JPH0316023A - Magnetic disk and method and apparatus for producing the disk - Google Patents

Abstract

PURPOSE:To prevent the reflocculation of the magnetic powder in a magnetic coating compd. and to obtain the magnetic disk having excellent electromagnetic characteristics, such as S/N by orienting the magnetic powder by applying surface waves to magnetic recording films while maintaining the undried state of the magnetic recording films. CONSTITUTION:The magnetic powder is dispersed and oriented by applying the surface waves to the magnetic recording films of a substrate 3 while the undried state of the magnetic recording films 100, 100' is maintained. For example, the material coating compd. is applied by spin coating on the aluminum disk 3 and the disk is immediately mounted to the device for producing the magnetic disk, then the impression of the surface waves of random mode and the intrasurface orientation of magnetic fields are simultaneously executed. The coating compd. is thereafter cured at 210 deg.C to form the magnetic recording films 100, 100'. A surface wave oscillator 4 having a flat con tact surface with the magnetic disk 3 is used and the surface waves of the random type mode are generated by adjusting the degree of clamping of the surface wave oscillator 4 and a receiving base 2. The reflocculation of the above-mentioned powder in the magnetic coating compd. is prevented in this way and the magnetic disk having the excellent electromagnetic characteristics is obtd.

Description

[Detailed description of the invention]

[Industrial Field of Application 1] The present invention relates to a coated magnetic disk used in storage devices and the like, a method for manufacturing the same, and a magnetic disk manufacturing apparatus for manufacturing the magnetic disk. [Prior Art] A conventional method for manufacturing coated magnetic disks involves applying magnetic powder to a binder (polymer binder) or its solution in a sand mill.
The magnetic paint is dispersed using a secondary machine, a ball mill, etc., and then applied to the substrate, oriented as necessary, cured with heat, and polished. lubricant is applied to the The binder is usually a thermosetting resin and is cured by heat, but it may also be used simply by heating. Magnetic paints are also typically stirred during storage to prevent reagglomeration of the magnetic powder. In recent years, as the density of magnetic disks has increased, finer particles of magnetic powder have come to be used. Magnetic paints using such magnetic powders tend to re-agglomerate, and it is difficult to completely prevent re-agglomeration with the above-mentioned stirring alone.
In order to solve such problems, Japanese Patent Laid-Open No. 62-186
The technique described in No. 29 involves applying a magnetic paint uniformly to a predetermined thickness on a support, immediately after applying ultrasonic vibration, and applying a uniform magnetic field to orient the magnetic powder. By this method, even if reaggregation occurs, the structure is destroyed, the magnetic powder is easily rotated by the orienting magnetic field, and the magnetic powder is highly oriented and the squareness ratio can be improved. Furthermore, Japanese Patent Laid-Open No. 60-83224 describes manufacturing a vertically oriented magnetic disk by applying a perpendicular magnetic field and ultrasonic vibration to the substrate.

Problems to be Solved by the Invention The prior art described in JP-A-62-18629 has excellent electromagnetic properties as an in-plane oriented magnetic recording medium.
For example, no consideration is given to manufacturing high-density magnetic disks with good S/N ratios and squareness ratios. That is, the above-mentioned prior art relates to the manufacture of magnetic tapes rather than magnetic disks. Magnetic tape has a thick coating (1 μm or more), and the material for such coating is magnetic paint. It has a high viscosity of about 1000 cp to 2000 cp. Therefore, once the paint is highly dispersed by ultrasonic vibration, it will not re-agglomerate in a short period of time, so it can be easily oriented by applying magnetic field orientation immediately after dispersing the magnetic powder in the form of a paint film. In contrast, the coating film of a high-density magnetic disk is thin (0.75 μm or less). Therefore, it is necessary to use a magnetic coating material with a low viscosity of 500 cp or less as the material for such a coating film. Since low-viscosity paints tend to coagulate more easily than high-viscosity paints, the degree of aggregation in the applied state is much greater than that of high-viscosity paints. This agglomeration is difficult to break even when the ultrasonic vibrations used in the above-mentioned conventional technology are applied, and the coating film does not become highly dispersed. If there is time before the magnetic field alignment is performed, even if it is for a short time, the particles will aggregate again. As a result, the manufactured magnetic disks have a problem in that various electromagnetic properties, such as squareness ratio, etc., deteriorate. Furthermore, the vertically oriented magnetic disk described in JP-A No. 60-83224 has a unique property that the magnetic powder is oriented in the vertical direction, so it is 1 liter smaller than the in-plane oriented magnetic disk.
There were problems with its magnetic properties, such as a low S/N ratio, a high tendency for defects to occur, and a low output. An object of the present invention is to provide the following advantages when manufacturing a magnetic disk with in-plane alignment using a magnetic paint in which at least magnetic powder is dispersed.
Method for manufacturing a high-density in-plane oriented magnetic disk with excellent electromagnetic properties by preventing re-agglomeration of magnetic powder, the manufactured in-plane oriented magnetic disk, and a magnetic disk manufacturing apparatus for manufacturing such a magnetic disk Our goal is to provide the following. [Means for Solving the Problems 1] The above objectives are as follows: (1) After applying a magnetic paint in which at least magnetic powder is dispersed in a polymer binder onto a non-magnetic substrate, the magnetic powder is oriented to form a magnetic recording film. A magnetic disk manufacturing method for manufacturing a magnetic disk, characterized in that the magnetic powder is oriented by applying a surface wave to the magnetic recording film while maintaining the magnetic recording film in an undried state. (2) The method for manufacturing a magnetic disk according to (1) above, wherein the surface wave is a vibration under conditions that generate a standing wave or an interference wave; (3) at least magnetic powder is dispersed in a polymeric binder. In a method for manufacturing a magnetic disk, in which a magnetic recording film is manufactured by coating a magnetic paint on a non-magnetic substrate,
A method for producing a magnetic disk, characterized in that, while maintaining the magnetic recording film in an undried state, a surface wave is applied to the substrate and its magnetic recording film to disperse and orient the magnetic powder, (4) the surface of the magnetic disk; The method for producing a magnetic disk according to 3 above, in which the waves are vibrations under conditions that generate standing waves or interference waves; In a magnetic disk placed on a magnetic substrate, the magnetic powder is distributed concentrically with respect to the center of the disk, has a density gradient in the radial direction, and is oriented in the plane. a magnetic disk, (6) a pedestal for attaching a non-magnetic substrate having at least magnetic powder dispersed in a polymeric binder and having an undried magnetic recording film; and a surface wave on the magnetic recording film on the non-magnetic substrate. This is achieved by a magnetic disk manufacturing apparatus characterized by having a surface wave vibrator for providing This manufacturing apparatus may further include means for applying a magnetic field to the magnetic recording film, such as providing at least one pair of magnets above and below the nonmagnetic substrate. The feature of the present invention is the application of surface waves to the magnetic recording film. There are three main types of surface wave modes: concentric, random, and interference. Magnetic powder is dispersed by applying surface waves to a substrate, for example. In this case, the dispersion takes place in particular by interfacial or surface tension waves, which are a type of surface wave. The concentric type example will be explained later with reference to FIG. 3, the random type example with FIG. 4, and the interference type example with FIGS. 5(B) and (D), respectively. In the present invention, by applying a magnetic field for orientation or rotating a magnetic disk when applying a surface wave, the dispersion and orientation of the magnetic powder can be further enhanced compared to when only applying a surface wave. The magnetic field may be a non-uniform magnetic field. The magnetic field may be applied after the surface wave is applied. It is preferable that the magnetic paint in the present invention further contains a filler (reinforcing agent) such as alumina in addition to magnetic powder.
The non-magnetic substrate is preferably made of a material such as a metal or ceramic substrate through which surface wave vibrations propagate directly and effectively. The magnetic disk is, for example, a 5-inch aluminum disc (
Inner diameter 40.0+++mφ, outer diameter 130mmφ, thickness 1
．． Approximately 0.55±0.0 at the inner circumference of 9 +++r++)
Approximately 0.65±0 at the outer periphery for a film thickness of 6μm or less
．． After shaping the coating film to a thickness of 0.05 μm or less, the coating film is heat cured and ground to a thickness of about 0.35 μm or less at the inner circumference and 0.45 μm or less at the outer circumference. In this way, for example, a magnetic disk with a recording density of 30 and OOOBPI or higher can be obtained. [Operation] By applying surface waves to a non-magnetic substrate coated with magnetic paint, the magnetic powder in the paint film is dispersed and oriented. JP-A-62-18629 and JP-A-60-8322 of the above-mentioned prior art
The ultrasonic vibration described in 4 merely vibrates the substrate (tape or disk) up and down in the direction of its thickness.
On the other hand, since the surface wave of the present invention is a wave that travels in the plane direction of the substrate, it is effective for dividing magnetic powder aggregates into smaller pieces and separating aggregates that are in contact with each other, and has the effect of dispersing and orienting the magnetic powder. big. In the present invention, ultrasonic vibration is merely a means for generating surface waves. The state of dispersion and orientation of magnetic powder differs depending on how the surface waves are applied. Surface wave modes can be roughly divided into three types: concentric mode, random mode, and interference mode. The differences between these modes are shown in Figure 3, Figure 4@I, and Figure 5 (B).
This will be explained using an example. An example will be explained in which the surface wave vibrator 4 has three protrusions arranged at equal angles on one circumference, as shown in FIG. 5(B). Figure 3 is a plan view of a magnetic disk showing the dispersion and distribution of magnetic powder when a concentric mode surface wave is applied. The reason why such a concentric mode is generated is that the frequency, phase, and amplitude of the surface waves applied to the three protrusions are equal, and that the frequency is the same as that of the substrate (strictly speaking, that including the magnetic coating). is obtained when the natural resonant frequency is The surface wave in this case is a standing wave. For example, when using an aluminum disk with a diameter of 5.25 inches and a thickness of 2 mm (inner diameter 40+m, outer diameter 130 mm) as a substrate, if it is vibrated at a frequency of about 37 KHz (variable from IW to 100 W is also possible) or an integral multiple thereof. good. The surface wave in this case is a standing wave. The a-type powder is distributed concentrically with respect to the center of the disk, and a density gradient exists in the radial direction. The number of repetitions of the density gradient can be changed depending on the frequency of the surface waves. Also, rotating the disc is more effective. FIG. 4 is a plan view of a magnetic disk showing the state of dispersion and orientation of magnetic powder when a random mode surface wave is applied. This is obtained when the phase of the surface wave applied to at least one of the three protrusions is different from the other two.
Magnetic powder is distributed with uniform density, but its direction is random. Further, FIG. 5(B) is a diagram showing the state of surface waves within the surface of the magnetic disk when surface waves in an interference mode are applied. This is obtained when the balance of surface wave transmission from the three protrusions mentioned above is lost. For example, there are differences in the pressures that the three protrusions apply to the substrate. The magnetic powder is unevenly distributed in the thickness direction of the magnetic recording film. For this reason, its density is large. Note that a protrusion here refers to a part within the surface wave transducer that transmits surface waves to the substrate, and in addition to artificially processed protrusions, certain parts within the surface wave transducer result in surface waves being transmitted to the substrate. Including cases where it is given. The conditions for the protrusions to generate surface waves include the arrangement of the protrusions (distance from the disk center, protrusion spacing, etc.), the number of protrusions, the contact area of the protrusions, and the pressure. The arrangement of the protrusions is preferably at the center of the disk and around the center. Further, there is no need to make the arrangement symmetrical to the center of the disk, and various arrangements are possible. For example, it is also possible to arrange the protrusions in a biased manner in some areas, so that only desired areas of the disk are oriented in-plane. Since vibration propagation speed is high in metal bodies (aluminum, etc.) and ceramics (glass, etc.), it is sufficient to have at least one disc of these. In particular, in the case of one surface wave vibrator, it is preferable to provide a certain pseudo protrusion from an elastic body (rubber, Teflon, etc.) in order to stably hold the surface wave vibrator against the disk surface. In this case, the surface wave mode is a standing wave among the concentric modes. The contact area of the protrusion must be small enough to be considered a point compared to the area of the disk.
Otherwise, no surface waves will be generated and the ultrasonic vibration will be the same as in the conventional technology. The contact pressure of the protrusion is set to a value that allows the surface wave to proceed. A magnetic recording film is formed by applying a magnetic paint in which magnetic powder is dispersed in a polymeric binder onto a non-magnetic substrate, and when a surface wave is applied to the substrate while the recording film is not dry, the recording antagonism occurs. Even if the magnetic powder reagglomerates inside, its structure is destroyed. Furthermore, by applying a magnetic field, the magnetic powder can be aligned in a desired direction. When the magnetic powder is a fine powder, for example, when it is a fine powder with an average particle size of 0.4 μm or less, more preferably 0.2 μm or less, the resistance of the magnetic powder to move and rotate in the coating film is small, so the magnetic powder When the resonant frequency of the powder and the frequency of the surface wave applied to the substrate almost match, it is possible to align the magnetic powder in a certain direction, preferably in the in-plane direction, without applying a magnetic field. A schematic sectional view of an example of the magnetic disk manufacturing apparatus of the present invention is shown in FIG. 1 and FIG. 5(A). A pedestal 2 made of a material such as Teflon that moderates phase variable ultrasonic waves is rotatably arranged by a motor. An aluminum substrate 3 is mounted on the pedestal 2, and a metallic surface wave vibrator 4 is screwed so that surface waves are directly transmitted to the aluminum substrate 3. An ultrasonic transducer 6 is fixed to the surface wave transducer 4, and the two are mechanically coupled. A coil 7 is wound around the ultrasonic vibrator 6, and a fighting current is supplied to the coil 7 from an ultrasonic oscillator 10 via electrode slip rings 8, 8' and energizing sliding pins 9, 9'. Ru. In order to increase the efficiency of ultrasonic vibration, the inside of the container 1 is filled with resin. Note that the pedestal 2 may be placed above the substrate 3 and the surface wave vibrator 4 may be placed below the substrate 3. A pair of magnets for applying the magnetic field were placed above and below the disk 3 as shown in the figure. In this case, a repulsive magnetic field is obtained. The magnets can be arranged in various ways, for example, as described in Japanese Patent Publication No. Sho 56-45210, the magnets may be arranged vertically at different positions. In addition, the 11th! The surface wave oscillator 4 of I is annular, but
Surface wave vibrator 4 shown in FIG. 5(B) and FIG. 5(C)
In order to further enhance the effect, a protrusion 103 is provided on the portion of the disc that comes into contact with the substrate. In addition, Fig. 5 (B
) and Figure 5(C) show the three-point interference mode, and Figure 5(D) shows the three-point interference mode.
) and Figure S (E) show the eight-point interference mode, but of course other multi-point interference modes are also effective. The device shown in FIG. 1 is an example suitable for dispersing and orienting magnetic paint after applying it to a substrate using another coating device.
Coating, dispersing, orientation and magnetic field orientation can also be carried out with this device. In that case, it is preferable to make the magnet movable, apply the magnetic paint with the magnet removed, and then arrange the magnet at a predetermined position to perform dispersion, orientation, and magnetic field orientation. [Examples] The present invention will be explained below based on Examples. Example 1 70g of polyvinyl butyral powder and acicular γ-Fe,O
, magnetic powder (average particle size: 0.35X0.06pi, coercive force Hc: 3300e, specific surface area BET: 22m"/
g) Pour 700g into the two-dar mixer and mix the two-dars for about 15 minutes. Next, 250 g of cellosolve acetate was gradually added, and kneader mixing was performed for about 4 hours. Take 480 g of this kneaded material and put it in a Pall mill pot with a volume of 3Q, along with 3 g of alumina + 700 g of cellosolve acetate.
Ball mill kneading was performed for 7 days to disperse the magnetic powder and alumina well. Next, 120g of phenolic resin, 40%
Add and mix 300 g of the epoxy resin butyl cellosolve solution and 500 g of butyl cellosolve to prepare a magnetic paint. Next, apply this paint to a 5.2mm diameter
5 inches, thickness 2n+m (inner diameter 40If!l, outer diameter 13
After rotating and coating the aluminum disk 3 (0 mm), immediately attach it to the magnetic disk manufacturing apparatus shown in FIG.
Random mode surface wave application and in-plane magnetic field orientation at the same time 3
This was carried out for 0±5 seconds. Thereafter, it was cured at 210° C. to form magnetic recording films 100, 100'. The surface wave vibrator 4 used was one that was not provided with any artificial protrusions and had a flat surface in contact with the magnetic disk substrate 3. Random mode surface waves were generated by adjusting the tightness between the surface wave vibrator 4 and the pedestal 2. Next, the squareness ratio of the magnetic recording film was measured. Surface wave vibrator 4
The relationship between the frequency of the ultrasonic waves applied to the magnetic recording film and the squareness ratio (Br/Bm) of the obtained magnetic recording film is shown in curve a in FIG. As shown in the figure, by applying a surface wave, a magnetic disk with a squareness ratio exceeding the quality standard of 0.55 can be obtained. In addition, when the ultrasonic frequency is increased, the squareness ratio increases,
If you raise it even higher, it will become saturated. A more preferable ultrasonic frequency is 30 KHz or more, and an even more preferable ultrasonic frequency is 35
KHz or higher. After measuring the squareness ratio, the magnetic recording film was processed to have a film thickness of 0.32 μm at the inner peripheral portion. When the recording and reproducing characteristics of this magnetic disk were measured using a MnZn ferrite head with a gap length of 0.5 μm, the S/N ratio was improved by 5 to 10% compared to that of a conventional magnetic disk obtained without applying surface waves. Example 2 Acicular CO-γ-Fe20, magnetic powder (average particle size: 0.40
The same treatment as in Example 1 was carried out except that X0.07 μm, coercive force Ha: 6500e, specific surface area BET: 22.5 m"/g). As a result, the squareness ratio was small. A similar experiment was carried out by changing the wave mode from a random mode to a concentric mode.In this case, three protrusions 103 were used as the surface wave oscillator 41 at equal intervals on one circumference, as shown in the diagram. The S/N ratio was improved by 5 to 10% compared to the conventional one.Example 3 Acicular fine powder Co-γ-Fe2O3 magnetic powder (average particle size: 0.
The same treatment as in Example 1 was performed except that 2×0.02 μm, coercive force Hc: 7700e, specific surface area BET: 50 m2/g) were used. As a result, the squareness ratio was small. The same result was obtained even when the concentric mode surface wave used in Example 2 was applied. Therefore, an interferometric mode surface wave was applied. The surface wave vibrator 4 used was the same as in the case of the concentric mode in Example 2. (See Figures 5(B) and 5(C)) The results are as shown in the second drawing. The S/N ratio was improved by S~10% compared to the conventional one. As shown in Examples 1 to 3, as the coercive force Ha and specific surface area BET value of the magnetic powder used increase, the dispersion occurs in the order of random mode and concentric mode.
It was found that the effect of orientation becomes small and that it is necessary to use an interference mode that has the greatest dispersion and orientation effects. Example 4 25 parts by weight of powdered epoxy resin and plate-shaped hexagonal barium ferrite magnetic powder (diameter approximately 0.1 μm, coercive force Hc:
6550e, specific surface area BET: 30 m''/g) and 5 parts by weight of single-crystal alumina were thoroughly mixed, 10 parts by weight of cyclohexanone was added, and two-dar mixing was performed in a kneader mixing machine. Thereafter, 5 parts by weight of cyclohexanone was further added and the two-dar mixture was continued for about 4 hours.The above mixture was placed in a three-volume Pall mill pot, and 140 parts by weight of a mixed solvent of cyclohexanone and isophorone was added. was added and kneaded in a ball mill for 3 days to disperse the magnetic powder.Next, 25 parts by weight of phenolic resin and 6 parts by weight of vinyl resin were added.
A magnetic paint was prepared by adding a solution in which parts by weight were dissolved in 490 parts by weight of a mixed solvent of cyclohexanone, isophorone, and dioxane. Next, the diameter 5.2 with the surface cleaned in advance
The above paint was spin-coated onto an aluminum substrate 3 having a size of 5 inches and a thickness of 21 ml. Thereafter, this was attached to the magnetic disk manufacturing apparatus shown in FIG. 1, and an interferometric mode surface wave was applied. The surface wave vibrator 4 is the same as the one used in Examples 2 and 3.
A material with several protrusions was used. (Refer to Figure 5 (B) and Figure 5 (C)) In this case, there are two sets: one that is oriented by applying only an interferometric mode surface wave, and one that is oriented in-plane by simultaneously applying a magnetic field. Manufactured. Furthermore, the coating film was cured at 210°C to form a magnetic recording film 100,1.
00' was measured, and then the squareness ratio was measured. Next, surface processing (roughness Ra<0.02 μm) of the shite magnetic recording film 10
0.100' was made to have a thickness of 0.3 μm (7), and a fluorine-based liquid lubricant was applied to form a magnetic disk. When the electrical characteristics of this magnetic disk were measured using a metal-in-gap head with a gap length of 0.3 μm, it was found that the S/N ratio was lower than that of a conventional magnetic disk manufactured without applying surface waves. The improvement was achieved by 2 to 6% in the case without the alignment, and 7 to 12% in the case of the alignment. Further, the relationship between the squareness ratio of the magnetic recording film produced by magnetic field orientation and the ultrasonic frequency applied to the surface wave transducer 4 is as shown in Example 3 shown in curve C in FIG.
It was almost the same as in the case of That is, when a surface wave in an interference mode is applied, [Effects of the Invention] The method for manufacturing a magnetic disk of the present invention prevents reagglomeration of magnetic powder in the magnetic paint and has excellent electromagnetic properties such as S/N ratio. You can obtain a magnetic disk with

[Brief explanation of drawings]

Fig. 1 is a schematic cross-sectional view of a magnetic disk manufacturing apparatus of the present invention, Fig. 2 is a diagram showing the relationship between the frequency of ultrasonic waves applied to a surface wave vibrator and the squareness ratio of the medium, and Fig. 3 is a concentric circular type Figure 4 is a plan view of a magnetic disk showing the state of dispersion and orientation of the magnetic powder when a random mode surface wave is applied. FIG. 5(A) is a schematic sectional view of a surface wave vibrator portion having protrusions of a magnetic disk manufacturing apparatus of the present invention, and FIGS. 5(B) and 5(C) are a plan view of a magnetic disk shown in FIG. A diagram showing the state of the interferometric mode surface wave in the plane of the magnetic disk when using surface wave vibrators each having three protrusions, and a schematic cross-sectional view of the surface wave vibrator portion, FIG. 5(D) and Figure 5 (
E) is a diagram showing the state of interferometric mode surface waves in the plane of the magnetic disk when surface wave vibrators each having eight protrusions are used, and a schematic cross-sectional view of the surface wave vibrator portion. 1
...Motor, 2...Base, 3...Board, 4...
・Surface wave transducer, 6... Ultrasonic transducer, 7... Coil, 8, 8'... Electrode slip ring, 9, 9'...
・Sliding pin, 10...Ultrasonic oscillator, 1l...Container, 12.12'...Magnet, 100,100'...
Magnetic recording film, 101...Circular standing wave, 102...
Random mode, 103...Protrusion, 104...Surface wave, 105...Interference mode. A---#7k eye movements

Claims (1)

[Claims] 1. In a method for manufacturing a magnetic disk, in which a magnetic coating material in which at least magnetic powder is dispersed in a polymeric binder is coated on a non-magnetic substrate, the magnetic powder is orientated to produce a magnetic recording film. A method for manufacturing a magnetic disk, characterized in that a surface wave is applied to the magnetic recording film while maintaining a dry state. 2. 2. The method of manufacturing a magnetic disk according to claim 1, wherein the surface wave is a vibration under conditions that generate a standing wave or an interference wave. 3. 2. The method of manufacturing a magnetic disk according to claim 1, wherein the magnetic powder is plate-shaped hexagonal barium ferrite. 4. In a method of manufacturing a magnetic disk, in which a magnetic recording film is manufactured by applying a magnetic paint in which at least magnetic powder is dispersed in a polymeric binder onto a non-magnetic substrate, the magnetic recording film is maintained in an undried state. , a method of manufacturing a magnetic disk characterized by applying surface waves to a substrate and its magnetic recording film. 5. 5. The method of manufacturing a magnetic disk according to claim 4, wherein the surface wave is a vibration under conditions that generate a standing wave or an interference wave. 6. 5. The method of manufacturing a magnetic disk according to claim 4, wherein a magnetic field is applied in an in-plane direction when applying the surface wave. 7. 7. The method of manufacturing a magnetic disk according to claim 6, wherein the magnetic powder is plate-shaped hexagonal barium ferrite. 8. In a magnetic disk in which a magnetic recording film in which at least magnetic powder is dispersed and oriented in a polymeric binder is disposed on a non-magnetic substrate, the magnetic powder is distributed concentrically and a density gradient exists in the radial direction. What is claimed is: 1. A magnetic disk characterized in that it is oriented in a plane and has an in-plane orientation. 9. 9. The magnetic disk according to claim 8, wherein said density gradient exists repeatedly in said radial direction. 10. A pedestal for attaching a non-magnetic substrate having at least magnetic powder dispersed in a polymeric binder and having a undried magnetic recording film; and a surface wave vibration for applying a surface wave to the magnetic recording film on the non-magnetic substrate. A magnetic disk manufacturing apparatus characterized by having a child. 11. 11. The magnetic disk manufacturing apparatus according to claim 10, wherein the surface wave vibrator is mechanically coupled to an ultrasonic vibrator. 12. 12. The magnetic disk manufacturing apparatus according to claim 11, wherein the surface wave vibrator has a protrusion, and the protrusion is installed so as to be in contact with the nonmagnetic substrate. 13. 13. The magnetic disk manufacturing apparatus according to claim 12, wherein there are a plurality of said protrusions, and said protrusions are arranged at equal intervals on one circumference. 14. 12. The magnetic disk manufacturing apparatus according to claim 11, further comprising at least one pair of magnets arranged above and below the non-magnetic substrate for applying a magnetic field to the magnetic recording film.