JPH0766484B2 - Magnetic recording method - Google Patents

Description

BACKGROUND OF THE INVENTION 1. Technical Field The present invention relates to a magnetic recording medium, and more particularly to a magnetic recording method using a metal thin film type magnetic recording medium.

2. Description of the Related Art Prior Art and Problems There are active developments of magnetic recording media for video, audio, etc., which have a metal thin film type magnetic layer because of their compactness when formed into a tape and wound.

As a magnetic layer of such a metal thin film type medium, characteristically, vapor deposition is performed by a so-called oblique vapor deposition method in which vapor deposition is performed at a predetermined inclination angle with respect to the normal to the substrate, and vapor deposition of Co-based, Co-Ni-based, etc. Membranes are preferred.

For such media, research has been made on media such as 10 μm or less for miniaturization and long-time recording. However, runnability, durability, strength of ferromagnetic metal thin film, etc. There is a problem with.

Therefore, in order to eliminate these disadvantages, it is proposed to provide a metal thin film reinforcing layer on the back surface of the film (JP-A-56-16939).
No. 58-97131, No. 57-78627, No. 57-37737)
Alternatively, it is proposed that fine particles are provided on the film surface to improve the head touch and the running surface (Japanese Patent Laid-Open No. 58-68227).
No. 58-100221).

Further, in order to improve durability and electromagnetic conversion characteristics, various proposals have been made that the ferromagnetic metal thin film layer has a multilayer structure of two or more layers (Japanese Patent Laid-Open No. 141608/54, Japanese Patent Publication No. 56/56). −
26892, JP-A-57-130228, etc.). In particular, the reduction of high frequency output above 5MHz is a big problem.

However, at present, in a medium using a film having a thickness of 10 μm or less, a technique that has good running property, durability, ferromagnetic metal thin film strength, and causes no inconvenience in terms of electromagnetic conversion characteristics has not yet been realized.

II Object of the Invention The object of the present invention is to use a film support having a film thickness of 8 μm or less, to achieve good running properties of the medium, to reduce cracks and scratches in the magnetic layer due to running, and to reduce head wear and dropout to reduce electromagnetic interference. It is an object of the present invention to provide a magnetic recording method capable of significantly improving high frequency output by using a metal thin film type magnetic recording medium having good conversion characteristics.

IV Disclosure of the Invention Such an object is achieved by the present invention described below.
That is, the present invention has a ferromagnetic metal thin film layer containing Co as a main component on a plastic film substrate having a thickness of 8 μm or less, and this ferromagnetic metal thin film layer has a multi-layer structure composed of two or more layers. However, the minimum incident angle of the deposited material with respect to the normal to the substrate when depositing the ferromagnetic metal thin film layer is 50 ° or less when the lowermost layer is formed on the substrate side, and 20 ° when the uppermost layer is formed on the opposite side of the substrate. Is 90 °, and the value C ₂ / C _{1 obtained} by dividing the oxygen concentration C ₂ in the vicinity of the substrate side interface of the lowermost layer by the oxygen concentration C ₁ in the vicinity of the surface of the uppermost layer substrate is 0.3 or less. the value C ₃ / C ₁ obtained by dividing the oxygen concentration C ₃ in the vicinity of the interface between the top layer of the adjacent layers in the oxygen concentration C ₁ at the opposite side surface near the top layer of the substrate to have a 0.2 to 3.0, most A magnetic recording medium having an upper layer thickness smaller than that of the lowermost layer is used, and the upper layer mainly holds signals in a high frequency region of 5 MHz or more, and 0.75 MHz or more. Mainly with magnetic recording to the lower layer holds the signal of the low frequency region near the a magnetic recording method of improving the signal output.

V Specific Structure of the Invention Hereinafter, a specific structure of the present invention will be described in detail.

The ferromagnetic metal thin film layer as the magnetic layer in the present invention has a multi-layer structure including at least two layers. The ferromagnetic metal thin film layer used in the present invention has a composition containing Co as a main component, containing O, and optionally containing Ni and / or Cr.

That is, in a preferred embodiment, it may consist of Co alone or Co and Ni. If Ni is included,
The Co / Ni weight ratio is preferably 1.5 or more.

Further, Cr may be contained in the ferromagnetic metal thin film layer.

In such a case, the weight ratio of Cr / Co or Cr / (Co + Ni) is 0.1 or less, particularly 0.001 to 0.1, and more preferably 0.005.
It is preferably ˜0.05.

Further, the ferromagnetic metal thin film of the present invention contains O.

The average oxygen content of the whole layer in the ferromagnetic metal thin film is an atomic ratio, especially the atomic ratio of O / (Co or Co + Ni), and the average oxygen content in the uppermost layer is about 0.1 to 0.5, preferably about 0.1 to 0.4. .

If the average oxygen content is less than 0.1, it is insufficient in terms of corrosion resistance, running properties, cracks in the magnetic layer, scratches, etc.If it exceeds 0.5, the surface oxide layer increases and the output decreases due to spacing with the head. Cause problems.

The oxygen concentration C ₂ near the interface with the lowermost plastic film, in particular the O / (Co or Co + Ni) atomic ratio, is the oxygen concentration C ₁ near the surface opposite to the uppermost plastic film, especially O / The value divided by (Co or Co + Ni) atomic ratio is
It is preferably 0.3 or less, more preferably 0.15 or less.

In this case, these oxygen concentrations are determined by Auger spectroscopic analysis, SIMS (secondary ion mass spectrometry), etc. while Ar or the like ion-milling or ion-etching the ferromagnetic metal thin film.
Can be measured.

That is, while performing ion etching, O, Co, Ni
Etc. are counted and the profiles in the film thickness direction are compared.

The O / (Co or Co + Ni) on the surface of the ferromagnetic metal thin film opposite to the plastic film is C ₁ . As for the bottom layer, the plastic film is etched until the O / (Co or Co +
Ni) is C ₂ .

Ion etching, Auger spectroscopic analysis, and SIMS may be measured by conventional methods.

By increasing the oxygen concentration on the surface of the uppermost layer in this way, the coercive force Hc is increased, and by decreasing the oxygen concentration on the lowermost layer, the maximum residual magnetic flux Φr and the squareness ratio SQ are increased. As a result, the magnetic layer becomes high and the electromagnetic conversion characteristics are extremely good.

Further, in the ferromagnetic metal thin film layer as the magnetic layer of the present invention,
Oxygen concentration near the interface between the uppermost layer and the adjacent uppermost layer
C _3, in particular O / (Co or Co + Ni) oxygen concentration C ₁ of the atomic ratio near the surface opposite the top layer of plastic film, the value C ₃ / C, in particular divided by O / (Co or Co + Ni) atomic ratio ₁ is 0.
It is preferably 0.2 to 2.0, more preferably 2 to 3.0.

In this case, O / (Co or Co + Ni) C _{1 on the} surface of the ferromagnetic metal thin film opposite to the plastic film can be measured in the same manner as described above. The oxygen concentration C ₃ near the interface between the uppermost layer and the uppermost layer is O / (Co or Co or Co from the count during etching corresponding to the thickness of the uppermost layer.
+ Ni) and calculate this as C ₃ . However, in each layer, the oxygen concentration is maximum on the surface opposite to the film substrate under normal film forming conditions. Therefore, normally, when O is counted while performing ion etching,
The maximum value in the film may be C ₃ .

In this way, by relatively increasing the oxygen concentration C ₁ on the surface of the uppermost layer, the coercive force Hc increases, and the oxygen concentration of the portion below the surface of the uppermost layer and in the vicinity of the layer adjacent to the uppermost layer is also increased. Is relatively lower than the above C ₁ , the maximum residual magnetic flux Φr and the squareness ratio SQ are increased, and the magnetic layer has extremely good electromagnetic conversion characteristics. Therefore, the center frequency is 5M
A signal with a relatively shallow magnetic field of about Hz is effectively retained in the uppermost layer.

Further, the oxygen concentration C ₃ in the vicinity of the interface between the uppermost layer adjacent to the uppermost layer, the relationship between the C ₁ is is C ₃ / C _1, as described above zero.
In the range of 1 to 3.0, by relatively increasing the coercive force Hc in this portion, the oxygen concentration in the portion below the interface with the uppermost layer of the layer adjacent to the uppermost layer and below is also increased. By making it relatively lower than C ₃ , the maximum residual magnetic flux Φr and the squareness ratio SQ become high, and a magnetic layer having extremely good electromagnetic conversion characteristics is obtained. Therefore, the center frequency is 0.
A signal with a comparatively deep magnetic field of about 7 MHz is effectively retained below the layer next to the uppermost layer.

And, as described above, the relationship between C ₁ and C ₃ is C ₃ / C ₁ is 0.
When it is 1 to 3.0, it becomes an excellent magnetic layer having the best balance of electromagnetic conversion characteristics and corrosion resistance of the magnetic layer.

The O / (Co or Co + Ni) C ₁ near the surface is generally 0.
2 to 0.7, preferably 0.3 to 0.6.

Therefore, O / (Co or Co + Ni) C ₂ near the film interface is
It is 0.06 to 0.21, preferably 0.09 to 0.18.

Also, O / (Co or
Co + Ni) C ₃ is 0.07 to 0.6 is preferably 0.1 to 0.5.

Furthermore, O / (Co or Co + Ni) C ₁ ^{* in the} entire top layer
Is preferably 0.1 to 0.5, more preferably 0.1 to 0.4. In addition, O / (Co or Co + Ni) in the entire bottom layer
C ₂ ^* is preferably 0.5 or less, more preferably 0.3 or less.

In addition, O / (Co or Co + in the entire layer adjacent to the uppermost layer
Ni) is preferably 0.5 or less, more preferably 0.3 or less.

At this time, electromagnetic conversion characteristics, corrosion resistance, running durability, magnetic film strength, etc. are extremely good.

In this case, in the case of a multi-layered structure having three or more layers, the O / (Co or Co + Ni) of each layer is generally 0.5 or less, preferably 0.3 or less.

In this case, on the surface of each layer of the ferromagnetic metal thin film layer,
Oxygen forms an oxide with ferromagnetic metals (Co, Ni).

That is, in the range of 100 to 2000 Å, more preferably 500 to 1000 Å thickness from the surface of each layer, a peak showing an oxide is recognized by Auger spectroscopic analysis.

In the present invention, the oxygen concentration between the ferromagnetic metal thin film layer surface and the film-side interface is regulated, and the oxygen concentration between the ferromagnetic metal thin film layer surface and the uppermost layer adjacent to the uppermost layer is regulated. In that case, the predetermined effects of the present invention are realized.

Therefore, regarding the oxygen concentration profile in the thickness direction of the ferromagnetic metal thin film, there is usually a peak of oxygen distribution at least at the interface between the uppermost layer and the layer adjacent to the uppermost layer.

It should be noted that the ferromagnetic metal thin film usually has two layers, but if necessary, it may have three or more layers, particularly 3 to 5 layers.

Incidentally, in such a ferromagnetic metal thin film, further other trace components, especially transition elements such as Fe, Mn, V, Zr, Nb, T
It may contain a, Ti, Zn, Mo, W, Cu and the like.

In a preferred embodiment, such a ferromagnetic metal thin film layer is composed of an aggregate of columnar crystal grains containing Co as the main component.

In this case, the total thickness of the ferromagnetic metal thin film layer is 0.05 to 0.
The thickness is 5 μm, preferably 0.07 to 0.3 μm.

And, in each layer of such a ferromagnetic metal thin film layer,
The thickness of the top layer is less than the thickness of the bottom layer.

In this case, the ratio of the thickness of the top layer to the thickness of the bottom layer is 0.2-
It is preferably 0.9, more preferably 0.4 to 0.9.

The thickness of the intermediate layer in the structure of three or more layers is usually within the range of the thickness of the uppermost layer and the thickness of the lowermost layer.

Then, the columnar crystal grains have a length extending over substantially the entire region in the thickness direction of each layer, the longitudinal direction, the minimum angle to the normal line of the main surface of the substrate is 20 ~ 90 ° in the uppermost layer, more preferably 20.
In the range of up to 50 °, the lowermost layer is 50 ° or less, more preferably 0
It is preferable to incline in the range of -40 °.

In this case, in each of the intermediate layers in the structure of three or more layers, the tilt angle of the columnar crystal grains with respect to the normal to the main surface of the substrate is
Usually, the uppermost layer and the lowermost layer are not particularly limited as long as they are within the inclination angle range.

In this case, the directions of inclination of the crystal grains of the magnetic layers adjacent to each other with respect to the normal to the main surface of the substrate may be the same in the longitudinal direction of the medium, but are preferably opposite to each other. preferable.

FIG. 1 and FIG. 2 schematically show such a tilt direction of the crystal grains by taking a two-layer structure as an example.

In FIGS. 1 and 2, a magnetic recording medium 1 has a ferromagnetic metal thin film lower layer portion 3 and a ferromagnetic metal thin film upper layer portion 4 on a substrate 2. Then, the tilt direction of the lower layer crystal grains 5 in the ferromagnetic metal thin film lower layer portion 3, the ferromagnetic metal thin film upper layer portion 4
The upper layer crystal grains 6 in the inside are inclined in opposite directions in the medium length direction a in FIG. 1, and are in the same direction in the medium length direction a in FIG.

In the present invention, the crystal grain gradient shown in FIG. 1 or 2 may be used, but the crystal grain gradient shown in FIG. 1 is preferable.

It should be noted that oxygen is present in the form of a compound on the surface of the columnar crystal grains in the surface portion as described above.

There is no particular limitation on the oxygen concentration gradient of the ferromagnetic metal thin film layer.

Further, the minor axis of the crystal grains preferably has a length of about 50 to 500Å.

As described above, by forming the ferromagnetic metal thin film layer in a multi-layered structure, the length of the columnar crystal grains becomes small, so that the film strength of the ferromagnetic metal thin film layer is improved.

Further, since the columnar crystal grains of the uppermost layer having a small layer thickness have an inclination of 20 to 90 ° with respect to the normal line to the main surface of the substrate, particularly those having an inclination of 50 ° or more, for example, have a relatively shallow magnetic field. A signal having a center frequency of about 5 MHz can be effectively held in the uppermost layer.

In addition, since the columnar crystal grains in the lowermost layer with a large layer thickness have an inclination of 50 ° or less with respect to the normal to the main surface of the substrate and stand upright with respect to the substrate, A signal with a frequency of about 0.75 MHz can be effectively held in the lowest layer such as the lowest layer.

Further, as described above, by increasing the oxygen concentration in the uppermost layer, oxides such as Co, Ni, etc., which have excellent wear resistance, are formed in the uppermost layer, and therefore, due to the synergistic effect with the multilayer structure, The film strength of the ferromagnetic metal thin film layer becomes higher.

The material of the substrate used in the magnetic recording medium of the present invention is not particularly limited as long as it is a non-magnetic plastic, but usually polyethylene terephthalate, polyethylene 2,6
-Polyester such as naphthalate, polyamide, polyimide, polyphenylene sulfide, polysulfone,
Wholly aromatic polyester, polyetheretherketone,
Polyether sulfone, polyether imide, etc. are used.

The thickness is preferably 8 μm or less, and particularly preferably about 5 to 7 μm.

If the thickness exceeds 8 μm, the objectives of downsizing the medium, recording for a long time, etc. cannot be achieved. Further, if this thickness is too thin, the effect of improving the film strength by forming the magnetic layer as a multilayer structure as described above is offset, and the running property and the output decrease,
Problems such as head wear occur.

The effect of improving the electromagnetic conversion characteristics by the multilayer structure of the ferromagnetic metal thin film layer of the present invention is more remarkable when the thickness of the substrate is thin.

For example, taking a two-layer structure as an example, when the thickness of the substrate is 10 μm, the ferromagnetic metal thin film layer is changed from the conventional single-layer structure to that of the present invention.
The improvement in electromagnetic conversion characteristics due to the layered structure is 0.75
The signal in the low frequency region of MHz is about +6 (dB), and the signal in the high frequency region of 5 MHz is about +6 (dB). However, when the thickness of the substrate is 7 μm, the improvement is 0.75 MHz in the low frequency region. The signal is about +6 (dB), which is similar to the case where the substrate thickness is 10 μm, but it increases to about +7.5 (dB) for a signal in the high frequency region of 5 MHz.

In this way, the improvement in electromagnetic conversion characteristics of the 7 μm-thick substrate is remarkable when compared with the 10 μm-thick substrate.
When the thickness is changed from 7 μm to 7 μm, the head touch abruptly deteriorates due to the lack of rigidity of the substrate, and this effect is greater in a high frequency region such as 5 MHz, and the effect of the present invention is exhibited in such a case.

It is preferable to form various known backing layers on the other surface of the plastic film on which the magnetic layer is not provided.

The material of the backing layer is not particularly limited, but a material containing a pigment and a radiation curable resin is particularly preferable. The thickness of the lining layer is 0.05 to 1.5 μm, more preferably 0.07 to 1.0 μm
m.

Fine protrusions may be provided at a predetermined density on the surface of the magnetic recording medium of the present invention.

The fine protrusions have a height of 30 to 300Å, more preferably 50 to 250Å.

That is, the protrusions of the present invention can be observed with an optical microscope and can be observed with a scanning electron microscope, not with a stylus type surface roughness meter.

If the projection height exceeds 300Å and can be observed with an optical microscope, electromagnetic conversion characteristics will deteriorate and running stability will decrease.

Also, if it is less than 50Å, the improvement of physical properties is not effective.

And, the density is 10 ⁵ or more per 1 mm ^{2 on} average, more preferably 10 ⁵ to 10 ⁹ , and especially 10 ⁶ to 10 ⁸ .

When the density of the protrusions is less than 10 ⁵ pieces / mm ² , noise increases and the physical properties such as the deterioration of the still characteristics are deteriorated, which is not practical.

On the other hand, if it exceeds 10 ⁹ pieces / mm ² , the effect on the physical properties will be reduced.

The diameter of the protrusion is generally about 200 to 1000Å.

In order to provide such protrusions, it is usually sufficient to dispose fine particles on the substrate. The particle size should be 30 to 1000Å,
As a result, fine protrusions corresponding to the particle diameter are formed.

The fine particles to be used are generally known as colloidal particles, for example, SiO ₂ (colloidal silica), Al ₂ O ₃ (alumina sol), MgO, TiO ₂ , ZnO, Fe.
₂ O ₃ , zirconia, CdO, NiO, CaWo ₄ , CaCO ₃ , BaCO ₃ , Co
CO ₃ , BaTiO ₃ , Ti (titanium black), Au, Ag, Cu, N
i, Fe, various hydrosols, resin particles and the like can be used. In this case, it is particularly preferable to use an inorganic substance.

Such fine particles may be used as a coating solution using various solvents, which may be coated on a substrate and dried, or a coating solution to which a resin component such as various aqueous emulsions is added may be coated and dried. Good.

Further, when a resin component is used, it is possible to form a gentle protrusion by superposing on the fine protrusions based on these fine particles, but it is not usually necessary to do so.

If necessary, a non-magnetic metal thin film layer may be interposed between the uppermost layer and the lowermost layer of the ferromagnetic metal thin film layer.

In the present invention, the magnetic layer is preferably formed by a so-called oblique vapor deposition method.

In this case, the minimum incident angle of the vapor deposition substance with respect to the normal to the substrate is 50 ° or less when the lowermost layer is formed, 20 to 90 ° when the uppermost layer is formed, and the middle in the case of the structure of three or more layers. When the layers are formed, the angle is preferably 20 to 50 °.

If the minimum incident angle deviates from the above-mentioned incident angle, the electromagnetic conversion characteristics deteriorate.

Further, the magnetic layer may be formed by continuously forming two or more layers in one step, but it is usually preferable to form each layer after the vapor deposition step.

In this way, by dividing the layers of the magnetic layer into layers,
As described above, the direction of the inclination of the magnetic columnar crystal grains with respect to the normal to the substrate is such that the adjacent layers are opposed to each other in the longitudinal direction of the medium.

With such a magnetic layer structure, electromagnetic conversion characteristics become extremely good.

The vapor deposition atmosphere is usually an atmosphere containing oxygen gas in an inert atmosphere such as argon, helium, or vacuum, and 10 ^-5
And to 10 ⁰ Pa pressure of about, also deposition distance, a substrate transfer method, the structure of the can and the mask, arrangement and the like may be the same as known conditions.

Then, a metal oxide film is formed on the surface by vapor deposition in an oxygen atmosphere. The oxygen gas partial pressure at which the metal oxide is formed can be easily obtained from experiments.

Various oxidation treatments can be performed to form a metal oxide film on the surface.

The following oxidation treatments can be applied.

1) Dry treatment a Energy particle treatment As described in Japanese Patent Application No. 58-76640, in the latter stage of vapor deposition,
An ion gun or neutral gun that causes oxygen to enter the magnetic layer as energetic particles.

b. Glow treatment O ₂ , H ₂ O, O ₂ + H ₂ O etc. and an inert gas such as Ar, N ₂ are used,
Glow discharge is performed to generate plasma, and the surface of the magnetic film is exposed to this plasma.

c. Oxidizing gas A gas that blows an oxidizing gas such as ozone or heated steam.

d. Heat treatment A substance that is oxidized by heating. Heating temperature is 60-150 ℃
degree.

2) Wet treatment a. Anodizing b. Alkali treatment c. Acid treatment Chromate treatment, permanganate treatment, phosphate treatment, etc. are used.

d. Oxidant treatment H ₂ O ₂ or the like is used.

Further, in the medium of the present invention, a surface layer may be provided on the magnetic layer to further improve the running property.

As the surface layer, various known ones can be applied, for example, various polymer material coatings, or those containing a lubricant, an antioxidant, a surfactant, inorganic fine particles or the like, or various lubricants. There is a coating film or a vapor deposition film.

The thickness of the surface layer is about 5 to 300 liters.

VI Specific Action and Effect of the Invention According to the present invention, since a substrate having a thickness of 8 μm or less is used,
It is possible to downsize the medium and record for a long time.

Further, since the magnetic layer has a layered structure of two or more layers, the length of the magnetic columnar crystal grains becomes small, so that the film strength of the magnetic layer is improved. Therefore, driving stability is extremely high,
Further, the occurrence of cracks in the magnetic layer and scratches on the magnetic surface due to running is extremely small, and the amount of head wear is also extremely small.

In addition, the columnar crystal grains in the uppermost layer are 20 to 90
Some have a slope of 50 ° or more, and in particular have a slope of 50 ° or more, and at the same time, the ratio C ₂ / C ₁ of the oxygen concentration C ₂ of the lowermost layer and the oxygen concentration C ₁ of the uppermost layer is 0.3 or less, and , Oxygen concentration C _{3 in the} vicinity of the uppermost layer interface of the layer adjacent to the uppermost layer and oxygen concentration of the uppermost layer
Since the ratio with C ₁ is 0.1 to 0.3, the coercive force Hc becomes relatively high in the uppermost layer, the signal having a relatively shallow magnetic field with a center frequency of about 5 MHz is effectively held, and the resolution is good. Will be things.

Further, the columnar crystal grains in the bottom layer are 50
It has a slope of less than ° and stands upright with respect to the substrate. Also, the maximum residual magnetic flux Φr and the squareness ratio are high in the lowermost layer at the same time, and in the vicinity of the uppermost layer interface of the layer adjacent to the uppermost layer. Since the coercive force Hc has a peak, it effectively holds a signal having a relatively deep magnetic field and a center frequency of about 0.75 MHz.

The above-mentioned effect is more remarkably exhibited when the ratio of the thickness of the uppermost layer of the magnetic layer to the thickness of the lowermost layer is 0.2 to 0.9.

VII Specific Examples of the Invention Hereinafter, the present invention will be described in more detail by showing specific examples of the invention.

Example 1 A polyester (PET) film having a thickness shown in Table 1 below was moved along the circumferential surface of a cylindrical cooling can to remove O ₂ + Ar.
(Volume ratio 1: 1) flows at a speed of 800cc per minute and the degree of vacuum is 1.0 x 1
Co80, Ni20 (weight ratio) in a chamber set to 0 ^-4 Torr
Was melted and the incident angle was set to the incident angle shown in Table 1 to form a Co-Ni-O two-layer thin film shown in FIG. 1 by oblique vapor deposition.

As a comparison, only a portion having an incident angle of 30 to 90 ° was obliquely vapor-deposited to form a 0.15 μm thick Co—Ni—O single-layer thin film.

A large amount of oxygen was unevenly distributed on the interface between the lower layer and the upper layer and on the surface opposite to the base. Also, the surface opposite to the base was almost entirely covered with oxide.

Hc = 1000 Oe. The average oxygen content in the film is the atomic ratio to Co and Ni. Was 40%.

Table 1 shows that among the O / (Co or Co + Ni) atomic ratios obtained by performing Auger spectroscopic analysis while performing ion etching with Ar, C ₁ (surface), C ₁ ^* (upper layer average), and C ₂ ( The interface between the lower layer and the substrate), C ₂ ^* (average of the lower layer), and C ₃ (in the vicinity of the interface between the lower layer and the upper layer) are also described.

In addition, on the magnetic layer thin film, a surface layer of isopropyl myristate was formed with a film thickness of 25 Å, and 0.
A backing layer containing carbon, silica and a radiation curable resin was formed to a thickness of 5 μm.

The following measurements were carried out for each of the samples formed in this way and shown in Table 1 below. The medium running direction and the direction of the inclination with respect to the base layer normal to the lower layer were the same direction.

1) Durability Conditions of temperature 20 ℃, humidity 60% RH and temperature 40 ℃, humidity 80
A continuous running test was performed under each condition of% RH, and the number of passes until the output decreased by 2 dB was obtained.

Deck used: SONY A-300 Head: Spatter Sendust 2) Still durability The time required for the output to drop by 6 dB at a temperature of 0 ° C was determined.

Deck used: SONY A-300 (I used it without the still-release mechanism) Head: Spatter sendust 3) Electromagnetic conversion characteristics Measure the output at center frequencies 0.75MHz and 5MHz and sample N
The value was calculated when the output of o.15 was set to 0 dB.

Deck used: SONY A-300 Head: Sputter sendust mode: SP mode Sample No. 1 to 10 (base thickness 7 μm, base thickness 7 μm, magnetic layer configuration: single layer) output
The improvement range of the output of the magnetic layer structure: 2 layers is shown in () in Table 1.

From the results shown in Table 1, the effect of the present invention is clear.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a schematic view of a cross section parallel to the medium direction of one embodiment of a magnetic recording medium of the present invention. FIG. 2 is a schematic view of a cross section parallel to the medium direction of another embodiment of the magnetic recording medium of the present invention. Explanation of symbols 1 ... Magnetic recording medium 2 ... Substrate 3 ... Ferromagnetic metal thin film lower layer 4 ... Ferromagnetic metal thin film upper layer 5 ... Lower crystal grain 6 ... Upper crystal grain Arrow a ... Medium length direction

Claims (4)

[Claims] 1. A ferromagnetic metal thin film layer containing Co as a main component on a plastic film substrate having a thickness of 8 μm or less, and the ferromagnetic metal thin film layer has a multi-layer structure composed of two or more layers. , The minimum incident angle of the deposited material with respect to the normal to the substrate when depositing the ferromagnetic metal thin film layer is 50 ° or less when the lowermost layer is formed on the substrate side, and 20 ° when the uppermost layer is formed on the opposite side of the substrate. 90 °, and the value C ₂ / C _{1 obtained} by dividing the oxygen concentration C ₂ in the vicinity of the substrate side interface of the lowermost layer by the oxygen concentration C ₁ in the vicinity of the surface of the uppermost layer substrate is 0.3 or less. the value C ₃ / C ₁ obtained by dividing the oxygen concentration C ₃ in the vicinity of the interface between the top layer with an oxygen concentration C ₁ at the opposite side surface near the top layer of the substrate adjacent layers is 0.2 to 3.0, the top layer Magnetic recording medium whose thickness is smaller than the thickness of the bottom layer, the upper layer mainly holds signals in the high frequency range of 5MHz or more, and 0.75MHz or its vicinity. The signal of the low-frequency region, primarily the lower holds with magnetic recording, magnetic recording method of improving the signal output. 2. The magnetic recording method according to claim 1, wherein the ferromagnetic metal thin film layer has a two-layer structure. 3. The magnetic recording method according to claim 1, wherein the thickness of the uppermost layer is 0.2 to 0.9 of the thickness of the lowermost layer. 4. The magnetic recording method according to any one of claims 1 to 3, wherein the output of a signal in the high frequency region is improved.