Magnetic recording medium

A magnetic recording medium having an excellent sliding durability, and including a magnetic layer of a metal magnetic thin film formed on a non-magnetic support body and a carbon protection film formed on the magnetic layer. Recording and/or reproduction is carried out by sliding a magnetic head. The carbon protection film shows, in a Raman spectrum obtained by Raman spectrum analysis using an argon ion laser having a wavelength of 514.5 nm, an intensity ratio A/B of 2 or above for a main peak intensity A appearing in the vicinity of wave number 1550 cm.sup.-1 with respect to a background intensity B.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention relates to a metal magnetic thin film type magnetic 
recording medium onto/from which recording and/or reproduction is carried 
out by sliding of a magnetic head and in particular, to an improvement of 
a carbon protection film. 
2. Description of the Prior Art 
Conventionally, the magnetic recording medium widely used is a so-called 
paint-type magnetic recording medium which is prepared by coating a 
non-magnetic support body with a magnetic paint made from a powder 
magnetic material such as an oxide magnetic powder or an alloy magnetic 
powder dispersed in an organic binder such as vinyl chloride--vinyl 
acetate copolymer, polyester resin, urethane resin, and the like. 
In contrast to this, with increasing requirement for a high-density 
recording, attention is paid on a so-called metal magnetic thin film type 
recording medium which is prepared by directly applying a metal magnetic 
material such as a Co-Ni alloy, Co-Cr alloy, Co-O alloy by way of plating 
or vacuum thin film formation means (vacuum deposition, sputtering, ion 
plating method, and the like). 
This metal magnetic thin film type magnetic recording medium has various 
merits such as an excellent anti-magnetization force and rectangular ratio 
and enables to obtain an extremely thin magnetic layer, which in turn 
suppresses the recording magnetization loss and the thickness loss during 
reproduction, enabling to obtain an excellent electromagnetic conversion 
characteristic in a short wavelength. Moreover, because there is not need 
of mixing a non-magnetic material such as a binder in the magnetic layer, 
it is possible to increase the magnetic material filling density. 
Consequently, it is considered that the metal magnetic thin film type 
magnetic recording medium will become a main stream of the high-density 
magnetic recording because of its excellent magnetic characteristic. 
Furthermore, in order to improve the electro-magnetic conversion 
characteristic of this type of magnetic recording medium so as to obtain a 
high output in a shorter wavelength, a so-called oblique-deposited medium 
has been suggested and used in practice. This oblique-deposited medium is 
a magnetic metal thin film type magnetic recording medium having a 
magnetic layer formed by way of so-called oblique deposition, i.e., a 
magnetic metal is deposited in an oblique direction onto a traveling 
non-magnetic support body. 
On the other hand, in order to answer to a further higher density 
recording, there is a tendency that the magnetic recording medium is made 
flat so as to reduce a spacing loss. The flattening of the magnetic 
recording medium is accompanied by increase of the friction between a head 
and a medium, increasing the shearing stress generated in the medium. 
Here, in order to improve the sliding durability, there has been studied a 
technique to form a protection film on the surface of the magnetic layer. 
As such a protection film, a carbon film, quartz (SiO.sub.2) film, a 
zirconia (ZrO.sub.2) film, and the like have been studied and implemented 
in practice for a hard disc. As for the carbon protection film, a 
diamond-lie carbon (hereinafter, referred to as DLC) film is studied as a 
harder film. The DLC film is formed by using a sputtering method, a 
chemical vapor phase epitaxy (hereinafter, referred to as CVD) method, and 
the like. 
In the sputtering method, firstly, an electric field and a magnetic field 
are used for plasmatizing an inert gas such as argon gas and the 
plasmatized argon ion is accelerated so that its kinetic energy strikes 
out a target atom. The struck out atom is accumulated on a opposing 
substrate, thus forming a film as a purpose. This sputtering method has a 
poor productivity from an industrial viewpoint because the aforementioned 
DLC film is formed with a low speed. 
On the contrary, the CVD method is a chemical process in which the plasma 
energy generated by an electric field and a magnetic field causes a 
chemical reaction of a material gas such as decomposition and synthesis. 
The DLC film formation speed is faster than in the sputtering method. 
Thus, by providing the carbon protection film by way of the aforementioned 
methods, the magnetic recording medium significantly increases its sliding 
durability. 
However, these years, the magnetic recording apparatus tends to become 
smaller and to have a greater capacity. Especially for the data storage 
use which requires a high reliability, a further improved sliding 
durability is required. 
Moreover, the magnetic head of the magnetic recording/reproduction 
apparatus has a lower floating amount. As the extreme case of the floating 
amount, a so-called contact method is also suggested in which recording 
and reproduction are carried out with the magnetic head always in contact 
with the surface of a magnetic recording medium. 
In the magnetic recording medium in which recording and/or reproduction is 
carried out by way of contact sliding between a magnetic head and a 
magnetic recording medium surface, it is necessary to further reduce the 
friction and improve the sliding durability. 
SUMMARY OF THE INVENTION 
It is therefore an object of the present invention to provide a magnetic 
recording medium having a further improved sliding durability. 
The inventors of the present invention has made various studies and found 
that a carbon protection film showing in a Raman spectrum obtained by 
Raman spectrum analysis, an intensity ratio of 2 or above for a main peak 
intensity appearing in the vicinity of wave number 1550 cm.sup.-1 with 
respect to a background intensity shows a low friction and an excellent 
sliding durability. 
That is, the magnetic recording medium according to the present invention 
includes a magnetic layer of a metal magnetic thin film formed on a 
non-magnetic support body and a carbon protection film formed on the 
magnetic layer, wherein recording and/or reproduction is carried out by 
sliding of a magnetic head, and the aforementioned protection is 
characterized in that in a Raman spectrum obtained by a Raman spectrum 
analysis using an argon ion laser having a wavelength of 514.5 nm, the 
ratio of the main peak intensity appearing in the vicinity of wave number 
of 1550 cm.sup.-1 with respect to a background intensity is 2 or above. 
The magnetic recording medium according to the present invention includes a 
carbon protection film showing a Raman spectrum pattern (intensity ratio) 
obtained by a Raman spectrum analysis which is defined in a specific range 
and enables to obtain a carbon protection film having an excellent shuttle 
characteristic and still characteristic. As a result, it is possible to 
provide a magnetic recording medium having an excellent sliding durability 
.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
Hereinafter, description will be directed to a magnetic recording medium 
according to the present invention. 
The magnetic recording medium according to the present invention includes a 
magnetic layer formed from a metal magnetic thin film on a non-magnetic 
support body, and the magnetic layer is covered with a carbon protection 
film which satisfies a condition as follows. 
That is, the aforementioned carbon protection films, as shown in FIG. 1, 
shows a Raman spectrum obtained by a Raman spectrum analysis using an 
argon ion laser of wavelength 514.5 nm, in which Raman spectrum intensity 
ratio A/B is 2 or above, assuming A is a main peak intensity appearing in 
the vicinity of the wave number 1550cm.sup.-1 and B is a background 
intensity. 
Here, the background intensity B is a base line connecting the minimum 
values at both ends of the main peak appearing in the vicinity of the wave 
number 1550 cm.sup.-1. The inclination of this base line is changed 
depending on a hydrogen concentration in the carbon protection film 
(material gas composition) and a voltage condition applied to an 
electrode. 
The aforementioned intensity ratio A/B is 2 or above and more preferably, 
in a range from 2.5 to 4.2. If the intensity ratio A/B is below 2, it is 
considered that almost no amorphous carbon is generated. Moreover, if the 
intensity ratio A/B exceeds 4.2, the voltage applied to the CVD apparatus 
is increased and there is a danger that the non-magnetic support body may 
be damaged by heat. 
This carbon protection film is preferably formed, for example, by way of a 
vacuum thin film formation technique such as the CVD method and the 
sputtering method, in which a material is subjected to vapor phase 
chemical reaction and then accumulated on a non-magnetic support body. In 
particular, the CVD method is preferably used because it enables a 
high-speed film formation. 
Moreover, in this case, the carbon protection film is preferably formed as 
a hard carbon film of amorphous configuration, i.e., so-called DLC film. 
The DLC film enables to realize a high hardness and low friction in a thin 
film area (in the order of 10 nm) and accordingly, the DLC film is 
preferable as a protection film of a sliding medium such as a tape. 
The carbon protection film preferably has a thickness from 1 to 20 nm. If 
the carbon protection film has a thickness below 1 nm, it is impossible to 
obtain a sufficient effect of providing the protection film. Moreover, if 
the carbon protection film has a too great thickness, the coercive force 
of the magnetic layer is lowered. Besides, in order to increase the length 
of the magnetic tape which can be contained in a cassette, the thickness 
of the carbon protection film is preferably equal to or below 20 nm. 
Thus, the carbon protection film having the intensity ratio A/B equal to or 
above 2 for the main peak intensity A in the vicinity of wave number 1550 
cm.sup.-1 with respect to the background intensity B exhibits a low 
fiction and an excellent sliding durability. 
Consequently, the magnetic recording medium according to the present 
invention is a magnetic recording medium in which recording and/or 
reproduction are carried out by sliding of a magnetic head and is 
appropriate to be used as a magnetic recording medium for a special use 
requiring a high reliability such as a data streamer and a video library. 
Moreover, the magnetic recording medium according to the present invention 
may be a disc-shaped or tape-shaped medium but can preferably be used as a 
magnetic recording medium such as a tape-shaped medium which, unlike a 
disc-shaped medium contained in a case, is often exposed outside and 
should have a strict environment-proof characteristic. 
Now, the present invention relates to a so-called metal magnetic thin film 
type magnetic recording medium but does not limit to particular materials 
the non-magnetic support body and the metal magnetic thin film. 
The non-magnetic support body may be made from any of materials which are 
usually used for a non-magnetic support body in this type of magnetic 
recording medium. For example, it is possible to form the non-magnetic 
support body by using polyethylene terephthalate, polyethylene-2, 
6-naphthalate, and other polyesters: polyethylene, polypropylene, and 
other polyolefins; cellulose triacetate, cellulose diacetate, cellulose 
triacetate butylate, and other cellulose derivatives; polyvinyl chloride, 
polyvinylidene chloride, and other vinyl resins; polycarbonate, polyamide, 
polyimide, and other high molecule materials; alumina glass, ceramics, and 
the like. 
The magnetic layer is formed by way of film formation using a ferromagnetic 
metal material. The ferromagnetic metal material may be any of the 
materials which are used for an ordinary deposited tape. For example, it 
is possible to use Fe, Co, Ni, and other ferromagnetic metals; and Fe-Co, 
Co-Ni, Fe-Co-Ni, Fe-Cu, Co-Cu, Co-Au, Co-Pt, Mn-Bi, Mn-Al, Fe-Cr, Co-Cr, 
Ni-Cr, Fe-Co-Cr, Co-Ni-Cr, Fe-Co-Ni-Cr, and other ferromagnetic alloys. 
The magnetic layer made from these ferromagnetic metals may be a 
single-layered film or multi-layered film. 
The magnetic layer may be formed by using a vacuum deposition method in 
which a ferromagnetic metal material is heated and evaporated in a vacuum 
so as to be deposited on a non-magnetic support body; the ion plating 
method in which evaporated particles are ionized and accelerated by an 
electric field so as to adhere onto a non-magnetic support body; the 
sputtering method in which atoms are struck out of a target surface by 
argon ion generated by glow discharge in an atmosphere containing argon as 
a main content; and other PVD techniques. 
It should be noted that when the magnetic layer is formed by way of the 
vacuum deposition method, in order to improve the characteristic in a 
high-density recording range, it is preferable to employ an oblique 
deposition method in which a magnetic metal material as a deposition 
source is used so that magnetic metal particles are deposited in an 
oblique direction onto a traveling non-magnetic support body. 
Here, the magnetic layer preferably has a magnetization axis with an 
inclination of 20 to 90 degrees with respect to the surface of the 
non-magnetic support body. As this inclination angle of the magnetization 
axis increases, the characteristic in a high-density recording range is 
improved. It should be noted that the inclination of the magnetization 
axis of the magnetic layer can be controlled by changing the incoming 
angle of deposition particles with respect to the non-magnetic support 
body when depositing the particles. 
It should be noted that in this deposition method, it is preferable to 
introduce an oxygen gas into the deposition atmosphere so as to obtain a 
film containing oxygen such as a Co-O thin film and a Co-Ni-O thin film. 
This enables to reduce the crystal particle size in the magnetic layer, 
reducing the medium noise. Moreover, when the crystal particle has a 
columnar configuration, it is possible to increase a configuration 
anisotropy in the oblique direction. 
The aforementioned is the basic configuration of the magnetic recording 
medium according to the present invention. In this magnetic recording 
medium, it is possible to provide an additional configuration which is 
normally used in this type of magnetic recording medium, so as to further 
improve its characteristics. 
For example, an under layer or an intermediate layer may be provided 
between the aforementioned non-magnetic support body and the magnetic 
layer or between multiple layers, so as to improve the adhesion between 
the layers and control the anti-magnetization force. Furthermore, it is 
possible to carry out a surface treatment on the non-magnetic support body 
so as to form fine convex and concave shapes so as to control the surface 
state. 
Moreover, it is possible to provide a top coat layer made from a lubricant 
and anti-corrosive agent on the metal magnetic thin film serving as the 
magnetic layer. Moreover, on the opposite side of the non-magnetic support 
body not having the magnetic layer, it is possible to form a back coat 
layer from a non-magnetic pigment, binder, lubricant, and the like, in 
order to improve the magnetic recording medium running durability and to 
prevent charging and transfer. These materials may be those which are 
conventionally used. 
EXAMPLES 
Hereinafter, description will be directed to experiment results of the 
embodiment of the present invention. 
EXAMPLE 1 
Firstly, a vacuum deposition was carried out onto a polyethylene 
terephthalate (PET) film having a thickness of 6 .mu.m by using Co as a 
deposition source while introducing an oxygen gas, so as to obtain a metal 
magnetic thin film (Co-O single-layered) having a film thickness of 200 
nm. The metal magnetic thin film was formed under a condition as follows. 
&lt;Metal magnetic thin film formation condition&gt; 
Particle deposition angle: 45 to 90 degrees 
Vacuum degree during deposition: 2.times.10.sup.-2 Pa 
Next, a protection film consisting of a DLC film was formed on the 
aforementioned metal magnetic thin film. Explanation will be given on a 
plasma CVD apparatus used for this film formation. 
As shown in FIG. 2, this plasma CVD apparatus includes a vacuum chamber 2 
from which air has been removed by a vacuum exhaust system 1. In this 
vacuum chamber, there are arranged a feed roll 3 and a winding-up roll 4 
so that a tape 5 travels from this feed roll 3 toward the winding-up roll 
4. During this travel, as has been described above, the metal magnetic 
thin film is formed on the film. 
A cylindrical can 6 having a greater diameter than the rolls 3 and 4 is 
provided in the middle of the tape travel from the feed roll 3 to the 
winding-up roll 4. 
This cylindrical can 6 is provided so that the tape 5 is pulled downward in 
the figure. This cylindrical can has a cooling apparatus (not depicted) 
inside so as to suppress deformation of the tape 5 due to temperature 
increase. 
In the plasmas CVD apparatus having the aforementioned configuration, the 
tape 5 is successively fed from the feed roll 3 so as to travel along the 
circumference of the cylindrical can 6 and is wound up by the winding-up 
roll 4. 
Moreover, guide rolls 7 and 8 each having a diameter smaller than the rolls 
3 and 4 are provided between the feed roll 3 and the cylindrical can 6 and 
between the cylindrical can 6 and the winding-up roll, respectively. The 
guide rolls 7 and 8 serve to apply a predetermined tension to the tape so 
that tape can travel smoothly from the feed roll 3 to the cylindrical can 
6 and from the cylindrical can 6 to the winding-up roll 4. 
Moreover, below the cylindrical can 6, there is provided a gas reaction 
pipe 9 having a curved opening so as to be in parallel to the 
circumference of the cylindrical can 6. Inside the gas reaction pipe 9, 
there is provided a metal mesh-shaped electrode 10. This electrode 10 is 
supplied with current from a predetermined DC power source so that a 
predetermined DC voltage is applied between the electrode 10 and the 
cylindrical can 6. Moreover, the gas reaction pipe 9 has a gas supply port 
11 for supplying a gas inside the gas reaction pipe 9. It should be noted 
that the gas reaction pipe 9 has a width almost identical to that of the 
cylindrical can 6 opposing to the gas reaction pipe 9. 
In the plasma CVD apparatus having the aforementioned configuration, the 
pressure inside the vacuum chamber 2 was set to 30 Pa, and a voltage of 
0.8 kV was applied between the electrode 10 and the cylindrical can 6 so 
as to cause discharge. In this state, a gas mixture containing ethylene 
and argon (ethylene: argon =85:15) was supplied as a raw material gas from 
the gas supply port 11. Thus, a carbon protection film having a thickness 
of 8 nm was formed on a metal magnetic thin film of a tape traveling 
around the circumeference of the cylindrical can 6. 
After forming in this way a carbon protection film on the metal magnetic 
thin film, the tape was cut into a width of 6.35 mm, thus completing a 
magnetic tape. This is the sample of Example 1. 
Examples 2 to 5 and Comparative Examples 1 and 2 
Sample tapes were prepared by forming a carbon protection film in the same 
way as Example 1 except for the difference in the reaction gas composition 
and the voltage conditions shown in Table 1. 
TABLE 1 
______________________________________ 
Reaction gas 
composition 
Ethylene:Argon 
Voltage [kV] 
______________________________________ 
Example 1 85:15 0.8 
Example 2 80:20 1.0 
Example 3 80:20 1.2 
Example 4 70:30 1.5 
Example 5 70:30 2.0 
Comparative 80:20 0.6 
Ex. 1 
Comparative 80:20 0.7 
Ex. 2 
Comparative 70:30 2:5 
Ex. 3 
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Evaluation of Characteristics 
The sample tapes of the Examples and the Comparative Examples were 
subjected to a Raman spectrum analysis, a shuttle travel test, still 
durability test, and friction test. The tests were carried out as follows. 
Raman spectrum analysis: An argon ion laser having a wavelength of 514.5 nm 
was applied to a carbon film to excite a scattered light which was 
subjected to a spectrum measurement. A representative method for 
evaluation of the carbon protection film was employed to determine the 
intensity ratio A/B for the main peak intensity A appearing in the 
vicinity of 1550 cm.sup.-1 with respect to the background intensity B. 
Shuttle travel test: A digital video camera (trade name DVC-700 produced by 
Sony Co., Ltd.) was used to carry out one recording for 10 minutes in an 
environment of temperature 40.degree. C. and relative humidity 30%. After 
this, reproduction was carried out 99 times to check a 100-th pass output 
with respect to an initial output. If this level down amount is within -3 
dB, a signal amplifier circuit built in a digital video tape recorder will 
not affect the picture quality. 
Still durability test: A digital video camera (trade name: DVC-700 produced 
by Sony Co., Ltd.) was used and a still state was maintained in an 
environment of temperature of -5 C, so as to check the time for becoming 
-3 dB with respect to an initial output. 
Friction test: A sliding friction test was carried out in an environment of 
temperature 40.degree. C. and relative humidity 80%. Results were 
expressed in friction coefficients. 
Table 2 shows results of these tests. Moreover, FIG. 3 shows a Raman 
spectrum of the carbon protection film. 
TABLE 2 
______________________________________ 
Raman Shuttle 
intensity durability 
Still 
ratio A/B 
Friction [dB] durability 
______________________________________ 
Example 1 2.1 0.29 -2.8 100 min. 
Example 2 2.5 0.26 -2.7 &gt;120 min. 
Example 3 2.7 0.26 -2.0 &gt;120 min. 
Example 4 3.1 0.26 -2.0 &gt;120 min. 
Example 5 4.2 0.25 -1.8 &gt;120 min. 
Comparative 
1.5 0.48 -4.2 10 min. 
Ex. 1 
Comparative 
2.0 0.42 -3.5 30 min. 
Ex. 2 
Comparative 
4.5 -- -- -- 
Ex. 3 
______________________________________ 
As can be understood from the results of Table 2, the sample tapes having a 
carbon film whose Raman intensity ratio is equal to or above 2 exhibited a 
preferable shuttle characteristic, still characteristic, low friction, and 
excellent sliding durability. 
On the contrary, the sample tapes of Comparative Examples 1 and 2 having a 
carbon protection film whose Raman intensity ratio is below 2 were not 
able to exhibit preferable characteristics. On the other hand, the sample 
tape of Comparative Example 3 having a carbon protection film whose Raman 
intensity ratio was 4.5 was damaged by heat during the film formation and 
could not be subjected to the tests. 
As is clear from the aforementioned, according to the present invention, a 
carbon protection film has the intensity ratio of 2 or above for the main 
peak intensity appearing in the vicinity of wave number 1550 cm.sup.-1 
with respect to the background intensity, thus enabling to provide a 
magnetic recording medium having an excellent sliding durability and a 
high reliability.