Stock material or miscellaneous articles – Web or sheet containing structurally defined element or... – Including a second component containing structurally defined...
Reexamination Certificate
2002-08-29
2004-08-03
Resan, Stevan A. (Department: 1773)
Stock material or miscellaneous articles
Web or sheet containing structurally defined element or...
Including a second component containing structurally defined...
C428S336000, C428S690000
Reexamination Certificate
active
06770359
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to a magnetic recording medium such as a magnetic tape, in particular, to a coating type magnetic recording medium (i.e., a magnetic recording particulate medium) comprising a support having provided thereon a magnetic layer formed by coating a magnetic coating solution containing a ferromagnetic powder and a binder as main components, further, the present invention relates to a magnetic recording medium for high density recording having a magnetic layer containing a hexagonal ferrite powder which is particularly suited for use in a system using an MR head utilizing magneto-resistance effect.
BACKGROUND OF THE INVENTION
As magnetic recording media such as a video tape, an audio tape, a computer tape and a flexible disc, those comprising a support having provided thereon a magnetic layer by coating a ferromagnetic iron oxide, a Co-modified ferromagnetic iron oxide, a CrO
2
, a ferromagnetic metal powder or a hexagonal ferrite powder dispersed in a binder have so far been used. Of these magnetic powders, a hexagonal ferrite powder is well known to have excellent high density recording characteristics (e.g., JP-A-60-157719 (the term “JP-A” as used herein means an “unexamined published Japanese patent application”), JP-A-62-109226 and JP-A-3-280215). JP-A-5-12650 discloses to restrict the layer thickness of a magnetic layer containing the ferrite powder to the range of from 0.1 to 0.6 &mgr;m and to provide a nonmagnetic layer thicker than the magnetic layer between a support and the magnetic layer, to thereby improve surface property, short wavelength output, erasure property and durability. JP-A-5-225547 discloses a magnetic recording medium comprising a support having thereon a nonmagnetic layer and a magnetic layer containing a magnetic powder having a thickness of 0.1 &mgr;m or less on the nonmagnetic layer, which is excellent in every point of high frequency characteristics, overwriting of signals and durability.
The fact that the electromagnetic characteristics of magnetic recording media are affected by the anisotropic magnetic field Hk of a hexagonal ferrite is described in JP-A-3-286420 and
IEEE. Trans. Mag
., Vol. 24 (6), p. 2850 (November, 1988). JP-A-3-286420 discloses a magnetic recording medium having two magnetic layers on a nonmagnetic layer, wherein the lower magnetic layer has an axis of easy magnetization in the longer direction, and the upper magnetic layer contains a magnetic powder having anisotropic magnetic field (Hk) of 239 kA/m or less, and the same patent discloses that the magnetic recording medium is capable of high output in a wide range of from long wavelength to short wavelength. JP-A-8-115518 discloses a magnetic recording medium for high density recording which has a coercive force (Hc) of from 103.5 to 398 kA/m, an Hc/Hk of from 0.30 to 1.0, and the squareness ratio (SQ) in the direction of in-plane of from 0.65 to 1.00. The same patent is characterized in that the Hc, Hc/Hk and SQ in the in-plane direction of the magnetic layer containing a hexagonal ferrite powder are specified, by which short wave length output necessary for high density recording is conspicuously improved. However, when a system using an MR head is used, high noise is unfavorably generated.
A high sensitivity reproducing head (an MR head) utilizing magneto-resistance effect has come to be used in a data recording system for computer in recent years, and the system noise is governed by the noise coming from a magnetic recording medium. JP-A-7-182646 suggests to use an MR head for the reproduction of a magnetic recording medium in which a Ba ferrite is used. Okabe et al., suggest in
IEEE. Trans. Mag
., Vol. 32 (5), pp. 3404 to 3406 (1996) that it is preferred to use a magnetic recording medium using a Ba ferrite in combination with an MR head, because saturation of an MR head can be avoided. For reducing medium noise, fining of ferromagnetic particles has been advanced, however, it is presumed that the stability in magnetization transition region may come into problem by the influence of thermal fluctuation with the progress of fining of ferromagnetic particles. The stability of magnetization is evaluated KuV/kT (Ku is a magnetic anisotropy constant, V is a particle volume, k is a Boltzmann's constant, and T is an absolute temperature). With respect to the particle volume and the thermal fluctuation of a metal tape, Toshiyuki Suzuki et al.,
Shingaku Giho
, MR 97-55, pp. 33 to 40 (Nov. 21, 1997) can be referred to. Regarding a Ba ferrite medium and thermal fluctuation, Toshiyuki Suzuki et al., also report in
Shingaku Giho
, MR 2000-45 (Nov. 14, 2000).
Since saturation magnetization of a hexagonal ferrite is about ⅓ to ½ of that of ferromagnetic metal powder, it is difficult to make Ku great and thermal fluctuation becomes large. In a magnetic recording medium using a hexagonal ferrite, it is said that the interaction between particles is large and that fact influences the noise level of the medium. It is said that magnetization stability is excellent when interaction between particles is large, but when a particle is subjected to magnetic flux revolution due to any reason, it is possible that surrounding particles are also subjected to magnetic flux revolution together. Perhaps for that reason, there is a problem that it is difficult to ensure sufficient C/N ratio when a magnetic recording medium for high density recording formed by a fined hexagonal ferrite powder is reproduced with an MR head.
SUMMARY OF THE INVENTION
The present invention has been done in view of the problems in the prior art techniques, and an object of the present invention is to provide a magnetic recording medium comprising a fine hexagonal ferrite powder which shows good short wavelength output and C/N ratio and recorded magnetization is stable when reproduced with an MR head.
The above object of the present invention has been attained by a magnetic recording medium comprising a support having provided thereon a nonmagnetic layer containing a nonmagnetic powder dispersed in a binder and a magnetic layer containing a ferromagnetic powder dispersed in a binder provided on the nonmagnetic layer, wherein the magnetic layer comprises a hexagonal ferrite powder having an average tabular diameter of from 10 to 35 nm, a coercive force (Hc) of from 135 to 400 kA/m, SFD (Switching Field Distribution) (25° C.) of from 0.40 to 0.90, and delta SFD (i.e., &Dgr; SFD) [SFD (100° C.)−SFD (25° C.)] of from −0.15 to 0.20.
It is also preferred that the magnetic recording medium according to the present invention has a magnetic layer thickness of from 0.01 to 0.5 &mgr;m, a value of a residual magnetic flux density×a magnetic layer thickness of from 0.5 to 100 mT.&mgr;m.
DETAILED DESCRIPTION OF THE INVENTION
The present invention specifies each value of the particle size, Hc, SFD (25° C.) and delta SFD (i.e., i.e., &Dgr; SFD: the change of SFD by the change of temperature or the temperature dependency of SFD) of a hexagonal ferrite powder, by which the short wave length output necessary for high density recording and the stability of magnetization can be markedly improved.
The hexagonal ferrite powder for use in the magnetic layer in the present invention preferably has an average tabular diameter of from 10 to 35 nm, a coercive force (Hc) of from 135 to 400 kA/m, and the coercive force (Hc) of the magnetic layer containing the hexagonal ferrite powder is preferably from 135 to 440 kA/m. A great Hc heightens an anisotropy constant (Ku) and a great Hc is preferred for achieving the object of the present invention of increasing thermal stability of magnetization.
Setting of the Hc of a magnetic layer largely depends upon the performance of the head for use in recording. Heads which are produced out of materials having a high saturation magnetic flux density (Bs), e.g., Fe—Ta—N, are preferably used. The upper limit of the Hc depends upon the materials of heads, but when a thin magnetic layer is used, it is supposed to be poss
Fuji Photo Film Co. , Ltd.
Resan Stevan A.
Sughrue & Mion, PLLC
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