CN118888256A

CN118888256A - Method and device for demagnetizing local area of permanent magnet and magnet

Info

Publication number: CN118888256A
Application number: CN202411324901.6A
Authority: CN
Inventors: 王进东; 付国安; 钮萼; 王湛; 刘风乔; 饶晓雷
Original assignee: Beijing Zhong Ke San Huan High Tech Co Ltd
Current assignee: Beijing Zhong Ke San Huan High Tech Co Ltd
Priority date: 2024-09-23
Filing date: 2024-09-23
Publication date: 2024-11-01

Abstract

The application provides a method and a device for demagnetizing a local area of a permanent magnet and the magnet. The demagnetizing method comprises the following steps: placing a demagnetizing coil adjacent to a local region of the permanent magnet, wherein the demagnetizing coil comprises an upper coil and a lower coil, the upper coil is adjacent to the upper side of the local region, and the lower coil is adjacent to the lower side of the local region; and applying a current to the demagnetizing coils to generate a demagnetizing field having a predetermined angle with respect to the magnetization direction of the local region, wherein the directions of the magnetic fields generated by the upper and lower coils are the same.

Description

Method and device for demagnetizing local area of permanent magnet and magnet

Technical Field

The application relates to the technical field of magnetic materials, in particular to a method and a device for demagnetizing a local area of a permanent magnet and the magnet.

Background

Permanent magnets have been widely used in the aspects of people's production and life due to their unique properties. In particular, neodymium iron boron permanent magnets are widely used in various fields such as automobiles, electronics, machinery, energy sources, medical devices, and the like because of their excellent magnetic properties.

For mass production of permanent magnet assemblies such as motors and wireless charging of mobile phones, a production mode of firstly assembling the assemblies or the components in a non-magnetic mode and then magnetizing the whole assemblies or the components in a magnetic mode is generally adopted.

In some special application fields, in order to meet the magnetic field requirement in the whole magnetic circuit, the magnetic field intensity of the magnet assembly is greatly lower in a certain local area than in other areas, and the magnetic field intensity of each magnet or each part of the magnet outside the local area is uniform. To achieve the above effect, in the prior art, after the magnets are assembled into a whole or after the magnets are integrally formed, separate magnetizing is performed, that is, the magnets in local areas are not magnetized, and other areas are magnetized.

However, since permanent magnets such as sintered neodymium iron boron have easy magnetization characteristics, local area magnets which are not desired to be magnetized tend to be affected by surrounding magnetizing magnetic fields during the above operation, so that the area magnets have weak magnetic fields, and the surface magnetic field strength may exceed the design magnetic field strength, which is not in accordance with the design requirements.

Disclosure of Invention

In order to solve the problems in the prior art, the application provides a method and a device for demagnetizing a local area of a permanent magnet and the magnet.

According to an aspect of the present application, there is provided a method of demagnetizing a local region of a permanent magnet, comprising:

Placing a demagnetizing coil adjacent to a local region of the permanent magnet, wherein the demagnetizing coil comprises an upper coil and a lower coil, the upper coil is adjacent to the upper side of the local region, and the lower coil is adjacent to the lower side of the local region; and

And current is introduced into the demagnetizing coils to generate a demagnetizing field with a preset angle with the magnetization direction of the local area, wherein the directions of the magnetic fields generated by the upper coil and the lower coil are the same.

According to another aspect of the present application, there is also provided an apparatus for demagnetizing a partial region of a permanent magnet, comprising: an upper coil arranged close to the upper side of the local area; and a lower coil arranged near the lower part of the local area and opposite to the upper coil, wherein the magnetic fields generated by the upper coil and the lower coil have the same direction and form a preset angle with the magnetization direction of the local area.

According to another aspect of the present application, there is also provided a magnet obtained by the method or apparatus as described above, wherein the residual magnetic moment of a local region of the magnet has a magnitude of 40% M _max or less, and the region of the magnet other than the local region has a magnetic moment magnitude of 90% M _max or more, wherein M _max is the saturation magnetic moment of the local region.

Thus, in order to obtain a magnet or magnet assembly having a substantially lower magnetization in a particular localized area than in other areas, the entire magnet or magnet assembly may be fully magnetized (i.e., the particular localized area is also magnetized) and then demagnetized for that particular localized area. The direction of the demagnetizing magnetic field is basically perpendicular to the magnetization direction of the local area to be demagnetized, so that the demagnetizing effect on the local area can be realized, and the influence on the magnetization intensity of the peripheral area can be avoided; on the other hand, the demagnetizing method can also avoid the problems that the strength of the reverse magnetic field is required to be higher and difficult to accurately control when the demagnetizing is performed by using the magnetic field which is completely reverse to the magnetization of the magnet in the prior art.

Drawings

In order to more clearly illustrate the technical solutions of the embodiments of the present application, the drawings required for the description of the embodiments will be briefly described below, and it is apparent that the drawings in the following description are only some embodiments of the present application, and other drawings may be obtained according to these drawings without inventive effort for a person skilled in the art.

Fig. 1 shows a flow chart of a method of demagnetizing a localized region of a permanent magnet, according to one embodiment of the present application.

Fig. 2 shows a schematic diagram of a demagnetization operation according to this embodiment.

Fig. 3 shows a schematic view of a gap arrangement between an upper coil and a lower coil according to an embodiment of the application.

Fig. 4 shows a flow chart of a method of demagnetizing a localized region of a permanent magnet according to another embodiment of the present application.

Fig. 5 shows a schematic diagram of a demagnetizing coil according to this embodiment.

Fig. 6A shows a simulated plot of the demagnetizing field of a demagnetizing coil in a localized region a and adjacent regions of a magnet assembly, in accordance with one embodiment of the present application.

FIG. 6B shows demagnetizing field attenuation slopes at different lateral positions according to an embodiment of the present application) Is a simulation graph of (2).

Fig. 7 shows a schematic diagram of demagnetizing a localized region of a toroidal neodymium-iron-boron permanent magnet sample, according to one embodiment of the present application.

Fig. 8A shows a simulated plot of the demagnetizing field of a demagnetizing coil in a localized region and adjacent region of a circular magnetic ring, in accordance with one embodiment of the present application.

FIG. 8B shows demagnetizing field attenuation slopes at different circumferential positions according to an embodiment of the present application) Is a simulation graph of (2).

Detailed Description

For a better understanding of the technical solutions and advantages of the present application, the following description of the present application refers to the accompanying drawings and specific examples. The specific embodiments described herein are to be considered in an illustrative sense only and are not intended to limit the application. Further, technical features of the embodiments of the present application described below may be used in combination, except for the case where they collide with each other, to thereby constitute other embodiments within the scope of the present application.

The following description provides many different embodiments, or examples, for implementing different features of the application. In order to simplify the present disclosure, components and arrangements of specific examples are described below. They are, of course, merely examples and are not intended to limit the application. Furthermore, the present application may repeat reference numerals and/or letters in the various examples, which are for the purpose of brevity and clarity, and which do not themselves indicate the relationship between the various embodiments and/or arrangements discussed.

The flowcharts in the figures illustrate the operation of possible implementations of methods in accordance with one or more embodiments of the present application. It should be noted that in some alternative implementations, the steps noted in the blocks may occur out of the order noted in the figures. For example, two or more blocks shown in succession may in fact be executed substantially concurrently or the blocks may sometimes be executed in the reverse order, depending upon the functionality involved. And all such embodiments are intended to fall within the scope of the present application.

Terms such as up, down, left, right, front, rear, etc. used in the present application, which represent directions in the drawings, do not represent directions in actual application scenarios.

FIG. 1 shows a flow chart of a method of demagnetizing a localized region of a permanent magnet, according to one embodiment of the present application; fig. 2 shows a schematic diagram of a demagnetization operation according to this embodiment. As shown in fig. 1 and 2, the demagnetization method 100 of the local area of the permanent magnet may include steps S110 and S120.

Prior to the operation of step S110, the permanent magnet 200 (e.g., without limitation, a neodymium-iron-boron magnet) has been fully magnetized as a whole, and the whole has been provided with the magnetization and direction required for the design. The permanent magnet 200 may be a magnet assembly formed by splicing a plurality of small magnets (which may be considered as magnets A, B, C in fig. 2, each having substantially the same magnetic properties); alternatively, the permanent magnet 200 may be a single magnet that is integrally formed (the single magnet may be considered to include regions A, B, C as shown in fig. 2, each region having substantially the same magnetic properties). In the permanent magnet 200, a partial area a, which is an area demagnetized according to design requirements in the following description of the present application, needs to be demagnetized. The partial region a may be a part of the permanent magnet 100 formed integrally, or may be one or more magnets of a magnet assembly formed by splicing.

In step S110, a demagnetizing coil is placed close to the local region a of the permanent magnet 200. As shown in fig. 2, the demagnetizing coils may include an upper coil 310 and a lower coil 320, the upper coil 310 being close to the upper side of the local region a, and the lower coil 320 being close to the lower side of the local region a. In the present application, the permanent magnet 200 may be elongated, ring-shaped, or any other shape required for design.

In step S120, a current is applied to the demagnetizing coil to generate a demagnetizing field having a predetermined angle with respect to the magnetization direction of the local region a. As shown in fig. 2, the magnetic fields generated by the upper coil 310 and the lower coil 320 have the same direction. According to one embodiment of the present application, the above-mentioned predetermined angle may be in the range of 90 ° ± 8 °, preferably in the range of 90 ° ± 3 °. That is, referring to fig. 2, the magnetization direction of the local area a of the permanent magnet 200 is a direction substantially perpendicular to the paper surface before demagnetization is performed using the method 100. According to an embodiment of the present application, the demagnetizing field generated by the demagnetizing coil may be a dc pulsed magnetic field or an ac attenuated magnetic field, preferably an ac attenuated magnetic field.

According to one embodiment of the present application, the maximum magnetic field strength of the demagnetizing field generated by the demagnetizing coils acting together in the local region a satisfies the following equation:

2.1×HcJ/μ_r≤H≤3.5×HcJ/μ_r

where H is the magnetic field strength of the demagnetizing field, hcJ is the intrinsic coercive force of the local region a of the permanent magnet 200, and μ _r is the relative permeability of the local region a.

Therefore, when the magnitude of the demagnetizing field generated by the demagnetizing coil is set, the method can ensure that the generated field intensity is large enough (more than or equal to 2.1 XH _cJ/μ_r) so that the local area A can achieve better demagnetizing effect, and on the other hand, if the demagnetizing field intensity is too large, the demagnetizing area is increased, and the transition area between the demagnetizing area and the non-demagnetizing area is increased, so that the precise control of the demagnetizing area is weakened, and therefore, the situation that H is less than or equal to 3.5 XHcJ/mu _r can be effectively avoided.

Fig. 3 shows a schematic view of a gap arrangement between an upper coil and a lower coil according to an embodiment of the application. As shown in fig. 3, there is a gap D between the upper coil 310 and the lower coil 320 for accommodating the local area a to be demagnetized, the gap D (in mm) satisfying the following equation:

D≤10/μ_r

Where mu _r is the relative permeability of the local area a of the permanent magnet 200.

The gap between the upper and lower coils is thereby set as small as possible, thereby preventing the magnet from vibrating during application of the demagnetizing field and/or deflecting in the demagnetizing field. Further, if the gap between the upper coil and the lower coil is too large, the demagnetizing field at the coil edge also easily affects the magnetization of the non-demagnetizing region other than the local region a to be demagnetized, and for example, there is a possibility that the non-demagnetizing region may be partially demagnetized.

According to one embodiment of the application, the maximum decay slope of the demagnetizing field generated by the demagnetizing coil is greater than or equal to 0.5T/mm, preferably greater than or equal to 0.7T/mm, at the edge region of the demagnetizing coil. The demagnetizing field generated by the demagnetizing coil in the area outside the projection area of the surface of the magnet or the magnet assembly is small, and in order to reduce the influence of the demagnetizing field to the area outside the local area a to be demagnetized as much as possible, the attenuation slope of the demagnetizing field needs to be large with the increase of the distance from the edge of the projection area. That is, the magnetic field strength generated by the demagnetizing coil at the edge thereof will decrease sharply with distance from the local area to be demagnetized.

For example, using a sintered neodymium-iron-boron magnet as an example of a permanent magnet, the inventors found that a strong demagnetizing effect is achieved when a demagnetizing field H is applied perpendicular to the magnetization direction after the sintered neodymium-iron-boron magnet is saturated and magnetized, and the field strength value of H is equal to or greater than 2.1×HcJ, that is, an effect of M/M _max < 30% is achieved after the local region A of the magnet is demagnetized (M is the magnetic moment after the local region A is demagnetized, and M _max is the saturated magnetic moment of the local region A). However, since the magnetic circuit is open, the demagnetizing field will also act on the non-demagnetizing region B around the partial region a to be demagnetized. When the field strength value of the demagnetizing field applied to the surface of the non-demagnetizing region B reaches 1.5×hcj, the effect is such that the surface magnetization of the magnetized non-demagnetizing region B is reduced, which is undesirable, that is, the demagnetizing field needs to be concentrated to the local region a, and the non-demagnetizing region B is affected as little as possible by the demagnetizing field. Therefore, it is necessary to control the decay slope of the demagnetizing field with the demagnetizing coil edge distance (i.e., local region edge distance) so as to ensure that the region affected by the demagnetizing field is defined in the local region a to be demagnetized.

Fig. 4 shows a flow chart of a method of demagnetizing a localized region of a permanent magnet, according to another embodiment of the present application; fig. 5 shows a schematic diagram of a demagnetizing coil according to this embodiment. As shown in fig. 4, the demagnetization method 100' of the local area of the permanent magnet may further include step S130 in addition to steps S110 and S120. In the method 100' shown in fig. 4, the operations of steps S110 and S120 are similar to those of fig. 1, and are not repeated here for brevity.

In step S130, a core rod 330 is provided in the upper coil 310 and the lower coil 320. The portion I of the core rod 330 remote from the partial region a (not shown in fig. 4) is made of magnetically permeable material, and the portion II of the core rod 330 adjacent to the partial region a is made of magnetically non-permeable material. Therefore, the magnetic field generated by the demagnetizing coil can be enhanced by the magnetic conduction material part I, and the partial area A can be prevented from being adsorbed on the demagnetizing coil during or after the demagnetizing operation, so that the demagnetizing coil is inconvenient to remove.

According to another embodiment of the present application, there is also provided an apparatus for demagnetizing a localized region of a permanent magnet. Referring to fig. 2 and 3, the apparatus 300 may include an upper coil 310 and a lower coil 320. The upper coil 310 is disposed near the upper side of the partial area a to be demagnetized of the permanent magnet, and the lower coil 320 is disposed near the lower side of the partial area a to be demagnetized and opposite to the upper coil 310. The upper coil 310 and the lower coil 320 generate the same magnetic field direction and form a predetermined angle with the magnetization direction of the local area a according to the demagnetization requirement of the local area to be demagnetized.

Referring again to fig. 5, the apparatus 300 may further include a mandrel 330. The core rod 330 includes an upper core rod and a lower core rod, which are disposed in the upper coil 310 and the lower coil 320, respectively. The portion of the core rod 330 remote from the partial region a to be demagnetized is made of a magnetically conductive material to strengthen the magnetic field generated by the upper coil 310 and the lower coil 320. The portion of the core rod 330 adjacent to the partial region a to be demagnetized is made of a non-magnetically conductive material to avoid the partial region a from being adsorbed on the upper coil 310 and/or the lower coil 320 during or after the demagnetization operation, which is inconvenient to remove.

Example 1:

a long-strip-shaped neodymium-iron-boron permanent magnet sample (shown in fig. 2) is selected, a plurality of magnets are spliced and assembled into a magnet assembly, and the sizes of the single magnets are as follows: thickness 2.0mm, width 4.5mm (magnetization direction), length 8.3mm, μ _r =1.226 for each individual magnet, hcj=12 kOe, magnetization direction of the magnet assembly perpendicular to the paper surface. The sample was magnetized and then the local area a of the sample was demagnetized and tested in the following manner:

step 1: the magnet assembly was first integrally magnetized using a 2.5T magnetizing field, wherein each individual magnet had a saturation moment value of M _max at the 2.5T magnetizing field.

Step 2: the demagnetizing coil is placed near a local area A of the permanent magnet and comprises an upper coil and a lower coil, the upper coil is near the upper side of the local area A, the lower coil is near the lower side of the local area A, the upper coil and the lower coil can respectively generate alternating current attenuation magnetic fields with the same magnetic field direction, a gap D=3.0 mm between the upper coil and the lower coil, the magnetic field direction is 90 degrees with the magnetization direction of a single magnet, and the maximum magnetic field strength H of the demagnetizing field of the demagnetizing area is designed to be 2.7T.

Step 3: a current is applied to the demagnetizing coils to generate a demagnetizing field that is 90 ° from the magnetization direction of the local region a of the magnetized magnet assembly. Fig. 6A shows a graph of the demagnetizing field simulation of the demagnetizing coil in the local area a and the adjacent area of the magnet assembly, the abscissa is the lateral position L of the magnet assembly in mm, and the ordinate is the demagnetizing field strength generated by the demagnetizing coil in T, where TR represents the edge area of the demagnetizing coil; FIG. 6B shows the demagnetizing field attenuation slope at different lateral positions) Is a simulation graph of ||_max=0.8T/mm。

Step 4: after the partial demagnetization is completed, the individual magnets A, B, C (see fig. 2) in the magnet assembly are removed to measure the magnetic moments M, respectively. Table 1 below shows the M/M _max ratio of the measurement results.

TABLE 1

Example 2:

A circular neodymium iron boron permanent magnet sample (shown in fig. 7) is selected, a plurality of magnets are spliced and assembled into a magnet assembly, the outer diameter d1=54 mm and the inner diameter d2=46 mm of the circular magnet ring assembly, the corresponding center angle of each single magnet is 18 degrees, the thickness=1.0 mm, the mu _r =1.022 of each single magnet is, the hcj=25 kOe is, and the magnetization direction of the magnet assembly faces the radial direction of the circular ring. The sample was magnetized and then the local area a of the sample was demagnetized and tested in the following manner:

Step 1: firstly, a 2.5T magnetizing field is adopted to integrally magnetize the circular magnetic ring assembly, wherein the saturation magnetic moment value of each single magnet in the 2.5T magnetizing field is M _max.

Step 2: the demagnetizing coil is placed near a local area A of the circular magnetic ring and comprises an upper coil and a lower coil, the upper coil is near the upper side of the local area A, the lower coil is near the lower side of the local area A, the upper coil and the lower coil can respectively generate direct current pulse magnetic fields with the same magnetic field direction, a gap D=2.0 mm between the upper coil and the lower coil, the magnetic field direction is 90 degrees with the magnetization direction of a single magnet, and the maximum magnetic field strength H of the demagnetizing field of the demagnetizing area is designed to be 5.8T.

Step 3: and (3) introducing current into the demagnetizing coil to generate a demagnetizing field which is 90 degrees to the magnetization direction of the local area A of the magnetized circular magnetic ring. Fig. 8A shows a demagnetizing field simulation graph of a demagnetizing coil in a local area a and an adjacent area of a circular magnetic ring, wherein the abscissa is a circumferential Angle of the circular magnetic ring along the circumference in degrees, and the ordinate is a demagnetizing field strength generated by the demagnetizing coil in T, and TR represents an edge area of the demagnetizing coil; FIG. 8B shows the demagnetizing field attenuation slope at different circumferential positions) Is a simulation graph of |I _max =0.7T/degree, converted to length units ||_max=1.5T/mm。

Step 4: after the partial demagnetization is completed, the single magnets A, B, C in the magnet assembly are taken down to measure the magnetic moment M respectively, wherein the magnet A is a single magnet to be demagnetized, the magnet B is a magnet close to the magnet A, and the magnet C is a magnet close to the magnet B. Table 2 below shows the M/M _max ratio of the measurement results.

TABLE 2

From the above experimental results, it can be seen that the magnet demagnetized by the embodiments of the present application has a good demagnetizing effect in the target demagnetizing partial region, and the magnetization of the region other than the target demagnetizing partial region is less affected by the demagnetizing operation. Typically the magnitude of the remanent magnetic moment of the localized region of the magnet is less than or equal to 40% M _max, preferably less than or equal to 30% M _max; the magnetic moment of the magnet in the region other than the target demagnetizing local region is 90% M _max or more, preferably 98% M _max or more.

In the foregoing embodiments, the descriptions of the embodiments are emphasized, and for parts of one embodiment that are not described in detail, reference may be made to related descriptions of other embodiments. The technical features of the foregoing embodiments may be arbitrarily combined, and for brevity, all of the possible combinations of the technical features of the foregoing embodiments are not described, however, all of the combinations of the technical features should be considered as being within the scope of the disclosure.

The foregoing has outlined rather broadly the more detailed description of embodiments of the application in order that the detailed description of the principles and embodiments of the application may be implemented in conjunction with the detailed description of embodiments of the application that follows. Meanwhile, based on the idea of the present application, those skilled in the art can make changes or modifications on the specific embodiments and application scope of the present application, which belong to the protection scope of the present application. In view of the foregoing, this description should not be construed as limiting the application.

Claims

1. A method of demagnetizing a localized region of a permanent magnet, comprising:

Applying a current to the demagnetizing coils to generate a demagnetizing field having a predetermined angle with respect to the magnetization direction of the local region, wherein the directions of the magnetic fields generated by the upper and lower coils are the same,

Wherein an angle between a direction of a demagnetizing field generated by the demagnetizing coil and the magnetization direction of the local region is in a range of 90 ° ± 8 °.

2. The method of claim 1, wherein the demagnetizing field generated by the demagnetizing coil cooperates to provide a maximum magnetic field strength at the local region that satisfies the following equation:

2.1×HcJ/μ_r≤H≤3.5×HcJ/μ_r

wherein H is the magnetic field strength of the demagnetizing field, hcJ is the intrinsic coercivity of the local region, and mu _r is the relative permeability of the local region.

3. The method of claim 1, wherein a gap between the upper coil and the lower coil satisfies the following formula:

D≤10/μ_r

Wherein D is the gap between the upper coil and the lower coil, and mu _r is the relative permeability of the local area in mm.

4. The method of claim 1, wherein a maximum decay slope of a demagnetizing field generated by the demagnetizing coil at an edge region of the demagnetizing coil is greater than or equal to 0.5T/mm.

5. The method of claim 1, further comprising:

And core rods are arranged in the upper coil and the lower coil, wherein the part, far away from the local area, of the core rods is made of magnetic conductive materials so as to strengthen the magnetic field generated by the demagnetizing coil, and the part, close to the local area, of the core rods is made of non-magnetic conductive materials so as to avoid the local area from being adsorbed on the demagnetizing coil.

6. The method of claim 1, further comprising:

and magnetizing the whole permanent magnet before demagnetizing.

7. The method of claim 1, wherein the demagnetizing field generated by the demagnetizing coil is a dc pulsed magnetic field or an ac decaying magnetic field.

8. An apparatus for demagnetizing a localized region of a permanent magnet, comprising:

An upper coil arranged close to the upper side of the local area; and

A lower coil arranged near the lower part of the local area and opposite to the upper coil, wherein the magnetic fields generated by the upper coil and the lower coil have the same direction and form a preset angle with the magnetization direction of the local area,

Wherein the angle between the direction of the magnetic field generated by the upper and lower coils and the magnetization direction of the local region is in the range of 90 ° ± 8 °.

9. The apparatus of claim 8, wherein a maximum magnetic field strength of the magnetic fields generated by the upper and lower coils acting together on the local region satisfies the following formula:

2.1×HcJ/μ_r≤H≤3.5×HcJ/μ_r

wherein H is the magnetic field intensity, hcJ is the intrinsic coercive force of the local area, and mu _r is the relative magnetic permeability of the local area.

10. The apparatus of claim 8, wherein a gap between the upper coil and the lower coil satisfies the following formula:

D≤10/μ_r

11. The apparatus of claim 8, wherein a maximum decay slope of a magnetic field strength of the magnetic field generated by the upper and lower coils at edge regions of the upper and lower coils is greater than or equal to 0.5T/mm.

12. The apparatus of claim 8, further comprising:

the core rod is respectively arranged in the upper coil and the lower coil, the part of the core rod, which is far away from the local area, is made of magnetic conductive materials so as to strengthen the magnetic fields generated by the upper coil and the lower coil, and the part of the core rod, which is close to the local area, is made of non-magnetic conductive materials so as to avoid the local area from being adsorbed on the upper coil and/or the lower coil.

13. The apparatus of claim 8, wherein the magnetic fields generated by the upper and lower coils are dc pulsed magnetic fields or ac decaying magnetic fields.

14. A magnet obtainable by a method according to any one of claims 1 to 7 or by a device according to any one of claims 8 to 13, wherein the residual magnetic moment of a local region of the magnet has a magnitude of less than or equal to 40% M _max, and the magnetic moment of a region of the magnet other than the local region has a magnitude of greater than or equal to 90% M _max, wherein M _max is the saturated magnetic moment of the local region.

15. The magnet of claim 14, wherein a partial region of the magnet has a remanent magnetic moment of 30% m _max or less and a region of the magnet other than the partial region has a magnetic moment of 98% m _max or more.