Stock material or miscellaneous articles – Composite – Of inorganic material
Reexamination Certificate
2001-06-28
2003-04-01
Le, H. Thi (Department: 1773)
Stock material or miscellaneous articles
Composite
Of inorganic material
C428S690000, C428S690000, C360S135000
Reexamination Certificate
active
06541132
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to a magnetic disk and, more particularly, to a high-density recording magnetic disk having a very thin magnetic layer.
BACKGROUND OF THE INVENTION
As to magnetic disks, it has become standard to install 2MB MF-2HD floppy disks using Co-modified iron oxide in personal computers. Lately, however, there has been a rapid increase in the amount of data to be processed, so it can be said that the capacity of floppy disks of the foregoing type is no longer sufficient. Thus, it is expected to develop large-capacity floppy disks.
In the field of magnetic tapes, as office computers, such as mini-computers, personal computers and work-stations, have come into wide use, lively researches have been done on computer data-recording magnetic tapes (backup tapes) as external storage media. In putting magnetic tapes to practical use for such an application, a step-up in recording capacity is strongly required for achievement of size reduction and high-capacity recording coupled in particular with downsizing of computers and increase of information processing capabilities.
Magnetic recording media hitherto used widely are materials in which a non-magnetic support is coated with a magnetic layer comprising iron oxide, Co-modified iron oxide, CrO
2
, ferromagnetic metal powder or hexagonal ferrite powder dispersed in a binder. Of such magnetic substances, ferromagnetic metal powder and hexagonal ferrite powder are known to be superior in high-density recording characteristics.
In the case of magnetic disks, 10MB MF-2TD and 21MB MF-2SD are available as large-capacity disks using ferromagnetic metal powder with excellent high-density recording characteristics, and 4MB MF-2ED and 21MB Floptical are available as large-capacity disks using hexagonal ferrite. However, it cannot be said any longer than the capacity and performance of those disks are sufficiently high. Under these circumstances, many attempts have been made to improve high-density recording characteristics.
For instance, JP-A-5-290354 (the term “JP-A” as used herein means an “unexamined published Japanese patent application) discloses the magnetic recording disk having on a non-magnetic layer a magnetic layer controlled so as to have an average thickness of 0.5 &mgr;m or below and a thickness variation within ±15% and thereby ensuring consistent output and overwrite characteristics. However, the magnetic substances used in the examples of the reference are a ferromagnetic powder having a major axis length of 0.2 &mgr;m and a ferromagnetic hexagonal ferrite powder having a particle diameter of 0.06 &mgr;m, and the magnetic layer thickness is adjusted to the relatively thick range of 0.39 to 0.45 &mgr;m. As a result, when the track width and the bit length are reduced in order to attain high-density recording, such a magnetic disk becomes insufficient to meet the desired levels of noise characteristic and half width of isolated waveform (PW50), and cannot attain the needed error rate.
In JP-A-5-298653 is proposed the magnetic recording medium having an average thickness of its magnetic layer in the range of 0.01 to 0.3 &mgr;m and a (standard deviation of thickness)/(average thickness) ratio in the range of 0.05 to 0.5, and thereby achieving high output and small PW50 in digital recording. However, in this reference also, the magnetic substance used in each example is a ferromagnetic metal powder having a major axis length of 0.2 &mgr;m or tabular particles of Co-doped barium ferrite having an average tabular diameter of 0.06 &mgr;m, so such a magnetic recording material also cannot ensure a sufficiently low signal-to-noise ratio in narrow track width-adopted high-density recording.
In addition, JP-A-10-228629 discloses a high-density magnetic recording disk having a surface recording density of 0.2 to 2 Gbit/inch
2
, wherein the magnetic layer has a thickness of 0.05 to 0.25 &mgr;m, &phgr;m of 10.1 to 1.3 &mgr;Tcm (8.0×10
−3
to 1.0×10
−3
emu/cm
2
) and a coercive force of at least 143 kA/m (1,800 oersted) and comprises a ferromagnetic metal fine powder having a major axis length of 0.12 &mgr;m or below or fine particles of ferromagnetic hexagonal ferrite having their sizes in the range of 10 to 100 nm. The magnetic substance used in the examples of the reference is particulate magnetic substance, specifically ferromagnetic metal particles having an average major axis length in the range of 0.048 to 0.20 &mgr;m and a crystallite size of 12 to 22 nm or ferromagnetic hexagonal ferrite particles having an average tabular diameter of 35 nm. As a result, the magnetic recording medium achieves a greatly improved error rate and high performance.
However, as the advance of recording density, it becomes necessary to further reduce the thickness of a magnetic layer. Herein, the relation between a track width and a magnetic particle size and the relation between a bit length and a magnetic particle size are of importance. In the reference cited above, however, the magnetic substance used for the magnetic layers thinner than 0.15 &mgr;m has a rather great major axis length as 0.1 &mgr;m, so that it cannot ensure a sufficiently low signal-to-noise ratio. Moreover, when the thickness of a magnetic layer is decreased to less than 0.15 &mgr;m, the variation coefficient thereof comes to have a noticeable influence. And much noise is produced so far as variations in thickness are not controlled.
SUMMARY OF THE INVENTION
An object of the invention is to provide a large-capacity disk-shaped magnetic recording medium having extensive improvements in electromagnetic characteristics, particularly in high-density recording characteristic, an especially high signal-to-noise ratio and a significantly improved error rate.
In embodiments of the invention, magnetic disks are provided which have the following constitutions respectively, and thereby the aforesaid object is attained.
1. A magnetic disk comprising a support having provided thereon a non-magnetic layer having substantially no magnetism and a magnetic layer having a thickness of less than 0.15 &mgr;m and containing as ferromagnetic powder, fine particles of ferromagnetic metal or ferromagnetic hexagonal ferrite dispersed in a binder, in this order, wherein
(I) the disk has a track width of less than 5 &mgr;m and is able to record signals having a bit length of less than 0.5 &mgr;m,
(II)-(i) when the ferromagnetic powder is fine particles of ferromagnetic metal, the fine particles of ferromagnetic metal have an average major axis length satisfying conditions that a ratio of the track width to the average major axis length is from 50 to 100 and a ratio of the bit length to the average major axis length is from 3 to 20,
(II)-(ii) when the ferromagnetic powder is fine particles of ferromagnetic hexagonal ferrite, the fine particles of ferromagnetic hexagonal ferrite have an average tabular diameter satisfying conditions that a ratio of the track width to the average tabular diameter is from 50 to 100 and a ratio of the bit length to the average tabular diameter is from 3 to 20, and
(III) the thickness of the magnetic layer has a variation coefficient less than 0.5, expressed in terms of a ratio of the standard deviation to the average thickness of the magnetic layer.
2. The magnetic disk according to the above embodiment 1, wherein the ferromagnetic powder is fine particles of ferromagnetic hexagonal ferrite having an average tabular diameter of 35 nm or below.
3. The magnetic disk according to the above embodiment 1, wherein the ferromagnetic powder is fine particles of ferromagnetic metal having an average major axis length of 150 nm or below.
4. The magnetic disk according to any one of the above embodiments 1 to 3, wherein the non-magnetic layer and the magnetic layer are those formed by a wet-on-wet coating method.
DETAILED DESCRIPTION OF THE INVENTION
The invention is described in detail below.
The present magnetic disk comprises a support on which are provided a non-magnetic layer having substantially no magnetism and a
Harasawa Takeshi
Noguchi Hitoshi
Saito Shinji
Fuji Photo Film Co. , Ltd.
Le H. Thi
Sughrue & Mion, PLLC
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