Stock material or miscellaneous articles – Composite – Of inorganic material
Reexamination Certificate
2000-03-10
2004-03-16
Hudspeth, David (Department: 2651)
Stock material or miscellaneous articles
Composite
Of inorganic material
C360S046000
Reexamination Certificate
active
06706426
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention concerns a longitudinal magnetic recording medium, such as magnetic recording drums, magnetic recording tapes, magnetic recording disks and magnetic recording cards, as well as a magnetic storage apparatus and, more particularly, the invention relates to a longitudinal magnetic recording medium which enables super high density recording at 3 Gbit or more per one square inch, and a magnetic storage apparatus using the longitudinal magnetic recording medium.
In recent years, the sizes of recording bits formed on a magnetic recording medium have been reduced more and more along with a remarkable increase in the capacity and recording density of magnetic recording disks. In the magnetic recording medium known at present, it is difficult to attain a super high density recording of 3 Gbit or more per one square inch, and there is a need to further decrease the medium noises. For this purpose, it is important to decrease the crystal grain size of the magnetic layers. However, when the volume of the magnetic particles is reduced extremely by the refinement of the magnetic crystal grains, the effect of the thermal energy becomes remarkable even at normal temperatures, which raises a concern that the recorded magnetization will decay. Actually, it has been reported by Y. Hosoe, et al, that information recorded at a density of 225 kFCI (FCI: Flux Change/Inch) is decayed by as much as 10% or more after 96 hours in a noise-reduced medium (IEEE Trans. Magn. 33, pp, 3028-3030, September 1997).
For making the reduction of the medium noises compatible with an improvement of the heat resistant fluctuation performance, it is effective to decrease the average crystal grain size of the magnetic membrane and, at the same time, suppress the growing of extremely small magnetic particles.
As an example of a magnetic recording medium of this type, it has been proposed, for example, in U.S. Pat. No. 5,693,426, by CMU (Carnegie Mellon University), to produce a magnetic recording medium using an under layer having a B2 (CsCl) structure laminated directly thereon or by way of a Cr underlying film, the magnetic layer thereby making the magnetic crystal grains into a non bi-crystal structure.
FIG. 2
is a view illustrating an epitaxial relationship between an underlayer and a magnetic layer of a magnetic recording medium according to the technique proposed by CMU, which will be explained.
FIG. 2
shows a crystal structure for an NiAl underlayer, a Cr underlayer and a Co magnetic layer from below. In
FIG. 2
, the group on the left illustrates the shape of the crystals in which a meshed plane represents a portion growing in parallel with a substrate, and the group on the right shows a representative size of the meshed plane.
The crystal structure for each of the layers is: B2 for the NiAl underlayer, (b.c.c.) for the Cr underlayer and (h.c.p.) for the Co magnetic layer. When the NiAl underlayer is formed on the substrate while optimizing the deposition condition, crystals grow preferentially such that (
211
) is in parallel with the substrate. The Cr underlayer formed thereon shows substantial orientation (
211
) and, further, the magnetic layer shows substantial orientation (
10
.
0
).
When atoms are located at lattice points of crystals possessed by each of the layers, when each of the layers has the orientation as described above, a rectangle is formed in a film plane as shown on the left of FIG.
2
. As a result, when each of the layers is formed successively on the substrate, a layer structure is obtained in which meshed portions in
FIG. 2
are stacked successively. When the sizes of the rectangles are compared, while using the bulk value for the lattice constant of each of the layers, it can be seen that they are substantially of the same size in the [
0001
] direction of the magnetic layer (direction of c-axis), that is, in the direction of the axis of easy magnetization. On the other hand, when the length of the sizes of the rectangles formed with the respective layers are compared in the direction perpendicular thereto, that is, in the [
1
-
210
] direction of the magnetic layer, it can be seen that the sizes are different.
According to the result of an experiment conducted by the present inventors, it has been found that the orientation of the axis of easy magnetization to the in-plane direction can be improved particularly by making the sizes of the respective rectangles formed by the underlayer adjacent to the magnetic layer and the magnetic layer substantially equal to each other. For a medium having the structure proposed by CMU, when the rectangles formed by the Co magnetic layer and the Cr underlayer adjacent to the magnetic layer are compared, the length of the sizes are substantially equal in the [
0001
] direction of the magnetic layer, but the length for the side of the rectangle formed by the underlayer is excessively small in the [
1
-
210
] direction perpendicular thereto. In a case where such a size difference exists, the in-plane orientation of the axis of easy magnetization of the magnetic layer is remarkably deteriorated, resulting in a decrease of the coercivity and an increase in the media noise. Further, for the purpose of increasing the coercivity and the reduction of the media noise, elements such as Pt, Ta, Ti, Nb are added generally to the magnetic layer. Therefore, the unit lattice (lattice constant) of the magnetic layer having the h.c.p. structure, that is, the size of the rectangle formed by the alloy magnetic layer is greater than that of Co, and, in the longitudinal magnetic recording medium having a structure in which a Cr underlayer is formed on the underlayer having the B2 structure of NiAl, etc. proposed by CMU, lattice matching between the Cr underlayer and the Co alloy magnetic layer is further deteriorated, thereby to worsen the in-plane orientation of the axis of easy magnetization.
Since the magnetic recording medium proposed by CMU as described above is a longitudinal recording medium, it is preferred that the axis of easy magnetization of the medium is oriented within a plane for attaining high coercivity and reduced noise. Generally, since the magnetic layer comprises Co as the main ingredient, the crystal structure has a substantially hexagonal closed packed lattice with the direction of the axis of easy magnetization being in the direction of the c axis. Then, in the magnetic recording medium in which a magnetic layer is formed on the B2 (mainly comprising NiAl) underlayer directly or by way of the Cr underlayer proposed by CMU, the axis of easy magnetization of the medium shows an in-plane orientation when the c-axis length of the magnetic layer has a size nearly equal to that of Co. However, in the usual magnetic layer, elements such as Pt, Ta, Ti or Nb are added as described above with an aim of improving the coercivity and reducing the media noise. In this case, the lattice constant of the magnetic layer is made greater compared with that of Co, thereby to bring about a problem in that the matching property with the lattice of the B2 underlayer or the Cr underlayer is deteriorated and the in-plane orientation of the axis of easy magnetization is worsened.
A first object of the present invention is to provide a longitudinal magnetic recording medium of high coercivity, reduced noise and which has an excellent thermal decay resistance, by developing the magnetic recording medium of the structure proposed by CMU and improving the in-plane orientation of the axis of the easy magnetization also for the magnetic layer with the addition of an element such as Pt, Ta, Ti or Nb.
A second object of the present invention is to provide a magnetic storage apparatus having a recording density of 3 Gbit or more per square inch, while fully talking an advantage of the performance of the longitudinal magnetic recording medium.
SUMMARY OF THE INVENTION
At first, an explanation will be given of the basic concept of the present invention for solving the problem that the lattice mat
Ishikawa Akira
Nakamura Atsushi
Yamamoto Tomoo
Yamanaka Kazusuke
Antonelli Terry Stout & Kraus LLP
Hitachi , Ltd.
Hudspeth David
Kapadia Varsha A.
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