Stock material or miscellaneous articles – Coated or structually defined flake – particle – cell – strand,... – Particulate matter
Reexamination Certificate
2001-12-20
2004-06-22
Le, H. Thi (Department: 1773)
Stock material or miscellaneous articles
Coated or structually defined flake, particle, cell, strand,...
Particulate matter
C428S690000, C428S900000
Reexamination Certificate
active
06753084
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates to spindle-shaped magnetic alloy particles for magnetic recording, and a magnetic recording medium, and more particularly, to a high-density magnetic recording medium having an excellent output characteristics in a short wavelength region, a considerably reduced noise and an excellent storage stability; spindle-shaped magnetic alloy particles containing Fe and Co as main components, which are contained in the magnetic recording medium and are fine particles exhibiting not only a high coercive force, an adequate saturation magnetization value and an excellent oxidation stability in spite of a small crystallite size, but also a small rotational hysteresis integral value when formed into a coating film; and a high-density magnetic recording medium having an excellent output characteristics in a short wavelength region, a considerably reduced noise and an excellent storage stability.
With recent development of miniaturized and lightweight magnetic recording and reproducing apparatuses for use in audio, video and computer as well as increase in recording time and memory capacity thereof, magnetic recording media used therefor have increasingly required to have a high performance and a high recording density.
In particular, in the field of computer tapes, with rapid development of high performance computers, it has been strongly required to enhance a memory capacity thereof for achieving miniaturization and large capacity thereof.
Namely, the magnetic recording media have been required to exhibit a high recording density, high output characteristics and improved frequency characteristics, especially excellent output characteristics in a short wavelength region. For this purpose, it has been required to reduce a noise of the magnetic recording media, and enhance a coercive force Hc thereof.
Also, in recent magneto resistive-type head as a reproduction head for computer tapes instead of conventional induction-type magnetic heads. Since the MR head can readily produce a considerably high reproduction output as compared to the conventional induction-type magnetic heads, it has been expected to further improve a high-density recording performance of the magnetic recording media.
In particular, the MR head is free from impedance noise due to use of induction coil and, therefore, is capable of reducing a system noise such as device noise to a large extent, resulting in reduced noise and high C/N ratio of the magnetic recording media. Accordingly, it has been strongly required to further reduce the noise of the magnetic recording media as compared to conventional ones.
In addition, in order to achieve a high-density recording, especially reduce a recording wavelength, it has been required to narrow a magnetization transition region and sharpen a digital signal reproduction waveform from the standpoints of high output and low noise. For this purpose, it has also been desired to approach the magnetization reversal to coherent rotation relative to a magnetic field and lessen the width of magnetization reversal.
These properties of the magnetic recording media have a close relation to magnetic particles used therein. In recent years, magnetic alloy particles containing iron as a main component have been noticed, because such particles can show a high coercive force and a large saturation magnetization (&sgr;s) as compared to those of conventional magnetic iron oxide particles. Further, magnetic alloy particles containing iron as a main component have been already used as magnetic particles for external memory devices such as computer tapes, e.g., DDS, DLT and TRAVAN.
Therefore, it has been strongly required to further improve properties of the magnetic alloy particles containing iron as a main component in order to satisfy the above requirements for magnetic recording media.
Specifically, in order to obtain magnetic recording media having a high coercive force, a less noise and a small width of magnetization reversal, it has been strongly required to provide fine magnetic alloy particles containing iron as a main component which are fine particles and are capable of not only exhibiting a small crystallite size, a high coercive force and an excellent dispersibility, but also having a magnetization reversal mechanism for ensuring substantially coherent rotation of the magnetization relative to a magnetic field.
As to the reduction in particle size of the magnetic alloy particles, in Japanese Patent Application Laid-Open (KOKAI) No. 2000-251243, it is described that “. . . When the size of a magnetic particles used becomes as large as compatible with a length of a recording region for signals, a clear magnetization transition region is no longer available, so that it becomes substantially impossible to record signals thereon. For this reason, it has been longtime demanded to provide the fine particles for achieving high-density recording upon use . . . ” Thus, in order to obtain magnetic recording media having a high output characteristics in a short wavelength region, and a less noise, it is required to reduce the particle size of the magnetic alloy particles for obtaining fine particles, i.e., reduce the major axis diameter thereof.
Also, as to the crystallite size of the magnetic alloy particles, in Japanese Patent Application Laid-Open (KOKAI) No. 7-126704 (1995), it is described that “. . . In order to reduce the noise level due to magnetic recording media, it is also effective to reduce the X-ray-measured size to as low a level as possible . . . ”. Thus, in order to obtain magnetic recording media having a less noise, the magnetic alloy particles containing iron as a main component have been strongly required to have a smaller crystallite size D
110
.
In addition, in order to further reduce the noise of the magnetic recording media, it is insufficient to lessen merely the major axis diameter and crystallite size of the magnetic alloy particles. For this purpose, it has been strongly required to clearly determine what factors should be concerned with the noise to be reduced.
In particular, there has been recently studied the activation volume obtained by measuring a time decay of magnetization due to thermal fluctuation and magnetic after-effect. On the basis of such studies, the reduction in noise of magnetic recording media has been attempted by optimizing the activation volume.
Also, it has been required to approach a magnetization reversal mode of the magnetic alloy particles to coherent rotation.
However, it is very difficult to obtain magnetic alloy particles containing Fe and Co as main components, which are fine particles, and exhibit a small crystallite size, a high coercive force and a magnetization reversal mode close to coherent rotation, because of its production process.
The above fact is explained below.
That is, the magnetic alloy particles containing iron as a main component have been generally produced by heat-reducing in a reducing gas atmosphere (i) spindle-shaped goethite particles obtained by passing an oxygen-containing gas such as air through an aqueous solution containing an iron-containing precipitate produced by reacting an aqueous ferrous salt solution with an aqueous alkali solution to conduct an oxidation reaction thereof, (ii) spindle-shaped hematite particles obtained by heat-dehydrating the spindle-shaped goethite particles, or (iii) spindle-shaped particles obtained by incorporating different elements other than iron into these particles.
As to the relationship between crystallite size and coercive force, in Japanese Patent Application Laid-Open (KOKAI) No. 4-61302 (1992), it is described that “. . . There is a tendency that the smaller the crystallite size, the lower the coercive force. Therefore, . . . it has been strongly required to provide magnetic particles exhibiting a small crystallite size while maintaining as high a coercive force as possible . . . ”. Thus, in the case of the spindle-shaped magnetic alloy particles, the crystallite size and the coercive force thereof have a reverse interrela
Kurokawa Haruki
Maekawa Masaaki
Okinaka Kenji
Ota Yasutaka
Yamamoto Akinori
Le H. Thi
Nixon & Vanderhye P.C.
Toda Kogyo Corporation
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