Stock material or miscellaneous articles – Composite – Of inorganic material
Reexamination Certificate
2001-11-09
2004-05-04
Rickman, Holly (Department: 1773)
Stock material or miscellaneous articles
Composite
Of inorganic material
C428S336000, C428S611000, C428S667000, C428S697000, C428S699000, C428S702000, C428S690000, C428S900000
Reexamination Certificate
active
06730421
ABSTRACT:
TECHNICAL FIELD
The present invention relates to a magnetic recording medium which is suitable for high density recording. In particular, the present invention relates to a magnetic recording medium which makes it possible to record bit information in an extremely minute area of a magnetic layer. The present invention also relates to a method for producing the magnetic recording medium and a magnetic recording apparatus.
BACKGROUND ART
Recent development of the advanced information society is remarkable. The multimedia technology, with which various types of information can be dealt with, is quickly popularized. A magnetic recording apparatus, which is installed, for example, to a computer, is known as one of those based on the multimedia technology. At present, the development of the magnetic recording apparatus is advanced along with a course to realize a small-sized apparatus while improving the recording density.
In order to realize high density recording with the magnetic recording apparatus, it is demanded, for example, that (1) the distance between a magnetic disk and a magnetic head is narrowed, (2) the coercive force of a magnetic recording medium is increased, (3) the speed of the signal processing process is increased, and (4) a medium, which suffers from less thermal fluctuation, is developed.
The magnetic recording medium has a magnetic layer in which magnetic particles or magnetic grains are aggregated on a substrate. Information is recorded thereon by magnetizing a certain group of several magnetic grains in an identical direction by the aid of a magnetic head. Therefore, in order to realize the high density recording, it is necessary to decrease the minimum area which may be magnetized in the identical direction at once in the magnetic layer, i.e., the unit area in which the inversion of magnetization may occur, in addition to the increase in coercive force of the magnetic layer. In order to decrease the unit area of inversion of magnetization, it is necessary that individual magnetic grains are allowed to have a fine and minute size, or it is necessary to decrease the number of magnetic grains for constructing the unit of inversion of magnetization. For example, in order to achieve a recording density above 40 Gbits/inch
2
(6.20 Gbits/cm
2
), it is necessary that the diameter of the magnetic grain is suppressed to be not more than 10 nm. Further, it is also necessary to make countermeasures in order to decrease the dispersion of the grain diameter when the magnetic grain is allowed to have a fine and minute size, and decrease the thermal fluctuation. As a trial to realize the demands as described above, it has been suggested that a seed film is provided between a substrate and a magnetic layer, as disclosed, for example, in U.S. Pat. No. 4,652,499.
However, the method, in which the magnetic layer is provided on the substrate with the seed film intervening therebetween as described above, has had a limit to control the magnetic grain diameter and the distribution thereof in the magnetic layer. For example, even when the material for the seed film, the film formation condition, the structure of the seed film, and other factors were adjusted in order to obtain magnetic grains having a grain diameter of about 10 nm in the magnetic layer, the grain diameter distribution was broad, in which considerable amounts of grains having a size coarsely increased to several tens nm and grains inversely having a size finely decreased to about a half of 10 nm were present in a mixed manner. As for such magnetic grains, magnetic grains having a grain diameter larger than the average cause the increase in noise upon recording/reproduction. On the other hand, magnetic grains having a grain diameter smaller than the average cause the increase in thermal fluctuation upon recording/reproduction. As a result of the presence of the magnetic grains having a variety of sizes in a mixed manner, the boundary line between an area in which the inversion of magnetization occurs and an area in which the inversion of magnetization does not occur provides a coarse zigzag pattern as a whole. This fact was also a factor to increase the noise. Further, the inversion of magnetization hitherto occurred in a unit composed of a number of 5 to 10 individuals of magnetic grains in the magnetic layer of the conventional magnetic recording medium.
As for the spacing distance between the magnetic head and the magnetic layer of the magnetic recording medium for the high density recording, it is investigated that the spacing distance is narrowed to be not more than 15 nm. In general, scratches and rough irregularities exist on the substrate surface. For this reason, rough irregularities originating from the substrate have hitherto appeared on the surface of the magnetic recording medium prepared by stacking a film on the substrate. If the distance between the magnetic recording medium and the magnetic head is narrowed, it is impossible for the magnetic head to stably fly due to the rough irregularities as described above, resulting in the occurrence of the following problems. That is, the recording and reproducing characteristics are deteriorated, and the magnetic head collides with the magnetic recording medium to cause damages of the both. Therefore, it is demanded to realize a technique for forming a flat film without being affected by the surface roughness of the substrate.
On the other hand, as the spacing distance between the magnetic head and the magnetic layer is narrowed, it is more necessary to protect the magnetic layer from the shock exerted by the magnetic head and the environment of use. Therefore, it is required to form a protective film for protecting the magnetic film so that the protective film is more uniform without causing any deficiency. However, in order to realize the spacing distance of not more than 15 nm between the magnetic head and the magnetic layer, it is necessary that the protective film to be formed on the magnetic layer has a film thickness of not more than 5 nm. Even when it is intended to form a carbon protective film with a thickness of not more than 5 nm by using the conventional DC sputtering method or the magnetron sputtering method, it has been impossible to completely cover the surface of the magnetic recording medium with the protective film, because the carbon protective film is formed only in an island form, or any defect such as hollow hole and crack occurs in the protective film. If the surface of the magnetic recording medium is not completely covered with the protective film, then any corrosion occurs in the magnetic layer, and the magnetic layer suffers from any physical damage due to the head crash or the like.
In order to allow the magnetic head to fly over the magnetic disk, it is necessary to provide a texture provided with a concave/convex structure on the surface. However, it has not been easy to control the concave/convex structure to have an appropriate size.
Japanese Patent No. 2704957 discloses a magnetic recording medium having a keeper layer. The keeper layer is an auxiliary film having soft magnetization. The keeper layer is arranged so that it makes tight contact with the surface of a magnetic layer (recording layer) for performing recording. When the magnetic layer is in a recording magnetization state, a portion of the keeper layer, which contacts with a recording magnetization portion of the magnetic layer, is magnetized in a direction opposite to the magnetic layer, because the keeper layer has the soft magnetization. An annular magnetic path is formed by the recording magnetization portion of the magnetic layer and the portion of magnetization in the opposite direction of the keeper layer. Even when the film thickness of the magnetic layer is thinned, the recording magnetization is stably maintained without being demagnetized. Further, owing to such a situation, the diamagnetic field, which acts on the magnetic layer of the recording magnetization portion, is decreased. Therefore, even when the recording density is increased by allowing t
Inaba Nobuyuki
Kirino Fumiyoshi
Takeuchi Teruaki
Wakabayashi Kouichirou
Hitachi Maxell Ltd.
Oliff & Berridg,e PLC
Rickman Holly
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