Process for producing hexagonal-system ferrite powder

A novel hexagonal-system ferrite powder having surprisingly high coercive force as well as other excellent performance characteristics is disclosed. The ferrite powder is prepared by co-precipitating ferrous hydroxide and a metal carbonate in a liquid phase under specified conditions, followed by calcinating the resulting co-precipitated product under specified temperature conditions. The ferrite powder thus obtained has a good miscibility with a plastic material and is suitable for making a composite plastic-ferrite magnet.

The present invention relates to a hexagonal-system ferrite powder, a 
composite plastic-ferrite magnet comprising the same and processes for the 
production of them. 
In one aspect, the present invention relates to a hard-ferrite powder 
having excellent performance characteristics and a process for the 
production thereof. More particularly, the invention relates to a 
hexagonal-system ferrite powder having particularly improved coercive 
force and a process for the production thereof, said process comprising 
the two steps: the first step in which a co-precipitated product of 
ferrous hydroxide and a metal carbonate is formed in a liquid phase under 
specified conditions; and the second step in which the co-precipitated 
product thus obtained is subjected to calcination treatment under 
specified temperature conditions. 
Heretofore, a hexagonal-system ferrite powder has been prepared mostly by a 
dry process which depends on a solid phase reaction. For example, barium 
ferrite is usually prepared by the following process, although some 
processing conditions are different depending on whether isotropic or 
anisotropic magnet is wanted. Namely, in the first step iron oxide and 
barium carbonate are mixed with each other in accordance with the 
predetermined formulation. Then, the mixture is calcinated at a 
temperature in the range of 1,000.degree.-1,300.degree. C., followed by 
grinding the calcined product and molding the resulting powder to any of 
the desired shapes by a conventional molding method such as "pressure 
molding", "in-magnetic field molding" and the like before the molded 
material is sintered to obtain the end product. When ferrite is prepared 
by the process as mentioned above which is based on a solid phase 
reaction, the calcination temperature has to be at least 1,000.degree. C. 
or so to ensure the fair progress of ferrite-forming reaction. As a result 
of carrying out the calcination at such a high temperature, a sintered 
mass forms due to the agglomeration of molten grains and the individual 
grains grow into coarser size during the progress of calcination. All of 
these are considered to exert an adverse effect on the magnetic properties 
of the resulting ferrite. 
On the other hand, a wet process is also known, in which the production of 
ferrite is carried out in a liquid phase. However, the conventional wet 
process has a defect in that the co-precipitated product obtained in the 
liquid phase is too minute, and accordingly, the filtration thereof for 
the separation from liquid is very difficult. In addition, the 
precipitated product is generally accompanied by considerable amounts of 
alkalis and other contaminants. For these reasons, it is rather difficult 
to obtain the precipitated product having a composition which meets the 
predetermined molar proportions of respective components. Because of these 
defects, the commercialization of a wet process has not been realized yet. 
In addition, these defects may also be a bar to the production of a 
hexagonal-system ferrite powder having a high level coercive force which 
is one of the important characteristic properties of a hard ferrite. 
In the conventional dry process, the adjustment of grain size is often 
carried out by mechanical crushing, which generally gives impact strain to 
individual ferrite grains. On the other hand, in the conventional wet 
process, the grain size of the reaction product obtained is unacceptably 
fine. It is known that the coercive force of the produced ferrite is 
adversely affected by either of the matters mentioned above. Namely, the 
strength of coercive force of a ferrite powder largely depends on the 
grain size of the ferrite. For example, if a ferrite powder contains 
coarse grains 1.0 micron or greater in diameter, as is often the case with 
the conventional dry process, the coefficient of diamagnetic field 
increases and the remarkable reduction in magnetic value is observed. On 
the other hand, if the grain size is extremely fine, for example as fine 
as 0.2 micron or less, the grains exhibit "super-paramagnetism", which may 
constitute one important reason for the reduction of coercive force. It is 
generally considered that a hard ferrite powder having the maximum 
coercive force is obtained when the grain size is adjusted to be within 
the range of 0.5-1.0 micron. 
The present invention is to provide a novel process for the production of a 
hexagonal-system ferrite powder which is entirely different from any of 
the above mentioned conventional dry and wet processes. As will be 
explained in more detail later, a ferrite powder having the preferred 
range of grain size as mentioned above is easily obtained according to 
this process, which of course means that the production of a ferrite 
powder having high coercive force is ensured. 
The process of the present invention comprises two major steps: the first 
step in which a co-precipitated product is formed in a liquid phase under 
controlled conditions; and the second step in which the co-precipitated 
product thus obtained is subjected to heat treatment at a relatively low 
temperature to carry out the ferrite-forming reaction. The temperature in 
the second step at which said heat treatment is carried out is much lower 
than the temperature generally employed in the prior art dry process. This 
process enables the easy production of a hard ferrite powder having a 
proper range of grain size, free from strain due to crushing and having 
excellent coercive force. 
The outline of the process of the present invention can be explained as 
follows by the reaction formulas. 
##STR1## 
Namely, to carry out the reaction in the first step which is represented 
by the numerical identification (1) as given above, a solution of 
FeCl.sub.2 and BaCl.sub.2 containing equimolar Fe.sup.2+ and Ba.sup.2+ 
ions is added to a solution of equimolarly mixed alkalis such as, for 
example, a solution of NaOH or KOH and Na.sub.2 CO.sub.3. Then, air is 
bubbled into the mixed solution at a constant rate (preferably at a flow 
rate of 4.0-5.0 liters/min., providing the reaction is carried out in a 10 
liter-capacity beaker, i.e. air being blown at a rate of 29-36 
kg/m.sup.3.Hr), while the solution is agitated and kept at a pH in the 
range of 9-12, preferably 9-10 and a temperature in the range of 
50.degree.-80.degree. C., preferably 50.degree.-70.degree. C. throughout 
the reaction. By this, the oxidation of the precipitated product, 
particularly the oxidation of Fe.sup.2+ to Fe.sup.3+ is carried out. When 
part of the Fe.sup.2+ ions in the solution are oxidized to the Fe.sup.3+ 
ions, Fe(OH).sub.2 and Fe(OH).sub.3 react with each other to gradually 
form Fe.sub.3 O.sub.4. The Fe.sub.3 O.sub.4 thus formed in the solution 
co-precipitates with BaCO.sub.3 which is simultaneously formed in the 
solution, at a good efficiency. 
In a conventional wet process, the precipitated product thus formed in a 
liquid phase consists of extremely fine grains and accordingly the 
filtration thereof is almost impossible, and in addition, because of the 
concomittant precipitation of various coexisting ions and alkalis to be 
associated with said fine grains, the removal of such contaminants can not 
be done by the conventional washing. Thus, it is very difficult to obtain 
a co-precipitated product having a composition which satisfies the 
predetermined molar fractions. In contrast, if the co-precipitated product 
is formed in a liquid phase under controlled conditions according to the 
present invention, a product free from alkalis and other coexisting ions 
and consisting of proper and relatively coarse size grains such as in the 
range of 0.5-0.7 micron is easily obtained. The precipitate thus obtained 
has good filtering characteristics. If the process is carried out under 
the conditions more severely controlled with respect to the liquid 
temperature, the reaction time and the like, a co-precipitated product 
having a composition approximately equal to the predetermined formulation 
for obtaining hexagonal-system ferrite and consisting of grains having the 
proper grain size in the range of 0.2-1.0 micron can be obtained rather 
easily. The process of the present invention makes possible the formation 
in a liquid phase of a precipitate which meets the predetermined 
formulation, something considered very difficult heretofore, and also 
provides a co-precipitated product, from which co-precipitated ions and 
alkalis as contaminant can be removed rather easily only a washing several 
times with water. 
In the first step of the process of the present invention, the 
co-precipitated product having the characteristic properties as mentioned 
above is obtained. The product is further treated in the second step for 
the formation of almost complete hexagonal-system ferrite powder. Namely, 
in the second step, the co-precipitated product obtained in the first step 
is subjected to calcination treatment, which is carried out in a 
heat-treating furnace (2 inch-tube furnace) at a temperature in the range 
of 400.degree.-900.degree. C., preferably in the range of 
500.degree.-900.degree. C., or alternatively, with O.sub.2 gas being blown 
at a rate of 0.2-0.5 liter/min. (1.6-4.0 kg/m.sup.3.Hr), at a temperature 
in the range of 400.degree.-900.degree. C., preferably 
400.degree.-600.degree. C. and most preferably 500.degree.-550.degree. C. 
for 30 minutes to 3 hours, preferably for 30 minutes to 2 hours, to 
thereby carry out the ferrite-forming reaction. The heat-treating 
temperature employed in the second step according to the present invention 
is much lower than the calcinating temperature generally employed in the 
prior art dry process. However, since this heat treatment is carried out 
not singly but in combination with the first step mentioned above, almost 
complete hexagonal-system ferrite can be formed. When a sample of the 
ferrite thus obtained is identified by the X-ray diffractometry, a 
diffraction pattern typical of magnet plumbite is confirmed, which shows 
that the ferrite thus obtained has a good crystallizing in spite of the 
fact that the calcination is carried out at a temperature range much lower 
than that employed in the prior art dry process. The ferrite powder thus 
obtained had a coersive force (.sub.I Hc) in the range of 4800-5300 
oersteds (Oe), and a saturation magnetization in the range of 50.0-65.0 
emu/g. The average grain size of the powder was in the range of 0.5-0.9 
micron. Since the process of the present invention contains a liquid phase 
reaction in the first step, the reactivity of the powdered material to be 
calcined in the second step is remarkably high as compared with that 
obtained in the prior art dry process which depends on a solid phase 
reaction. For this reason, the ferrite-forming reaction proceeds 
satisfactorily within temperature range as low as 400.degree.-600.degree. 
C. Thus, the process is almost free from such defects as unacceptable 
growth of grains and the appearance of strains in the grains, both of 
which are closely related to the reduction in coercive force. The ferrite 
powder of the present invention thus obtained is available as a raw 
material for preparing a hard ferrite powder to be incorporated in a 
plastic material for obtaining a plastic-ferrite composite magnet. When 
used for such purpose, the ferrite powder exhibits improved magnetic 
properties as compared with the prior art product. It has a high coercive 
force and a good miscibility with a plastic material, which ensures the 
preparation of a composition containing high percentage of the ferrite 
powder. 
The following examples are given to illustrate but not to limit the present 
invention.

EXAMPLE 1 
A mixed solution consisting of 275 ml of an FeCl.sub.2 solution having a 
concentration of 1.0 mol/liter and 25 ml of a BaCl.sub.2 solution having a 
concentration of 1.0 mol/liter was added to a mixed alkali solution 
consisting of 550 ml of an NaOH solution having a concentration of 1.0 
mol/liter and 50 ml of an Na.sub.2 CO.sub.3 solution having a 
concentration of 1.0 mol/liter, and air was blown into the resulting 
mixture at a rate of 4.0-5.0 (29-36 kg/m.sup.3.Hr.) liters/min. to produce 
fine bubbles, while the solution was kept at a pH of 9-10 and at a 
temperature of 50.degree.-60.degree. C. for the purpose of oxidizing part 
of the Fe.sup.2+ ions of the Fe(OH).sub.2 precipitate formed in the 
solution into the Fe.sup.3+ ions to allow the formation of Fe.sub.3 
O.sub.4 precipitate in the same solution. The co-precipitated product of 
this Fe.sub.3 O.sub.4 containing the concomittantly precipitated 
BaCO.sub.3 was separated from the solution and was washed with water 
several times followed by filtering and drying. Then, the dried product 
was placed in an electric muffle furnace to be subjected to calcination 
treatment at a temperature of 550.degree. C. for 1 hour, while a stream of 
gaseous O.sub.2 was introduced into the furnace at a rate of 0.2-0.5 
liter/min. (1.6-4.0 kg/m.sup.3.Hr.). The powder thus obtained was examined 
by X-ray diffractometry and observed to exhibit the diffraction pattern of 
magnet plumbite. The saturation magnetization (.delta..sub.s) thereof 
determined by a magnetic balance was 60.5 emu/g, the coersive force 
(.sub.I Hc) of the same was 4700 Oe, the molar ratio of Fe.sub.2 O.sub.3 
to BaO (Fe.sub.2 O.sub.3 /BaO) determined by chemical analysis was 5.53 
and the average grain size of the powder was 0.5 micron. 
EXAMPLE 2 
A mixed solution consisting of 5.5 liters of an FeCl.sub.2 solution having 
a concentration of 1.0 mol/liter and 0.5 liter of an SrCl.sub.2 solution 
having a concentration of 1.0 mol/liter was added to the separately 
prepared mixed alkali solution consisting of 11 liters of NaOH solution 
having a concentration of 1.0 mol/liter and 1.0 liter of an Na.sub.2 
CO.sub.3 solution having a concentration of 1.0 mol/liter and air was 
blown into the resulting mixture at a rate of 4.5 liters/min. (33 
kg/m.sup.3.Hr.) to form fine bubbles, while the solution was kept at a pH 
of 10 and at a temperature of 70.degree. C. for the purpose of oxidizing 
part of the Fe.sup.2+ ions of the Fe(OH).sub.2 precipitate formed in the 
solution into the Fe.sup.3+ ions to allow the formation of a 
co-precipitated product of Fe.sub.3 O.sub.4 and SrCO.sub.3 in the same 
solution. The co-precipitated product thus obtained was separated from the 
solution and was washed with water several times, followed by filtering 
and drying. Then the dried product was placed in an electric muffle 
furnace to be calcined therein at a temperature of 600.degree. C. for 1 
hour, while a stream of gaseous O.sub.2 was introduced into the furnace at 
a rate of 0.2-0.5 liter/min. (1.6-4.0 kg/m.sup.3.Hr). The powder thus 
obtained was identified as SrO.6Fe.sub.2 O.sub.3 by X-ray diffractometry. 
Its coercive force was 5100 Oe and the saturation magnetization 
(.delta..sub.s) was 59.0 emu/g. 
EXAMPLE 3 
A mixed solution consisting of 5.5 liters of an FeCl.sub.2 solution having 
a concentration of 1.0 mol/liter, 0.375 liter of a BaCl.sub.2 solution 
having a concentration of 1.0 mol/liter and 0.125 liter of an SrCl.sub.2 
solution having a concentration of 1.0 mol/liter was added to the 
separately prepared mixed alkali solution consisting of 11 liters of an 
NaOH solution having a concentration of 1.0 mol/liter and 1.0 liter of an 
Na.sub.2 CO.sub.3 solution having a concentration of 1.0 mol/liter, and 
air was blown into the resulting mixture at a rate of 5.0 liters/min. (36 
kg/m.sup.3.Hr) to form fine bubbles, while the solution was kept at a pH 
of 11 and at a temperature of 80.degree. C. for the purpose of oxidizing 
part of the Fe.sup.2+ ions of the Fe(OH).sub.2 precipitate formed in the 
solution into the Fe.sup.3+ ions to allow the formation of a 
co-precipitated product of Fe.sub.3 O.sub.4, BaCO.sub.3 and SrCO.sub.3 in 
the same solution. The co-precipitated product thus obtained was separated 
from the solution and washed with water, followed by filtering and drying. 
Then the dried product was placed in an electric muffle furnace to be 
calcined therein at a temperature of 600.degree. C. for 1 hour, while a 
stream of gaseous O.sub.2 was introduced into the furnace at a rate of 
0.2-0.5 liter/min. (1.6-4.0 kg/m.sup.3.Hr). The powder thus obtained was 
identified by X-ray diffractometry as a solid solution of BaO.6Fe.sub.2 
O.sub.3 and SrO.6Fe.sub.2 O.sub.3. Its coercive force was 5500 Oe and the 
saturation magnetization (.delta..sub.s) thereof determined by a magnetic 
balance was 60.5 emu/g. The composition was determined by chemical 
analysis as being Ba.sub.0.75.Sr.sub.0.25 O.5.53Fe.sub.2 O.sub.3. 
EXAMPLE 4 
A mixed solution consisting of 5.5 liters of an FeCl.sub.2 solution having 
a concentration of 1.0 mol/liter, 0.45 liter of an SrCl.sub.2 solution 
having a concentration of 1.0 mol/liter and 0.05 liter of a CaCl.sub.2 
solution having a concentration of 1.0 mol/liter was added to the 
separately prepared mixed alkali solution consisting of 11.0 liters of an 
NaOH solution having a concentration of 1.0 mol/liter and 1.0 liter of an 
Na.sub.2 CO.sub.3 solution having a concentration of 1.0 mol/liter, and 
air was blown into the resulting mixture at a rate of 5.0 liters/min. (36 
kg/m.sup.3.Hr.) to form fine bubbles, while the solution was kept at a pH 
of 10.5 and at a temperature of 75.degree. C. After 4 hours, a 
co-precipitated product of Fe.sub.3 O.sub.4, SrCO.sub.3 and CaCO.sub.3 was 
obtained. Then, the co-precipitated product thus obtained was treated in 
the same manner as in Example 1. The powder eventually obtained was 
identified by X-ray diffractometry as the Ca-replaced solid solution of 
SrFe.sub.12 O.sub.19 and had a coercive force of 4900 Oe and a saturation 
magnetization (.delta..sub.s) of 61.5 emu/g. The chemical analysis showed 
the composition of Sr.sub.0.9 Ca.sub.0.1 O.5.54Fe.sub.2 O.sub.3. 
EXAMPLE 5 
A mixed solution consisting of 5.5 liters of an Fe(NO.sub.3).sub.2 solution 
having a concentration of 1.0 mol/liter, 0.255 liter of an 
Sr(NO.sub.3).sub.2 solution having a concentration of 1.0 mol/liter, 0.05 
liter of a Pb(NO.sub.3).sub.2 solution having a concentration of 1.0 
mol/liter and 0.9 liter of a Ba(NO.sub.3).sub.2 solution having a 
concentration of 0.25 mol/liter was added to the separately prepared mixed 
alkali solution consisting of 11.0 liters of an NaOH solution having a 
concentration of 1.0 mol/liter and 1.0 liter of an Na.sub.2 CO.sub.3 
solution having a concentration of 1.0 mol/liter, and air was blown into 
the resulting mixture at a rate of 5.0 liters/min. (36 kg/m.sup.3.Hr.) to 
form fine bubbles, while the solution was kept at a pH of 11.0 and a 
temperature of 70.degree. C. After 4 hours, a co-precipitated product of 
Fe.sub.3 O.sub.4, BaCO.sub.3, SrCO.sub. 3 and PbCO.sub.3 was obtained. The 
co-precipitated product thus obtained was treated in the same manner as in 
Example 1. The powder thus obtained was identified as the Sr, and 
Pb-replaced solid solution of BaFe.sub.12 O.sub.19 and had a coercive 
force of 4950 Oe, a saturation magnetization (.delta..sub.s) determined by 
the magnetic balance of 63.5 emu/g and a composition determined by 
chemical analysis of Ba.sub.0.45 Sr.sub.0.45 Pb.sub.0.1 O.5.52Fe.sub.2 
O.sub.3. 
The above-mentioned novel hexagonal-system ferrite powder of the present 
invention is particularly useful as a raw material for preparing a 
plastic-ferrite composite magnet. 
Generally, a plastic-ferrite composite magnet is prepared by kneading a 
thermoplastic resin with a ferrite powder. The ferrite powder herefore 
used for this purpose was in most cases one which was prepared by a dry 
process depending on a solid phase reaction. However, the ferrite powder 
prepared by the conventional dry process has various defects as already 
explained in detail hereinbefore. One important defect is that such prior 
art product does not have a satisfactory coercive force, which is one of 
the important characteristic properties of a plastics composite-magnet. In 
contrast, the novel hexagonal-system ferrite powder according to the 
present invention has a surprisingly high coercive force and a good 
miscibility with a thermoplastic resin and accordingly is very suitable 
for making a plasic-ferrite composite magnet. 
When a plastic-ferrite composite magnet is prepared from a hexagonal-system 
ferrite powder by mixing with any of the various plastic materials, the 
magnetic properties of the product are most strongly affected by the 
magnetic properties of the hard ferrite powder used as a raw material and 
its proportion in the resulting mixture. As regards the currently 
available plastic-ferrite composite magnet, the maximum proportion of a 
ferrite powder in the composite is considered to be in the range of 
85-90%. For this reason, the magnetic properties of such plastic-ferrite 
composite magnet are limited to such values as, for example, a maximum 
coercive force (.sub.I Hc) in the range of 2500-3000 Oe and a maximum 
surface magnetic flux density in the range of 700-850 G. The present 
inventors investigated ways to exceed such limits and developed a novel 
plastic-ferrite composite magnet having improved characteristic 
properties. Their investigation was directed to two goals: one was to 
improve the magnetic properties of the ferrite powder itself, and the 
other was to develop a raw material which has improved miscibility with 
various plastic materials. 
As already mentioned, the ferrite powder obtained by a conventional dry 
process has to be mechanically crushed in the later step to reduce the 
grain size to the order of several microns, and this causes remarkable 
impact strains in the grains. Because of these strains, the magnetic 
properties of the product, particularly its coercive force, are greatly 
decreased. For this reason, post-treatment such as annealing is often 
required. In addition, mechanical crushing is accompanied by other 
defects, such as unevenness in the shape and size of the grain. 
A wet process is also known in the art. The ferrite powder obtained in a 
liquid phase is free from the defects peculiar to the above mentioned dry 
process product, and it is known that the orientation (which is important 
in mixing with a plastic material) of the grains of the ferrite powder 
obtained by a wet process is remarkably superior to that obtained by a dry 
process. However, the ferrite powder obtained by the conventional wet 
process had a defect in that the grain size is too fine and its 
miscibility with plastic material is not as good as that of the powder 
obtained by a dry process. 
In contrast, the novel process of the present invention comprises a 
combination of a wet process and a dry process. The ferrite powder 
obtained by this process is free from the defects of both the conventional 
dry and wet processes, and is particularly suitable for use as a raw 
material for the production of a plastic-ferrite composite magnet. The 
ferrite powder has a superior coercive force and has good miscibility with 
various plastic materials. The process for preparing such ferrite powder 
is given in detail hereinbefore. 
The ferrite powder which is used for preparing a plastic-ferrite 
composite-magnet according to the present invention is prepared by a 
process comprising the first and second steps as explained in detail 
hereinbefore. 
The term "co-precipitated product obtained in a liquid phase" used in the 
specification and the attached claims means, without specific reference 
thereto, the co-precipitated product obtained in the first step of the 
process of the present invention. If the hexagonal-system ferrite powder 
obtained by carrying out both the first and the second steps is subjected 
to further calcination treatment in a heat-treating furnace at a 
temperature in the range of 700.degree.-900.degree. C. for 1-3 hours, the 
grain size of the hexagonal ferrite will grow until it reaches 1.0-7.0 
microns. This gives an improved ferrite powder having improved miscibility 
and a narrower distribution of grain size. The product thus obtained 
exhibits almost completely the diffraction pattern of magnet plumbite, 
thereby providing the satisfactory completion of the ferrite-forming 
reaction. The powdered product thus obtained can be reduced by a simple 
means into individual grains to be used as a raw material for ferrite 
product. 
Plastic materials which can be used in the practice of the present 
invention for preparing a plastics composite-magnet by mixing with ferrite 
powder include any of the known thermoplastic resins which can be molded 
by compression molding, extrusion molding or injection molding. 
The mixing ratio of the ferrite powder to a plastic material is arbitrary, 
depending on the intended purpose. However, as already explained, when 
ferrite powder prepared by the prior art process was used, the limit of 
ferrite powder in the mixture ranged from 85-90%. In contrast, when the 
novel ferrite powder according to the present invention is used, the 
proportion of ferrite powder in the mixture of the ferrite powder and a 
plastic material can range as high as from 92-95%. This is one very 
important advantage brought about by the present invention. The numerical 
limitation with respect to the mixing proportion given in the claim thus 
refers to the maximum and does not mean that a lower proportion cannot be 
employed. 
Kneading of the ferrite powder and a plastic material can be carried out 
with any of the known types of kneaders. The mixture of a plastic material 
and the ferrite powder of the present invention has a good fluidity. Thus, 
if such mixture is molded by, for example, extrusion molding, either a 
plunger type or screw type machines can be used. As regards molding 
conditions, a cylinder temperature in the range of 150.degree.-250.degree. 
C. and an extrusion pressure in the range of 500-1500 kg/cm.sup.2 can be 
employed conveniently. 
In one preferred embodiment of the present invention, a plastic-ferrite 
composite magnet of a barium-ferrite power and a methacrylate resin is 
made in the manner as mentioned below. First, as a raw material, a ferrite 
powder which has been prepared in a liquid phase by the manner mentioned 
above is used. Namely, the ferrite powder to be used as a raw material is 
one which has been prepared by the process comprising the steps of 
producing a co-precipitated product for preparing the hexagonal-system 
ferrite in a liquid phase, and calcining the resulting co-precipitated 
product at a temperature in the range of 400.degree.-600.degree. C., which 
is much lower than the temperature employed in a conventional dry process 
which depends on a solid phase reaction, or alternatively the 
hexagonal-system ferrite powder thus obtained is further calcined in a 
heat-treating furnace at a temperature in the range of 
700.degree.-900.degree. C. for 1-3 hours to obtain a product having an 
average grain size in the range of 1.0-7.0 microns and an improved 
miscibility. 
On the other hand, 0.1-0.5% by weight of sulfurous acid used as a catalyst 
is added to methyl methacrylate which is polymerized into a viscous syrup 
having a degree of polymerization in the range of about 10-30% by heat 
polymerization, and the resulting syrup is admixed with a ferrite powder 
to make a composite plastic-ferrite magnet. 
Based on the amount of the syrup thus obtained, 92% by weight or more of 
the ferrite powder was mixed therewith and the mixture was fully kneaded 
by a conventional kneading machine One example of the plastic-ferrite 
composite magnet thus prepared exhibits the following characteristic 
properties: a proportion of ferrite powder in the mixture in the range of 
92-95% by weight based on the amount of the mixture of the ferrite powder 
and the plastic material component, a coercive force (.sub.I Hc) in the 
range of 3200-3800 Oe, a surface magnetic flux density in the range of 
850-1200 G and a saturation magnetization (.delta..sub.s) in the range of 
50-65 emu/g. In contrast, one example of the prior art plastic-ferrite 
composite magnet having incorporated therein 90% by weight of a ferrite 
powder (maximum value) was observed to have a coercive force (.sub.I Hc) 
of 2800 Oe, a surface magnetic flux density of 750-800 G, and a saturation 
magnetization (.delta..sub.s) of 45-50 emu/g. Thus, obviously, the 
plastics composite-magnet of the present invention is substantially 
superior to the prior art product of a plastic-ferrite composite magnet. 
The novel ferrite powder prepared by the process including said first and 
second steps has improved magnetic properties including a coercive force 
(.sub.I Hc) of as high as 4800-5300 Oe and a saturation magnetization 
(.delta..sub.s) of as high as 50.0-65.0 emu/g. These extremely high 
numerical values were never achieved by prior art processes. Such 
plastic-ferrite composite-magnet containing at least 92% by weight of 
ferrite powder is an excellent plastic composite-magnet which could not 
exist heretofore. 
The following examples are given to illustrate but not to limit the present 
invention. 
EXAMPLE 6 
A co-precipitated product for making hexagonal-system ferrite was produced 
in a liquid phase, and the precipitate was then calcined at 450.degree. C. 
for 2 hours to provide BaO.6Fe.sub.2 O.sub.3 powder, and it was calcined 
again at 800.degree. C. for l hour to obtain a ferrite powder having a 
good miscibility with a plastic material. The ferrite powder thus obtained 
was disintegrated by a simple means before it was added to a syrupy 
polymer to admix therewith which had been prepared separately by heat 
polymerizing methyl methacrylate in the presence of 0.4% by weight of 
sulfurous acid as catalyst added thereto. The resulting mixture was molded 
by a screw extrusion molding machine into a plate 15 mm thick of a 
plastic-ferrite composite magnet. 
The proportion of the ferrite powder mixed with a plastic material was 93% 
by weight based on the amount of the plastic-ferrite composite magnet. 
Magnetic properties determined by measuring a test specimen taken out of 
the plastic plate mentioned above incuded a coercive force (.sub.I Hc) of 
3500 Oe, a surface magnetic flux density of 1050 G, and a saturation 
magnetization (.delta.s) of 55.5 emu/g. 
EXAMPLE 7 
A powder of SrO.6FeO.sub.3 prepared in a manner similar to that used in 
Example 6 was kneaded with a methacrylate resin as mentioned in Example 6 
to make therefrom a plastic composite-magnet plate 15 mm thick. The 
proportion of ferrite powder mixed with the plastic material was 92% by 
weight based on the amount of the plastic-ferrite compsite magnet. A test 
specimen was taken out of the plastic plate and the magnetic properties 
thereof were measured. The coercive force was 4100 Oe, the surface 
magnetic flux density was 1000 G and the saturation magnetization 
(.delta..sub.s) was 53.8 emu/g. 
EXAMPLE 8 
A powder of Ba.sub.0.75.Sr.sub.0.25 0.6Fe.sub.2 O.sub.3 prepared in a 
mannersimilar to that used Example 6 was kneaded with a methacrylate resin 
as in Example 6 and a plastic-ferrite compsite magnet. plate 15 mm thick 
was made therefrom. The proportion of the ferrite powder mixed with a 
plastic material was 93% by weight based on the amount of the 
plastic-ferrite composite magnet A test specimen taken out of the 
resulting magnet plate exhibiting magnetic properties including a coercive 
force (.sub.I Hc) of 4600 Oe, a surface magnetic flux density of 1110 G 
and a saturation magnetization (.delta..sub.s) of 56.1 emu/g. 
EXAMPLE 9 
A powder of Ba.sub.0.45 Sr.sub.0.45 Pb.sub.0.10 0.6Fe.sub.2 O.sub.3 
prepared in a manner similar to that used in Example 6 was kneaded with a 
methacrylate resin as mentioned and a similr plastic composite magnet 
plate was prepared therefrom. The proportion of the ferrite powder based 
on the amount of plastic-ferrite composite magnet 93%. Magnetic 
properties, determined in similar manner included a coercive force of 4200 
Oe a surface magnetic flux density of 1050 G and 58.5 emu and a saturation 
magnetization (.delta..sub.s) of 58.5 emu/g.