal plate 5 made of SUS having a thickness of about 0.2 mm is adhered to the head surface of the lower carriage 2 to form a receiving surface for steel balls, and the environment is set at 20 ° C. and 50% RH. The sample disk incorporated in the cartridge is set below, and a weight is placed on the balance so that a load of 10 g is applied to the steel ball. In this state, the disk is rotated at 720 mm ^-1 for 10 seconds at a position having a radius of 30 mm. Let it.

Next, the steel ball is moved to the outer periphery of 1 mm while applying a load and rotated for 10 seconds, and then the steel ball is moved to the outer periphery of 1 mm and rotated for 10 seconds. Repeat and spin for a total of 60 seconds to stop the disk. Thereafter, the steel ball is removed and observed with an optical microscope (100 times). From the radius (r) of the circular wear surface generated by sliding and the radius (R) of the steel ball, the following equation is obtained. Accordingly, the steel ball wear volume (V) of the magnetic layer is calculated.
In FIG. 1, 6 is a drive board, and 7 is a drive control.
Reference numeral 8 denotes a spindle, and 9 denotes a stator. V = (π / 3) × ｛2R ³ -√ (R ² -r ² ) × [2R
² + r ^2]}

The structure of the magnetic recording medium of the present invention will be described below in the order of the magnetic layer, the undercoat layer and the non-magnetic support. First, the constituent components of the magnetic layer include a ferromagnetic iron-based alloy powder containing Co and a rare earth element and carbon black, as well as a binder, a lubricant, and an abrasive for the alloy powder. In order to enhance the dispersibility of the alloy powder, a dispersant such as an alcohol, a fatty acid, an aliphatic amine or a surfactant is used.

The binder used for the magnetic layer includes vinyl chloride resin, vinyl chloride-vinyl acetate copolymer resin, vinyl chloride-vinyl alcohol copolymer resin, vinyl chloride-vinyl acetate-maleic anhydride copolymer resin, nitrocellulose. -There is a combination of at least one kind selected from polyurethane and a polyurethane resin. Among them, it is preferable to use a vinyl chloride-vinyl acetate-vinyl alcohol copolymer resin and a polyurethane resin in combination. Polyurethane resins include polyester polyurethane, polyether polyurethane, polyether polyester polyurethane, polycarbonate polyurethane, polyester polycarbonate polyurethane, and the like.

These binders improve the dispersibility of ferromagnetic iron-based alloy powder containing Co and rare earth elements,
What has a functional group is preferable in order to raise filling property. The functional group, -COOM, -SO ₃ M, -OS
_{O 3 M, -P = O (} OM) 3, -O-P = O (OM) 2
(In each case, M is a hydrogen atom or an alkali metal base),
—OH, —NR ₂ , —N ⁺ R ₃ (in each case, R is a hydrocarbon group), an epoxy group and the like. When two or more resins are used in combination, it is preferable that the functional groups are the same, and among them, a —SO ₃ M group is preferable.

These binders are used in an amount of 5 parts by weight based on 100 parts by weight of a ferromagnetic iron alloy powder containing Co and a rare earth element.
It is used in an amount of from 50 to 50 parts by weight, preferably from 10 to 35 parts by weight. In particular, when a vinyl chloride resin is used as the binder, the range is 5 to 30 parts by weight, and when a polyurethane resin is used, the range is 2 to 20 parts by weight. preferable.

It is desirable to use together with these binders a thermosetting crosslinking agent which binds to and crosslinks a functional group contained in the binder. Examples of the crosslinking agent include tolylene diisocyanate, hexamethylene diisocyanate
, Methylene diisocyanate, etc., the reaction products of these isocyanates with those having a plurality of hydroxyl groups such as trimethylolpropane, and various polyisocyanates such as condensation products of the above isocyanates. Is preferred. These crosslinking agents are based on 100 parts by weight of the binder.
Usually, it is used in a proportion of 15 to 70 parts by weight.

As the lubricant used in the magnetic layer, conventionally known fatty acids, fatty acid esters, hydrocarbons and the like are used alone or in combination of two or more. Among them, it is preferable to use a fatty acid having 10 or more carbon atoms, preferably 12 to 30 carbon atoms, and a fatty acid ester having a melting point of 35 ° C. or less, preferably 10 ° C. or less. Some of these lubricants adsorb to ferromagnetic iron-based alloy powders to help disperse the magnetic powders, soften the medium-to-head contact during initial wear, lower the wear coefficient and reduce head contamination. To contribute.

The fatty acids having 10 or more carbon atoms include straight-chain,
Any of isomers such as branched and cis-trans may be used, but a linear type having excellent lubricating performance is preferred. Examples of such fatty acids include lauric acid, myristic acid, stearic acid, palmitic acid, behenic acid, oleic acid, linoleic acid and the like. Among these, myristic acid, stearic acid, palmitic acid and the like are preferable.

Fatty acid esters having a melting point of 35 ° C. or lower include n-butyl oleate, hexyl oleate, n-octyl oleate, 2-ethylhexyl oleate, oleyl oleate, n-butyl laurate, heptyl laurate, myristine N-butyl acid, n-butoxyethyl oleate, trimethylolpropane trioleate, n-butyl stearate, s-butyl stearate, isoamyl stearate, butyl cellosolve stearate and the like. These fatty acid esters can control the oil film strength and the amount of extraction by the difference in molecular weight, structure, and melting point, and may be optimized by combination. By having the above melting point, even when exposed to low temperature and low humidity, the magnetic layer easily oozes and migrates to the surface of the magnetic layer at the time of high-speed sliding contact between the magnetic layer and the magnetic head, and effectively exerts its excellent lubricating action. it can.

In order to include these lubricants in the magnetic layer, when mixing the magnetic powder and the binder, they are added together with the two components, before or after the mixing, or A lubricant solution or the like may be applied or sprayed on the surface of the formed magnetic layer. At this time, the amount of the lubricant used is 1 to 15 parts by weight, preferably 2 to 12 parts by weight, more preferably 4 to 10 parts by weight, based on 100 parts by weight of the ferromagnetic iron-based alloy powder containing Co and the rare earth element. It is good to use parts by weight. Fatty acids with 10 or more carbon atoms, melting point 35 ° C
When the following fatty acid esters are used in combination, the addition ratio of the two is 10/90 to 80/20, preferably 20 by weight.
/ 80 to 60/40 is preferable.

The abrasive used for the magnetic layer is preferably used for adjusting the wear volume of the steel ball of the magnetic layer. Examples of the abrasive include granular, angular, and needle-like α-alumina (Al ₂ O ₃ ) having an α-rate of 90% or more, β-alumina, γ-alumina,
There are chromium, α-iron oxide, silicon carbide, titanium oxide, cerium oxide, silicon dioxide, diamond and the like. Among them, α-alumina, which has a high hardness and exists on the surface of the magnetic layer and has a large effect of reducing the wear coefficient, is particularly preferable. The particle size of the abrasive is 0.3 μm or less, particularly 0.2 μm or less, in order to reduce the spacing between the medium and the head and to keep the steel ball wear volume within the range of the present invention. Is preferred. However, if the particle size is too small, the coefficient of wear with the magnetic head increases, and the running durability decreases. Therefore, the particle size is preferably at least 0.02 μm, particularly preferably at least 0.1 μm.

In order to incorporate the abrasive into the magnetic layer, the magnetic powder and the binder are added in a step of kneading with a kneader or the like or in a preliminary stirring step, or an abrasive dispersion is prepared in advance and prepared. May be added to the magnetic paint. In terms of productivity, the former, which does not require a separate step, is more preferable. From the viewpoint of electromagnetic conversion characteristics and head contamination, the amount of the abrasive added is 100 parts by weight of the ferromagnetic iron-based alloy powder containing Co and the rare earth element.
The content is 5 to 25 parts by weight, preferably 10 to 20 parts by weight.

In forming the magnetic layer, any of the conventionally used solvents can be used as a solvent for preparing a magnetic paint, a lubricant solution and the like. For example, aromatic solvents such as benzene, toluene and xylene; ketone solvents such as acetone, cyclohexanone and methyl ethyl ketone; ester solvents such as ethyl acetate and butyl acetate; alcohol solvents such as ethanol and isopropanol. And hexane, tetrahydrofuran, dimethylformamide and the like.

Next, the constituent components of the undercoat layer include inorganic powders, binders, lubricants, carbon black and the like. Both non-magnetic powder and magnetic powder can be used as the inorganic powder.
Non-magnetic powders include α-alumina, β-alumina, γ-alumina, α-iron oxide, and TiO having an α-conversion rate of 90% or more.
₂ (rutile, anatase), TiO _x , cerium oxide,
Tin oxide, tungsten oxide, ZnO, ZrO ₂ , Si
O ₂ , Cr ₂ O ₃ , gateite, corundum, silicon nitride, titanium carbide, magnesium oxide, boron nitride,
Molybdenum disulfide, copper oxide, MgCO ₃ , CaCO ₃ ,
BaCO ₃ , SrCO ₃ , BaSO ₄ , silicon carbide, titanium carbide and the like are used alone or in combination. As the magnetic powder, γ-Fe ₂ O ₃ , Co-γ-Fe
Coercive force of 23.9 kA / m such as ₂ O ₃ and Ba ferrite
The following low coercivity magnetic powder is used.

These inorganic powders may be in any of spherical, acicular, and plate shapes. The acicular inorganic powder enhances the surface properties of the undercoat layer and thereby gives good results to the surface properties of the upper magnetic layer, and the granular inorganic powder disperses the stress from the magnetic head in the undercoat layer. Works effectively. Therefore, it is preferable to use the acicular inorganic powder and the granular inorganic powder together. If the particle size of the inorganic powder (the major axis diameter in the case of needles, the maximum diameter in the case of particles, and the plate diameter in the case of plates) is too large, the surface properties of the undercoat layer will decrease and the surface of the magnetic layer will decrease. To 0.5 μm or less. On the other hand, if the particle size of the inorganic powder is too small, the filling property of the inorganic powder in the undercoat layer increases, the number of pores capable of holding the lubricant decreases, and the cushioning effect also decreases. It is preferably at least 05 μm. The amount of the inorganic powder used is 60 to 90% by weight of the entire undercoat layer for the same reason as the above particle size,
In particular, it is preferably 70 to 80% by weight.

As the binder used for the undercoat layer, the same resin as the above-mentioned binder constituting the magnetic layer is used, and preferably, the same resin as the binder of the magnetic layer is used. In particular, if they are matched in the combined use of vinyl chloride resin and polyurethane resin, the elasticity of the undercoat layer and the magnetic layer becomes closer,
The load from the magnetic head can be satisfactorily dispersed. The binder of the undercoat layer preferably has the same type of functional group as the binder of the magnetic layer. In particular, when the functional groups of the undercoat layer and the magnetic layer are matched to each other in a combined system of a vinyl chloride resin and a polyurethane resin, the adhesion between both layers is improved, and the leaching of the lubricant from the undercoat layer to the magnetic layer is smooth. Is preferable.

The amount of the binder used in the undercoat layer was determined based on the amount of the inorganic powder 1
20 to 45 parts by weight, especially 25 to
Preferably it is 40 parts by weight. In addition, in order to increase the strength of the undercoat layer, similarly to the case of the magnetic layer, together with the binder, a thermosetting crosslinking agent that is combined with a functional group contained in the binder and crosslinked is used in combination. Is also desirable. The amount of the crosslinking agent used is preferably 15 to 70 parts by weight based on 100 parts by weight of the binder.

As the lubricant used for the undercoat layer, the same lubricant as that for the magnetic layer can be used. However, since fatty acids are inferior to the upper layer more than fatty acid esters, fatty acid esters are used alone or when fatty acid esters are added. It is desirable to use a larger ratio. The amount of lubricant added to the undercoat layer is
The amount is usually 4 to 18 parts by weight, preferably 5 to 16 parts by weight, more preferably 6 to 14 parts by weight based on 100 parts by weight of the inorganic powder. The weight ratio of the fatty acid and the fatty acid ester to the undercoat layer is preferably 0/100 to 40/60, particularly preferably 0/100 to 30/70. The lubricant may be included in the undercoat layer by adding it at the time of mixing the undercoat layer paint with a kneader or the like, before or after the mixing, or by adding the surface of the undercoat layer formed beforehand. , A lubricant solution or the like may be applied or sprayed.

As the carbon black used for the undercoat layer, a carbon black having a particle size of 0.01 to 0.03 μm and a carbon black having a particle size of 0.05 to 0.3 μm are used in combination. preferable. The former carbon black is used to secure holes for holding the lubricant, as in the case of the magnetic layer. The latter carbon black improves the coating strength of the undercoat layer and improves the cushioning effect. It is to balance the two. The amount of carbon black to be added to the undercoat layer is 5 to 70 parts by weight, based on 100 parts by weight of the inorganic powder, inclusive of both carbon blacks.
It is particularly preferable to use 15 to 40 parts by weight.

In the present invention, the particle size of the carbon black in the undercoat layer is 0.01 to 0 times the particle size of the carbon black having a particle size larger than the thickness of the magnetic layer contained in the magnetic layer. It is preferably 0.5 times, particularly preferably 0.05 to 0.25 times. By setting the particle size of the carbon black in the undercoat layer to 0.5 times or less of the particle size of the carbon black having a particle size larger than the thickness of the magnetic layer contained in the magnetic layer, the carbon black protrudes from the undercoat layer. The overlap at the interface between the carbon black to be formed and the carbon black having the large particle size in the upper magnetic layer can be suppressed, and the surface of the magnetic layer can be made smoother. Accordingly, when the carbon black having the large particle size contained in the upper magnetic layer receives a load from the magnetic head, the load can be more uniformly dispersed in the undercoat layer.

The carbon black having a particle size of 0.01 to 0.03 μm includes “BLACK PEA” manufactured by Cabot.
RLS 800 "," Mogul-L "," VULCA "
NXC-72 "," Regal 660R "," Raven 1255 "," C
ondtex SC ". Particle size 0.05
For carbon blacks of up to 0.3 μm, “BLACK PEARLS 130” and “Mona” manufactured by Cabot
rch 120 "and" R "manufactured by Columbian Carbon Co., Ltd.
aven 450 "," Raven 410 "," TermaxPowder N-991 "manufactured by Kancarb.
and so on.

In forming the undercoat layer, the same aromatic solvents, ketone solvents, ester solvents, alcohol solvents, and the like as those for the magnetic layer may be used as solvents for preparing the undercoat layer paint and the lubricant solution. Solvents such as hexane and tetrahydrofuran are used.

In the present invention, the above-mentioned undercoat layer is not limited to one layer, but may be two or more layers as required. For example, as a multilayer structure of two or more layers having different types and amounts of inorganic powders and binders, a total of three or more layers including the upper magnetic layer may be formed.

Next, as the non-magnetic support, any conventionally used non-magnetic support for magnetic recording media can be used. Specifically, polyethylene terephthalate
Polyesters such as polyethylene naphthalate,
A film having a thickness of usually 3 to 100 μm, which is composed of polyolefins, cellulostriacetate, polycarbonate, polyamide, polyimide, polyamideimide, polysulfone, aramid, aromatic polyamide and the like is used.

The tracking servo mechanism used to achieve a high track density on a high-density magnetic disk is designed to follow the use environment, especially when the anisotropy of shrinkage of the non-magnetic support that occurs in a test under a high temperature environment is large. And tracking errors easily occur. For this reason, the non-magnetic support has a heat shrinkage of 105 ° C. for 30 minutes, ie, 105 ° C.
The heat shrinkage after heat treatment for 30 minutes at room temperature and cooling after cooling is preferably 1.5% or less in the vertical direction and 1.0% or less in the horizontal direction. The above thermal shrinkage rate was determined by taking six test pieces of 10 mm in width and 300 mm in length of the non-magnetic support from MD / TD, respectively.
After heat treatment for 30 minutes in hot air of 05 ° C. and cooling, the length is measured and [(original length−length after shrinkage) / original length]
It is obtained as an average value of × 100 (%).

In the production of the magnetic recording medium of the present invention, the formation of the magnetic layer and the undercoat layer can be carried out by using a conventionally known paint production process, and particularly by using a kneading process using a kneader or the like and a primary dispersion process together. Is preferred. In the primary dispersion step, the use of a sand mill is preferable because the dispersibility of the magnetic powder can be improved and the strength of the coating film can be adjusted, whereby the wear volume of the steel ball of the magnetic layer can be set.

In the above-mentioned primary dispersion step, it is preferable to use zirconia beads having high hardness as a dispersion medium. The zirconia beads are preferably obtained by normal-temperature isostatic pressing (CIP method) or high-temperature isostatic pressing (HIP method). Among them, zirconia beads by the HIP method, which are close to the theoretical density, hardly crack the beads even when strongly dispersed by a sand mill or the like, and cause uniform wear, are most preferable. Specific examples of such a zirconia bead include "Treceram" manufactured by Toray Industries, Inc. and "Zirconia Ball" manufactured by Nippon Kagaku Ceramics Co., Ltd. The dispersion time is preferably adjusted as appropriate in the range of 30 to 100 minutes as the residence time of the paint.

In the coating step, conventionally known coating methods such as gravure coating, roll coating, blade coating and extrusion coating can be used. At that time, the method of applying the undercoat layer and the magnetic layer is to apply the magnetic layer after coating and drying the undercoat layer on the non-magnetic support, or to sequentially coat the undercoat layer and the magnetic layer, Any of the simultaneous multilayer coating methods may be employed.

In manufacturing the magnetic recording medium of the present invention,
After coating and drying, it is desirable to perform a surface treatment with a calendar using a plastic roll or a metal roll. By performing the calendar treatment, the surface of the magnetic layer can be smoothed, the filling degree of the ferromagnetic iron-based alloy powder can be improved, and the residual magnetic flux density can be improved. The temperature for such calendaring is preferably from 60 to 150 ° C.
~ 120 ° C is more preferred. Further, the linear pressure of the calendar processing is preferably 98 to 294 kN / m, and 118 to 196 kN / m.
kN / m is more preferred.

In the magnetic recording medium of the present invention, the wear volume of the steel ball of the magnetic layer also changes depending on the surface roughness of the magnetic layer. It can be adjusted appropriately by surface polishing treatment. The surface roughness of the magnetic layer is determined by the roll pressure or the like in the calendering step so that the average surface roughness (Ra) in the optical interferometer three-dimensional surface roughness is 1 to 8.5 nm, preferably 3 to 7 nm. It is preferable to perform a mirror finishing process and a final coating film surface polishing process by adjusting the temperature.

It is preferable that the surface of the coating film is polished with a polishing wheel such as alumina or chromium oxide having an average surface roughness of 0.4 μm or less or a polishing tape. The polishing conditions were such that the air pressure was 0.01 to 1.0MPa.
a, polishing time is 0.5 to 5 seconds, disk rotation speed is 500
It is preferable to adjust appropriately within the range of 5,000 min ⁻¹ . If the Ra is less than 1 nm, the magnetic layer becomes too smooth, the wear coefficient increases, and sticking to the magnetic head occurs, and the decrease in temperature cycle durability is remarkable due to an increase in the real contact area with the magnetic head. Become. Ra is 8.
If it is larger than 5 nm, the unevenness of the surface of the magnetic layer becomes conspicuous, so that powder fall-off due to scraping of the magnetic layer similarly occurs, so that the seek durability is easily deteriorated and the S / N ratio is also deteriorated.

When the magnetic recording medium of the present invention has a disk shape, a hub made of metal or plastic is used as a hub for mounting the disk. In this case, it is preferable to use a hub having a surface runout width of 80 μm or less. Since high-density magnetic disks are used at high disk rotation speeds, the disk run-out width of the hub tends to directly affect the disk run-out width in a disk clamp mechanism that fixes and rotates the disk inserted in the drive to the spindle. In addition, the output fluctuates, and a local load is easily applied to the disk at the time of rotation, whereby the seek durability is easily reduced. Therefore, the runout width is 80
By using a hub having a diameter of not more than μm, preferably not more than 60 μm, it is possible to further improve the seek durability at a high output.

The above-mentioned surface runout width in the present invention is determined by measuring the hub alone with an electromagnetic conversion characteristic evaluation device described below at 1,000 min ^-1.
The dynamic fluctuation width in the direction of the rotation axis when the rotation is performed is detected by a photo sensor (spot diameter 0.5 μm), and the maximum value is shown.

In the magnetic recording medium of the present invention, a servo signal is provided in a certain area, and the servo signal may be provided before recording the data signal as described above. ―
May be provided at the same time as recording the data signal. The servo signal includes a magnetic servo signal and an optical servo signal conventionally used. The area occupied by the servo signal is preferably 7% or less for the disk-shaped medium and 10% or less for the tape-shaped medium with respect to the total area of the magnetic layer of the medium. Outside this range, the tracking accuracy is improved, but the proportion of the data area on the effective recording surface is reduced, making it difficult to secure a recording capacity.

As a method of providing a general sector servo signal in a magnetic servo signal, for example, each sector can be provided by using a dedicated drive for a servo light which has been modified so that a magnetic head can be track-transmitted with high precision. The servo signal A is shifted by half a track to the head of
And then shift the position by half a track in the opposite direction
The signal B is recorded. The servo signal A and the servo signal B use different frequencies so that the servo signal processing circuit can easily identify them. The number of bits of each servo signal is determined according to the ratio of the servo signal area to the recording surface, the relative speed between the head and the medium, the configuration of the servo circuit, and the like. Further, in a system in which off-tracks are assumed to cover a plurality of tracks, for example, four servo signals of a servo signal A, a servo signal B, a servo signal C, and a servo signal D having different frequencies are used. Is used to make it easier to detect the correct track position.

For example, an optical servo signal is provided by forming a broken-line groove having a duty ratio of 50:50 between tracks and detecting the position of this groove with an optical sensor to position the head. The method of doing is taken. The width of the groove at this time is appropriately determined according to the ratio of the servo signal area to the recording surface, the required S / N ratio of the servo signal, and the like.
The method of forming the groove includes a method of engraving by a stamp method,
There is a method of burning off a part of the magnetic layer surface with a laser beam. In the case of a disk-shaped medium, the latter method, in which the roundness of the formed groove is high, can provide higher tracking accuracy. In the case of a tape-shaped medium, an auxiliary servo signal may be provided on a back coat surface not involved in magnetic recording. For such a servo signal, for example, a method of providing an optical servo signal by printing or the like is used. However, it is difficult to ensure mutual positional accuracy with the magnetic data track, and thus the servo signal is provided on the recording surface. It is necessary to use it together with the b signal.

In the present invention, when the above-mentioned disk-shaped magnetic recording medium is used in a high-speed rotating drive, the wear of the nonwoven fabric causes an increase in the error. Is preferred.

[0093]

The present invention will now be described more specifically with reference to examples. In each example, “parts” means “parts by weight”.

Example 1 <Coating Component for Undercoat Layer> 65 parts of α-iron oxide (average major axis length: 0.14 μm, average acicular ratio: 7) 10 parts of granular α-alumina (particle diameter: 0.4 μm) Carbon black (particle size: 0.024 μm) 18 parts Carbon black (particle size: 0.075 μm) 7 parts Vinyl chloride-vinyl acetate-vinyl alcohol copolymer resin 16 parts (contains -SO ₃ Na group: 0.1 part) 07 mmol / g) Polyurethane resin (containing -SO ₃ Na group: 0.1 mmol / g) 7 parts Oleyl oleate (melting point: 0 ° C or less) 6 parts n-butyl stearate (melting point: 28 ° C) 2 parts Cyclohexanone 200 parts 200 parts of methyl ethyl ketone

<Magnetic paint component> 100 parts of ferromagnetic iron-based alloy powder containing Co and Y (Co / Fe: 0.2, Y / Fe: 0.03, Al / Fe: 0.02, pH: 7) , Average major axis length: 0.12 μm, Hc: 159.2 kA / m, BET specific surface area: 50 m ² / g, σs: 17.6 μWb / g) Carbon black (particle size: 0.02 μm) 1 part Bomb black (particle size: 0.35 μm) 2.5 parts Granular α-alumina (particle size: 0.3 μm) 15 parts Vinyl chloride-vinyl acetate-vinyl alcohol copolymer resin 13 parts (contained -SO ₃ Na group: 0) .07mmol / g) polyurethane resin (containing -SO ₃ Na group: 0.1 mmol / g) 4 parts myristic acid (carbon number: 15), 1 part oleyl oleate (mp: 0 ° C. or less) 5 parts cyclohexanone 250 parts methyl ethyl Ketone 250 parts

After the above-mentioned coating composition for undercoat layer was kneaded with a kneader, it was dispersed with a sand mill for a residence time of 60 minutes, and 6 parts of polyisocyanate was added thereto to prepare a coating for undercoat layer. Separately, the above magnetic paint components are kneaded by kneading with a kneader, and the residence time is 6 minutes by a sand mill.
The dispersion was dispersed in 0 minutes, and 7 parts of polyisocyanate was added to the mixture to prepare a magnetic coating material. The above-mentioned undercoat layer paint is coated on a support (1) made of polyethylene terephthalate film.
The film was applied on both sides so that the heat shrinkage at 05 ° C. for 30 minutes was 0.8% in the vertical direction and 0.6% in the horizontal direction, and the thickness after drying was 1 μm on one side.

After the undercoat layers were formed on both sides of the support in this manner, the magnetic coating was calendered on the undercoat layer to a thickness of 0.18 on one side.
It was applied on both sides to a thickness of μm and dried. The particle size of the carbon black in the undercoat layer is 0.07 to 0.21 of the particle size of the large carbon black in the upper magnetic layer.
It was double. Next, the magnetic sheet is mirror-finished with a 5-stage calendar (temperature: 80 ° C., linear pressure: 147 kN / m).
After aging at 60 ° C. for 24 hours, it was punched into a 3.5-inch size. Then, an alumina polishing tape (WA-6)
000: subjected to a surface polishing treatment (air pressure: 0.25 MPa, polishing time: 1 second, disk rotation speed: 2,000 min ^-1 ) by an alumina particle size of 2 μm and surface roughness of 0.4 μm, and further performed at 70 ° C. for 24 hours. -A magnetic disk was prepared by a zing process.

This magnetic disk was mounted on a 3.5-inch floppy hub having a runout width of 60 μm, and a groove width of 2.2 μm was formed on the back surface of the disk by a laser cut method.
m, 1,500 grooves per lap, duty
Ratio 50:50, servo track pitch 20.4 μm,
An optical servo groove is provided at an area ratio of 5.4% to the magnetic layer area within a track radius of 23 mm to 39.5 mm, and for a 3.5-type floppy disk with a non-woven fabric without a lifter. The final magnetic disk was stored in a cartridge.

Example 2 In the magnetic paint component, the magnetic powder was a ferromagnetic iron-based alloy powder containing Co and Y (Co / Fe: 0.2, Y / Fe: 0.1.
03, Al / Fe: 0.03, pH: 7, average major axis length:
0.07 μm, Hc: 159.2 kA / m, BET specific surface area: 50 m ² / g, σs: 17.0 μWb / g) 10
0 parts, 15 parts of granular α-alumina having a particle size of 0.15 μm are used in place of 15 parts of granular α-alumina having a particle size of 0.3 μm, and carbon black having a particle size of 0.35 μm. Instead of 5 parts, use 2.5 parts of carbon black having a particle size of 0.27 μm (the particle size of the carbon black in the undercoat layer is the same as the particle size of the carbon black having a large particle size in the upper magnetic layer). 0.12 to 0.38 times), and the thickness of the magnetic layer after the calendering treatment was changed to 0.10 μm on one side, and the surface polishing treatment was further performed using an alumina polishing tape (WA-
6000) instead of alumina polishing tape (WA-800).
0: alumina particle size 1.2 μm, surface roughness 0.07 μm)
A magnetic disk having an optical servo group was produced in the same manner as in Example 1 except that the above procedure was performed.

Example 3 2.5 parts of a carbon black having a particle size of 0.27 μm were used in place of 2.5 parts of a carbon black having a particle size of 0.35 μm in the magnetic coating composition (the amount of the carbon in the undercoat layer was changed). The particle size of the black layer is 0.09 to 0.28 times the particle size of the large carbon black particles in the upper magnetic layer), and the thickness of the magnetic layer after calendering is 0.1 mm on one side. 16 μm, the amount of oleyl oleate added in the undercoat layer coating composition was changed from 6 parts to 13 parts, and the dispersion condition of the undercoat layer coating composition by a sand mill was changed from a residence time of 60 minutes to 50 parts.
A magnetic disk having an optical servo group was produced in the same manner as in Example 1 except that the number of minutes was changed.

Example 4 In the magnetic paint component, the magnetic powder was changed to a ferromagnetic iron alloy powder containing Co and Ce (Co / Fe: 0.2, Ce / Fe:
0.04, Al / Fe: 0.05, pH: 7, average major axis length: 0.14 μm, Hc: 159.2 kA / m, BET
(Specific surface area: 50 m ^{2 /} g, σs: 16.3 μWb / g)
100 parts, and the thickness of the magnetic layer after calendering was changed to 0.20 μm on one side, and the dispersing conditions of the undercoat layer paint component and the magnetic paint component by a sand mill were set to a residence time of 60 minutes to 90 minutes, respectively. And a magnetic disk having a groove for an optical servo in the same manner as in Example 1 except that the mirror finishing treatment using a five-stage calendar is performed at a temperature of 70 ° C. and a linear pressure of 118 kN / m. Was prepared.

Example 5 In the magnetic coating composition, the magnetic powder was changed to a ferromagnetic iron-based alloy powder containing Co and La (Co / Fe: 0.3, La / Fe:
0.03, Al / Fe: 0.02, pH: 9, average major axis length: 0.12 μm, Hc: 151.2 kA / m, BET
(Specific surface area: 55 m ^{2 /} g, σs: 18.8 μWb / g)
Change the dispersion conditions of the undercoat layer paint component and the magnetic paint component by a sand mill from a residence time of 60 minutes to 90 minutes, and perform a mirror finishing treatment using a five-stage calendar at a temperature of 90 ° C. and a linear pressure of 100 parts. 245kN /
A magnetic disk having an optical servo group was produced in the same manner as in Example 1 except that the operation was carried out at m.

Example 6 In the magnetic paint component, the magnetic powder was changed to a ferromagnetic iron-based alloy powder containing Co and Nd (Co / Fe: 0.4, Nd / Fe:
0.03, Al / Fe: 0.05, pH: 7, average major axis length: 0.10 μm, Hc: 175.1 kA / m, BET
(Specific surface area: 55 m ^{2 /} g, σs: 18.2 μWb / g)
While changing to 100 parts, 1 part of myristic acid was changed to 1 part of stearic acid (carbon number: 18), and 5 parts of oleyl oleate was changed to 8 parts of 2-ethylhexyl oleate (melting point: 0 ° C. or less). Further, in the same manner as in Example 1 except that the addition amount of oleyl oleate in the coating composition for the undercoat layer was changed from 6 parts to 3 parts, a magnetic disk having a groove for an optical servo was used. Was prepared.

Example 7 In the magnetic paint component, the magnetic powder was changed to a ferromagnetic iron alloy powder containing Co and Tb (Co / Fe: 0.3, Tb / Fe:
0.05, Al / Fe: 0.03, pH: 7, average major axis length: 0.10 μm, Hc: 167.1 kA / m, BET
(Specific surface area: 55 m ^{2 /} g, σs: 18.8 μWb / g)
100 parts, and the thickness of the magnetic layer after calendering was changed to 0.17 μm on one side, and γ-Fe ₂ O was used instead of 65 parts of α-iron oxide in the undercoat layer coating composition.
₃ Magnetic powder (particle size: 0.12 μm, Hc: 23.9 kA /
m, σs: 9.4 μWb / g, BET specific surface area 25 m ²
/ G) 65 parts, and 1 non-magnetic support
A polyethylene terephthalate film having a heat shrinkage of 0.3% in the vertical direction and 0.1% in the horizontal direction at a temperature of 05 ° C. for 30 minutes is used, and a hub having a surface runout of 70 μm is used. In the same manner as in Example 1, the optical servo group
A magnetic disk having a magnetic disk was manufactured.

Comparative Example 1 In the magnetic coating composition, the magnetic powder was changed to a Fe—Ni—Zn-based ferromagnetic iron-based alloy powder (Al /
Fe: 0.03, pH: 11, average major axis length: 0.35μ
m, Hc: 131.3 kA / m, BET specific surface area: 45
m ^{2 /} g, σs: 15.1 μWb / g) Same as Example 1 except that the thickness was changed to 100 parts, the thickness of the magnetic layer was changed to 0.20 μm on one side, and no undercoat layer was provided. In this way, a magnetic disk having an optical servo group was produced.

Comparative Example 2 In the magnetic coating composition, 2.5 parts of carbon black having a particle size of 0.27 μm was used instead of 2.5 parts of carbon black having a particle size of 0.35 μm. The particle size of the carbon black is 0.09 to 0.28 times the particle size of the large carbon black in the upper magnetic layer.
The amount of alumina (particle size: 0.3 μm) was changed from 15 parts to 30 parts, the thickness of the magnetic layer was changed to 0.30 μm, and a nonmagnetic support was used at 105 ° C. for 30 minutes. Example 1 was the same as Example 1 except that a polyethylene terephthalate film having a heat shrinkage of 2.0% in the vertical direction and 1.2% in the horizontal direction was used, and a hub having a surface runout width of 100 μm was used. Similarly, a magnetic disk having an optical servo groove was manufactured.

Comparative Example 3 In the magnetic paint component, the magnetic powder was a ferromagnetic iron-based alloy powder containing Co and Y (Co / Fe: 0.2, Y / Fe: 0.
01, Al / Fe: 0.02, pH: 7, average major axis length:
0.12 μm, Hc: 159.2 kA / m, BET specific surface area: 50 m ² / g, σs: 17.0 μWb / g) 10
0 parts and 3.0 parts of carbon black of 0.075 μm in place of 2.5 parts of carbon black of 0.35 μm and 1 part of carbon black of 0.02 μm. (The particle size of the carbon black in the undercoat layer is 0.32 to 1.00 times the particle size of the carbon black having a large particle size in the upper magnetic layer). Was changed to 0.20 μm, and in the coating components for the undercoat layer, the vinyl chloride-vinyl acetate-vinyl alcohol copolymer resin and the polyurethane resin each had -SO ₃ Na groups as functional groups, and only -OH groups were used. And 6 parts of oleyl oleate were changed to 18 parts of cetyl stearate (melting point: 40 ° C.), and the conditions for dispersing the undercoat layer coating component and the magnetic coating component by a sand mill were changed. Was changed to respectively residence time of 60 minutes to 90 minutes, in the same manner as in Example 1, the optical service - to prepare a magnetic disk having a blanking - board for Group.

Comparative Example 4 In the magnetic paint component, the magnetic powder was changed to a ferromagnetic Fe—Ni alloy powder containing no Co and rare earth elements (pH: 11, average major axis length: 0.25 μm, Hc: 123.3 kA / m). , BE
T specific surface area: 55 m ^{2 /} g, σs: 15.1 μWb /
g) Change to 100 parts.
Instead of 2.5 parts of carbon black, use 5 parts of carbon black having a particle size of 0.075 μm (the particle size of the carbon black in the undercoat layer is the same as the particle size of the carbon black having a large particle size in the upper magnetic layer). The diameter was 0.32 to 1.00 times), and the amount of oleyl oleate was changed from 5 parts to 0.5 part, and the addition of myristic acid was omitted. The optical servo was performed in the same manner as in Example 1 except that the addition amount of oleyl oleate was changed from 6 parts to 0.5 part and the addition amount of n-butyl stearate was changed from 2 parts to 1 part. A magnetic disk having a working group was produced.

For each of the magnetic disks of Examples 1 to 7 and Comparative Examples 1 to 4, the average major axis length of the magnetic powder, the amount of lubricant extracted with n-hexane, the extraction of fatty acids and fatty acid esters The weight ratio, the wear volume of the steel ball of the magnetic layer, and the residual magnetization were measured by the methods described in the text. These results were as shown in Table 1.

The average major axis length of the magnetic powder was shown in relation to the shortest recording wavelength (1 μm) used for measuring the reproduction output described later. In extracting the lubricant with n-hexane, the magnetic disk was cut out at 600 cm ^{2 on} both sides. Further, in measuring the steel ball wear volume, a drive manufactured by Matsushita Hisashi Electronics Co., Ltd. (LKM-F
434-1) (3.5 inch cartridge is used).

[0111]

Next, for each of the magnetic disks of Examples 1 to 7 and Comparative Examples 1 to 4, the reproduction output, resolution,
S / N ratio, temperature cycle durability and seek durability
It was measured by the following method. These results were as shown in Table 2.

<Reproduction output> Spindle motor SPU-MUX158GV3 manufactured by NTN Corporation for disk rotation and precision stage MM-40XY, M manufactured by Chuo Seiki for head position adjustment.
An MIG type head having a track width of 8 μm and a gap length of 0.3 μm was attached to an electromagnetic conversion characteristic evaluation device composed of M-40Z, MM-40GU and MM-40GL, with a rotation speed of 1,000 min ⁻¹ and a recording frequency of 7,215 kHz.
After recording at a position with a radius of 35 mm with (recording wavelength 1
μm), and the peak-to-peak value of the output of the reproducing amplifier is set to an oscilloscope 5450 manufactured by Hewlett Packard.
Measured at 4A. The measured values are shown as relative values with the magnetic disk of Comparative Example 2 taken as 100%.

<Resolution> The measured absolute value of the reproduction output is represented by H
The F output is used, and the measured absolute value when the recording frequency is set to 1,804 kHz is used as the LF output. The above HF output and LF
The output ratio (HF output / LF output) was defined as the resolution. The measured values are shown as relative values with the magnetic disk of Comparative Example 2 taken as 100%.

<S / N ratio> The above HF output signal is converted to a spectrum analyzer 3588 manufactured by Hewlett-Packard Company.
A, frequency span 0-7MHz, RBW4.6
The spectrum was displayed at a frequency of kHz and 64 averaging times. At this time, a peak level value (dBm) of 3,608 kHz is measured as a signal level S, and an average value (dBm) of a noise floor level around 2.5 MHz and a noise floor level around 4.5 MHz is measured as a noise level N. , S / N ratio (dB) = S (dBm) -N (dBm). The measured value was 0 dB for the magnetic disk of Comparative Example 2.
As relative values.

<Temperature Cycle Durability> Using the apparatus for measuring the reproduction output described above, 5 ° C., 20% RH and 50 ° C., 80%
Under a RH temperature cycle environment, the vehicle was continuously driven at a position with a radius of 35 mm, and the total running time until the output voltage was reduced to 85% of the initial value was examined.

<Seek Durability> Using the above-described apparatus for measuring reproduction output, at 20 ° C. and 65% environment, r = 30 to 4
A seek operation is performed in a range of 0 mm at a speed of 10 reciprocations / minute.
The total running time until the output voltage at the position of 35 mm decreased to 85% of the initial value was examined.

[0118]

As is clear from Table 2 above, each of the magnetic disks of Examples 1 to 7 of the present invention has a high reproduction output, a high resolution and a high S / N ratio, and has a temperature cycle durability and a seek durability. It is understood that both the electromagnetic conversion characteristics and the running durability are excellent. On the other hand, each of the magnetic disks of Comparative Examples 1 to 4 is clearly inferior in either the electromagnetic conversion characteristic or the running durability.

[0120]

As described above, the present invention relates to a magnetic recording medium provided with a servo signal, wherein an undercoat layer is provided between a nonmagnetic support and a magnetic layer, and a magnetic powder contained in the magnetic layer is provided. By using a ferromagnetic iron-based alloy powder having a specific elemental composition and average major axis length as well as specifying the thickness of the magnetic layer and the particle size of the carbon black contained in the magnetic layer,
Excellent electromagnetic conversion characteristics, high output, high resolution and high S
/ N ratio and excellent running durability.
A high-density magnetic recording medium having particularly satisfactory seek durability and temperature cycle durability can be provided.

[Brief description of the drawings]

FIG. 1 is an explanatory diagram showing an apparatus for measuring a steel ball wear volume of a magnetic layer.

[Explanation of symbols]

 1 Drive 2 Lower Carriage 3 Steel Ball 4 Balance 5 Metal Plate

──────────────────────────────────────────────────続 き Continued on the front page (51) Int.Cl. ⁷ Identification symbol FI Theme coat ゛ (Reference) G11B 5/706 G11B 5/706 5/708 5/708 23/00 601 23/00 601C H01F 1/047 H01F 1/06 J (72) Inventor Teruhisa Miyata 1-1-88 Ushitora, Ibaraki City, Osaka Prefecture Inside Hitachi Maxell Co., Ltd. (72) Inventor Yoshiyuki Makita 1-188 Ushitora, Ibaraki City, Osaka Prefecture Hitachi Maxell shares In company

Claims (7)

[Claims] In a magnetic recording medium provided with a servo signal, at least one undercoat layer and a magnetic layer are formed in this order on a nonmagnetic support, and the thickness of the magnetic layer is 0.20.
μm or less, the magnetic powder contained in the magnetic layer is composed of a ferromagnetic iron-based alloy powder containing Co and a rare earth element, the average major axis length L of which is 0.15 μm or less, the average major axis length L and the shortest recording wavelength λ. And L / λ ≦ １ ／, and the magnetic layer contains carbon black having a particle size larger than the thickness of the magnetic layer. 2. The non-magnetic support has a heat shrinkage of 1.5% or less in a longitudinal direction and 1.0% or less in a transverse direction after heat-treated at 105 ° C. for 30 minutes and allowed to cool. 3. The magnetic recording medium according to 1. 3. A component extracted with n-hexane from the magnetic layer and the undercoat layer contains at least one lubricant selected from fatty acids and fatty acid esters, and the amount of the lubricant extracted is 10 to 250 kg. ^3. The magnetic recording medium according to claim 1, wherein the ratio is / m ³ . 4. A lubricant comprising a fatty acid having 10 or more carbon atoms and a fatty acid ester having a melting point of 35 ° C. or less is contained in a component extracted with n-hexane from the magnetic layer and the undercoat layer. The extraction weight ratio of both components is 0.3 / 9
4. The magnetic recording medium according to claim 3, wherein the ratio is 9.7 to 10/90. 5. An undercoat layer containing carbon black, wherein the particle size of the carbon black is 0.1% of the particle size of the carbon black having a particle size larger than the thickness of the magnetic layer contained in the magnetic layer. The magnetic recording medium according to claim 1, wherein the ratio is from 01 to 0.5. 6. The steel ball wear volume of the magnetic layer is 0.5 × 10
The magnetic recording medium according to claim 1, wherein the magnetic recording medium has a size of ^{−4 to} 5.0 × 10 ⁻⁴ mm ³ . 7. A disk-like shape having a surface runout width of 8
7. The magnetic recording medium according to claim 1, wherein the magnetic recording medium is attached to a hub having a diameter of 0 [mu] m or less.