Stock material or miscellaneous articles – Web or sheet containing structurally defined element or... – Including a second component containing structurally defined...
Reexamination Certificate
2002-03-28
2004-02-10
Resan, Stevan A. (Department: 1723)
Stock material or miscellaneous articles
Web or sheet containing structurally defined element or...
Including a second component containing structurally defined...
C428S403000, C428S690000, C252S062560, C423S594120
Reexamination Certificate
active
06689455
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to a method of producing hexagonal (system) ferrite and a magnetic recording medium, such as a magnetic tape, using the hexagonal (system) ferrite produced thereby. More specifically, the present invention relates to a method of producing a specific hexagonal (system) ferrite and a particulate-type magnetic recording medium which has a magnetic layer formed on a support by coating a magnetic coating composition containing a ferromagnetic powder and a binder as main components. 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 for reproduction.
BACKGROUND OF THE INVENTION
Hitherto, recording materials having on a support magnetic layers formed by application of binder-dispersed ferromagnetic iron oxide, cobalt-modified ferromagnetic iron oxide, CrO
2
, ferromagnetic metal powder or hexagonal (system) ferrite have been widely used as magnetic recording media including video tapes, audio tapes, tapes for computer use and flexible disks. Of those magnetic powders, hexagonal (system) ferrite is known to have excellent high-density recording characteristics (by, e.g., Japanese Patent Application (Laid-Open) No. 157719/1985, Japanese Patent Application (Laid-Open) No. 109226/1987 and Japanese Patent Application (Laid-Open) 280215/1991).
In Japanese Patent Application (Laid-Open) 12650/1993 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, Japanese Patent Application (Laid-Open) 225547/1993 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-frequency characteristics but also a satisfactory signals-overwriting property and sufficient durability.
Further, Japanese Patent Application (Laid-Open) No. 290929/1994 describes as a hexagonal (system) ferrite having excellent magnetic characteristics and good dispersibility in a coating composition a hexagonal Ba ferrite having an Sm/Sc ratio of 1 or below wherein Sc represents a calculated specific surface area calculated from an average particle size determined by observation under a transmission electron microscope and Sm represents a specific surface area determined by the BET method.
Furthermore,
IEEE. Trans. Mag.,
vol. 23(6), p. 3125 (September 1987) reports a Ba ferrite of 0.70 to 0.82 in ratio of an actually measured specific surface area to a surface area calculated from an average diameter determined by observation under a transmission electron microscope.
In recent years, high-sensitive reproduction heads (MR heads) utilizing magnetoresistance effect have come to be used in the data recording systems for computers. Therein, noise originated from a magnetic recording medium occupies a dominant position in system noise. Okabe et al. suggests that the combined use of a barium ferrite-containing magnetic medium and an MR head is advantageous since it can avoid saturation of the MR head (
IEEE. Trans. Mag.,
volume 32(5), pages 3404-3406 (1996)).
With the intention of reducing the medium noise, 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 a stability problem is caused in magnetization transition regions. 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 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. In addition, minute hexagonal (system) ferrite particles are single crystals and have a tabular shape. Therefore, once the so-called stacking is formed by mutual approach of particles, it becomes difficult to pull apart the particles. For this reason, a high-density recording medium produced by using a hexagonal (system) ferrite magnetic powder minute in particle size has a problem that, even in the case of reproduction with an MR head, the recording medium makes noises above the noise level expected from the volume and the number of particles therein and thereby it becomes difficult to secure sufficient C/N.
SUMMARY OF THE INVENTION
The present invention has been made in view of the problems concerning the related arts. Specifically, the first object of the present invention is to provide a method of producing fine particles of hexagonal (system) ferrite which are suitable for magnetic recording media, in which the particles are prevented from stacking and have good dispersibility. Further, our attention has been focused on the hexagonal (system) ferrite produced by such a specific method, and the second object of the present invention is to provide a magnetic recording medium which enables achievements of high short-wavelength output, high C/N and low medium noise even in the case of using an MR head for reproduction.
As a result of our intensive studies, it has been found that, when hexagonal (system) ferrite particle surfaces are treated with a polar group-containing organic compound and/or a salt thereof in a ferrite production process, the particles can have dispersiblity sufficiently enhanced to ensure high suitability for magnetic recording media. Moreover, it has been found that, by using the hexagonal (system) ferrite produced via the surface treatment as a magnetic powder incorporated in a magnetic layer, ultrashort-wavelength output necessary for high-density recording and media noise or noise level can be greatly improved, thereby achieving the present invention.
More specifically, the first invention relates to a method of producing hexagonal (system) ferrite, wherein when the hexagonal (system) ferrite having an average tabular diameter of 10 to 40 nm and a coercive force (Hc) of 135 to 400 kA/m is recovered and washed, the hexagonal (system) ferrite is treated with an organic compound containing a polar group having an acid dissociation constant (pKa) of 4 or below and/or a salt thereof.
The second invention relates to 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 comprises a hexagonal (system) ferrite magnetic powder that has an average tabular diameter of 10 to 40 nm and a coercive force (Hc) of 135 to 400 kA/m and is subjected to treatment with an organic compound containing a polar group having an acid dissociation constant (pKa) of 4 or below and/or a salt thereof.
In the present invention, it is preferred that the magnetic layer further contains a fatty acid, and besides, the suitable thickness thereof is from 0.01 to 0.5 &mgr;m and the product of a residual magnetic flux density and a magnetic layer thickness is appropriately from 0.5 to 100 mT·&mgr;m.
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