Compositions – Magnetic – Iron-oxygen compound containing
Patent
1997-07-28
1999-01-12
Bonner, Melissa
Compositions
Magnetic
Iron-oxygen compound containing
252 6255, 252 6257, 264613, G11B 5706
Patent
active
058582657
DESCRIPTION:
BRIEF SUMMARY
BACKGROUND OF THE INVENTION
This invention relates to hexaferrite materials and is more particularly, though not exclusively, concerned with permanent magnet hexaferrite materials for high density recording media and permanent magnet applications.
DESCRIPTION OF THE BACKGROUND ART
The properties required of magnetic materials for high density recording media depends inter alia upon the type of recording media. For most recording media, high saturation magnetization, high structural and chemical stability and low cost are important criteria. In the case of recording media such as are used in hard disks, floppy disks, and video tapes, high coercivities should be avoided so as to facilitate re-recording. However, in recording media such as are used in identification cards and credit cards, high coercivity is required so as to reduce the risk of data becoming corrupted when such cards are exposed to stray magnetic fields. Also for fabrication of permanent magnets, high saturation magnetisation, sufficient coercivity and structural/chemical stability are the important criteria.
Barium hexaferrite (BaFe.sub.12 O.sub.19) and strontium hexaferrite (SrFe.sub.12 O.sub.19) in powder form have excellent chemical stability and are promising materials for perpendicular and longitudinal high density recording media. For increasing the suitability of hexaferrite powders for recording media such as hard disks, floppy disks and video tapes, it has been proposed to substitute Co--Ti, Co--Ni--Zn and Co--Zr for some of the iron in the hexaferrites in order to lower the coercivity somewhat. However, the saturation magnetization is also decreased markedly to an unacceptable extent. Conversely, in order to increase the coercivity to make these hexaferrites suitable for recording media in identification cards and credit cards or for the fabrication of permanent magnets, it has been previously proposed either to substitute the hexaferrites with elements such as aluminium and chromium or to produce the hexaferrites using chemical methods which are more expensive than conventional ceramic mixed oxide production methods. Increasing the amount of these substituting elements such as aluminium and chromium results in a significant and unacceptable decrease in the saturation magnetization.
DESCRIPTION OF THE INVENTION
It is an object of the present invention to provide a novel way of processing hexaferrites in order to produce the desired coercivity in a relatively economical manner without deleteriously affecting the saturation magnetization to an unacceptable extent, and in some cases actually increasing saturation magnetisation.
According to a first aspect of the present invention, barium, strontium and/or rare earth metal hexaferrite powders having suitably low coercivity for use in recording media such as hard disks, floppy disks and video tapes, can be prepared by heat treating barium, strontium and/or rare earth metal hexaferrite powder in the presence of carbon, nitrogen, carbon and nitrogen, hydrogen or carbon and hydrogen. Such heat treatment typically involves treatment of the hexaferrite powder under the conditions generally employed for carburising, nitriding, carbonitriding, hydriding or carbohydriding, as the case may be. Thus, for convenience, the respective heat treatments will be referred to hereinafter as "carburising", "nitriding", "carbonitriding", "hydriding" or and "carbohydriding" .
The resultant treated powder has a suitably low coercivity (not more than about 1 kOe) for use as recording media in hard disks, floppy disks and video tapes.
In order to produce powders of higher coercivity, such treated powders can be calcined to recover the ferrite structure. Surprisingly, it has been found that carburising, nitriding, carbonitriding, hydriding and carbohydriding are reversible reactions, and that the hexaferrite powders thus treated have a relatively low coercivity without the saturation magnetization being deleteriously affected to an unacceptable extent for use as recording media in hard disks, floppy disks
REFERENCES:
patent: 3502584 (1970-03-01), Denes
patent: 4699840 (1987-10-01), Yashiro et al.
Patent Abstracts of Japan; vol. 10, No. 352 for JP 61152003, Nov. 27, 1986.
Ataie Abolghasem
Harris Ivor Rex
Ponton Clive Brian
Bonner Melissa
The University of Birmingham
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