Compositions – Magnetic – Iron-oxygen compound containing
Patent
1998-10-26
2000-10-31
Koslow, C. Melissa
Compositions
Magnetic
Iron-oxygen compound containing
252 6263, 310 46, 428694T, 428694GT, H01R 100
Patent
active
061397660
DESCRIPTION:
BRIEF SUMMARY
TECHNICAL FIELD
This invention relates to a magnet powder and a sintered magnet each comprising ferrite, a bonded magnet and a magnetic recording medium each comprising the magnet powder, a magnetic recording medium having a thin film magnetic layer containing a ferrite phase, and a motor using such a magnet.
BACKGROUND ART
Oxide permanent magnet materials include hexagonal strontium ferrite and barium ferrite. Currently, strontium or barium ferrites of the magnetoplumbite type (M type) are mainly used, and they are manufactured into sintered magnets and bonded magnets.
Of magnet properties, a remanence or residual magnetic flux density (Br) and an intrinsic coercivity (HcJ) are most important.
The Br of a magnet is determined by the density and the degree of orientation of the magnet, and the saturation magnetization (4.pi.Is) which is determined by its crystal structure, and expressed by the equation: ferrites of the M type have a 4.pi.Is value of about 4.65 kG. The density and the degree of orientation have upper limits of about 98% even in the case of sintered magnets affording the highest values. Therefore, the Br of these magnets is limited to about 4.46 kG. It was substantially impossible in the prior art to achieve Br values of 4.5 kG or higher.
The inventors found in JP-A 115715/1997 that the inclusion of appropriate amounts of La and Zn enables to increase the 4.pi.Is of M type ferrite by about 200 G at maximum, thereby achieving a Br of at least 4.5 kG. In this case, however, since the anisotropy field (HA) to be described later lowers, it is difficult to acquire a Br of at least 4.5 kG and a HcJ of at least 3.5 kOe at the same time.
HcJ is in proportion to the product (H.sub.A xfc) of the anisotropy field (H.sub.A =2K.sub.1 /Is) multiplied by a single magnetic domain grain fraction (fc). Herein, K.sub.1 is a constant of crystal magnetic anisotropy which is determined by the crystalline structure like Is. M type barium ferrite has a K.sub.1 =3.3.times.10.sup.6 erg/cm.sup.3, and M type strontium ferrite has a K.sub.1 =3.5.times.10.sup.6 erg/cm.sup.3. It is known that M type strontium ferrite has the highest K.sub.1 although it is difficult to achieve a further improvement in K.sub.1.
On the other hand, if ferrite particles assume a single magnetic domain state, the maximum HcJ is expectable because to reverse the magnetization, the magnetization must be rotated against the anisotropy field. In order that ferrite particles be single magnetic domain particles, the size of ferrite particles must be reduced to or below the critical diameter (dc) given by the following expression:
Herein, k is the Boltzmann constant, Tc is a Curie temperature, and a is a distance between iron ions. Since M type strontium ferrite has a dc of about 1 .mu.m, it is necessary for the manufacture of sintered magnets, for example, to control the crystal grain size of a sintered body to 1 .mu.m or less. Although it was difficult in the prior art to realize such fine crystal grains at the same time as increasing the density and the degree of orientation for achieving higher Br, the inventors proposed a new preparation method in JP-A 53064/1994 and demonstrated the obtainment of better properties which were not found in the prior art. Even in this method, however, HcJ approximates to 4.0 kOe when Br is 4.4 kG, and it is difficult to simultaneously achieve a high HcJ of at least 4.5 kOe while maintaining a high Br of at least 4.4 kG.
In order to control the crystal grain size of a sintered body to 1 .mu.m or less, the particle size at the molding stage should preferably be controlled to 0.5 .mu.m or less when grain growth in the sintering stage is taken into account. The use of such fine particles generally gives rise to a productivity decline because the molding time is extended and more cracks occur upon molding. It is thus very difficult to find a compromise between property enhancement and high productivity.
Still further, it was known in the prior art that the addition of Al.sub.2 O.sub.3 and Cr.sub.2 O.sub.3 is effecti
REFERENCES:
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Iida Kazumasa
Masuzawa Kikoyuki
Minachi Yoshihiko
Taguchi Hitoshi
Koslow C. Melissa
TDK Corporation
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