Stock material or miscellaneous articles – All metal or with adjacent metals – Having metal particles
Reissue Patent
2000-06-30
2004-09-14
Rickman, Holly (Department: 1773)
Stock material or miscellaneous articles
All metal or with adjacent metals
Having metal particles
C428S561000, C428S611000, C428S668000, C428S690000, C428S690000, C428S450000, C428S457000, C428S900000
Reissue Patent
active
RE038587
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a magnetic recording drive, a magnetic recording medium and a method for manufacturing the same and, more particularly, to a magnetic recording drive for use in an external memory device of an information processing apparatus, etc., a magnetic recording medium used therein and a method for manufacturing the same.
2. Description of the Prior Art
In the magnetic recording drive, improvement of the recording density has been demanded more and more with an increase of an amount of information in proportion. When recording density of the conventional magnetic recording medium is increased, a S/N ratio is degraded to cause reduction of a reproducing output and increase of noise. Therefore, the magnetic recording medium enabling a large reproducing output and low noise has been demanded.
In particular, the problem is to achieve noise reduction in the magnetic recording medium since reading sensitivity has been extremely improved by practical use of the magnetoresistance head.
As a main factor of generating the noise in the magnetic recording medium, there is unclear boundary of the magnetization transition regions due to variation in magnetization in magnetization transition regions. The variation in magnetization is caused by magnetic interaction between crystal grains of the ferromagnetic film constituting the ferromagnetic layer.
In order to reduce noise in the magnetic recording medium, it is required to weaken the magnetic interaction between crystal grains of the ferromagnetic film.
In general, as the recording layer of the conventional magnetic recording medium, a thin film which is formed of cobalt (Co)-based ternary or quaternary alloy by sputtering may be used. By adjusting composition of the thin film and manufacturing conditions, segregation in the ferromagnetic portion and the nonmagnetic portion can be facilitated to reduce the noise.
In the conventional magnetic recording medium, as shown in
FIG. 1
, for example, a chromium layer
2
, a magnetic recording layer
3
consisting of CoCr
12
Ta
2
, and a protection layer
4
consisting of a carbon film are formed in that sequence on a nonmagnetic substrate
1
which is formed of an Al substrate covered with a NiP film.
However, since a cobalt system alloy constituting the recording layer
3
is inherently a solid solution, it is difficult to isolate crystal grain of the ferromagnetic film perfectly even if segregation is accelerated by adjusting composition and manufacturing conditions.
As the way of isolating crystal grains of the magnetic substance. Patent Application Publication (KOKAI) JP59-42642 and Patent Application Publication (KOKAI) P59-220907 have set forth manufacturing methods such that a binary or ternary alloy layer comprising the nonmagnetic substance such as silver and copper and the ferromagnetic film which is insoluble in this nonmagnetic substance is once formed by sputtering, and then the alloy layer is heated.
In the manufacturing methods set forth in these publications (KOKAIs), the ferromagnetic layer is heated at a temperature of less than 400° C. to accomplish high coercive force. Since a glass or polymer film is utilized as a substrate for supporting the magnetic recording film, the heating temperature of less than 400° C. is preferable.
In both Publications, the manufacturing methods for forming the magnetic recording film which has film thickness of 130 to 150 nm and t•Br value of 2000 Gauss•&mgr;m have been set forth. The t•Br value is denoted as a product of residual magnetization Br and a film thickness t of the magnetic recording medium (magnetic recording film).
However, in the magnetic recording medium for use in the magnetoresistance head, it is requested that the thickness of the magnetic recording layer would be set to be lower than or equal to 30 nm and also the t•Br value would be set to be lower than or equal to 150 Gauss•&mgr;m. In the case of manufacturing of the magnetic recording layer, the techniques set forth in these publication cannot be applied as they are. This is because of the following reasons.
First, the relation between recording density and effective output voltage in the magnetic recording medium for use in the magnetoresistance head has been well known as described in FIG.
2
.
In
FIG. 2
, in case the recording density is small like about 10, 20 kFRPI, the effective output is increased if the t•Br value is increased. But, in case the recording density is large like about 50, 100 kFRPI, the effective output voltage is decreased when the t•Br value is increased.
For this reason, if the recording density is increased up to about 50, 100 kFRPI, the t•Br value of the magnetic recording layer must be set to be lower than 150 Gauss•&mgr;m.
However, even if, under the conditions set forth in the above publications, it has been tried to accomplish the t•Br value of less than 150 Gauss•&mgr;m by forming the magnetic recording layer of less than 30 nm in thickness. However, even under these conditions, noise reduction and large coercive force have not been achieved since crystal grains in the magnetic recording layer are small and further partially continuous with each other in such circumstances.
Next, it can be considered that the magnetic recording layer
3
formed of CoCr
12
Ta
2
is formed thinner. For example, as shown in
FIG. 1
, the chromium layer
2
of 100 nm in thickness and the magnetic recording layer
3
of 20 nm in thickness are formed in that order on the nonmagnetic substrate
1
formed of a two-layered structure consisting of Al and NiP, and then the protection layer
4
formed of carbon is formed thereon to have a thickness of 20 nm. At this time, the t•Br value is about 100 Gauss•&mgr;m.
While the relation between the recording signal frequency and noise power in the magnetic recording layer has been investigated using the magnetoresistance reproducing head, the result has been derived as shown by the broken line in FIG.
9
. It has been appreciated that noise power is increased linearly in proportion as the recording signal frequency is increased. As a result, it has been found that the thinned CoCr
12
Ta
2
is not fit for the magnetic recording layer used for high recording signal frequency.
In the examination in
FIG. 2
, a relative velocity between the magnetic head and the magnetic recording medium is elected as 10 m/s, the recording signal frequency is set to 20 MHz, and the recording density is selected as about 100 kFRPI.
As has been stated above, regarding a granular magnetic film (Fe-SiO
2
) in which magnetic fine grains are dispersed into the SiO
2
film, the following problem is caused in addition to the problem of the magnetic characteristic due to crystal property of the magnetic recording film.
It has been recited in Applied Physics Letter, 52 (6), 512 (1988) and U.S. Pat. No. 4,973,525 that, in the above granular magnetic film, crystal property of the magnetic substance fine grains has been improved and also more preferable magnetic characteristics and recording/reproducing characteristics could be achieved by controlling the substrate temperature appropriately at the time of film-formation.
Thereby, there is a tendency that, in the granular magnetic film, segregation of the magnetic substance has too small size in the state of as-grown to show enough coercive force. It has been seen that, in order to increase coercive force, the annealing must be effected after growing the film to increase the volume of each segregation.
On the contrary, conventionally a NiP plated substrate has been used mainly as the substrate for the magnetic recording medium. But, the NiP layer formed on a surface of the substrate has been crystallized by heating at a temperature in excess of 300° C. Thus, there are caused some problems that flatness of the surface of the layer is damaged, the layer is magnetized, or the like. It is evident that such substrate is not adequate for heating process at a high temperature
Kaitsu Isatake
Okamoto Iwao
Shinohara Masayoshi
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