Magnetic disk comprising a magnetic layer having specific...

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Reexamination Certificate

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C428S690000

Reexamination Certificate

active

06632548

ABSTRACT:

FIELD OF THE INVENTION
The present invention relates to a magnetic particulate disk capable of high density-recording, which has a magnetic layer where a ferromagnetic powder is dispersed an a binder. More specifically, the invention is concerned with a magnetic disk for high-density recording, which has a substantially non-magnetic lower layer and a magnetic layer containing a ferromagnetic metal powder or a hexagonal ferrite powder.
BACKGROUND OF THE INVENTION
In the field of magnetic disks, 2 MB of MF-2HD floppy disks utilizing cobalt-modified iron oxide have become standard with personal computers. In these days of upsurge in data volume to be processed, however, it can no longer be said that such a capacity is sufficient, but it is desirable to further enlarge the capacities of floppy disks.
Magnetic recording media comprising a non-magnetic support coated with a magnetic layer containing iron oxide, cobalt-modified iron oxide, CrO
2
, a ferromagnetic metal powder or a hexagonal ferrite powder dispersed in a binder have so far been used widely. Of these magnetic substances, the ferromagnetic metal powder and hexagonal ferrite powder are known to have excellent high-density recording characteristics.
In the case of disk-shaped magnetic recording media, the available large-capacity disks utilizing ferromagnetic metal powders excellent in high-density recording characteristics are 10 MB of MF-2TD and 21 MB of MF-2SD, and those utilizing hexagonal ferrite powders are 4MB of MF-2ED and 21MB of Fl optical. However, it can be said that the capacities and performances of these disks are no longer sufficient. Under these circumstances, many attempts to improve high-density recording characteristics have been made. Examples thereof are described below.
For improving characteristics of disk-shaped magnetic recording media, JP-A-64-84418 (the term “JP-A” as used herein means an “unexamined published Japanese patent application”) proposes using vinyl chloride resin having acidic groups, epoxy groups and hydroxyl groups, JP-B-3-12374 (the term “JP-B” as used herein means an “examined Japanese patent publication”) proposes using a metal powder having Hc of at least 79.6 kA/m (1,000 oersted (Oe)) and a specific surface area of 25 to 70 m
2
/g, and JP-B-6-28106 proposes specifying the specific surface area and the magnetization intensity of a magnetic powder and further incorporating abrasives.
For improving durability of disk-shaped magnetic recording media, JP-B-7-85304 proposes using an unsaturated fatty acid ester and an ether linkage-containing fatty acid ester, JP-B-7-70045 proposes using a branched fatty acid ester and an ether linkage-containing fatty acid ester, JP-A-54-124716 proposes incorporating non-magnetic powder having a Mohs hardness of at least 6 and a high fatty acid ester, JP-B-7-89407 discloses controlling the volume of pores containing a lubricant to the specified range and the surface roughness to the range of 0.005 to 0.025 &mgr;m, JP-A-61-294637 discloses using fatty acid esters having low and high melting points, JP-B-7-36216 discloses using an abrasive having a grain size equal to from one-fourth to three-fourth the magnetic layer thickness and a fatty acid ester having a low melting point, and JP-A-3-203018 discloses using a ferromagnetic iron-aluminum alloy powder and particulate chromium oxide.
As to the structure of disk-shaped magnetic recording media having a non-magnetic lower or intermediate layer, JP-A-3-120613 proposes the structure comprising a conductive layer and a magnetic layer containing a metal powder, JP-A-6-290446 proposes the structure comprising a magnetic layer having a thickness of 1 &mgr;m or below and a non-magnetic layer, JP-A-62-159337 proposes the structure comprising a carbon interlayer and a magnetic layer containing a lubricant, and JP-A-5-290358 proposes the structure having a non-magnetic layer in which carbon particles having the limited size are incorporated.
Recently, on the other hand, disk-shaped magnetic recording media comprising a thin magnetic layer and a functional non-magnetic layer have been developed, and 100 MB class floppy disks have made their debut. As to magnetic disks having these characteristics, JP-A-5-109061 proposes the magnetic disk comprising a magnetic layer having Hc of at least 111.4 kA/m (1,400 Oe) and a thickness of at most 0.5 &mgr;m and a non-magnetic layer containing conductive particles, JP-A-5-197946 proposes the magnetic disk containing abrasives having a size greater than the magnetic layer thickness, JP-A-5-290354 proposes the magnetic disk wherein a magnetic layer has a thickness of at most 0.5 &mgr;m, variation in the magnetic layer thickness is controlled within ±15% and the surface electric resistance is adjusted to the specified range, and JP-A-6-68453 proposes the magnetic disk wherein two types of abrasives differing in grain size are incorporated and the their quantity at the surface is controlled to the specified range.
JP-A-6-52541 discloses the magnetic recording media containing as an abrasive at least one powder chosen from alumina, chromium oxide or diamond powder, and reports that the addition of such a highly hard powder has improved running stability.
For the purpose of preventing a drop in reproduction power caused by thickness loss of a magnetic layer, it is known to make the magnetic layer thin. For instance, JP-A-5-182178 discloses the magnetic recording media comprising a non-magnetic support, a non-magnetic lower layer containing an inorganic powder dispersed in a binder, and an upper magnetic layer having a thickness of at most 1.0 &mgr;m and containing a ferromagnetic powder dispersed in a binder, which is formed while the non-magnetic lower layer is in a wet state.
With even the aforementioned arts, however, it has become difficult to obtain satisfactory characteristics with the rapid increases in magnetic disk capacity and recording density. In the case of performing high-density recording on a magnetic disk in particular, minute destruction occurs in the magnetic layer by movements of the magnetic disk over a head, and makes it impossible to read data, thereby causing errors. On the other hand, when the magnetic disk surface is smoothened and the spacing between the head and the magnetic disk is made small with the intention of increasing the recording density, a friction increases between the head and the magnetic disk and as a result, the magnetic layer is further liable to be destroyed. Therefore, it has been desired to develop means to overcome those problems.
SUMMARY OF THE INVENTION
An object of the invention is to provide a magnetic disk for high-density recording which has greatly improved electromagnetic characteristics, especially high-density recording characteristics and which is prevented from the data loss caused by destruction of a magnetic layer resulting from head movement.
As a result of our intensive studies to produce a magnetic disk having excellent electromagnetic characteristics and satisfactory durability, it has been found that the foregoing object can be achieved by the magnetic disk described below.
More specifically, the present invention provides the following magnetic disks: 1. A magnetic disk comprising a support, a substantially non-magnetic lower layer formed on the support and a magnetic layer which is formed on the lower layer and contains a ferromagnetic powder dispersed in a binder, wherein the magnetic layer has a thickness of 0.02 to 0.5 &mgr;m and projections on the surface thereof, the number of the projections standing at least 20 nm high measured by an atomic force microscope is at least 1×10
4
per mm
2
and the number of the projections standing at least 70 nm high measured by an atomic force microscope is at most 1×10
3
per mm
2
.
2. The magnetic disk as described in 1 above, wherein the number of the projections standing at least 20 nm high on the magnetic layer surface measured by an atomic force microscope is at least 1×10
4
per mm
2
and the number of the projections standing at least 70

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