Magnetic recording powder having a high coercive force at room temperatures and a low curie point

A magnetic recording powder for use as a material for magnetic recording media including magnetic tapes and magnetic cards, the powder having a high coercive force at room temperatures and a low Curie point, consists of at least one element selected from the group consisting of Y and rare-earth elements (R): 5-20 atomic %, B: 5-20 atomic %, Mn: 4-20 atomic %; and Fe and inevitable impurities: the balance. If required, the powder may further contain Al: 1-10 atomic %, and/or Cr: 1-10 atomic %. The powder has a mean particle size of 0.5-2 .mu.m and a mean crystal grain size of 0.1-0.4 .mu.m.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
This invention relates to a magnetic recording powder for use as a material 
for magnetic recording media such as magnetic tapes and magnetic cards, 
and more particularly to a magnetic recording powder of this kind which 
has a high coercive force at room temperatures and a low Curie point. 
2. Description of the Prior Art 
Conventionally known magnetic recording powders of this kind include 
.gamma.-Fe.sub.2 O.sub.3 powders, Ba ferrite powders, RCo.sub.5 powders (R 
is an element selected from rare-earth elements including yttrium. Sm is 
mainly used as R), and Nd-Fe-B alloy powders as disclosed in Japanese 
Provisional Patent Publication (Kokai) No. 59-229461. The coercive force 
(iHc) at room temperatures and Curie point (Tc) of these conventional 
magnetic recording powders are shown in Table 1. 
TABLE 1 
______________________________________ 
Kind of Magnetic 
Coercive Force 
Curie Point 
Recording Powder 
iHc (Oe) Tc (.degree.C.) 
______________________________________ 
.gamma.-Fe.sub.2 O.sub.3 powder 
Appox. 400 Approx. 600 
Ba Ferrite Approx. 3000 Approx. 450 
Powder 
RCo.sub.5 Powder 
10000 or more 
Approx. 700 
Nd--Fe--B Alloy 
10000 or more 
Approx. 320 
Powder 
______________________________________ 
Magnetic recording media such as magnetic tapes and magnetic cards are 
manufactured by applying magnetic recording powders over surfaces of tapes 
or sheets of synthetic resin. To magnetically write or erase a magnetic 
recording medium thus manufactured, usually the medium is heated to a 
temperature immediately below the Curie point of the magnetic recording 
powder forming the medium to have its coercive force reduced, and then the 
medium is subjected to magnetic writing or erasing while it has a thus 
reduced coercive force. 
However, the aforesaid conventional magnetic recording powders have high 
Curie points, requiring a great deal of energy to heat to their high Curie 
Many studies have been made in order to develop magnetic recording powders 
having low Curie points. However, a problem has been encountered that if a 
magnetic recording medium in general is improved to have a lower Curie 
point, it also has a correspondingly reduced coercive force at room 
temperatures. Particularly, the conventional Nd-Fe-B alloy powders have 
their coercive forces drastically reduced when they are finely crushed. 
SUMMARY OF THE INVENTION 
It is therefore the object of the invention to provide a magnetic recording 
powder having a fine particle size which has a high coercive force at room 
temperatures and at the same time has a low Curie point. 
To attain the object, the present invention provides a magnetic recording 
powder for use as a material for magnetic recording media including 
magnetic tapes and magnetic cards, the powder having a high coercive force 
at room temperatures and a low Curie point, consisting of: 
at least one element selected from the group consisting of Y and rare-earth 
elements (R): 5-20 atomic %; 
B: 5-20 atomic %; 
Mn: 4-20 atomic %; and 
Fe and inevitable impurities: the balance, the powder having a mean 
particle size of 0.5-2 .mu.m and a mean crystal grain size of 0.1-0.4 
.mu.m. 
If required, the magnetic recording powder according to the invention may 
further contain Al: 1-10 atomic %;, and/or Cr: 1-10 atomic %. 
The above and other objects, features, and advantages of the invention will 
be more apparent from the following detailed description.

DETAILED DESCRIPTION 
Under the aforesaid circumstances, the present inventors have made studies 
in order to develop a magnetic recording powder having a fine particle 
size which has a lower Curie point and at the same time has a high 
coercive force at room temperatures. As a result, they reached the 
following finding: 
An alloy powder which is formed by adding Mn in an amount of 4-20 atomic % 
to an alloy consisting essentially of 5-20 atomic % of at least one 
element selected from the group consisting of Y and rare-earth elements 
(hereinafter abbreviated as R), 5-20 atomic % of B, and the balance of Fe 
and inevitable impurities, or an alloy powder which is formed by adding Mn 
in an amount of 4-20 atomic %, and one or both of Al in an amount of 1-10 
atomic %, and Cr in an amount of 1-10 atomic % has a high coercive force 
at room temperatures and at the same time has a low Curie point, if the 
mean particle size of the alloy powder is adjusted to a range of 0.5 to 
2.mu.m and the means crystal grain size to a range of 0.1 to 0.4.mu.m. 
The present invention is based upon the above finding. 
Magnetic recording powders according to the invention have the aforesaid 
chemical compositions. R includes at least one of Nd, Pr, La, Ce, Tb, Dy, 
Ho, Er, Eu, Sm, Gd, Pm, Tm, Yb, and Y. Nd and Dy are particularly 
preferable. 
The reasons why the percentages of the component elements, the mean 
particle size, and the mean crystal grain size have been limited to the 
aforesaid ranges will be explained hereinbelow: 
(1) R: 
R acts to enhance the coercive force at room temperatures of the powder. 
However, if the R content is less than 5 atomic %, a cubic structure which 
is identical with that of .alpha. iron appears in large quantities, which 
hinders the powder from having a sufficient coercive force. On the other 
hand, if R is contained in an amount exceeding 20 atomic %, many 
nonmagnetic phases which are rich with R are formed, so that the resulting 
powder has reduced saturation magnetization and can be oxidized. 
Therefore, the R content has been limited to 5-20 atomic %. The preferable 
R content is 10-15 atomic %. 
(2) B: 
B acts to enhance the coercive force at room temperatures of the powder, 
like R. However, if the B content is less than 5 atomic %, the resulting 
powder has a rhombohedral structure which hinders the powder from having a 
sufficient coercive force, whereas if B is contained in excess of 20 
atomic %, the resulting powder will have reduced saturation magnetization 
and hence a reduced coercive force. Therefore, the B content has been 
limited to 5-20 atomic %, preferably 5-10 atomic %. 
(3) Mn: 
Mn acts to lower the Curie point of the powder when it is contained in an 
R-B-Fe magnet alloy. However, if the Mn content is less than 4 atomic %, 
the above action cannot be obtained, whereas if the Mn content exceeds 20 
atomic %, the resulting powder will have a reduced coercive force at room 
temperatures. This is why the Mn content has been limited to 4-20 atomic 
%, preferably 4-12 atomic %. 
(4) Al and Cr: 
Al and/or Cr acts to further lower the Curie point when Al and/or Cr is 
added to an R-B-Fe magnet alloy together with Mn. However, if Al and/or Cr 
is contained in an amount less than 1 atomic %, the above action cannot be 
performed to an appreciable degree, whereas if Al and/or Cr is contained 
in excess of 10 atomic %, the resulting powder will have a reduced 
coercive force at room temperatures. Therefore, the Al and/or Cr content 
has been limited to 1-10 atomic %, preferably 3-10 atomic %. 
However, the addition of one or both of Al and Cr together with Mn must 
satisfy a condition of 5 atomic % .ltoreq.Mn+Al+Cr.ltoreq.20 atomic % to 
secure the action of further lowering the Curie point and at the same time 
prevent a decrease in the coercive force at room temperatures. 
(5) Mean Particle Size: 
If the magnetic recording powder according to the invention has a mean 
particle size of less than 0.5 .mu.m, it will have a drastically reduced 
coercive force. Further, if the mean particle size is less than 0.5 .mu.m, 
the powder will also have degraded properties due to oxidization with 
aging and even can kindle. On the other hand, if the mean particle size 
exceeds 2 .mu.m, a magnetic card or a magnetic tape formed of the powder 
will suffer from so-called medium noise during reproduction and even have 
degraded recording density. Therefore, the mean particle size has been 
limited to 0.5-2 .mu.m, preferably 0.7-1.5 .mu.m. 
(6) Mean Crystal Grain Size: 
If the mean particle size is within a range of 0.5 to 2 .mu.m, the magnetic 
recording powder according to the invention has a fine crystal grain 
structure. However, if the mean crystal grain size of the fine crystal 
grains is less than 0.1 .mu.m, the powder will have degraded 
magnetizability such that even if heated to a temperature immediately 
below its Curie point, it cannot accomplish recording with ordinary 
magnetic writing, whereas if the mean crystal grain size exceeds 0.4 
.mu.m, there can occur transcrystalline cracks, resulting in an 
insufficient coercive force, if the mean particle size is decreased to the 
range mentioned in the paragraph (5) above. Therefore, the mean crystsl 
grain size has been limited to 0.1-0.4 .mu.m, preferably 0.2-0.4 .mu.m. 
Examples of the invention will now be explained. 
Examples 
Prepared as starting materials were pure iron, a metal Nd, a metal Dy, an 
Fe-B alloy (B: 20%), an Fe-Mn alloy (Mn: 75%), an Fe-Al alloy (Al: 50%), 
and an Fe-Cr alloy (Cr: 60%). These starting materials were melted in a 
high-frequency smelting furnace and cast into rare-earth alloy ingots 
having chemical compositions shown at respective Examples Nos. 1-18 
according to the present invention and Comparative Examples Nos. 1-10 in 
Table 2. 
These ingots were coarsely crushed by a stamping mill in an Ar gas 
atmosphere, and then finely crushed by a vibrating ball mill into fine 
rare-earth alloy powders. The fine rare-earth alloy powders were each 
charged in an appropriate amount into a boat, which was in turn charged 
into a heat-treating furnace. The interior of the furnace was evacuated to 
a vacuum of 0.1.times.10.sup.-5 Torr, followed by introducing a hydrogen 
gas under 1 atm into the furnace. Then, the interior of the furnace was 
heated from a room temperature to 850.degree. C. while the hydrogen gas 
pressure was maintained at 1 atm. After 850.degree. C. was reached, the 
furnace interior was evacuated for 30 minutes to 2 hours while the furnace 
temperature was maintained at 850.degree. C. so that the furnace interior 
atmosphere was again brought into a vacuum of 1.0.times.10.sup.-5 Torr. 
Thereafter, an Ar gas was introduced into the heat-treating furnace until 
the furnace interior pressure increased to 1 atm, whereby the fine powders 
were rapidly cooled. The resulting agglomerate powders were crushed by a 
mortar, and then finely crushed by a vibrating ball mill into magnetic 
recording powders having mean particle sizes and mean crystal grain sizes 
shown in Table 2. 
The coercive force (iHc) at a room temperature and Curie point (Tc) of the 
magnetic recording powders thus obtained, i.e. Examples Nos. 1-18 
according to the present invention and Comparative Examples Nos. 1-10 were 
measured, the results of which are shown in Table 2. 
It will be learned from Table 2 that all the magnetic recording powders of 
Examples Nos. 1-18 according to the present invention are lower in Curie 
point than the be conventional magnetic recording powders shown in Table 
1, and further the former powders are so high in coercive force at room 
temperatures that they can be satisfactorily used as magnetic recording 
powders. On the other hand, satisfactory results cannot be obtained with 
magnetic recording powders of Comparative Examples Nos. 1-8 wherein R, Mn, 
Al or Cr are contained in amounts falling outside the range of the present 
invention, and/or one or both of the mean particle size and the mean 
crystal grain size shown values falling outside the range of the present 
invention (In Table 2, the asterisked content values fall outside the 
range of the present invention). 
TABLE 2 
__________________________________________________________________________ 
MAGNETIC 
COMPOSITION OF RARE- MEAN MEAN CRYS- 
PROPERTIES 
EARTH ALLOY INGOT (ATOMIC %) 
TICLE 
TAL GRAIN 
COER- 
Fe & IN- 
SIZE OF M.R. 
SIZE OF CIVE CURIE 
EVITABLE 
POWDER M.R. POW- 
FORCE 
POINT 
SPECIMEN Nd Dy B Mn Al Cr IMPURITIES 
(.mu.m) DER (.mu.m) 
iHc 
Tc 
__________________________________________________________________________ 
(.degree.C.) 
EXAMPLES AC- 
CORDING TO 
PRESENT 
INVENTION 
1 11 3 5 12 -- -- Bal. 1.5 0.4 8,000 
100 
2 15 -- 8 4 -- -- Bal. 1.0 0.4 9,000 
200 
3 11 3 5 8 -- -- Bal. 0.7 0.3 5,000 
150 
4 8 8 8 16 -- -- Bal. 0.5 0.3 1,500 
80 
5 8 8 8 20 -- -- Bal. 0.5 0.4 1,000 
60 
6 14 -- 5 8 1 -- Bal. 1.2 0.4 7,500 
130 
7 14 -- 5 6 3 -- Bal. 0.8 0.3 9,000 
180 
8 14 -- 5 6 6 -- Bal. 0.8 0.4 6,000 
150 
9 10 5 5 6 9 -- Bal. 0.6 0.3 5,000 
100 
10 11 3 5 4 -- 2 Bal. 2.0 0.4 9,000 
150 
11 11 3 5 8 -- 4 Bal. 1.5 0.4 8,000 
120 
12 11 3 5 5 -- 7 Bal. 1.0 0.2 6,000 
100 
13 10 5 5 4 -- 10 Bal. 0.5 0.1 4,000 
100 
14 11 4 5 5 5 5 Bal. 0.8 0.2 6,000 
100 
15 11 4 5 5 3 7 Bal. 0.8 0.3 4,000 
80 
16 11 4 5 5 7 5 Bal. 0.6 0.1 5,000 
60 
17 14 -- 5 4 4 1 Bal. 1.0 0.2 7,000 
150 
18 14 -- 5 8 2 2 Bal. 0.8 0.2 5,000 
100 
COMATIVE 
EXAMPLES 
1 4* 
-- 8 6 -- -- Bal. 100* 1.5 
700 
2 22* 
-- 8 8 -- -- Bal. 1.8 1.5 60 50 
3 15 -- 3* 
8 -- -- Bal. 1.2 1.0 40 30 
4 15 -- 23* 
8 -- -- Bal. 1.5 2.5 0 30 
5 15 -- 8 2* 
-- -- Bal. 0.8 0.3 10,000 
310 
6 14 -- 5 21* 
-- -- Bal. 1.0 0.3 100 
20 
7 15 -- 8 8 12* 
-- Bal. 0.8 0.4 0 20 
8 15 -- 8 8 -- 12* 
Bal. 2.0 0.4 50 20 
9 14 -- 5 6 3 -- Bal. 0.3* 0.3 300 
180 
10 14 -- 5 6 3 -- Bal. 0.8 0.6* 800 
180 
__________________________________________________________________________ 
(ASTERISKED VALUES FALL OUTSIDE RANGE OF PRESENT INVENTION) 
Then, to observe the structure of the magnetic recording powder according 
to the present invention, a powder of Example No. 7 according to the 
present invention obtained by crushing by a mortar was picked up and its 
surface structure was inspected and photographed with a scanning electron 
microscope (SEM) while the microscope was set to different magnifications, 
to obtain microphotographs of the surface structure of the powder as shown 
in FIGS. 1-3. 
FIG. 1 is a microphotograph of 600 magnifications, FIG. 2 a microphotograph 
of 10,000 magnifications, and FIG. 3 a microphotograph of 100,000 
magnifications. 
As is clear from the microphotographs, the fine crystal grains become 
visible more distinctly as the magnification is increased, making it 
evident that the magnetic recording powder according to the present 
invention is formed of fine crystal grains. 
Further prepared were specimens of a magnetic recording powder according to 
the present invention having the same chemical composition with that of 
Example No. 7 and in which hydrogen is occluded by the aforementioned 
introduction of hydrogen gas, specimens of a powder obtained by crushing 
an alloy ingot having the same chemical composition with that of Example 
No. 7, and specimens of a powder obtained by crushing a sintered magnet 
having the same chemical composition with that of Example No. 7, the 
specimens of each powder having different means particle sizes. The 
coercive force of the specimens was measured in relation to the mean 
particle size, the results of which are shown in FIG. 4. 
As is learned from FIG. 4, the magnetic recording powder according to the 
present invention maintains a required coercive force even if it is finely 
crushed to a means particle size of 0.5 .mu.m, whereas the powder obtained 
by crushing the alloy ingot and the powder obtained by crushing the 
sintered magnet show remarkably decreased coercive force values due to the 
crushing. More specifically, the powder obtained by crushing the alloy 
ingot has a coercive force almost equal to 0 KOe when its mean particle 
size is 30 .mu.m, while the powder obtained by crushing the sintered 
magnet has a coercive force equal to 0 KOe when its mean particle size is 
20 .mu.m. Therefore, they are not suitable for use as magnetic recording 
powders. The reason why the coercive force of the magnetic recording 
powder according to the present invention does not largely decrease even 
if the powder is crushed to 0.5-2 .mu.m presumably lies in the fact that 
its crystalline structure is formed of fine crystal grains. 
As a result, magnetic recording media such as magnetic tapes and magnetic 
cards formed from magnetic recording powders according to the present 
invention require only a small deal of energy to heat to a temperature 
immediately below the Curie point, which enables use of a compact and 
simple apparatus for heating the media to a temperature immediately below 
the Curie point. Thus, the invention can contribute to energy saving and 
is useful industrially.