JPH02108232A - Magnetic recording medium - Google Patents

Abstract

PURPOSE:To improve electromagnetic conversion characteristics and traveling durability by combining a nonmagnetic supporting substance having specific properties and a particle size of the magnetic material in a magnetic layer. CONSTITUTION:The nonmagnetic supporting substance is a film essentially comprising polyethylene terephethlate having the proper cutting aptitude index Z < 6 [Z = 383.3 - 2.76A - 2,000B + 840 n, where A represents Heis' factor, a coefft. of plane orientation B = {(nMD + nTD)/2}, and difference of refractive indices n = nMD - nTD]. The first and the second magnetic layer are successively formed thereon. The first magnetic layer has a specific surface area of 45m<2>/g or less by BET method and the grain size of 290 Angstrom or more, while the second magnetic layer has 30m<2>/g or more specific area and 400 Angstrom or less grain size. The specific surface area of the first magnetic layer is specified to be lower than that of the second magnetic layer by more than 5m<2>/g. The grain size of the first magnetic layer is specified to be larger than that of the second magnetic layer by 40 Angstrom or more.

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a magnetic recording medium, and more particularly to a magnetic recording medium that is easy to cut, has a sharp cut surface, and has excellent properties with no powder falling off.

[Conventional technology]

In general, tape-shaped magnetic recording media are often used, and these tape-shaped magnetic recording media are made by coating a magnetic paint containing ferromagnetic powder on the surface of a non-magnetic support such as a polyester film, and then aligning and It is manufactured by drying, calendering, etc., and then cutting it into a desired width.

If there are chips or cracks on the cut surface of the cut tape, or if there are chips left behind, it may cause powder to fall from that area, cause edge damage, or cause dropouts. This results in a significant deterioration in the performance of the magnetic recording medium. Therefore, it is extremely important to finish the cut surface of the tape neatly in the production of tape-shaped magnetic recording media.

For cutting magnetic recording media, a shear cutting method using a blade is usually adopted, and a blade with extremely high processing precision of 1OIII or less is used.

Additionally, various improvements have been proposed regarding the cutting device.

However, recently there has been a demand for the cut surfaces of tape-shaped magnetic recording media to be of extremely high quality, and improvements in cutting devices and methods cannot satisfactorily meet such demands.

In addition, in such a cutting process, wear of the blades used is unavoidable, and the blades must be periodically polished or replaced. Therefore, in order to increase the production efficiency of tape-shaped magnetic recording media, it is necessary to It is necessary to minimize the wear of this cutter.

In addition, with the increase in recording density of magnetic recording media, high-quality image quality,
As the requirements for sound quality are becoming higher and higher, it has become necessary to improve electromagnetic conversion characteristics, especially to increase C/N and lower bias noise.

For this purpose, it is necessary to make the ferromagnetic powder smaller to increase the number of magnetic substances per unit volume, and it is also necessary to improve the surface properties of the magnetic layer.

On the other hand, JP-A-54-145104, JP-A-58-56
No. 231 and JP-A No. 60-256917, the electromagnetic conversion characteristics can be improved by multilayering the magnetic layer and using ferromagnetic metal alloy powder in the upper layer and iron oxide-based ferromagnetic powder in the lower layer to separate the functions. There are also examples of improved performance.

For example, in JP-A No. 58-56231, B was added to the upper magnetic layer.
A metal magnetic powder with a specific surface area of 35 to 80 nf/g by the ET method is used to make Hc 500 to 9000e, while an oxide magnetic material powder with a specific surface area of 18 to 30 nf/g by the BET method is used for the lower magnetic layer. It is said that distributing magnetic powder reduces bias noise, and distributing oxide magnetic material powder with a small specific surface area inside the magnetic layer increases reproduction output over the entire band.

[Problem to be solved by the invention]

However, in such a multilayer magnetic layer, the roles of the first magnetic layer and the second magnetic layer are different, and their mechanical properties are also different, but when trying to cut using the shear cutting method, chips may occur on the cut surface. It was easy to cause powder falling, edge damage, dropouts, etc. to occur frequently.

Therefore, an object of the present invention is to provide a multi-layer tape-shaped magnetic recording medium cut by the shear cutting method with an extremely clean cut surface of high quality, less powder falling, edge damage, dropouts, etc., and with good electromagnetic conversion characteristics. Another object of the present invention is to provide a magnetic recording medium that can produce a tape-like multilayered magnetic recording medium that has excellent running durability.

[Means to solve the problem]

As a result of extensive research into the cutting suitability of multi-layered magnetic recording media, the present inventors found that by combining a new non-magnetic support with specific properties and the particle size of the magnetic material constituting the magnetic layer, the above-mentioned We have completed the present invention by discovering that the object can be achieved. That is, the present invention provides a magnetic recording medium comprising a non-magnetic support and a magnetic layer provided on the surface thereof, wherein the non-magnetic support is mainly made of polyethylene terephthalate having a cutting suitability index (Z) of 6 or less. In the magnetic recording medium, which is a film in which a first magnetic layer and a second Mi layer are provided in this order on the surface of the non-magnetic support, the specific surface area of the eleventh ifi layer magnetic material by the BET method is: 45 rrf/g or less, the crystallite size is 290 Å or more, the specific surface area of the second magnetic layer magnetic material by the BET method is 30 rrf/g or more, the crystallite size is smaller, and the difference therebetween is 5 rrf/g. The magnetic recording medium is characterized in that the crystallite size of the first magnetic layer magnetic material is larger than that of the second magnetic layer, and the difference therebetween is 40 Å or more.

Equation (1) in the present invention is expressed as 1/
This is an experimental formula determined by measuring the number of chips generated when cutting a 2-inch wide videotape and adhering to the cut surface.

In formula (1), the haze value A is a value related to the amount of microvoids in the film, and the larger the haze value, that is, the greater the amount of microvoids, the less load is applied to the cutting tool during cutting. It becomes easier to remove. However, when the amount of microvoids increases, the strength of the film decreases, making it unsatisfactory as a support for a magnetic recording medium. Therefore, the haze value A is preferably 2 to 10.

In addition, the haze value is 5pheria method H
This is the value of scattered light Td when using a TR meter (manufactured by Nippon Precision Engineering Co., Ltd.), inserting a G filter 550-μ, using liquid paraffin as a blank, and inserting a film.

The planar orientation coefficient B is a value that indicates the degree to which the molecular axes in the film are aligned in a direction parallel to the film surface. will be excellent.

When the plane orientation coefficient B is small, that is, when the degree to which the molecular axes are aligned in the direction perpendicular to the film plane increases, the number of chips on the cut surface of the film increases and the number of chips increases. The coefficient B is a value determined from the refractive index n in the longitudinal direction, the refractive index nte in the ID width direction, and the refractive index nZD in the thickness direction of the film as described above, and is a value of 0.15 to 0.17. is preferred.

Note that these refractive indexes are determined by Atsube's refractometer type 3 using the NaD ray as a light beam.

Also, the refractive index in the longitudinal direction is n. is 1.63 to 1.66,
The refractive index nTD in the width direction is 1.65 to 1.69, the refractive index nzo in the thickness direction is 1.48 to 1.51, and Δn
= null −nTD is preferably −0,02 to −0,05.

The smaller the cutting suitability index (Z) determined by equation (1) is, the better the cutting characteristics are.

[Actions and effects of the present invention]

As a result of intensive research into the cuttability of magnetic recording media, the present inventors have found that the above objective has been achieved by a magnetic recording medium that includes a novel combination of a non-magnetic support with specific properties and a magnetic layer with specific properties. We discovered that in a magnetic recording medium that includes a non-magnetic support and a magnetic layer provided on its surface, the non-magnetic support is mainly made of polyethylene terephthalate having a cutting suitability index (Z) of 6 or less. The magnetic layer has a yield elongation (L) of 10% or less and an energy to yield point (E) of 1.0 kg/■2 or less.
) (Patent Application No. 1983)
-218730) and a magnetic recording medium comprising a non-magnetic support and a magnetic layer provided on the surface thereof, wherein the non-magnetic support is made of polyethylene having a cut-through crystal index (Y) of 6 or more. It is a film mainly composed of terephthalate, and the magnetic layer is IO! Yield elongation (shi) below and 1.0k
Magnetic recording medium characterized by having an energy (E) up to the yield point of less than
No. 731) was filed.

However, in the case of multilayered magnetic recording media, because the mechanical properties of the upper and lower layers are different, our investigation has revealed that the cutting situation is different from that of conventional uniform magnetic layers.

That is, in the case of a multilayer magnetic recording medium, Japanese Patent Application No. 62218
Even if the mechanical properties of the magnetic layer and the pace cutting suitability index or the cutting suitability crystal index as shown in Patent Application No. 730 and Japanese Patent Application No. 62-218731 are satisfied, a good cut surface cannot be obtained due to powder dropout and edge cracking. In many cases there was none.

The magnetic recording medium of the present invention is a magnetic recording medium in which a first magnetic layer and a second magnetic layer are provided in this order on the surface of a non-magnetic support, in which the first magnetic layer has a specific surface area of 45 ia 8 or less by the BET method, The specific surface area of the second magnetic layer is 30% or more than 7g, and the specific surface area of the first magnetic layer measured by the BET method is smaller than that of the second magnetic layer, and the difference therebetween is 5rrf/g or more. .

As mentioned above, in the case of a multi-layered magnetic recording medium, the magnetic layer has a yield elongation (L) of 10% or less, preferably 6% or less as specified in Japanese Patent Application Nos. 62-218730 and 62-218731. and 1.0 kg/asz or less, which is better (c) A good cutting surface cannot be obtained just by providing the magnetic layer physical properties with an energy (E) up to the yield point of 0.7 kg/rd or less.

As a result of our analytical research into the cause of this, we discovered that when cutting using the shear cutting method, the physical properties of the surface layer of the magnetic layer are a very important factor in determining the quality of the cut surface.

That is, at the very early stage of the slitting operation, when the blade comes into contact with the surface of the magnetic layer and begins cutting, very fine cracks occur in the magnetic layer, and the way in which the cracks occur determines the quality of the cut surface. We found that this was a very large factor.

If a small number of these cracks form in the surface layer of the magnetic layer in a relatively stable manner, a good cut surface can be obtained, whereas if a large number of cracks form in the surface layer, a good cut surface cannot be obtained.

The manner in which the cracks form is determined by the relationship between the specific surface area and crystallite size of the first magnetic layer measured by the BET method and the specific surface area and crystallite size measured by the BET method of the second magnetic layer.

That is, the BET of the second magnetic layer magnetic material (upper layer: magnetic layer surface layer)
When the specific surface area determined by the method is 30 rd/g or more and the crystallite size is 400 Å or less, this crack will stably occur,
If it is less than that, the occurrence of cracks becomes unstable.

Also, the crystallite size of the magnetic material in the first magnetic layer is the second T! It is necessary that the crystallite size of the i-layer magnetic material is larger than that, and the difference is 40 Å or more, and the specific surface area by the BET method is 5n.
It is necessary to be smaller than f/g.

A crack in the second magnetic layer propagates to the first magnetic layer, but this crack propagates smoothly if the above requirements are met. Otherwise, number one. Cracks often become discontinuous at the interface of the second magnetic layer.

In addition, the occurrence of powder falling while the magnetic recording medium is running is caused by the first i
ffff The smaller the magnetic crystal grain size of the magnetic layer is, the more disadvantageous it becomes. In particular, when the specific surface area by the BET method exceeds 45 nf/g, it becomes difficult to prevent powder falling from the cut surface during running.

(Detailed Description of the Invention) The magnetic recording medium of the present invention has a characteristic configuration in combination of a specified novel non-magnetic support and a magnetic layer having specified properties, and Regarding the part, conventionally known techniques for magnetic recording media can be applied.For example, a back layer may be provided on the surface of the nonmagnetic support opposite to the surface on which the magnetic layer is provided. Also, as the ferromagnetic powder, binder, antistatic agent, abrasive, lubricant, and other materials used in the magnetic layer and bank layer, known materials may be used in appropriate amounts. Furthermore, as a method for manufacturing a magnetic recording medium, a method known per se can be adopted.

The non-magnetic support in the present invention is preferably a film mainly composed of polyethylene terephthalate and having a cutting suitability index (Y) of 6 or more.

Equation (2) defines cutting suitability as the sharpness of the cut edge when cutting a 1/2 inch wide videotape from the original magnetic recording medium, and scores are determined through a sensory test to determine the best cutting suitability. This is an experimental formula obtained as a point value Y, with 7 points and 1 point for the worst case.

In formula (2), X is a value indicating the degree of plane orientation of the crystal, which is the (110) plane peak intensity T (110) by X-ray diffraction of a nonmagnetic support and the (100) plane peak intensity + It is preferable that sXi, which is the ratio of (110)/(100) to
, -1°-0.3 m using a graphite monochromator.

Further, Xc is the crystallite size, which is determined from the half-width of the (200) plane in X-ray diffraction of the nonmagnetic support. That is, Xc = 1.15λ/βcosθ [λ is the wavelength, θ is the diffraction angle, and β is the width broadening of the diffraction peak (β-(B!)
bri l/! B is the half-width of the diffraction peak of the sample, and b is the half-width of the diffraction peak of a perfect crystal. Note that the X-rays were measured using Cukβ while rotating the sample. It is preferable that xc is 40-60.

Further, Δn includes the refractive index in the longitudinal direction null and the refractive index in the width direction of the film. (nsonto), and is preferably a value of -0.02 to -0.05.

Note that these refractive indices are determined by an Atsube refractometer using NaD rays as a light source.

Moreover, it is preferable that the refractive index nMI in the longitudinal direction is 1.63 to 1.66, and the refractive index null in the width direction is 1.65 to 1.69.

A film mainly composed of polyethylene terephthalate having a cutability index (Z) of 6 or less, preferably a cutability crystal index (Y) of 6 or more, as described above has not been previously known, and is a novel unstretched film. It is a film and can be manufactured, for example, by the following method.

That is, a film mainly made of polyethylene terephthalate produced by a conventional method is first rolled between a pair of rolls with different peripheral speeds at a temperature of 90 to 110°C in the longitudinal direction (extrusion from a molding machine) for 2 to 4 hours. Stretch the film twice, then send the film to a tenter and hold both ends with clips.
Stretched 3 to 5 times in the width (horizontal) direction at a temperature of 90 to 120°C, and finally stretched 5 to IO at a temperature of 200 to 250°C while relaxing 2 to 8χ in the width direction in the same tenter. It can be manufactured by heat treatment for seconds.

In the above manufacturing method, by increasing the temperature during stretching in the longitudinal direction or increasing the stretching ratio in each of the longitudinal and width directions, it is possible to increase the microvoids in the film and increase the haze value (A). . Also, the haze value (
A) can also be adjusted by changing the type and amount of filler added to the film.

Furthermore, the planar orientation coefficient (B) and Δn are in the longitudinal direction.

For example, when the stretching ratio in the width direction is increased, B becomes larger and Δn becomes smaller.

In the above production method, the cutting suitability crystal index (Y) increases both χ1 and Δn by increasing the stretching ratio in the width direction, and increases by increasing the addition rate of the polyester filler.

Therefore, by appropriately adjusting the various conditions for producing the above film, it is possible to easily produce a film mainly composed of polyethylene terephthalate having a desired cutting suitability crystal index (Y).

Other conditions for the non-magnetic support in the present invention, for example,
Regarding conditions such as thickness, tensile strength, mechanical properties such as elastic modulus, and thermal properties such as heat shrinkage rate, conventionally known conditions can be applied.

Preferred embodiments of the present invention are as follows.

(1) The non-magnetic support has a cutting suitability crystal index of 6 or more (
2. The magnetic recording medium according to claim 1, wherein the magnetic recording medium is a film mainly composed of polyethylene terephthalate.

(2) The specific surface area of the first magnetic layer magnetic material determined by the BET method is 20 to 45 M/g, and the specific surface area of the second magnetic layer magnetic material determined by the BET method is 30 to 65 rrr/g, with a difference of 5 r
rf/g or more and 20 rrr/g or less, and the magnetic recording medium according to (1) above, wherein the specific surface area of the first magnetic layer magnetic material by the BET method is smaller than that of the second magnetic layer magnetic material. .

(3) The first magnetic layer is based on 100 parts by weight of ferromagnetic powder,
The magnetic recording medium according to 1) or (2) above, characterized in that it contains an abrasive in an amount of 1 to 30 parts by weight.

(4) The magnetic recording medium (5) of (υ-(3)) above, wherein the second magnetic layer contains an abrasive in the range of 0.1 to 15 parts by weight based on 100 parts by weight of the ferromagnetic powder. The first
The magnetic layer contains 0.3 to 2 parts by weight per 100 parts by weight of ferromagnetic powder.
(1) to (4) above, characterized in that the second magnetic layer contains 0.1 to 10 parts by weight of carbon black per 100 parts by weight of the ferromagnetic powder.
) magnetic recording media.

(6) The first magnetic layer has a layer thickness of 3μ or more, and the second
The magnetic recording medium of (1) to (5) above, wherein the layer thickness of the magnetic coral is 1.5 μm or less.

(7) Applying the coating liquid for the first magnetic layer onto the surface of the non-magnetic support while it is running, and continuously applying the coating solution for the second magnetic layer on the coating layer while the coating layer is in a wet state. The method for manufacturing a magnetic recording medium according to (11) to (6) above, characterized in that a liquid is applied.

〔Example〕

Nonmagnetic support films having a thickness of 14 μm and mainly composed of polyethylene terephthalate having different cutting suitability indices were produced in the following manner.

(Film) Calcium acetate: 0.08% by weight Lithium acetate: 0.15% by weight Antimony acetate. ...0.04% by weight trimethyl phosphate...0.15 weight ff1X and average particle size 1. I
0.03% by weight of calcium carbonate (IIeam) was added, and polyester was produced by polycondensation using a conventional method.

The obtained polyester was dried, an unstretched sheet was produced using an extrusion molding machine, and this sheet was stretched 3.2 times in the longitudinal (extrusion) direction using a roll at a film temperature (measured with an infrared thermometer) of 100°C. Next, both ends were clipped in a tenku, and the film was heated at 110°C in the transverse direction for 3.
Stretched 7 times, then relaxed 5z in the transverse direction to 205
Polyester film-1 was obtained by heat treatment at a temperature of 'C for 10 seconds, and the cutting suitability index Z of film-1 was 1.8.

(Film-2) Among the same components as Film-1, only the amount of calcium carbonate added was changed to 0.5 weight χ to produce polyester,
Polyester film 2 was produced with a transverse stretching ratio of 3.9 times. The cutting passability index Z of Film-2 was 7.1.

(Film-3) Among the same components as Film-1, polyester was produced by changing only the added amount of calcium carbonate to 0.08% by weight, and the transverse stretching ratio was increased to 4.4 times to produce Polyester Film 3.
was manufactured.

The cutting passability index Z of Film-3 was 55.3.

The surface of these non-magnetic supports is coated with a first layer as shown below.
A coating solution was prepared from components of a magnetic layer coating solution and a second magnetic layer coating solution, and the coating solution was coated in this order to produce a magnetic recording medium.

Cor Fezes 1
00 parts vinyl chloride/vinyl acetate maleic anhydride copolymer (composition ratio 86:13:1. degree of polymerization 400)
12 parts polyester polyurethane resin 6 parts carbon black (18mμ, pH=8) 8 parts butyl stearate 1 part stearin 6
1 2 parts Butyl acetate
200 copies? Co 7 FeJz 10
0 parts Vinyl chloride/vinyl acetate maleic anhydride copolymer (composition ratio 86:13:1. degree of polymerization 400)
12 parts polyester polyurethane resin 6 parts carbon black (80-μ, pH=8) 1 part α-Affi to.

(Average particle size: 0.6μs) 6 parts butyl stearate 1 part stearic acid 2 parts butyl acetate
200 parts of each of the above two paints were cross-dispersed using a sand mill.

6 parts of polyisocyanate and 40 parts of butyl acetate were added to the obtained dispersion, and the mixture was filtered using a filter having an average pore size of 1 μι to form a coating solution for forming the first magnetic layer and the second magnetic layer. Each was adjusted.

The coating liquid for forming the first and second magnetic layers is applied to the first magnetic layer.
The following procedure was carried out using an extrusion coat for simultaneous multilayering having a slot for m-type N coating and a slot for coating the second magnetic layer.

The obtained coating liquid for the first magnetic layer has a thickness of 3.5 mm after drying.
While running a 14 μ-thick polyethylene terephthalate support at a speed of 60 m/sin, the first magnetic layer is coated on the surface of the support using an extrusion coater having slots for coating the first magnetic layer. Immediately (while the first magnetic layer is still wet), apply the coating liquid for the second magnetic layer using an extrusion coater having slots for coating the second magnetic layer so that the thickness after drying is 0.5 μm. Then, while the magnetic layer was still wet, it was oriented using a magnet, and after drying, it was subjected to a supercalender treatment, and each portion was slit to a width of 2 inches to produce a videotape.

Table 1 shows the results of evaluation of edge staining and powder falling on magnetic recording media in which the characteristics of the nonmagnetic support and the magnetic material of the magnetic layer were variously changed.

The measurement items were carried out as follows.

Measurement items: Edge stain: When rewinding the videotape, a non-woven fabric was placed on the edge end surface and the degree of stain was observed. Depending on the degree of stain, a sensory evaluation of XX, ×, Δ, O1◎ was given. .

xx: jill (linear stain, ×: linear stain, Δ:
Stained in pieces, O: Slightly stained, O: No smudge was visually observed.

Powder falling: For 1/2 inch slit. Using a VHS type VTR (AG-6200 manufactured by Matsushita Electric), the test tape was run for 2 hours and 300 passes, and the inside of the deck and half was measured.
The degree of dirt adhering to the audio control head and the ball was observed and scored on a 5-point scale.

In the case of 8m, the full length run is 30 with FIJIC5F-006.
0 bus repetitions, and the same evaluation as VH3 was performed.

Example-5 Film base used: Film-1 Coating solution number for the first magnetic layer was the same as Example-1, except that the magnetic material was Co.
-7-Fe・0・・H・“9800e・ Specific surface area by BET method: 45rrf/g Crystallite size: 29
Ferromagnetic metal powder (&l composition: Fe92χ, Zn4χ
, Ni4χ, Hc: 15300e Saturation magnetic flux density:
120emu/g, specific surface area by BET method: 5
2.5M/g) 100 parts by weight Vinyl chloride copolymer 5OJa group-containing 15 parts SO3Na-containing polyester polyurethane Number average molecular weight 60,000
10 parts polyisonanate compound
5α-A12os (average particle size 20.2μ
) 5 carbon fiber (70-μ)
2 Oleic acid 0.3
Stearic acid 1.5 Butyl stearate 1 Methyl ethyl ketone 250 A videotape was produced in the same manner as in Example 1 except that a magnetic coating having the composition described above was used, except that the slit width was 8 ms.
It was made into a videotape for M.

Example-6 The second Tli layer magnetic material of Example-5 was changed as follows.

Ferromagnetic metal powder (specific surface area by BET method: 260 Bo /
g) (Base film for JJM Film-1 Comparative Example-6 Same as Example-5 (base film used in the formulation of the magnetic layer) Film-2 results are shown in Table 2.

As shown in Table 2 above, when the magnetic recording medium of the present invention is cut into a tape-shaped magnetic recording medium by, for example, the shear cutting method, the cut surface is extremely clean and of high quality, and there is no smearing or edge damage. It is possible to produce a tape-shaped magnetic recording medium that is almost free of mud, dirt, etc. and has extremely excellent electromagnetic characteristics, running durability, etc., and has a remarkable effect.

In addition, the magnetic recording medium of the present invention has extremely little wear on the blades used for cutting it, so the period of continuous use of the blades can be extended, and tape-shaped magnetic recording media can be produced with high efficiency. It has a remarkable effect.

Claims (1)

[Claims] A magnetic recording medium comprising a non-magnetic support and a magnetic layer provided on the surface thereof, wherein the non-magnetic support has a cutting suitability index (Z) of 6 or less, provided that [Z=383 .3-2.76A-2000B+840△
n...(1) A is haze value, B={(n_M_D+n_T_D)/2}-n_Z_D
... plane orientation coefficient n_M_D: refractive index in the longitudinal direction n_T_D: refractive index in the width direction n_Z_D: refractive index in the thickness direction △n=n_M_D−n_T_D difference in refractive index], and the above-mentioned non-magnetic In a magnetic recording medium in which a first magnetic layer and a second magnetic layer are provided in this order on the surface of a support, the specific surface area of the first magnetic layer magnetic material by the BET method is 4.
5 m^2/g or less, the crystallite size is 290 Å or more, and the specific surface area of the second magnetic layer magnetic material by the BET method is 3
0 m^2/g or more, the crystallite size is 400 Å or less, and the specific surface area of the first magnetic layer magnetic material measured by the BET method is smaller than that of the second magnetic layer magnetic material, and the difference therebetween is 5 m^ 2/g or more, and the crystallite size of the first magnetic layer magnetic material is larger than that of the second magnetic layer, and the difference therebetween is 40 Å or more.