Stock material or miscellaneous articles – Composite – Of inorganic material
Reexamination Certificate
1999-11-19
2003-09-23
Rickman, Holly C. (Department: 1773)
Stock material or miscellaneous articles
Composite
Of inorganic material
C428S690000, C428S900000
Reexamination Certificate
active
06623873
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates to a magnetic recording apparatus used, for example, as an auxiliary storage of a computer and a magnetic recording medium for use with the apparatus, and in particular, to a thin-film magnetic recording medium suitable for a magnetic recording apparatus having a high surface recording density of at least five gigabits per square inch.
Due to development of the information-oriented society, the quantity of information daily used is rapidly increasing. Consequently, it is highly required to increase the recording density and capacity of magnetic recording apparatus. The magnetic heads employ an inductive head using a voltage change which appears in response to a change of a magnetic flux with respect to time. These heads are used for the data recording and reading operations. In contrast the heads of this type, there has been increasingly used at a higher speed a composite head including a recording head and a read-back or reproducing head in which the read-back head is a magnetoresistive read-back head having higher efficiency or sensitivity. The magnetoresistive head uses a change in its electric resistance in response to a change in leakage flux from a medium on which data to be read is recorded. Moreover, a giant magnetoresistive (GMR) head having still higher efficiency or sensitivity has been developed to be put to practice, the GMR head using a considerably great change in magnetic resistance (giant magnetoresistive effect or spin valve effect) due to a multilayered structure including a plurality of magnetic layers accumulated with a nonmagnetic layer therebetween. The giant magnetoresistive effect or spin valve effect is an effect in which relative directions of magnetization of the plural magnetic layers arranged with a nonmagnetic layer therebetween are changed by a leakage magnetic field from the medium, which resultantly varies the electric resistance.
The magnetic recording medium practically employed at present includes magnetic layers made of alloys primarily based on cobalt such as Co—Cr—Pt, Co—Cr—Ta and Co—Cr—Pt—Ta. These cobalt alloys have a hexagonal structure (hcp structure) in which a c axis is an axis for easy magnetization. Therefore, for an in-plane magnetic recording medium in which its magnetization is reversed in a magnetic film plane for the recording of information, the c axis of the cobalt alloys is set to (11.0) orientation for crystallization in which the c axis takes an in-plane direction. However, the (11.0) orientation is unstable and hence it is generally impossible to produce such a cobalt alloy directly on a substrate. To overcome this difficulty, there has been employed a method advantageously using a good matching property of a Cr(100) plane having a body-centered cubic (bcc) structure with the Co(11.0) plane. Namely, an underlayer film of chromium is first formed with (100) orientation on a substrate and then a magnetic layer of a cobalt alloy is epitaxially grown on the underlayer such that the c axis of the magnetic layer of cobalt alloy is oriented (11.0), namely, has an in-plane direction. To further increase the crystal lattice matching property in a boundary between the cobalt alloy magnetic film and the chromium underlayer, there has been adopted a method to add a second element to chromium to increase lattice spacing of the chromium underlayer. This enhances the (11.0) orientation in the cobalt alloy to resultantly increase coercivity thereof. JP-A-62-257618 and JP-A-63-197018 describe examples of the technology above in which elements such as vanadium and titanium are added to the chromium underlayer. In addition to the increase in coercivity of the recording medium, the decrease in noise is an essential factor for a higher recording density. The magnetoresistive head has quite a high reproducing efficiency and hence is suitable for high-density recording. However, the head is highly efficient not only to reproduced signals from a magnetic recording medium but also highly sensitive to the noise. Therefore, it is required to much more reduce the noise as compared with the recording medium. Japanese Patent No. 2650282 proposes alloy films, for example, a Cr—Mo film as an underlayer which increases coercivity and coercivity squareness and which decreases the medium noise. JP-A-10-228621 describes a combination of a Cr—Mo underlayer with a Co—Cr—Pt—Ta magnetic film. JP-A-4-221418 describes a magnetic recording medium in which a cobalt alloy magnetic layer includes at least platinum and boron to increase coercivity for the increase in the recording density. JP-A-9-293227 describes a combination of a Cr—Mo underlayer with a Co—Cr—Pt—B magnetic layer.
According to JP-A-10-74314, a Co-based nonmagnetic alloy layer is fabricated below the underlayer to decrease the medium noise. In accordance with JP-A-10-143865, chromium and zirconium which are relatively easily oxidized are added to the Co-based nonmagnetic alloy layer such that a surface of the layer is disposed to atmosphere of oxygen to be slightly oxidized so as to further decrease the medium noise in a stable state. JP-A-9-265619 and JP-A-10-214412 describe magnetic recording medium including a Cr-based underlayer in a b.c.c. structure on an alloy film (seed layer) including zirconium and titanium.
On the other hand, JP-A-1-303624 describes a magnetic recording medium having a high recording density. The medium includes two magnetic layers, i.e., a first magnetic layer is a Co-based recording film and a second magnetic layer is a layer which includes cobalt and chromium as primary elements and to which carbon, titanium, zirconium, niobium, and tungsten are added. However, this medium has a coercivity of 64 kA/m (800 oersted). This is less than the value, 160 kA/m (2000 oersted), required for the present invention. JP-A-5-114128 describes a method to lower the medium noise and to increase the recording density in which the medium includes two magnetic films, i.e., a lower layer is a Co—Cr—Ta alloy with a lower medium noise and a higher layer is a Co—Cr—Pt alloy with high coercivity.
It has been commonly known that the medium noise can be effectively lowered by reducing sizes of crystal grains of the magnetic film and possibly equalizing grain sizes to each other. The technologies above also use this advantageous effect. Such an example is written in pages 5351 to 5353 of J. Appl. Phys., vol. 79 (1996). Namely, the crystal grains become finer by using a Cr—Ti alloy underlayer when compared with the prior art employing a Cr underlayer, and the matching of the lattice constant with respect to the Co—Cr—Pt alloy is improved to resultantly increase coercivity. It is also known as described in this article that the medium noise is efficiently lowered by increasing the Cr concentration of the Co—Cr—Pt magnetic layer. On the other hand, JP-10-143865 describes a medium including a glass substrate. Namely, for example, when the Cr—Ti alloy underlayer and the Co—Cr—Pt magnetic film are fabricated after slightly oxidizing a surface of the Co—Cr—Zr seed layer, the c axis which is an axis for easy magnetization in the h.c.p. structure of the Co—Cr—Pt magnetic layer is oriented to be parallel to the film surface plane, i.e., in (11.0) orientation, and the crystal grains becomes finer, leading to a higher signal-to-noise (S/N) ratio. After fabricating a Co—Cr—Zr seed layer with composition of 60 at. % Co-30 at. % Cr-10 at. % Zr, a surface of the seed layer is slightly oxidized and an 80 at. % Cr-20 at. % Ti underlayer and a Co—Cr—Pt magnetic film are manufactured thereon to obtain a medium. Having producing samples of medium with different values of chromium concentration as 73 at. % Co-19 at. % Cr-8 at. % Pt, 71 at. % Co-21 at. % Cr-8 at. % Pt, and 69 at. % Co-23 at. % Cr-8 at. % Pt, read/write characteristics are evaluated using a magnetic recording disk having a surface recording density of five gigabits per square inch. As a result, the read/write characteristics are improved as the chromium concentration is increased in the Co—Cr—Pt
Hosoe Yuzuru
Kato Akira
Matsuda Yoshibumi
Sakamoto Koji
Yahisa Yotsuo
Hitachi , Ltd.
Mattingly Stanger & Malur, P.C.
Rickman Holly C.
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