Permanent Mn-Al-C alloy magnets and method for making same

A billet made of a polycrystalline Mn-Al-C alloy magnet which is obtained by plastically deforming a Mn-Al-C alloy for magnet such as by extrusion at a temperature of 530.degree. to 830.degree. C. is used for compressive working. When the billet is hollow, it is entirely or locally compressed along the axis of the hollow billet. On the other hand, when the billet is solid, an outer circumferential portion of the billet is compressed. By the compression, the anisotropic structure of the portion where compressed is changed into an anisotropic structure having a direction of easy magnetization in radial directions. The magnet obtained by the method is also disclosed. The magnet has a radially anisotropic structure or novel structures having two different types of anisotropies therein.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
This invention relates to permanent magnets and more particularly, to 
polycrystalline Mn-Al-C ally magnets of high performance suitable for use 
in multipolar magnetization. Also, it relates to a method for making the 
magnets of the just-mentioned type. 
2. Description of the Prior Art 
Mn-Al-C alloy magnets have mainly the ferromagnetic face-centered 
tetragonal phase structure (.tau. phase L1o type superstructure) and 
comprises carbon as their essential component element. The Mn-Al-C alloy 
magnets include those magnets of the ternary alloys free of any additive 
elements except for inevitable impurities and quaternary or multicomponent 
alloys which contain small amounts of additive elements. By the term 
"Mn-Al-C alloy magnet" used herein are meant magnets of the alloys 
including quaternary or multicomponent alloys as well as the ternary 
alloys. 
Known methods of making Mn-Al-C alloy magnets include, aside from those 
methods using casting and heat treatments, a method which comprises a warm 
plastic working process such as warm extrusion. The latter method is known 
as a method of making an anisotropic magnet which has excellent properties 
such as high magnetic characteristics, mechanical strength and 
machinability. 
On the other hand, Mn-Al-C alloy magnets for multipolar magnetization can 
be made by several methods including a method using isotropic magnets or 
compressive working, and a method in which a uniaxially anisotropic 
polycrystalline Mn-Al-C alloy magnet obtained by a known technique such as 
warm extrusion is subjected to warm free compressive working in a 
direction of easy magnetization, i.e. a compound working method. 
However, the compressive working method involves the drawbacks that 
although high magnetic characteristics are obtained in radial directions, 
a relatively high reduction rate is necessary, non-uniform deformation may 
take place, and occurrence of a dead zone is unavoidable. According to the 
compound working method, there can be obtained magnets which exhibit high 
magnetic characteristics in all the directions within a plane including 
radial and tangential directions in small compressive strains. The magnets 
obtained by the compound working method have such a structure that the 
direction of easy magnetization is parallel to a specific plane, and they 
are magnetically isotropic within the plane and are anisotropic within a 
plane including a perpendicular with respect to the first-mentioned plane 
and a straight line parallel to the first-mentioned plane. These magnets 
are hereinafter referred to as plane-anisotropic permanent magnet. 
Magnets for multipolar magnetization are generally in the form of a hollow 
cylinder and are magnetized as particularly shown in FIGS. 1 through 3 in 
which magnetic paths are indicated by broken lines. FIG. 1 is a schematic 
diagram of magnetic paths in a magnet body in case where a hollow 
cylindrical magnet undergoes multipolar magnetization in radial 
directions. FIG. 2 shows a case where a hollow cylindrical magnet is 
multipolarly magnetized around the outer circumferential surface and FIG. 
3 shows a case of multipolar magnetization around the inner 
circumferential surface of a cylindrical magnet. The magnetization shown 
in FIG. 1 is called radial magnetization throughout the specification. 
Similarly, those magnetizations shown in FIGS. 2 and 3 are called outer 
lateral or circumferential magnetization and inner lateral or 
circumferential magnetization. In FIG. 2, radial directions are indicated 
by r and a tangential direction with respect to one radial direction is 
indicated by .theta.. 
As shown in FIG. 1, with the radial magnetization, the magnetic paths 
substantially run along the radial directions and thus the structure of 
the above-mentioned plane-anisotropic permanent magnet may not necessarily 
be proper. On the other hand, according to the compressive working 
technique, high magnetic characteristics along radial directions can be 
obtained. However, as described before, this working technique involves 
the problems that a relatively high reduction rate is required, 
non-uniform deformation may occur and occurrence of a dead zone is 
unavoidable. 
Plane-anisotropic permanent magnets are magnets of versatile utility which 
exhibit excellent magnetic characteristics when magnetized in the manners 
shown in FIGS. 1 through 3. In this connection, however, if consideration 
is given, for example, to the outer circumferential magnetization, the 
plane-anisotropic permanent magnet has not necessarily a favorable 
anisotropic structure at its outer or inner circumferential portion. With 
regard to the outer circumferential portion of a magnet body, it should 
favorably have higher magnetic charcteristics in radial directions than in 
tangential directions. On the other hand, so far as an inner 
circumferential portion is concerned, an anisotropic structure having 
higher magnetic characteristics in tangential directions than in radial 
directions is more suitable for outer circumferential magnetization. It 
will be noted that the outer circumferential portion of a magnet body 
means a portion where magnetic paths run substantially along radial 
directions and the inner circumferential portion means a portion where the 
magnetic paths run substantially along tangential directions, as 
particularly seen in FIG. 2. 
SUMMARY OF THE INVENTION 
It is accordingly an object of the present invention to provide a method 
for making Mn-Al-C alloy magnets having different types of anisotropic 
structures suitable for multipolar magnetization. 
It is another object of the invention to provide a method for making 
anisotropic Mn-Al-C alloy magnets suitable for multipolar magnetization by 
compressing billets made of polycrystalline Mn-Al-C alloy magnets along 
the axis of the billets so that the billets are plastically deformed 
partially or entirely in section of the billet. 
It is a further object of the invention to provide anisotropic Mn-Al-C 
alloy magnets suitable for multipolar magnetization obtained by the 
just-mentioned method. 
The above objects can be achieved, according to the present invention, by a 
method which comprises providing a billet made of a polycrystalline 
Mn-Al-C alloy magnet which is rendered anisotropic and subjecting the 
billet to compressive working along the axis of the billet at a 
temperature ranging from 530.degree. C. to 830.degree. C. so that the 
billet is plastically deformed uniformly in radial directions. 
The billet is preferably in the form of a cylinder which may be hollow or 
solid. The cylindrical billet may be compressed entirely or locally along 
the inner or outer circumferential portion of the cylinder. 
Polycrystalline Mn-Al-C alloy magnets which are rendered anisotropic can 
be obtained by subjecting known Mn-Al-C alloys for magnet to known hot 
plastic working at temperatures ranging from 530.degree. to 830.degree. C. 
It will be noted that Mn-Al-C alloys usable in the practice of the 
invention include Mn-Al-C ternary alloys which are free of any additive 
elements and quaternary or multi-component alloys which contain, aside 
from Mn, Al and C, small amounts of additive elements such as Ni, Ti and 
the like. 
Further, magnets may be compressed at the inner or outer circumferential 
portion thereof to impart an anisotropic structure different from the 
non-compressed portion. These magnets have two different types of 
anisotropies in the body which are hitherto unknown.

DETAILED DESCRIPTION AND EMBODIMENTS OF THE INVENTION 
Known anisotropic magnets can be classified into three groups including a 
uniaxially anisotropic magnet which has high magnetic characteristics in 
one direction, a radially anisotropic magnet used in the field of 
multipolar magnetization, and the afore-mentioned plane-anisotropic 
magnet. The above three types of anisotropic structures are illustrated 
using a hollow cylindrical magnet. With uniaxially anisotropic magnets, a 
hollow cylindrical magnet has a direction of easy magnetization along its 
axis in which the direction of easy magnetization is parallel to the axis 
of the cylinder in any portions in the magnet. 
Radially anisotropic magnets have the direction of easy magnetization 
parallel to radial directions of the cylinder in which the direction of 
easy magnetization is parallel to a radius of the hollow cylinder in any 
portions of the magnet. 
Plane-anisotropic magnets have the direction of easy magnetization parallel 
to a plane vertical with respect to the axis of a hollow cylindrical 
magnet. The direction is not subject to preferred orientation in one 
direction within the plane, so that the magnet is magnetically isotropic 
within the plane. Any portions within the magnet have such a structure as 
described above. 
Multipolar magnetization can broadly be divided into three groups as 
particularly shown in FIGS. 1 through 3. A suitable anisotropic structure 
depends on the type of multipolar magnetization. For multipolar 
magnetization in radial directions shown in FIG. 1, magnets should 
preferably have a radially anisotropic structure. For the multipolar 
magnetization along the outer circumference shown in FIG. 2, the following 
three combinations were found to be suitable. 
(1) The outer circumferential portion of a cylindrical magnet is radially 
anisotropic and the inner circumferential portion is tangentially 
anisotropic. 
(2) The outer circumferential portion is radially anisotropic and the inner 
circumferential portion is plane-anisotropic. 
(3) The outer circumferential portion is plane-anisotropic and the inner 
circumferential portion is tangentially anisotropic. 
With the inner circumferential magnetization shown in FIG. 3, three 
combinations of anisotropic structures are considered suitable similar to 
the case of the outer circumferential magnetization but the anisotropic 
structures at the outer and inner circumferential portions are reversed. 
In FIG. 3, for example, the outer circumferential portion means a portion 
in which magnetic paths are formed substantially along tangential 
directions. On the other hand, the inner circumferential portion means a 
portion in which magnetic paths run substantially along radial directions. 
According to one aspect of the invention, there is obtained a radially 
anisotropic permanent magnet by a method which comprises subjecting a 
hollow cylindrical billet of a polycrystalline Mn-Al-C alloy magnet, which 
is rendered anisotropic, to compressive working along the axis of the 
billet at a temperature of 530.degree. to 830.degree. C. so that the 
billet is plastically deformed uniformly in radial directions. This magnet 
is suitably used for multipolar magnetization in radial directions. 
According to another aspect of the invention, there are obtained permanent 
magnets which have novel anisotropic structures as will not be experienced 
in hitherto known anisotropic magnets. 
Three novel types of anisotropic structures, for example, suitable for 
outer circumferential magnetization shown in FIG. 2 are illustrated using 
a magnet of a cylindrical form. 
That is, a permanent magnet suitable for the outer circumferential 
magnetization obtained in accordance with the present invention has not 
the same anisotropic structure throughout the magnet. For instance, the 
magnet has broadly two portions, i.e. outer and inner circumferential 
portions a, b, and includes two types of anisotropic structures in one 
magnet. 
In other words, the outer circumferential portion is rendered radially 
anisotropic or plane-anisotropic and the inner circumferential portion is 
rendered tangentially anisotropic or plane-anisotropic provided that both 
the outer and inner portions are not plane-anisotropic at the same time. 
As described before, this type of magnet can be divided into three classes 
including: a first class in which the outer circumferential portion of the 
magnet is radially anisotropic and the inner circumferential portion is 
tangentially anisotropic; a second class in which the outer 
circumferential portion is radially anisotropic and the inner 
circumferential portion is plane-anisotropic; and a third class in which 
the outer circumferential portion is plane-anisotropic and the inner 
circumferential portion is tangentially anisotropic. By the term 
"tangentially anisotropic" (i.e. anisotropy in .theta. direction) is meant 
an anisotropic structure similar to the radial anisotropy, in which when a 
magnet is in the form of a hollow cylinder, directions of easy 
magnetization are parallel to tangential directions (i.e. .theta. 
directions) of the cylinder in any portions within the magnet. In other 
words, the magnet is easily magnetized along tangential directions of the 
circumference. 
It was described before that two different types of anisotropic structures 
exist in one magnet. This may be considered as follows: at least one of 
the outer and inner circumferential portions on the magnetically isotropic 
plane of a plane-anisotropic magnet has a magnetically anisotropic 
structure provided that the magnet is rendered radially anisotropic at the 
outer portion or is rendered tangentially anisotropic at the inner 
portion. For instance, with the first class, the outer circumferential 
portion is radially anisotropic and the inner circumferential portion is 
tangentially anisotropic. 
When this type of magnet is subjected to the multipolar magnetization along 
the outer circumference as shown in FIG. 2, it exhibits more excellent 
magnetic characteristics than in the case of a mere plane-anisotropic 
magnet. This is considered as follow. The permanent magnet of the present 
invention having two different anisotropic structures therein has a 
structure whose [001] axes are arranged along magnetic paths in view of 
how the magnetic paths are formed in case where the multipolar 
magnetization is effected along the outer circumference as shown in FIG. 
2. In this sense, plane anisotropy permits [001] axes to be equally 
arranged in directions different from the directions of the magnetic paths 
and may thus be considered to be a wasteful anisotropic structure. 
In general, a preferred orientation of crystals in polycrystalline body is 
expressed by pole density P. The phase is tetragonal and the orientation 
of [001] axes can be taken as a distribution of (001) pole density. The 
(001) pole density in a given direction of polycrystalline body is 
determined as a ratio of an integral intensity of (00 n) plane diffraction 
of the body to an integral intensity for isotropic body in case where the 
normal direction of X-ray diffraction is caused to coincide with the given 
direction. With isotropic magnets, the pole density in all 
three-dimensional directions is 1. 
The permanent magnets obtained by the method of the invention have a pole 
density greater than 1 (P&gt;1) in a specific direction parallel to a 
specific plane within the magnet and P.ltoreq.1 in a perpendicular 
direction of the plane. 
With the first class, when the "within magnet" is assumed as the outer 
circumferential portion of a magnet, the specific direction is a radial 
direction (r direction). If the "within magnet" is considered as the inner 
circumferential portion, the specific direction is a tangential direction. 
For the second class, if the "within magnet" is taken as the outer 
circumferential portion of a magnet, the specific direction is a radial 
direction (r direction) similar to the first class and when taking as the 
inner portion of a magnet, the specific direction is an arbitrary 
direction. With the third class, the specific direction is an arbitrary 
direction when the "within magnet" means the outer circumferential portion 
of a magnet and is a tangential direction (.theta. direction) for the 
inner portion. 
All permanent magnets made by us according to the invention had a 
difference in (001) pole density between a specific direction and a normal 
direction over 3:1. When the direction parallel to the plane is an 
arbitrary direction, a change of the (001) pole density is less than about 
10%, which is within an ordinary accuracy in X-ray diffraction intensity 
measurements. If the direction is a specific direction, a ratio to a 
direction vertical to the specific direction exceeds 1.1:1. Larger ratios 
are more advantageous from the standpoint of magnetic characteristics. 
The permament magnets suitable for outer circumferential magnetization of 
the invention are considered as follows: plane-anisotropic magnets are 
subjected to preferred orientation in a specific direction at an outer 
and/or inner circumferential portion thereof within a plane of the 
plane-anisotropic magnet where [001] axes are equally arranged. From the 
standpoint of magnetic characteristics, it is a matter of choice as to 
whether the outer circumferential portion is rendered anisotropic radially 
or in r directions or the inner circumferential portion is rendered 
anisotropic tangentially or in .theta. directions. With the permanent 
magnets made by us, a ratio in residual magnetic flux density of a 
radially anisotropic magnet and a tangentially anisotropic magnet was 
found to exceed 1.1:1. 
The anisotropic structures have been described in detail with regard to 
magnets suitable for outer circumferential magnetization. The three 
anisotropic structures suitable for inner circumferential magnetization 
are the same as those for outer circumferential magnetization except that 
the outer and inner portions are reversed with respect to the anisotropic 
structures. 
Radially anisotropic magnets suitable for radial magnetization are 
obtained, according to the invention, only in small compressive strains 
imparted thereto without involving occurrence of non-uniform deformation 
and of dead zone. 
Broadly, the present invention provides a method in which a cylindrical 
billet made of a polycrystalline Mn-Al-C alloy magnet which is rendered 
anisotropic is subjected to compressive working along the axis of the 
cylindrical billet at a temperature ranging from 530.degree. to 
830.degree. C. so that the billet is plastically deformed uniformly in 
radial directions. As a result, a portion where compressed is converted 
from an initial anisotropic structure into an anisotropic structure having 
a direction of easy magnetization along radial directions, i.e. a radially 
anisotropic structure. The compressed portion may be an entire portion of 
the cylindrical billet or may be a circumferential portion of the billet. 
In the latter case, the cylindrical billet may be either hollow or solid 
whereas the cylindrical billet is hollow in the former case in the 
practice of the invention. 
The polycrystalline Mn-Al-C alloy magnets which are rendered anisotropic 
can be obtained by subjecting known Mn-Al-C alloys for magnets to known 
warm plastic deformation. 
By the compressive working in an axial direction of the billet, the 
compressed portion undergoes plastic deformation in radial directions. 
That is, the compressed portion is plastically deformed in radial 
directions and is thus rendered radially anisotropic. 
According to one embodiment of the invention, the cylindrical billet is 
entirely compressed or plastically deformed entirely with respect its 
section. In this case, the billet should be hollow. The billet after 
completion of the compressive working is a radially anisotropic magnet. 
Much higher magnetic characteristics in radial directions can be obtained 
in very small compressive strains than those attained by any known 
compressive working techniques. 
According to another embodiment of the invention, the cylindrical billet is 
compressed locally along its circumference and a portion where compressed 
is changed into an anisotropic structure having a direction of easy 
magnetization in radial directions. In this case, the billet may be either 
hollow or solid. Portions which undergo no compressive working have an 
initial anisotropic structure prior to the compressive working. 
For instance, where a billet prior to compressive working is a 
plane-anisotropic magnet and is intended to be magnetized along the inner 
circumference as shown in FIG. 3, only the inner circumferential portion 
where magnetic paths run almost along radial directions should be 
subjected to compressive working. By this, the portion is rendered more 
radially anisotropic, thereby improving the surface magnetic flux density 
when magnetized along the inner circumference. The billet obtained after 
the compressive working has two structures, i.e. radially anisotropic and 
plane-anisotropic structures. 
Alternatively, when a billet prior to compressive working is uniaxially 
anisotropic and is used for outer circumferential magnetization as shown 
in FIG. 2, the inner circumferential portion of the billet (where no 
magnetic paths run) is left uniaxially anisotropic. The resulting magnet 
is useful in detection of revolutions such as of motors. 
Still alternatively, when a billet prior to compressive working is 
tangentially anisotropic and is used for inner circumferential 
magnetization as shown in FIG. 3, only the inner circumferential portion 
where magnetic paths run approximately radially are compressed. The 
compressed portion is rendered radially anisotropic. Thus, there can be 
obtained a magnet which has the tangentailly anisotropic portion and the 
radially anisotropic portion and is suitable for inner circumferential 
magnetization. 
It will be noted that whether a compressed or plastically deformed portion 
is entire or local should be determined depending on whether or not the 
entire section of billet is compressed or plastically deformed. 
The manner of compressing or plastically deforming an entirety of a hollow 
cylindrical billet is described. 
According to one embodiment of the invention, a hollow cylindrical billet 
which is made of a polycrystallinee Mn-Al-C alloy magnet rendered 
anisotropic is axially compressed at a temperature of 530.degree. to 
830.degree. C. in such a state that the outer circumferential surface of 
the billet is held restrained while leaving at least a part of the inner 
circumferential surface free or non-restrictive. 
Polycrystalline Mn-Al-C alloy magnets which are rendered anisotropic can be 
obtained by subjecting to plastic working such as extrusion at a 
temperature of 530.degree. to 830.degree. C. known Mn-Al-C alloys for 
magnets which are composed, for example, of 68 to 73 wt% of Mn, (1/10 
Mn-6.6) to (1/3Mn-22.2) wt% of C and the balance of Al. Typical of the 
just-mentioned magnets are a uniaxially anisotropic magnet which is 
obtained by extrusion used as the plastic working and has a direction of 
easy magnetization along the extrusion direction, and the afore-described 
plane-anisotropic and tangentially anisotropic magnets. The anisotropic 
polycrystalline Mn-Al-C alloy magnet is shaped into a hollow billet. This 
billet is subjected to compressive working along the axis thereof in such 
a state that the billet is held restrained at the outer circumference 
thereof and at least a part of the inner circumference is left free 
thereby permitting the free portion to be plastically deformed inwardly 
and radially. The resulting magnet has high magnetic characteristics in 
the radial directions. When the hollow billet in which at least a part of 
the inner surface is set free is compressed in the axial direction while 
restraining the billet at the outer surface, the at least a part is 
plastically deformed inwardly and radially so that the cavity portion is 
reduced in sectional area. The compression strain in the axial direction 
may be imparted inwardly radially until no cavity is present. In this 
case, the billet is substantially solid after the compression working. As 
a matter of course, after a predetermined degree of compressive strain has 
once been imparted to the hollow billet, the inner circumference may be 
restrained such as by insertion of a die into the hollow billet in order 
to shape the billet along the inner circumference. 
It will be noted that the anisotropy of a magnet billet may vary depending 
on the degree of compressive working, e.g. when a tangentially anisotropic 
polycrystalline Mn-Al-C alloy magnet is axially compressed, its anisotropy 
changes to radial anisotropy through plane-anisotropy. Accordingly, proper 
control to the compressive working on a portion of the tangentially 
anisotropic magnet along its axis may result in a magnet having a 
tangentially anisotropic portion and a plane-anisotropic portion. 
When a billet is made of a polycrystalline Mn-Al-C alloy magnet having a 
direction of easy magnetization along its axis (i.e. uniaxially 
anisotropic magnet), the compressive strain should be 0.05 or more as 
expressed by an absolute value of logarithmic strain. As described in 
detail in examples, this is because a billet prior to plastic working is 
rendered anisotropic in a direction along which compressive strain is 
imparted and thus a compressive strain of at least 0.05 is necessary for 
changing the billet into a structure showing high magnetic characteristics 
in radial directions. 
A prior art technique is known in which a uniaxially anisotropic square 
pillar magnet is subjected to warm compressive working in axial 
directions. This is intended to change the direction of easy magnetization 
from one direction to another direction vertical to the one direction. 
Accordingly, the square pillar magnet still remains uniaxially anisotropic 
even after the compressive working. In addition, the change of the 
direction of easy magnetization in another direction by the prior art 
technique needs a working rate of over about 60 to 70% which corresponds 
to a value as high as about 0.9 to 1.2 calculated as an absolute value of 
logarithmic strain. 
Where a billet is made of a plane-anisotropic magnet, it exhibits, prior to 
plastic working, high magnetic characteristics in all directions within a 
plane including radial and tangential directions. When the billet is 
compressively worked along its axis while restraining the outer surface 
and setting free at least a part of the inner surface along the 
circumference of the hollow billet, it is plastically deformed at the free 
portion inwardly and radially. By this, the resulting magnet exhibits high 
magnetic characteristics in radial directions. 
The compressive working is not necessarily needed to be effected 
continuously but may be carried out as separated in several times. A 
billet which has once compressively worked may be subjected at a portion 
thereof to further compressive working along its axis. The further 
compressed portion will have higher magnetic characteristics in radial 
directions. This further compressive working may be effected in several 
times, not continuously. 
An example of the compressive working is illustrated using a billet of the 
cylindrical with reference to FIGS. 4(a) and 4(b). It will be noted here 
that like parts are designated by like reference numerals throughout the 
specification. 
In FIG. 4(a) there is shown part of a die D which includes a ring or 
outside die 1 and a pair of punches 2, 3. In the cavity of the ring die 1 
is placed a hollow cylindrical billet 4 prior to compressive working. As 
shown, the billet 4 is restrained at the outer circumferential surface 
thereof by means of the ring die 1 by which the billet suffers little or 
no change in outer diameter prior to and after the working. As a matter of 
course, in order to allow the billet to be readily inserted into the 
cavity, a suitable clearance between the billet 4 and the ring die 1 may 
be permitted. The billet 4 is not restrained at the inner circumferential 
surface thereof by the die and can be plastically deformed inwardly and 
radially. After the billet 4 has once been compressed and imparted with a 
predetermined degree of compressive strain, a core (not shown) may be 
inserted into the hollow cylinder of the billet so as to shape the inner 
circumference of the billet. In order to effect the compressive working, 
it is sufficient that at least a part of the inner surface is set free 
preferably along the axis thereof, by which the free portion can 
plastically uniformly deformed inward and radial directions. As described 
before, when the hollow cylindrical billet is made of a polycrystalline 
Mn-Al-C alloy magnet having the direction of easy magnetization along its 
axis, it should be compressed to a level of compressive strain of 0.05 or 
higher as expressed by an absolute value of logarithmic strain. If it is 
intended to leave a circumferential portion of the billet uniaxially 
anisotropic, the inner surface of the circumferential portion is 
restrained such as by insertion of a core. In this state, the billet is 
compressed thereby leaving the restrained inner portion free of any 
compressive strains produced in the axial direction. 
When the billet is axially compressed by the use of the punches 2, 3, it is 
plastically deformed inwardly and radially in a uniform manner as 
particularly shown in FIG. 4(b). 
According to another embodiment of the invention, the hollow cylindrical 
billet is subjected to free compressive working in an axial direction 
thereof at a temperature of 530.degree. to 830.degree. C. This is, the 
compressive working is effected in a state that at least parts of both the 
inner and outer circumferences are set free or in a non-restrained state. 
In this case, when the billet is made of an uniaxially anisotropic magnet, 
the compressive working is effected so that a compressive strain is 0.05 
or higher as expressed by an absolute value of logarithmic strain for the 
reason described with respect to the first embodiment. 
This free compressive working is particularly illustrated with reference to 
FIGS. 5(a) and 5(b). The hollow cylindrical billet 4 is is placed in a 
cavity C in such a state that it can be plastically deformed in radial 
directions both inwardly and outwardly. That is, inner and outer 
circumferential surfaces are set free without being restrained by dies. In 
this state, when the billet is freely compressed by means of the punches 2 
and 3, the radius of the billet increases until the outer surface of the 
billet comes into contact with the inner wall of the ring die 4 as shown 
in FIG. 5(b). In FIGS. 5(a) and 5(b), the free compressive working is 
effected while setting all the outer and inner circumferential surfaces 
free. If it is desirable that part of a final magnet has an anisotropy or 
a direction of easy magnetization prior to the plastic working of the 
invention, the part of the billet should be restrained at the outer and 
inner circumferences so that it undergoes no axially compressive strain. 
The freely compressed billet of FIG. 5(b) may be further compressed while 
restraining the outer periphery thereof as shown in FIG. 5(c). The 
resulting magnet exhibits higher magnetic characteristics in radial 
directions than the magnet of FIG. 5(b). In the case of FIG. 5(c), an 
amount of compressive strain should be determined as follows. When an 
amount of compressive strain produced on the free compressive working is 
given as .xi.zf and an amount of compressive strain produced under 
restraining conditions along the outer circumference is given as .xi.zr, 
the sum of .xi.zf and .xi.zr should be 0.05 or higher. 
By the procedures of the two embodiments described above, radially 
anisotropic magnets can be obtained. Upon comparing the magnets obtained 
by these two embodiments, magnetic characteristics in axial direction at a 
given level of compressive stain are higher in the magnets obtained by the 
first embodiment. In order to make a longer radially anisotropic magnet, 
the first embodiment is preferable. By the term "longer magnet" is meat a 
magnet of a larger ratio of h/D.sub.o in which when the magnet is 
cylindrical, D.sub.o represents an outer diameter and h represents a 
length. The procedure of the second embodiment is better than the 
procedure of the first embodiment with respect to readiness to working. 
This is because free compressive working is used in the second embodiment. 
It should be noted that the difference between the second embodiment using 
uniaxially anisotropic magnets and a known method of making 
plane-anisotropic magnets resides in the shape of billet used. If a billet 
used is a solid body such as, for example, a solid cylinder and is 
subjected to free compressive working along the axis of the solid body, a 
plane-anisotropic magnet can be obtained. On the other hand, when a billet 
which is a hollow body such as, for example, a hollow cylinder and is 
subjected to free compressive working along the axis of the hollow body, a 
radially anisotropic magnet can be obtained. That is, with hollow billets, 
the free compressive working proceeds while plastically deforming the 
billet in radial and inward directions thereby reducing the capacity of 
the cavity in the hollow billet. Accordingly, the magnet obtained after 
the compressive working working has not a plane-anisotropic structure but 
a radially anisotropic structure. 
A third embodiment of the invention is described in which a billet used is 
a hollow or solid body and is compressed in an axial direction so that a 
circumferential portion thereof is plastically deformed uniformly in 
radial directions. The circumferential portion may be either an outer or 
inner portion of the billet. 
The compressive working on a portion of a billet is particularly described 
with reference FIG. 6(a) and 6(b). In FIG. 6(a), the die D includes an 
under die 5, a fixed punch 6 having a core 6' through which a hollow 
billet 4 is set, and a movable working punch 7. The billet 4 is fixed and 
restrained using the fixed punch 6 and the under die 5. The fixed punch 6 
is so designed that an upper side of the billet 4 is partially covered or 
protected therewith as shown. When the working die 7 is moved downwards, 
an outer circumferential portion of the billet 4 is compressed along its 
axis and plastically deformed outwardly in radial directions as shown in 
FIG. 6(b). 
In order to plastically deform the hollow cylindrical billet at the inner 
circumferential portion thereof, another type of die is used as shown in 
FIGS. 7(a) and 7(b). In the figures, the hollow billet 4 is fixed and 
restrained entirely at the outer surface thereof while leaving the inner 
surface non-restrictive. The punch 2 partially contacts with the billet 4 
at the inner circumferential portion. When the punch 2 is moved downwards, 
the circumferential portion of the billet 4 is compressed along its axis 
as shown in FIG. 7(b). 
By these compressive workings, there can be obtained permanent magnets 
having such anisotropic structures suitable for the afore-described outer 
or inner circumferential magnetization. 
For example, when a tangentially anisotropic magnet is used as the billet, 
the compressive working at the outer or inner circumferential portion 
results in a magnet having directions of easy magnetization along its 
radius. That is, the magnet has the radially anisotropic portion which 
undergoes the compressive working and the original tangentially 
anisotropic portion. 
The procedures of the third embodiment may be applied to a billet which has 
once been compressed in an axial direction as shown in FIGS. 5(b) and 
5(c). When the magnets of FIGS. 5(b) and 5(c) are further compressed in an 
axial direction as shown in FIGS. 6(a) and 6(b) or 7(a) and (7b), the 
inner or outer circumferential portion of the magnet can be plastically 
deformed radially and have higher magnetic characteristics in the axial 
direction than the noncompressed portion. 
In any embodiments described above, the compressive working is not 
necessarily needed to be effected continuously but may be effected 
stepwise in two or more times until a desired level of compressive strain 
is attained. 
In the foregoing description, the compressive working of billet can be 
broadly divided into an entire working (first and second embodiments) and 
a local working (third embodiment). When a change in outer or inner 
circumferential length of a billet is taken into account, the method of 
the invention may be classified into two groups. One group involves little 
or no change in either of the outer and inner circumferential lengths and 
the other group involves changes in both the lengths. 
The case where a billet is entirely plastically deformed as in the first 
embodiment and the case where a billet is deformed locally along the outer 
or inner circumferential portion correspond to the one group. This group 
enables a billet to be rendered more radially anisotropic. The case of the 
second embodiment where a billet is subjected to the free compressive 
working corresponds to the other group. 
For instance, in the procedure illustrated in FIGS. 7(a) and 7(b), the 
outer length prior to and after the compressive working does not change 
with a change of the inner length. On the contrary, with the case shown in 
FIGS. 5(a) through 5(c), both the outer and inner lengths of the billet 
change prior to and after the compressive working. 
In the foregoing, the billet is illustrated as a cylinder but may be in 
other forms. 
As described before, the compressive working is effected at a temperature 
of 530.degree. to 830.degree. C. in the practice of the invention. 
However, temperatures exceeding 780.degree. result in lowering of magnetic 
characteristics to an extent. Accordingly, preferable temperatures range 
from 560.degree. to 760.degree. C. 
The present invention is more particularly described by way of examples. 
EXAMPLE 1 
A charge composition of 69.5 wt% (hereinafter referred to simply as %) of 
Mn, 29.3% of Al, 0.5% of C and 0.7% of Ni was melted and cast to make a 
solid cylindrical billet having a diameter of 70 mm and a length of 60 mm. 
This billet was kept at 1100.degree. C. for 2 hours and allowed to cool 
down to room temperature. The billet was extruded through a lubricant at 
720.degree. C. to a diameter of 45 mm, followed by further extrusion 
through a lubricant at a temperature of 680.degree. C. to a diameter of 31 
mm. The extruded rod was cut into pieces having a length of 20 mm. The 
pieces were machined to obtain several hollow cylindrical billets each 
having an outer diameter of 30 mm and an inner diameter of 15 to 24 mm. 
The billets were placed in a die of the type shown in FIG. 4(a) and 
compressed at different strains at a temperature of 680.degree. C. while 
retraining the outer circumferential surface but setting the inner 
circumferential surface free. In FIG. 4, the ring die 1 had an inner 
diameter of 30 mm. From the compressed billets were cut cubic samples 
having each side of about 4 mm, followed by measurement of magnetic 
characteristics in which the respective sides of each cube were arranged 
parallel to axial, radial and tangential directions. The variation of 
residual magnetic flux density, Br, in relation to compressive strain 
.xi.z is shown in FIG. 8. As will be seen from FIG. 8, when .xi.z is 0.05, 
the residual magnetic flux density is much greater in the radial direction 
than in the axial direction. Higher values of the compressive strain 
result in higher flux density in the radial direction. 
Moreover, the results shown in FIG. 8 reveal that a change of the 
direaction of easy magnetization from axially to radially is sharp in the 
range of .xi.z up to 0.05. Upon comparing with known compressive working 
techniques, higher magnetic characteristics can be obtained in very small 
compressive strains. 
In other words, in order to obtain high magnetic characteristics in radial 
directions by known compressive working techniques, great compressive 
strains are needed. However, in the practice of the invention, magnets of 
high magnetic characteristics can be obtained in small compressive 
strains. 
A billet which had been compressed to .xi.z=0.69 was machined to give a 
cylindrical magnet having an outer diameter of 28 mm, an inner diameter of 
14 mm and a length of 10 mm, followed by 6-pole magnetization in radial 
directions as shown in FIG. 1. The magnetization was effected using a 2000 
.mu.F by the pulse magnetization technique at 1500 V. The magnetic flux 
density of the outer circumferential surface was measured by the Hall 
element. For comparison, the aforeindicated extruded rod having a diameter 
of 31 mm was cut into a piece with a length of 20 mm and machined to give 
a solid cylindrical billet having a diameter of 20 mm and a length of 20 
mm, followed by free compressive working along the axis of the cylinder 
through a lubricant at a temperature of 680.degree. C. In the case, the 
compressive strain was 0.69. The billet obtained after the free 
compressive working was a plane-anisotropic magnet. This magnet was 
machined to have a form of a hollow cylinder in the same manner as 
described above and magnetized, followed by measurement of the surface 
magnetic flux density. 
As a result, it was found that the surface magnetic flux density of the 
magnet obtained according to the method of the invention had a value as 
high as about 1.4 times the density of the plane-anisotropic magnet. 
The magnetized magnet of the invention was subjected to compressive working 
along the outer circumferential portion at a temperature of 680.degree. C. 
using the die shown in FIGS. 6(a) and 6(b). The punch 6 had an outer 
diameter of 22 mm. The compressed portion had a length of 8 mm. After 
completion of the working, the billet was machined to give an outer 
diameter of 28 mm and an inner diameter of 14 mm, followed by 
magnetization in the same manner as described before. The magnet obtained 
after the local working had a surface magnetic flux density higher by 0.2 
kG than the magnet prior to the local compressive working. 
EXAMPLE 2 
The extruded rod with a diameter of 31 mm obtained in Example 1 was cut 
into a 50 mm long piece and extruded through a lubricant at a temperature 
of 680.degree. C. to a diameter of 22 mm. The extruded rod was cut into 20 
mm long pieces and subjected to free compressive working along the axis 
through a lubricant at a temperature of 680.degree. C. After the free 
compressive working, the billets were each machined to give a hollow 
cylinder having an outer diameter of 30 mm, an inner diameter of 22 mm and 
a length of 10 mm. Two hollow cylinders were put one on the top of another 
along their axis and subjected to compressive working using the die shown 
in FIGS. 4(a) and 4(b) at a temperature of 680.degree. C. while 
restraining the outer circumference of each cylinder with the inner 
circumference being non-restrictive. The worked billet had a length of 10 
mm and machined in the same manner as in Example 1, followed by 
magnetization and measurement of its surface magnetic flux density. 
Similar results as with the magnet obtained in Example 1 prior to the 
local compressive working were obtained. 
EXAMPLE 3 
The extruded rod obtained in Example 1 was cut into pieces with a length of 
20 mm and machined to give several hollow cylindrical billets each having 
an outer diameter of 30 mm, an inner diameter of 20 mm and a length of 20 
mm. 
These billets were subjected to free compressive working along the axis 
thereof through a lubricant at a temperature of 680.degree. C. at 
different strains. A cubic sample having each side of about 4 mm was cut 
off from each billet obtained after the working and subjected to 
measurement of magnetic characteristics. The measurement was effected such 
that the respective sides of the sample were parallel to axial, radial and 
tangential directions. In FIG. 9, there is shown a compressive strain 
(.xi.z) in relation to residual magnetic flux (Br) for different 
directions. 
As is seen from FIG. 9, when the compressive strain is 0.05, the residual 
magnetic flux density becomes greater in the radial direction than in the 
axial direction. A greater compressive strain results in a greater 
residual magnetic flux density in the radial direction. Furthermore, a 
change of the direction of easy magnetization from axial to radial 
directions becomes sharp within a range of .xi.z up to 0.05. Upon 
comparing with magnets obtained by known compressive workings, the magnets 
obtained according to the present invention exhibit high magnetic 
characteristics in much smaller compressive strains. 
The billet was further compressed as shown in FIG. 5(c) to have a 
compressive strain of 0.69 and machined to give a hollow cylindrical 
magnet having an outer diameter of 36 mm, an inner diameter of 25 mm and a 
length of 10 mm. The magnet was subjected to inner circumferential 
magnetization of 18 poles as shown in FIG. 3. The magnetization was 
effected using a 2000 .mu.F oil condenser by a pulse magnetization 
technique at 1500 V. The surface magnetic flux density of the inner 
magnetized portion was measured by the Hall element. 
For comparison, the afore-indicated extruded rod having a diameter of 31 mm 
was cut into a piece with a length of 20 mm and machined to give a solid 
cylindrical billet having a diameter of 30 mm and a length of 20 mm. The 
billet was subjected to free compressive working along its axis through a 
lubricant at a temperature of 680.degree. C. so that it was imparted with 
a compressive strain (.xi.z) of 0.69. The compressed billet was a 
plane-anisotropic magnet. The magnet was machined in the same manner as 
described above to obtain a hollow cylindrical magnet, followed by inner 
circumferential magnetization and measurement of a surface magnetic flux 
density. 
As a result, it was found that the magnet obtained according to the 
invention had a surface magnetic flux density as high as about 1.2 times 
the known plane-anisotropic magnet. 
Upon the free compressive working in the axial direction of the hollow 
cylindrical billet having an outer diameter of 30 mm, an inner diameter of 
20 mm and a length of 20 mm, the billet was first compressed to a 
compressive strain of 0.41. Then, the plastic working was stopped for 15 
seconds and then the compressed billet was subjected to further free 
compressive working at a temperature of 680.degree. C. so that a 
compressive strain reached 0.69 in total. After completion of the 
compressive working, the billet was machined in the same manner as 
described before to give a hollow cylinder. The hollow cylindrical magnet 
was magnetized at the inner circumferential portion and its surface 
magnetic flux density was measured in the same manner as described. As a 
result it was found that the density increased by 0.2 kG as compared with 
the case where the free compressive working was continuously effected. 
Moreover, the hollow cylindrical magnet which had been subjected to the 
continuous free compressive working and magnetized was further subjected 
to compressive working of the inner circumferential portion thereof using 
the die shown in FIG. 7. In this case, the punch 2 had a diameter of 30 
mm. After the compressive working, the compressed portion had a length of 
8 m. The resulting billet was machined to have an inner diameter of 36 mm 
and an inner diameter of 25 mm, followed by magnetization and measurement 
of a surface magnetic flux density in the same manner as described before. 
By the compressive working of the billet only at the inner circumferential 
portion thereof, the surface magnetic flux density increased by 0.2 kG. 
EXAMPLE 4 
The extruded rod having a diameter of 31 mm obtained in Example 1 was cut 
into a piece having a length of 20 mm and machined to give a hollow 
cylindrical billet having an outer diameter of 24 mm, an inner diameter of 
12 mm and a length of 20 mm. The billet was subjected to free compressive 
working along its axis at a temperature of 680.degree. C. in a manner as 
shown in FIGS. 5(a) and 5(b), followed by further compressive working in a 
manner as shown in FIGS. 5(b) and 5(c) in which the outer surface of the 
billet was restrained but the inner surface was not restrained so that the 
billet could be freely deformed inwardly. In this case, the ring die 1 of 
FIG. 5 had an inner diameter of 30 mm. After completion of the working, 
the billet had an outer diameter of 30 mm and a length of 10 mm. The 
billet was machined to have an outer diameter of 28 mm and an inner 
diameter of 14 mm and was then radially magnetized under the same 
conditions as in Example 3. The measurement was effected in the same 
manner as in Example 3. 
For comparison, a plane-anisotropic magnet made in the same manner as in 
Example 3 was machined to have an outer diameter of 28 mm and an inner 
diameter of 14 mm, followed by magnetization in the same manner as 
described above. 
The magnet obtained according to the method of the invention had a surface 
magnetic flux density as high as about 1.3 times the known 
plane-anisotropic magnet. 
The magnetized hollow cylindrical magnet of the invention was subjected to 
further compressive working of the outer circumferential portion alone at 
a temperature of 680.degree. C. using the die shown in FIGS. 6(a) and 
6(b). In this case, the punch 6 had an outer diameter, i.e., an inner 
diameter of the punch 7, of 20 mm. After completion of the working, the 
billet had a length of 8 mm at the compressed portion. The worked billet 
was machined to have an outer diameter of 28 mm and an inner diameter of 
14 mm, followed by magnetization and measurement of a surface magnetic 
flux density in the same manner as described above. As a result, it was 
found that the density increased by 0.2 kG. 
EXAMPLE 5 
Two plane-anisotropic magnets (i.e. solid cylindrical billets having a 
diameter of 42 and a length of 10 mm) made in Example 3 for comparison 
were machined to give a hollow cylindrical billet having an outer diameter 
of 30 mm, an inner diameter of 20 mm and a length of 20 mm. The billet was 
subjected to free compressive working along its axis through a lubricant 
at a temperature of 660.degree. C. The compressed billet had a length of 
10 mm. The billet was machined to give a hollow cylindrical magnet having 
an outer diameter of 36 mm, an inner diameter of 25 mm and a length of 10 
mm, followed by inner circumferential magnetization in the same manner as 
in Example 3. The surface magnetic flux density was measured in the same 
manner as in Example 3. As a result, it was found upon comparison with the 
magnet of the present invention obtained in Example 3, no appreciable 
difference in surface magnetic flux density was recognized. 
EXAMPLE 6 
A charge composition of 69.5% of Mn, 29.3% of Al, 0.5% of C and 0.7% of Ni 
was melted and cast to give a solid cylindrical billet having a diameter 
of 60 mm and a length of 50 mm. The billet was kept at 100.degree. C. for 
2 hours and allowed to cool down to room temperature. The billet was 
extruded through a lubricant at a temperature of 720.degree. C. to a 
diameter of 35 mm, followed by further extrusion through a lubricant at a 
temperature of 680.degree. C. to a diameter of 24 mm. The extruded rod was 
cut into a piece having a length of 20 mm, followed by free compressive 
working through a lubricant at a temperature of 680.degree. C. to a length 
of 10 mm. After the working, the billet was machined to have a diameter of 
32 mm and a length of 10 mm, thereby obtaining a solid cylindrical magnet 
(plane-anisotropic magnet). 
This magnet was further subjected to compressive working at its outer 
circumferential portion alone at a temperature of 680.degree. C. using a 
die shown in FIG. 10. In the figure, the working punch 7 had an inner 
diameter of 25 mm, i.e. an outer diameter of the fixed punch was 25 mm. 
The magnet after the compressive working had the compressed outer portion 
with a length of 8 mm. The magnet after the working was machined in the 
form of a hollow cylinder having an outer diameter of 32 mm and an inner 
diameter of 10 mm. The hollow cylindrical magnet was magnetized at 24 
poles along the compressed outer portion. The magnetization was carried 
out using a 2000 .mu.F oil condenser by the pulse magnetization technique 
at 1500 V. The surface magnetic flux density of the outer circumferential 
portion was measured by the Hall element. 
For comparison, a plane-anisotropic magnet made by the same procedure as 
described above was machined into a hollow cylinder having an outer 
diameter of 32 mm and inner diameter of 10 mm, followed by outer 
circumferential magnetization in a manner as mentioned above. 
As a result, it was found that the magnet obtained according to the method 
of the invention had a surface magnetic flux density as high as about 1.2 
times the known plane-anisotropic magnet. 
Two plane-anisotropic magnets made in the same manner as described above 
were machined to give hollow cylindrical magnets each having an outer 
diameter of 32 mm, an inner diameter of 16 mm and a length of 10 mm. One 
hollow cylindrical magnet was subjected to compressive working only at the 
inner circumferential portion thereof at a temperature of 680.degree. C. 
using a die of the type shown in FIG. 7. The compressed inner portion had 
a length of 8 mm. The punch 2 in FIG. 7 had a diameter of 23 mm. The 
worked magnet was machined to give a hollow cylinder having an outer 
diameter of 32 mm and an inner diameter of 16 mm. The magnet of the 
invention and the plane-anisotropic magnet, both of which had been 
compressed only at the inner circumferential portion thereof, were 
subjected to inner circumferential magnetization of 18 poles. 
As a result, it was found that the magnet obtained according to the 
invention had a surface magnetic flux density higher by about 1.2 times 
than the known plane-anisotropic magnet. 
EXAMPLE 7 
The extruded rod with a diameter of 31 mm obtained in Example 1 was cut 
into pieces having a length of 20 mm, followed by machining into hollow 
cylinders having an outer diameter of 31 mm, an inner diameter of 10 to 22 
mm and a length of 20 mm. These billets were subjected to compressive 
working only at the inner circumferential portion thereof at a temperature 
of 680.degree. C. using a die of the type shown in FIG. 7. In FIG. 7, the 
punch 2 had an outer diameter of 25 mm. 
A cubic sample having each side of about 5 mm was cut off from the 
compressed portion of the worked billet and its magnetic characteristics 
were measured. In the measurement, the cube was set so that the respective 
sides thereof were parallel to the axial, radial and tangential 
directions. The variation of residual magnetic flux density, Br, in 
relation to compressive strain, .xi.z, is shown in FIG. 11. When the 
compressive strain is 0.05, the flux density becomes greater in the radial 
direction than in the axial direction. A greater compressive strain 
results in a greater flux density in the radial direction. Furthermore, a 
change of the direction of easy magnetization from axial to radial 
directions sharply proceeds within a range of .xi.z up to 0.05. High 
magnetic characteristics can be obtained in very small strains. 
The extruded rod having a diameter of 31 mm as used above was cut into a 
piece with a length of 20 mm, followed by compressive working only at the 
outer circumferential portion thereof at a temperature of 680.degree. C. 
using a die of the type shown in FIG. 10. In this case, the inner diameter 
of the punch 7 was 14 mm. From the compressed portion of the billet was 
cut off a cubic sample having each side of about 5 mm, followed by 
measurement of its magnetic characteristics. 
Upon comparing with the magnet obtained in the former part of this example 
and compressed to the same degree of Ez, no substantial difference was 
recognized. 
EXAMPLE 8 
A charge composition of 69.5% of Mn, 29.3% of Al, 0.5% of C and 0.7% of Ni 
was melted and cast to give a solid cylindrical billet having a diameter 
of 80 mm and a length of 60 mm. The billet was kept at 1100.degree. C. for 
2 hours, followed by allowing to cool to room temperature. The billet was 
extruded through a lubricant at a temperature of 720.degree. C. to a 
diameter of 45 mm, followed by further extrusion through a lubricant at a 
temperature of 680.degree. C. to a diameter of 31 mm. The extruded rod was 
machined to give a hollow cylinder having an outer diameter of 30 mm, an 
inner diameter of 10 mm and a length of 20 mm. 
The hollow cylindrical billet was extruded through a lubricant at a 
temperature of 680.degree. C. using a die of the type shown in FIGS. 12(a) 
and 12(b). FIG. 12(a) show a state prior to the extrusion and FIG. 12(b) 
shows a state after the extrusion. In FIGS. 12(a) and 12(b), there is 
shown a die D which has a core 9 having a small-size section 9a, a 
frustoconical section 9b and a large-size section 9c and a ring die 1 
surrounding the core 9. Between the core and the ring die is established a 
cavity C having a container portion 11, a intermediate portion 12 and a 
bearing portion 13. In this example, the container portion had an outer 
diameter of 30 mm and an inner diameter of 10 mm. The bearing portion 13 
had an outer diameter of 63.2 mm and an inner diameter of 49 mm. The 
length of the intermediate portion along the axis of the billet was 40 mm. 
After completion of the extrusion, the billet had an outer diameter of 
63.2 mm, an inner diameter of 49 mm and a length of 10 mm. 
The thus extruded billet was subjected to compressive working along its 
axis only at the outer circumferential portion thereof at a temperature of 
680.degree. C. according to the procedure shown in FIGS. 6(a) and 6(b). 
That is, the billet 4 was set coaxially with the movable punch 7 and 
compressed only at the outer circumferential portion of the billet 4. 
After the compressive working, the compressed portion had a length of 8 mm 
and the non-compressed inner portion had a length of 10 mm. The billet was 
machined to have an outer diameter of 62 mm and an inner diameter of 50 mm 
and magnetized at 30 poles along the outer circumference. The 
magnetization was effected using a 2000 .mu.F oil condenser by the pulse 
magnetization technique at 1500 V. The surface magnetic flux density was 
measured by the use of the Hall element. 
In the same manner as in the above procedure, there was made a hollow 
cylindrical magnet having an outer diameter of 62 mm, an inner diameter of 
50 mm and lengths of 8 mm at the outer circumferential portion and 10 mm 
at the inner circumferential portion. From the outer and inner 
circumferential portions of the magnet were, respectively, cut off three 
rectangular parallelopipeds (six in total) so that the respective sides 
were parallel to radial (r direction), tangential (.theta. direction) and 
axial directions. The side parallel to the axial direction was 2 mm, the 
side parallel to the tangential direction was 4 mm and the side parallel 
to the axial direction was 5 mm. The three rectangular parallelopipeds 
were put one on the top of another to form a rectangular parallelopiped 
having sides of 6 mm, 4 mm and 5 mm. This sample was subjected to 
measurement of magnetic characteristics in the respective directions. 
As a result, it was found that as for the inner circumferential portion, 
Br=5.9 kG, Hc=2.7 kOe and (BH)max=6.2 MG.Oe in the tangential direction, 
Br=3.1 kG, Hc=2.3 kOe and (BH)max=2.0 in the radial direction, and Br=2.6 
kG, Hc=1.9 kOe and (BH)max=1.4 MG.Oe in the axial direction. With regard 
to the outer circumferential portion, Br=3.0 kG, Hc=1.9 kOe and 
(BH)max=1.4 MG.Oe in the tangential direction, Br=5.6 kG, Hc=2.5 kOe and 
(BH)max=5.4 MG.Oe in the axial direction, and Br=2.6 kG, Hc=1.9 kOe and 
(BH)max=1.4 MG.Oe in the axial direction. 
As will be understood from the above results, the magnet is an anisotropic 
magnet of the type which is suitable for outer circumferential 
magnetization. The inner circumferential portion is rendered tangentially 
anisotropic and the outer circumferential portion is rendered radially 
anisotropic. 
For comparison, the extruded rod with a diameter of 45 mm as used above was 
cut into a 20 mm long piece to give a solid cylindrical billet having a 
diameter of 45 mm and a length of 20 mm. Thereafter, the solid cylindrical 
billet was subjected to free compressive working through a lubricant along 
the axis thereof at a temperature of 680.degree. C. After the working, the 
billet had a length of 10 mm. This billet was a plane-anisotropic magnet 
and was machined to give a hollow cylinder having an outer diameter of 62 
mm and an inner diameter of 50 mm, followed by magnetization and 
measurement in the same manner as described above. 
From the plane-anisotropic magnet at a diameter of about 55 mm was cut off 
a cubic sample having each side of 5 mm so that the respective sides were 
parallel to radial, tangential and axial directions. The cubic sample was 
subjected to measurement of magnetic characteristics. The magnetic 
characteristics were as follows: Br=4.6 kG, Hc=2.8 kOe and (BH)max=4.0 
MG.Oe in the radial and tangential directions; and Br=2.6 kG, Hc=2.0 kOe 
and (BH)max=1.4 MG.Oe in the axial direction. 
The permanent magnet of the invention had a surface magnetic flux density 
as high as about 1.4 times the known plane-anisotropic magnet and was thus 
very excellent for outer circumferential magnetization. 
EXAMPLE 9 
The extruded rod obtained in Example 8 was cut into a 20 mm long piece and 
machined to give a hollow cylindrical billet having an outer diameter of 
30 mm, an inner diameter of 10 mm and a length of 20 mm similar to Example 
8. 
The hollow cylindrical billet was extruded in the same manner as in Example 
8 using such a die as shown in FIG. 12 to obtain a billet having an outer 
diameter of 63.2 mm, an inner diameter of 49 mm and a length of 10 mm. The 
billet was further subjected to compressive working only at the outer 
circumferential portion thereof along the axis at a temperature of 
680.degree. C. using a die as shown in FIGS. 7(a) and 7(b). That is, the 
billet was fixed using the restrictive die 8 and the under die 5 and the 
billet 4 was set substantially coaxially with the punch 2, followed by 
compressive working. 
The punch 2 had a diameter of 56 mm and after the working, the billet had a 
length of 8 mm at the compressed inner portion and a length of 10 mm at 
the outer portion. 
The billet was machined to have an outer diameter of 62 mm and an inner 
diameter of 50 mm and magnetized at the inner circumferential portion 
thereof at 30 poles by the pulse magnetization technique at 1500 V using a 
2000 .mu.F oil condenser. The surface magnetic flux density of the inner 
circumferential portion was measured by the Hall element. 
In a manner similar to the above procedure, there was made a hollow 
cylindrical magnet having an outer diameter of 62 mm, an inner diameter of 
50 mm and lengths of 8 mm at the inner portion and 10 mm at the outer 
portion. From the inner and outer portions were, respectively, cut off 
three rectangular parallelopipeds (six in total) so that the respective 
sides were parallel to radial (r direction), tangential (.theta.) and 
axial directions. The side parallel to the radial direction was 2 mm, the 
side parallel to the tangential direction was 4 mm, and the side parallel 
to the axial direction was 5 mm. The three parallelopipeds were put one on 
the top of another to give a rectangular parallelopiped having sides of 6 
mm, 4 mm and 5 mm, followed by measuring magnetic characteristics in the 
respective directions. As for the outer circumferential portion, Br=5.9 
kG, Hc=2.7 kOr and (BH)max=6.2 MG.Oe in the tangential direction, Br=3.1 
kG, Hc=2.3 kOe and (BH)max=6.2 MG.Oe in the radial direction, and Br=2.6 
kG, Hc=1.9 kOe and (BH)max=1.4 MG.Oe in the axial direction. With regard 
to the inner circumferential portion, Br=3.0 kG, Hc=2.0 kOe and 
(BH)max=1.7 MG.Oe in the tangential direction, Br=5.6 kG, Hc=2.5 kOe and 
(BH)max=5.4 MG.Oe in the radial direction, and Br=2.6 kG, Hc=1.9 kOe and 
(B)max=1.4 MG.Oe in the axial direction. 
As will be seen from the above, the magnet is tangentially anisotropic in 
the outer portion and is radially anisotropic in the inner portion. 
For comparison, the extruded rod with a diameter of 45 mm used above was 
cut into a 20 mm long piece and machined to give a solid cylinder having a 
diameter of 45 mm. This solid cylindrical billet was subjected to free 
compressive working through a lubricant along the axis thereof at a 
temperature of 680.degree. C. The compressed billet had a length of 10 mm 
and was plane-anisotropic. The billet was machined to give a hollow 
cylinder having an outer diameter of 62 mm and an inner diameter of 50 mm, 
followed by magnetization and measurement in the same manner as described 
before. 
The plane-anisotropic magnet obtained in the same manner as described above 
was cut off at a portion of about 55 mm in diameter to give a cube having 
each side of 5 mm. The respective sides were made parallel to radial, 
tangential and axial directions. The cubic sample was subjected to 
measurement of magnetic characteristics. The characteristics were as 
follows: Br=4.6 kG, Hc=2.8 kOe and (BH)max=4.0 MG.Oe in the radial and 
tangential directions and Br=2.6 kG, Hc=2.0 kOe and (BH)max=1.4 MG.Oe in 
the axial direction. 
The permanent magnet of the invention had a surface magnetic flux density 
as high as about 1.4 times the plane-anisotropic magnet and was thus very 
excellent for inner circumferential magnetization.