Oxide magnetic material, ferrite particles, bonded magnet,...

Details Oxide magnetic material, ferrite particles, bonded magnet,... Oxide magnetic material, ferrite particles, bonded magnet,...

1999-05-19

2002-06-11

Koslow, C. Melissa (Department: 1755)

Compositions

Magnetic

Iron-oxygen compound containing

C252S062570

Reexamination Certificate

active

06402980

ABSTRACT:

TECHNICAL FIELD
The present invention relates to magnet powder and a sintered magnet comprising a hexagonal ferrite, a bonded magnet and a magnetic recording medium comprising the magnet powder, and a magnetic recording medium having a thin film magnetic layer comprising a hexagonal ferrite phase.
TECHNICAL BACKGROUND
As a material for an oxide permanent magnet, a magnetoplumbite (M type) hexagonal strontium (Sr) ferrite or barium (Ba) ferrite has been mainly used. Calcium (Ca), which is one of the alkaline earth elements as similar to Ba and Sr, has not been used as a magnet material though it is not expensive because Ca does not form a hexagonal ferrite.
In general, a Ca ferrite has a stable structure of CaO-Fe
2
O
3
or CaO-2Fe
2
O
3
, and does not form a hexagonal ferrite (CaO-6Fe
2
O
3
), but it has been known that a hexagonal ferrite is formed by adding La. In this case, it is considered that the valence of a part of the Fe ion is changed (Fe
3+
to Fe
2+
) to compensate the difference in valence between La and Sr (La
3+
and Sr
2+
). However, the magnetic characteristics obtained in this case is those equivalent to a Ba ferrite at most, which is not considerably high. Furthermore, there has been no example in that an element forming a divalent ion and La are complexly added to a Ca ferrite.
What are important among characteristics of a magnet are a residual magnetic flux density (Br) and an intrinsic coercive force (HcJ).
Br is determined by the density of the magnet, the degree of orientation of the magnet, and the saturation magnetization (4&pgr;Is) determined by the crystal structure. Br is expressed by the following equation:
Br=4&pgr;Is×(degree of orientation)×(density)
The Sr ferrite and the Ba ferrite of M type has a 4&pgr;Is value of about 4.65 kG. The density and the degree of orientation each is about 98% at most in the sintered magnet, which provides the highest values. Therefore, Br of these magnets is limited to about 4.46 kG at most, and it has been substantially impossible to provide a high Br value of 4.5 kG or more.
The inventor of the invention have found that the addition of appropriate amounts of La and Zn in an M type ferrite raises its 4&pgr;Is value by about 200 G at most, and a Br value of 4.4 kG or more can be obtained, as described in U.S. patent application Ser. No. 08/672,848, now U.S. Pat. No. 5,846,449. In this case, however, since the anisotropic magnetic field (H
A
), which will be described later, is decreased, it is difficult to obtain a Br value of 4.4 kG or more and an HcJ of 3.5 kOe or more at the same time.
HcJ is in proportion to the product (H
A
×fc) of the anisotropic magnetic field (H
A
(=2K
1
/Is)) and a single magnetic domain grain fraction (fc), in which K
1
represents a crystal magnetic anisotropy constant, which is determined by the crystal structure as similar to Is. The M type Ba ferrite has K
1
of 3.3×10
6
erg/cm
3
, and the M type Sr ferrite has K
1
of 3.5×10
6
erg/cm
3
. It has been known that the M type Sr ferrite has the largest K
1
value, but it has been difficult to further raise the K
1
value.
On the other hand, in the case where ferrite grains are in a single magnetic domain condition, the maximum HcJ is expected because the magnetization must be rotated against the anisotropic magnetic field to reverse the magnetization. In order to make ferrite grains into single magnetic domain grains, the size of the ferrite grains must be smaller than the following critical diameter (dc) expressed by the following equation:
dc=2(k·Tc·K
1
/a)
½
/Is
2
wherein k represents the Boltzman constant, Tc represents a Curie temperature, and a represents a distance between iron ions. In the case of the M type Sr ferrite, since dc is about 1 &mgr;m, it is necessary for producing a sintered magnet that the crystal grain size of the sintered magnet must be controlled to 1 &mgr;m or less. While it has been difficult to realize such a fine crystal grain and the high density and the high degree of orientation to provide a high Br at the same time, the inventor has proposed a new production process to demonstrate that superior characteristics that cannot be found in the art are obtained, as described in U.S. Pat. No. 5,648,039. In this process, however, the HcJ value becomes 4.0 kOe when the Br value is 4.4 kG, and therefore it has been difficult to obtain a high HcJ of 4.5 kOe or more with maintaining a high Br of 4.4 kG or more at the same time.
In order to control a crystal grain size of a sintered body to 1 &mgr;m or less, it is necessary to make the grain size in the molding step 0.5 &mgr;m or less with taking the growth of the grains in the sintering step into consideration. The use of such fine grains brings about a problem in that the productivity is generally deteriorated due to increase in molding time and increase in generation of cracks on molding. Thus, it has been very difficult to realize high characteristics and high productivity at the same time.
It has been known that the addition of Al
2
O
3
and Cr
2
O
3
is effective to obtain a high HcJ value. In this case, Al
3+
and Cr
3+
have effects of increasing H
A
and suppressing the grain growth by substituting for Fe
3+
having an upward spin in the M type structure, so that a high HcJ value of 4.5 kOe or more is obtained. However, when the Is value is reduced, the Br value is considerably reduced since the sintered density is reduced. As a result, the composition exhibiting the maximum HcJ of 4.5 kOe can only provide a Br value of 4.2 kG.
A sintered magnet of the conventional anisotropic M type ferrite has a temperature dependency of HcJ of about +13 Oe/° C. and a relatively high temperature coefficient of about from +0.3 to +0.5%/° C., which sometimes bring about great reduction in HcJ on the low temperature side and thus demagnetization. In order to prevent such demagnetization, the HcJ value at room temperature must be a large value of about 5 kOe, and therefore it is substantially impossible to obtain a high Br value at the same time. Powder of an isotropic M type ferrite has a temperature dependency of HcJ of at least about +8 Oe/° C., although it is superior to the anisotropic sintered magnet, and a temperature coefficient of +0.15%/° C., and thus it has been difficult to further improve the temperature characteristics.
The inventors have proposed that the temperature dependency of HcJ is reduced by introducing distortion into ferrite grains by pulverization, as described in U.S. Pat. No. 5,468,039. In this case, however, HcJ at room temperature is also decreased, and thus the high HcJ at room temperature and its temperature characteristics cannot be improved at the same time.
DISCLOSURE OF THE INVENTION
An object of the invention is to realize a hexagonal ferrite having both a high saturation magnetization and a high magnetic anisotropy, so as to provide a ferrite magnet having a high residual magnetic flux density and a high coercive force, which cannot be realized by the conventional hexagonal ferrite magnet.
Another object of the invention is to provide a ferrite magnet excellent in temperature characteristics of the coercive force, where in particular, reduction of the coercive force in a low temperature region is small.
Further object of the invention is to provide a ferrite magnet having a high residual magnetic flux density and a high coercive force by using relatively coarse ferrite grains having a diameter exceeding 1 &mgr;m.
Still further object of the invention is to provide a magnetic recording medium having a high residual magnetic flux density and being thermally stable.
The objects of the invention can be attained by one of the constitutions (1) to (13) described below.
(1) An oxide magnetic material comprising a primary phase of a hexagonal ferrite containing Ca, R, Fe and M, where M represents at least one element selected from the group consisting of Co, Ni and Zn, and R represents at least one element selected from the group

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