JP2001067660A - Production of magnetic recording medium - Google Patents

Abstract

PROBLEM TO BE SOLVED: To enhance mass productivity by depositing a magnetic film on a nonmagnetic substrate and heating a heating element formed with ruggedness on a concentric circle, then pressing the heating element to the magnetic film. SOLUTION: After the magnetic film 3 is deposited on a substrate 1, a metal mold 11 alternately concentrically formed with projecting parts and recessed parts is heated and is pressed onto the magnetic film 3. The regions of the magnetic film 3 abutting on the projecting parts of the metal mold 11 are heated and are made nonmagnetic or higher in coercive force by pressing the metal mold 11 to the magnetic film 3. Consequently, the magnetical recording in the heated regions is made difficult. The heating temperature is preferably in a range of 70 to 500 deg.C. A protective film 4 is deposited on the magnetic film 3 after the heating to the magnetic film 3. The substrate 1 consists of a nonmagnetic material and the metal mold 11 consists of Ni or Ni alloy. For example, the depth of the recessed parts of the metal mold 11 is about 0.5 μm, the width in the radial direction of the recessed parts is about 1.8 μm and the width in the radial direction of the projecting parts is about 0.4 μm. The heating of the magnetic film 3 by pressing of the metal mold allows the formation of plural guard bands at a time and contributes to the shortening of a work period.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a magnetic recording medium applied to a hard disk drive and a method for manufacturing the medium.

In recent years, a hard disk drive that plays a central role in an information storage device has been required to be reduced in size and increased in capacity. These requirements are realized by improving the areal recording density of the magnetic disk medium. The areal recording density includes a linear recording density in the circumferential direction and a track density in the radial direction of the disk medium. A high areal recording density can be obtained by improving one or both of them.
The present invention is a technique that particularly contributes to improvement in track density.

[0003]

2. Description of the Related Art In the prior art, as shown in FIG. 1A, a chromium layer is formed on a nonmagnetic substrate 2 such as an aluminum substrate to form a base film 3, and cobalt is mainly formed on the base film 3. A magnetic disk medium 60 is provided in which a magnetic film 4 made of an alloy is formed and a protective film 6 such as amorphous carbon is formed on the magnetic film 4.

In order to increase the track density of the magnetic disk medium, it is necessary to reduce the width of the recording track by reducing the core width of the recording magnetic head. However, in a method using a magnetic head for recording information, extra recording is performed in a portion (guard band) between recording tracks due to a leakage magnetic field generated from the side of the recording head. Such an area is called side erase,
This causes noise during playback. Furthermore, even if the core width of the recording magnetic head is reduced by improving the track density, the width of the side erase hardly changes unless the gap length and the head flying height are reduced. S / when reproducing data
It becomes difficult to secure the N ratio.

Therefore, as shown in FIG. 1B, grooves 9 are formed in advance in the circumferential direction of the disk substrate 2 and are assigned to guard bands to physically separate tracks. A disk medium 61 has been proposed. In this magnetic disk medium 61, if the depth of the groove is sufficient, the leakage magnetic field from the write head does not reach the guard band. Therefore, since magnetic recording cannot be performed in the guard band, track edge noise is suppressed, and it is considered to be effective in increasing the track density.

[0006]

However, the medium on which the groove is formed has a height difference of several tens nm on the surface.
Unevenness of several hundred nm remains. Considering that the flying height of the magnetic head will be reduced to 30 nm or less with the increase in the density of magnetic disk media in the future, problems will arise in terms of reliability due to the following reasons.

In a hard disk drive, an air flow generated by the high-speed rotation of a disk causes a slider on which a magnetic head is mounted to fly, and the magnetic head performs recording and reproduction without contacting the disk. However, it is known that if there are irregularities on the surface of the disk, the flying height of the slider fluctuates due to the turbulence of the air flow, and becomes unstable. This means that the flying height is 50 nm to 100 nm.
Although it was not a problem when the diameter was relatively large as nm, it became a serious problem when an extremely low flying height of 30 nm or less required in the future.

Accordingly, a first object of the present invention is to provide a magnetic recording medium having a high recording density.

A second object of the present invention is to provide a magnetic recording medium with reduced track edge noise.

[0010] A third object of the present invention is to stabilize the flying of a magnetic head.

[0011] A fourth object of the present invention is to manufacture a magnetic recording medium at low cost.

[0012]

In the technique of allocating a groove to a guard band, the groove is spatially demagnetized between recording tracks to suppress side erase. On the other hand, in the present invention, magnetic writing is performed by using a chemical change or crystallization due to heating to demagnetize or increase the coercive force of a portion of the recording magnetic film serving as a guard band. Make it impossible.

In a first aspect of the present invention, a magnetic film is formed on a non-magnetic substrate, and a heating element having concavities and convexities formed on concentric circles is heated and pressed against the magnetic film, thereby forming a magnetic recording medium. To manufacture. In the region having the convex surface, the coercive force becomes too high due to heating, so that magnetic recording cannot be performed, and the region is allocated to a guard band. The area facing the concave surface can be magnetically recorded and is assigned to a recording track. First, by using this manufacturing method, the guard band is in a state where magnetic recording cannot be performed, so that track edge noise can be reduced and S / N can be improved. Secondly, since shaping such as the formation of grooves is not required, manufacturing costs can be reduced. Also, the media surface is kept smooth,
The flying height of the head can be further reduced, and the recording density (linear density) in the circumferential direction can be improved. Furthermore, since the width of the guard band can be further reduced, the recording density (track density) in the radial direction can be improved. Third, since a plurality of recording tracks and guard bands can be formed at once, the construction period can be shortened.

According to a second aspect of the present invention, a magnetic film is formed on a non-magnetic substrate, and a laser beam is condensed by a hemispherical or hyper-hemispherical lens and irradiated on the magnetic film, thereby obtaining a magnetic film. Manufacture a recording medium. The area irradiated with laser light
Heating causes the coercive force to be too high to perform magnetic recording, and is assigned to a guard band. The area not irradiated with the laser beam can be magnetically recorded and is assigned to a recording track. By using this manufacturing method, in addition to the first and second advantages described in the first invention, the laser spot diameter can be narrowed to the limit. The width can be further reduced, and the track density can be further increased.

Preferably, in the first and second inventions, specifically, an alloy of a transition metal and a rare earth metal is formed as the magnetic film. Thereby, by heating the magnetic film, the coercive force is increased to 10 kOe or more, and it is possible to create a state where recording by the magnetic head is difficult.

The heating temperature of the magnetic film by the heating element or the laser is preferably in the range of 70 ° C. to 500 ° C. By heating at 70 ° C. or more, the magnetic film can be heated in a normal temperature environment. Is prevented from being changed. By heating at 500 ° C. or lower, deformation of the substrate can be prevented.

Further, in the first invention, the heating element is Ni
Alternatively, it is desirable to be composed of a Ni alloy. In the field of optical discs, Ni or Ni is already used in a mold for forming grooves or pits for tracking.
Alloys are used. Then, by diverting this mold to the heating element of the first invention, it is possible to suppress a rise in manufacturing cost.

[0018]

FIG. 2 shows a magnetic recording medium 10 according to the present invention.
2 shows a cross section of FIG. That is, the base film 2, the magnetic film 3, and the protective film 4 are sequentially laminated on the substrate 1, and the lubricant 7 is formed on the protective film 6.
Is formed. Hereinafter, each film constituting the magnetic recording medium 1 will be described.

The substrate 2 is made of a non-magnetic material and has a disk shape. As a material forming the substrate 2, aluminum plated with NiP (including an aluminum alloy)
Discs, glass (including tempered glass) discs, silicon discs having a surface oxide film, SiC discs, carbon discs, plastic discs, ceramic discs and the like are included. The substrate 2 may or may not be textured in the circumferential direction. The size of the substrate 2 is determined according to the type of a desired medium, a magnetic disk device to which the substrate 2 is applied, and the like. Generally, the diameter is 1 inch to 3.5 inches, and the thickness is 0.5 mm to 0.5 mm. 1.0 mm.

The underlayer 3 is made of a non-magnetic metal material containing chromium as a main component. As a specific material, a metal material containing only chromium as a main component, or CrW, Cr
Chromium alloys such as V, CrTi, CrMo, and the like.
The base film 3 is formed, for example, by a magnetron sputtering method. The base film 3 has a substrate temperature of 200 ° C. to 300 ° C., an Ar gas pressure of 1 to 10 mTorr,
The film is preferably formed under the condition of a DC bias voltage of 00V. Further, as another film forming method instead of the magnetron sputtering method, for example, a vapor deposition method, an ion beam sputtering method and the like can be mentioned. It is preferable that the thickness of the base film 3 be set in the range of 10 nm to 100 nm in order to increase the S / N ratio. If the thickness of the underlayer is less than 10 nm, there arises a problem that magnetic properties cannot be sufficiently obtained.
If it exceeds nm, the flying of the head tends to be unstable. The base film 3 is not always necessary, and a structure in which the magnetic film 4 is stacked on the substrate 2 without the base film interposed therebetween may be used. Further, the base film 3 may have a multilayer structure.

The magnetic film 4 is made of a material that loses residual magnetization (non-magnetization) after cooling (room temperature) by heating up to about 500 ° C. or has a coercive force twice or more that before heating. . As a specific material of the magnetic film 4, Sm
There are compounds of transition metals such as Co and NdFe and heavy rare earth metals. When these materials are deposited at room temperature, they usually form an amorphous structure. The coercive force of the film is 1.5 to 3 kOe, which is equivalent to that of a magnetic film used for a normal hard disk. However, the magnetic film showing an amorphous state is crystallized when heated at 300 to 600 ° C., and the coercive force is dramatically increased to 10 kOe or more. The magnetic film 4 is formed, for example, by a magnetron sputtering method. The magnetic film 4 has a substrate at room temperature,
It is desirable to form a film under the condition of Ar gas pressure of mTorr. Further, as another film forming method instead of the magnetron sputtering method, for example, a vapor deposition method, an ion beam sputtering method, or the like can be given.

After the magnetic film 3 is formed on the substrate 1, the magnetic film 3 is heated concentrically to alternately form high and low coercive regions along the radial direction of the substrate. Specifically, there are a method in which a groove is formed on a concentric circle and a heated mold is pressed onto the magnetic film 3, and a method in which a laser beam is concentrically irradiated on the magnetic film 3. The steps of the heat treatment when each method is used will be described below.
A. FIG. 3 is a view showing a mold for heating a magnetic film, and FIG. 3A is a plan view of the mold, and FIG.
(B) is a cross-sectional view along the radial direction. The mold is made of Ni or a Ni alloy. The surface of the mold has convex portions (FIG. 3).
In (a), a hatched area) and a concave portion are formed concentrically, the depth of the concave portion is about 0.5 μm, the radial width of the concave portion is about 1.8 μm, and the radial direction of the convex portion. Is about 0.4 μm.

On the substrate of the optical disk, tracking grooves and pits are formed. This substrate is manufactured by pouring a liquid plastic resin into a mold on which a required pattern is formed and cooling it, that is, a technique called injection molding. This mold is manufactured by applying a pattern to a glass substrate or the like by using a photolithography technique, applying a metal material to the pattern by a plating method, curing the metal material, and then peeling the cured product. In general, Ni or a Ni-based alloy is used as the metal material because a substrate that is easy to handle and has high precision is manufactured.

The mold 11 of the present invention also utilizes the state-of-the-art technology as much as possible and aims to avoid cost increase.
Ni or an alloy containing Ni is used as the material, and is manufactured by the same method as the above-described mold for the optical disk substrate.

FIG. 4 is a view showing the heat treatment of the magnetic film by pressing the mold, and shows a cross section of the disk and the mold along the radial direction.

The heat treatment includes the steps shown in FIG.
That is, it is performed before the magnetic film 3 is formed and before the protective film 4 is formed.

Then, as shown in FIG. 4B, the heated mold 11 is pressed onto the magnetic film 3.

By pressing the mold 11 against the magnetic film,
As shown in FIG. 4C, a region of the magnetic film 3 which comes into contact with the convex portion of the mold 11 is heated to be demagnetized or have a high coercive force. As a result, magnetic recording on the heated area becomes difficult. The heating time for the magnetic film 3 is 30
Seconds are preferred.

Incidentally, in a hard disk drive during operation, the temperature of the disk rises to about 70 ° C. Therefore, at such a temperature, the magnetic disk is not allowed to have high coercive force or non-magnetization that cannot be recorded by the magnetic head. That is,
In a temperature range from room temperature (about 25 ° C.) to about 70 ° C., the magnetic film needs to be magnetically stable. Therefore, the heating temperature for the magnetic film is desirably 70 ° C. or higher. By heating the magnetic film to 70 ° C. or higher, it is possible to prevent a change in the magnetic characteristics of the magnetic film under a normal temperature environment.

On the other hand, the substrate is not allowed to be deformed by the heating. Recently, a glass substrate has been used as a 2.5-inch disk substrate because of its high rigidity and good flatness. Heating the magnetic film at too high a temperature is undesirable because glass deforms when heated above 500 ° C. Therefore, for the above reason, the heating temperature is 70 ° C. to 500 ° C.
It is desirable to select from the range of C.

After heating the magnetic film, a protective film 4 is formed on the magnetic film 3 as shown in FIG.

In the heating of the magnetic film by pressing the mold as described above, since a plurality of guard bands can be formed at one time, the productivity can be improved, the construction period can be shortened, and the manufacturing cost can be reduced. It is valid. B. Heating method by laser irradiation After the magnetic film 3 is formed, as shown in FIG. 5, while rotating the disk 10, the laser light emitted from the light source 13 is condensed by the solid immersion lens 12, and Apply to membrane 3. As the medium rotates, the magnetic film 3 is heated along the circumferential direction of the disk.

FIG. 6 is a view showing a heat treatment of the magnetic film by laser irradiation, and shows a cross section of the disk.

As shown in FIG. 6A, the heat treatment by laser irradiation is also performed before the magnetic film 3 is formed and the protective film 4 is formed, similarly to the heat treatment by pressing the mold. Is done.

Then, as shown in FIG. 6B, a laser beam narrowed by the condenser lens 12 is irradiated onto the guard band region of the magnetic film 3. The laser spot diameter is adjusted to be the same as the desired guard band width. The spot diameter can be made smaller than that of a normal lens by using, as the condenser lens 12, a hemispherical or hyperhemispherical lens having a part of a spherical lens called a solid immersion lens cut off. Here, the diameter of the condenser lens 12 is about 1 mm, and the thickness is about 0.5 mm.
mm, and BaF or the like is used as a material. For example, the wavelength λ of the laser beam is set to 400 nm, and the refractive index n
If the super hemispherical condenser lens 12 of = 2 is adopted, the spot diameter can be reduced to about 100 nm. The irradiation time of the laser beam is about 0.1 second per one guard band.

The laser spot moves in the radial direction of the disk, and the magnetic film 3 is heated at predetermined intervals in the radial direction. As a result, as shown in FIG. 6C, a nonmagnetic or high coercive force region is intermittently formed on the magnetic film along the radial direction.

After heating the magnetic film, a protective film 4 is formed on the magnetic film 3 as shown in FIG.

In the heating of the magnetic film by the laser light condensed by the solid immersion lens as described above, the radial width of the heated area is made smaller than the condensing by the normal lens and the pressing by the mold described above. Can be smaller. As a result, the width of the guard band becomes extremely small, which is effective for increasing the track density.

The protective film 6 is made of carbon alone or a compound containing carbon, for example, WC, SiC, B ₄
C, hydrogen-containing carbon, and diamond-like carbon (DLC), which has attracted attention because of its high hardness. The protective film 6 is preferably formed by a sputtering method such as a magnetron sputtering method. Suitable film forming conditions include, for example, a film forming temperature of 20 ° C. to 100 ° C., and an Ar gas of 1 to 10 mTorr. Pressure.
Further, instead of the sputtering method, another film forming method, for example, an evaporation method, an ion beam sputtering method, or the like may be used. The thickness of the protective film 6 depends on various factors and is determined within a wide range, but is preferably 5 nm to 20 nm.

The lubricating film 7 is made of a fluorocarbon resin material and has a thickness of 0.5 nm to 2 nm. Lubricating film 7
By immersing the medium 1 in a solution containing the above materials, a lubricant film is formed on the recording medium. The film thickness depends on the concentration of the material in the solution and the speed at which the medium is pulled out of the solution.

Further, the present invention resides in a disk device on which the above-mentioned disk medium is mounted. The disk device of the present invention basically includes a magnetic head including a recording head unit for recording information on a disk medium and a reproducing head unit for reproducing information. In particular, the reproducing head is a magneto-resistive head using a magneto-resistive element whose electric resistance changes according to the strength of the magnetic field, ie, M
It is preferable to have an R head.

The disk device of the present invention is as shown in FIGS. 8 is a plan view of the disk device (with the cover removed), and FIG. 10 is a line segment AA in FIG.
FIG.

The magnetic disk 10 has a base plate 51
It is driven by a spindle motor 52 provided above. In the present embodiment, three disk media are provided.

The driving actuator 53 for supporting the head slider is rotatably supported on the base plate 51. At one end of the actuator 53, a plurality of head arms 54 extending in a direction parallel to the recording surface of the disk 10 are formed. At one end of the head arm 54,
The suspension 55 is attached. The head slider 1 is attached to a flexure portion of the suspension 55 via an insulating film (not shown). A coil 57 is attached to the other end of the actuator 53.

A magnetic circuit 58 composed of a magnet and a yoke is provided on the base plate 51, and the coil 57 is disposed in a magnetic gap of the magnetic circuit 58. The magnetic circuit 58 and the coil 57 constitute a moving coil type linear motor (VCM: voice coil motor). The upper portions of the base plates 51 are covered with a cover 59.

Next, the operation of the disk device having the above configuration will be described. When the disk 10 is stopped, the slider 40 contacts the retraction zone of the disk 10 and stops.

Next, when the magnetic disk 10 is driven to rotate at a high speed by the spindle motor 52, the slider 40 flies from the disk surface at a minute interval due to the air flow generated by the rotation of the magnetic disk 10.
When a current is applied to the coil 57 in this state, a thrust is generated in the coil 57, and the actuator 53 rotates. As a result, the head (slider 40) can be moved to a desired track on the magnetic disk 10 to read / write data.

[0048]

DESCRIPTION OF THE PREFERRED EMBODIMENTS The present invention will be described below with reference to typical embodiments. 1. Preparation of Magnetic Recording Medium (Magnetic Disk) In the present embodiment, a magnetic disk A having a cross-sectional structure shown in FIG. It is.

The underlayer 2 was formed of a single layer of Cr, and was formed to a thickness of 20 nm on the substrate 1 under sputtering conditions of an Ar gas pressure of a sputtering chamber of 5 mTorr and a DC power of 300 W.

The magnetic film 3 is a single layer made of CoSm, and is formed to a thickness of 20 nm on the underlayer 3 under the sputtering conditions of an Ar gas pressure of 5 mTorr and a DC power of 200 W in the sputtering chamber. .

After the formation of the magnetic film 3, the mold 11 (see FIG. 4) heated to 400 ° C. was pressed onto the magnetic film 3, and the area of the magnetic film 3 to be a guard band was heated. . The material of the mold 11 is made of Ni, and the width of the convex portion is 0.4.
μm, the width of the recess is 1.8 μm, and the depth of the recess is 0.5 μm
It is.

The protective film 4 is made of C. After the magnetic film 3 is heated, the Ar gas pressure in the sputtering chamber is 5 mTorr,
Under the condition of a C power of 400 W, a thickness of 1
The film was formed to a thickness of 0 nm.

As a comparison target of the magnetic disk A,
A magnetic disk B was manufactured under the same manufacturing conditions as the magnetic disk A, except that the heat treatment for the magnetic film was omitted. 2. Evaluation of Magnetic Disk First, a magnetic signal is recorded on the magnetic disks A and B along the circumferential direction of the disk, and a radial width (effective track width) at which a predetermined level of reproduction output is obtained for the recording. Was measured. The measurement results are shown in FIG. The core width of the recording head in the magnetic head used in this evaluation was 2.0 μm, and the core width of the reproducing head was 1.5 μm.

As shown in FIG. 9, in the magnetic disk A in which the guard band has been subjected to the heat treatment, the effective track width is 1.81 μm, which is defined by the width of the concave portion of the mold 11. I understand. That is, it means that almost no magnetic signal was recorded in the area assigned to the guard band. On the other hand, in the case of the magnetic disk B, the effective track width is 2.23 μm, which is larger than the core width of the recording head, and it can be seen that the influence of side erasure appears. From these results, it can be seen that local erasing can suppress side erasure and increase track density.

Next, the effect of the coercivity ratio between the guard band and the recording track on the effective track width of the magnetic disk was examined. Here, a plurality of magnetic disks having different ratios of the coercive force between the guard band and the recording track were manufactured. The film structure and manufacturing method of these media are the same as those described above for the magnetic disk A.
But the temperatures of the molds used for heating the guard bands are different. FIG. 10 is a graph showing the effective track width with respect to the coercive force of the guard band / the coercive force of the track. From FIG. 10, it can be seen that the larger the coercive force ratio, the smaller the effective track width, and when the coercive force of the guard band is twice the coercive force of the recording track, the effective track width is equal to the width of the concave portion of the mold. The same is 1.8 μm. From the results shown in FIG. 10, the coercive force of the guard band magnetic film is:
It can be said that twice that of the recording track is necessary.

Further, in the present embodiment, the influence of the Sm content in the Sm of Co constituting the magnetic film on the coercive force was examined. Here, an SmCo alloy is formed by sputtering, and a plurality of substrates having different Sm contents are deposited on a 400
Annealing was performed at ° C, and coercive force before and after annealing was measured. FIG. 11A shows the coercive force before and after heating with respect to the Sm content, and FIG. 11B shows the ratio of the coercive force before and after heating with respect to the Sm content.

From the results shown in FIG. 10, it was stated that the coercive force of the guard band (heated portion) was required to be at least twice as large as the coercive force of the recording track (unheated portion).
According to the results shown in FIGS. 1A and 1B, in order to make the coercive force of the heated portion twice as large as that of the non-heated portion, the Sm content in the CoSm magnetic film is in the range of 10 to 24 at%. It is clear that you should choose from.

[0058]

As described above, according to the manufacturing method of the present invention, the guard band can be made in a state where magnetic recording cannot be performed, and the S / N can be improved and the track density can be increased.

Further, since the guard band is created by heating, the manufacturing cost is reduced as compared with the conventional manufacturing method of forming a groove. In addition, since a high surface smoothness can be obtained, the flying height of the head can be reduced, and the linear recording density can be improved. Further. The groove width can be reduced, and the track density can be further improved. By heating with a mold, the construction period can be shortened and the production cost can be reduced.

[Brief description of the drawings]

FIG. 1 is a sectional view of a conventional magnetic recording medium.

FIG. 2 is a sectional view of a magnetic recording medium according to the present invention.

FIG. 3 is a view showing a mold used for heating.

FIG. 4 is a view showing a step of heating a magnetic recording medium by pressing a mold.

FIG. 5 is a diagram showing a laser heating device.

FIG. 6 is a diagram showing a step of heating a magnetic recording medium by laser irradiation.

FIG. 7 is a plan view of the magnetic disk drive according to the embodiment of the present invention.

FIG. 8 is a sectional view of the magnetic disk drive according to the embodiment of the present invention.

FIG. 9 is a table showing effective track widths in the magnetic recording media of the related art and the present invention.

FIG. 10 is a graph showing a track effective width with respect to guard band coercive force / recording track coercive force.

FIG. 11 is a graph showing guard band coercive force / recording track coercive force with respect to Sm content.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 ... Substrate 2 ... Base film 3 ... Magnetic film 4 ... Protective film 11 ... Die 12 ... Solid immersion lens

 ──────────────────────────────────────────────────続 き Continued on the front page F term (reference) 5D006 BB01 BB07 DA03 DA04 EA03 5D112 AA05 AA24 BB01 GA00 GA19 GB01 GB04

Claims (5)

[Claims] 1. A step of forming a magnetic film on a non-magnetic substrate, a step of heating a heating element having concentric unevenness formed thereon, and pressing the uneven surface of the heating element onto the magnetic film. A method for manufacturing a magnetic recording medium, comprising: 2. A step of forming a magnetic film on a non-magnetic substrate, and focusing a laser beam with a hemispherical or super hemispherical lens.
Irradiating the magnetic film concentrically onto the magnetic film. 3. The method according to claim 1, wherein an alloy of a transition metal and a rare earth metal is formed as the magnetic film. 4. The method according to claim 1, wherein Ni or a Ni alloy is used as the heating element. 5. The method for manufacturing a magnetic recording medium according to claim 1, wherein the magnetic film is heated to 70 ° C. to 500 ° C.