Stock material or miscellaneous articles – Web or sheet containing structurally defined element or... – Including a second component containing structurally defined...
Reexamination Certificate
2002-02-27
2003-09-16
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...
C428S690000, C428S690000
Reexamination Certificate
active
06620499
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to a magnetic recording medium, such as a magnetic tape, and more specifically to a magnetic recording particulate medium (i.e., a coating-type magnetic recording medium) which has a magnetic layer formed by coating on a support a magnetic coating composition containing as main components a ferromagnetic powder and a binder. In particular, the present invention relates to a magnetic recording medium containing a specific hexagonal (system) ferrite in a magnetic layer and enabling high-density recording, which is especially suitable for systems using a magnetoresistance-utilized MR head (i.e., magneto resistive head) for reproduction.
BACKGROUND OF THE INVENTION
Magnetic recording media hitherto used widely as video tapes, audio tapes, tapes for computer use or flexible disks have magnetic recording layers containing ferromagnetic iron oxide, cobalt-modified ferromagnetic iron oxide, CrO
2
, ferromagnetic metal powder or hexagonal (system) ferrite in a state of being dispersed in binder. Therein, the magnetic recording layer is provided on a support by the use of coating techniques. Of those ferromagnetic powders, Hexagonal (system) ferrite is known to have excellent high-density recording characteristics (by, e.g., JP-A-60-157719, JP-A-62-109226 and JP-A-3-280215, wherein the term “JP-A” means an “unexamined published Japanese patent application).
In JP-A-5-12650 is disclosed a magnetic recording medium provided with a Hexagonal (system) ferrite-containing magnetic layer, wherein the magnetic layer has a thickness adjusted to the range of 0.1 &mgr;m to 0.6 &mgr;m and a non-magnetic layer having a thickness greater than the thickness of the magnetic layer is formed between the magnetic layer and a support, thereby achieving improvements in surface properties, short wavelength output, erasure characteristics and durability. On the other hand, JP-A-5-225547 discloses a magnetic recording medium having on a support a non-magnetic layer and on the non-magnetic layer a magnetic layer containing magnetic powders having a particle size of 0.1 &mgr;m or below, thereby attaining not only excellent high-wavelength region characteristics but also a satisfactory signals-overwriting property and sufficient durability.
Further, JP-A-3-286420 and
IEEE. Trans. Mag.
, vol. 24 (6), p. 2850 (Nov. 1988) disclose that the anisotropic magnetic field Hk of hexagonal (system) ferrite has an influence upon electromagnetic (conversion) characteristics of a magnetic recording medium. The magnetic recording medium disclosed in the former publication has two magnetic layers on a non-magnetic layer. The lower magnetic layer thereof has its axis of easy magnetization in a long length direction, and in the upper magnetic layer is incorporated a magnetic powder having an anisotropic magnetic field (Hk) of not more than 239 kA/m. And such a medium can provide high output over a wide range from long to short wavelengths. Furthermore, JP-A-8-115518 proposes a high-density recording medium having an Hc value of 103.5 to 398 kA/m, an Hc/Hk ratio of 0.30 to 1.0 and a squareness ratio SQ of 0.65 to 1.00 in the in-plane direction. This medium is characterized by having a Hexagonal (system) ferrite magnetic powder-containing magnetic layer whose Hc value, Hc/Hk ratio and squareness ratio SQ in the in-plane direction are within individually specified ranges. By specifying such factors, a marked improvement in very short wavelength output necessary for high-density recording is attained. Even by application of such methods to magnetic recording media, however, there still remains a problem that the recording media obtained cause high noise when they are used in combination with an MR head.
In recent years, magnetoresistance-utilized high-sensitivity reproduction heads (MR heads) have come to be used in data recording systems for computers. Therein, the magnetic recording medium-originated noise occupies a dominant position in system noise. Okabe et al. suggests that the combined use of a barium ferrite-containing magnetic medium and a MR head is advantageous since it can inhibit saturation of the MR head (in IEEE. Trans. Mg., volume 32 (5), pages 3404-3406 (1996)).
In order to reduce the media noise originated from magnetic recording media, it has been underway to make ferromagnetic particles finer. However, it is thought that an influence of thermal fluctuation on ferromagnetic particles is increased as the particles are made finer, and thereby stability of magnetization transition domain is lowered. Therefore, a solution to this problem is sought. The stability of magnetization is evaluated by KuV/kT (wherein Ku represents a magnetic anisotropic constant, V represents a particle volume, k represents Boltzmann constant and T represents an absolute temperature). In regard to the particle volume and thermal fluctuation of metallic tapes, there is a report presented by Toshiyuki Suzuki et al. (Shingaku Giho MR97-55, pages 33-40 (Nov. 21, 1996).
In the case of Hexagonal (system) ferrite, it is difficult to get a great Ku value because the saturation magnetization thereof is about ⅓ to about ½ of the saturation magnetization of a ferromagnetic metal powder; as a result, the thermal fluctuation tends to become great. Further, there is an indication that the inter-particle interaction in a magnetic recording medium using Hexagonal (system) ferrite is great and has an influence on a noise level of the medium. It can be said that the great inter-particle interaction secures high magnetization stability, but if magnetization of particles is once inverted through some cause, the possibility that a magnetic substance around the particles causes inversion of its magnetization under the influence of the particles is also increased. Probably, such magnetization inversion causes a problem that it is difficult to secure sufficient C/N in reproducing a prerecorded high-density recording medium using a fine powder of magnetic hexagonal (system) ferrite by means of an MR head.
SUMMARY OF THE INVENTION
The present invention has made in view of those problems concerning the related arts. And the object of the present invention is to provide a magnetic recording medium using an extremely fine powder of Hexagonal (system) ferrite and enabling achievement of high short-wavelength output and high C/N even when an MR head is used for reproduction in combination therewith, and besides, capable of ensuring high stability for magnetization recorded therein.
As a result of our intensive study of the problems wherein our attention has been focused on anisotropic magnetic field and rotational hysteresis loss integral (RH) in particular, it has been found that ultrashort-wavelength output necessary for high-density recording and magnetization stability can be greatly improved by using a special magnetic powder in a magnetic layer and adjusting a coercive force Hc, an anisotropic magnetic field Hk and a rotational hysteresis loss integral of the magnetic layer to respectively specified numerical ranges, thereby achieving the present invention.
More specifically, the present invention is embodied by a magnetic recording medium having on a support a non-magnetic layer comprising a non-magnetic powder dispersed in a binder resin and on the non-magnetic layer a magnetic layer comprising a ferromagnetic powder dispersed in a binder resin, wherein the ferromagnetic powder is a hexagonal (system) ferrite magnetic powder having an average tabular diameter of 10 to 35 nm and a coercive force Hc of 135 to 400 kA/m, and that the magnetic layer has a coercive force Hc of 135 to 440 kA/m, an anisotropic magnetic field of at least 358 kA/m and a rotational hysteresis loss integral of 0.5 to 0.95.
In addition to the constitutional requirements described above, the magnetic recording medium of the present invention can remarkably achieve its effects when the magnetic layer has a squareness ratio SQ of 0.50 to 0.65 in the in-plane direction, and the effects thereof can also be increased when the magneti
Fuji Photo Film Co. , Ltd.
Resan Stevan A.
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