Demagnetization method and device for local area of ​​permanent magnet and magnet

Abstract

The present application provides a method, device and magnet for demagnetizing a local area of ​​a permanent magnet. The demagnetization method comprises: placing a demagnetization coil near the local area of ​​the permanent magnet, wherein the demagnetization coil comprises an upper coil and a lower coil, wherein the upper coil is near the top of the local area, and the lower coil is near the bottom of the local area; and passing a current through the demagnetization coil to generate a demagnetization field at a predetermined angle to the magnetization direction of the local area, wherein the magnetic fields generated by the upper coil and the lower coil have the same direction.

Description

Technical Field

[0001] The present application relates to the technical field of magnetic materials, and in particular to a method and device for demagnetizing a local area of ​​a permanent magnet, and a magnet. Background Art

[0002] Permanent magnets have been widely used in all aspects of people's production and life due to their unique properties. In particular, NdFeB permanent magnets, for example, are widely used in many fields such as automobiles, electronics, machinery, energy, and medical equipment due to their excellent magnetic properties.

[0003] For example, permanent magnet components such as motors and mobile phone wireless charging are usually assembled into components or parts without magnetism and then magnetized in order to mass produce them.

[0004] In some special application fields, in order to meet the magnetic field requirements in the entire magnetic circuit, the magnetic field strength in a local area of ​​the magnet assembly is made much lower than that in other areas as needed, and the magnetic field strength of each magnet or each part of the magnet outside the local area is uniform. In order to achieve the above effect, in the prior art, the magnets are usually assembled into a whole or magnetized at intervals after a single magnet is integrally formed, that is, the magnets in a local area are not magnetized, and magnetization is performed in other areas.

[0005] However, since permanent magnets such as sintered NdFeB have the characteristic of easy magnetization, during the above operation, the magnets in the local area that do not want to be magnetized are often affected by the surrounding magnetizing magnetic field, resulting in weak magnetism on the surface of the magnets in this area. The surface magnetic field strength may exceed the design magnetic field strength, which does not meet the design requirements. Summary of the invention

[0006] In order to solve the above problems in the prior art, the present application provides a method and device for demagnetizing a local area of ​​a permanent magnet, and a magnet.

[0007] According to one aspect of the present application, a method for demagnetizing a local area of ​​a permanent magnet is provided, comprising:

[0008] placing a demagnetization coil near the local area of ​​the permanent magnet, wherein the demagnetization coil comprises an upper coil and a lower coil, the upper coil is near the upper part of the local area, and the lower coil is near the lower part of the local area; and

[0009] A current is supplied to the demagnetization coil to generate a demagnetization field which is at a predetermined angle to the magnetization direction of the local area, wherein the magnetic fields generated by the upper coil and the lower coil are in the same direction.

[0010] According to another aspect of the present application, a device for demagnetizing a local area of ​​a permanent magnet is also provided, comprising: an upper coil for being arranged above the local area; and a lower coil for being arranged below 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 are at a predetermined angle to the magnetization direction of the local area.

[0011] According to another aspect of the present application, there is also provided a magnet obtained by the method or device as described above, wherein the magnitude of the residual magnetic moment of a local area of ​​the magnet is ≤40%M max The magnetic moment of the magnet in the area other than the local area is ≥90%M max , where M max is the saturation magnetic moment of the local area.

[0012] Therefore, in order to obtain a magnet or magnet assembly whose magnetization intensity in a certain local area is much lower than that in other areas, the entire magnet or magnet assembly can be fully magnetized first (that is, the specific local area is also magnetized), and then the specific local area is demagnetized. Moreover, the direction of the magnetic field for demagnetization is basically perpendicular to the magnetization direction of the local area to be demagnetized. On the one hand, this can achieve the demagnetization effect for the local area and avoid affecting the magnetization intensity of the surrounding area; on the other hand, the demagnetization method of the present application can also avoid the problem in the prior art that the intensity of the reverse magnetic field is high and difficult to accurately control when demagnetizing using a magnetic field that is completely opposite to the magnetization intensity of the magnet. BRIEF DESCRIPTION OF THE DRAWINGS

[0013] In order to more clearly illustrate the technical solutions in the embodiments of the present application, the drawings required for use in the description of the embodiments will be briefly introduced below. Obviously, the drawings described below are only some embodiments of the present application. For ordinary technicians in this field, other drawings can be obtained based on these drawings without creative work.

[0014] Figure 1 A flow chart showing a method for demagnetizing a local area of ​​a permanent magnet according to one embodiment of the present application is shown.

[0015] Figure 2 A schematic diagram of a demagnetization operation according to this embodiment is shown.

[0016] Figure 3 A schematic diagram showing the gap setting between the upper coil and the lower coil according to an embodiment of the present application is shown.

[0017] Figure 4 A flow chart of a method for demagnetizing a local area of ​​a permanent magnet according to another embodiment of the present application is shown.

[0018] Figure 5 A schematic diagram of a demagnetization coil according to this embodiment is shown.

[0019] Fig. 6A A demagnetization magnetic field simulation curve diagram of a demagnetization coil in a local area A and adjacent areas of a magnet assembly according to an embodiment of the present application is shown.

[0020] Figure 6B The demagnetization magnetic field attenuation slopes at different lateral positions according to one embodiment of the present application are shown ( ) of the simulation curve.

[0021] Figure 7 A schematic diagram of demagnetizing a local area of ​​a ring-shaped NdFeB permanent magnet sample according to an embodiment of the present application is shown.

[0022] Fig. 8A A demagnetization magnetic field simulation curve diagram of a demagnetization coil in a local area and adjacent areas of a circular magnetic ring according to an embodiment of the present application is shown.

[0023] Figure 8B The demagnetization magnetic field attenuation slopes at different circumferential positions according to an embodiment of the present application are shown ( ) of the simulation curve. DETAILED DESCRIPTION

[0024] In order to better understand the technical solutions and advantages of the present application, the contents of the present application are further described in detail below in conjunction with the accompanying drawings and specific embodiments. However, the specific embodiments described herein are only used to explain the present application and are not intended to limit the present application. In addition, the technical features involved in the various embodiments of the present application described below can be used in combination except for situations where they conflict with each other, thereby constituting other embodiments within the scope of the present application.

[0025] The content described below provides many different embodiments or examples to realize the different structures of the present application. In order to simplify the disclosure of the present application, the parts and settings of specific examples are described below. Of course, they are only examples, and the purpose is not to limit the present application. In addition, the present application can repeat reference numerals and / or reference letters in different examples, and this repetition is for the purpose of simplification and clarity, and itself does not indicate the relationship between the various embodiments and / or settings discussed.

[0026] The flowchart in the accompanying drawings shows the possible implementation of the method according to one or more embodiments of the present application. It should be noted that in some alternative implementations, the steps marked in the frame may not occur in the order marked in the accompanying drawings. For example, depending on the functionality involved, two or more frames shown in succession may actually be executed roughly at the same time, or these frames are sometimes executed in the opposite order. These embodiments all belong to the protection scope of the present application.

[0027] Words indicating directions such as up, down, left, right, front, and back described in this application all refer to directions in the accompanying drawings and do not represent directions in actual application scenarios.

[0028] Figure 1 A flow chart showing a method for demagnetizing a local area of ​​a permanent magnet according to one embodiment of the present application is shown; Figure 2 FIG. 2 shows a schematic diagram of the demagnetization operation according to this embodiment. Figure 1 and 2 As shown, the method 100 for demagnetizing a local area of ​​a permanent magnet may include steps S110 and S120.

[0029] Before the operation of step S110, the permanent magnet 200 (such as, but not limited to, a neodymium iron boron magnet) has been fully magnetized as a whole, and has the magnetization strength and direction required by the design as a whole. The permanent magnet 200 can be a magnet assembly, which is composed of a plurality of small magnets ( Figure 2 The magnets shown in the figure can be regarded as magnets A, B, and C, and the magnetic properties of each magnet are basically the same); or the permanent magnet 200 can also be a single magnet, which is integrally formed ( Figure 2 As shown in the figure, a single magnet includes regions A, B, and C, and the magnetic properties of each region are basically the same). In the permanent magnet 200, the local region A needs to be demagnetized. In the following description of the present application, the local region A is the region that needs to be demagnetized according to the design requirements. The local region A can be a part of the integrally formed permanent magnet 100, or one or more magnets in a spliced ​​magnet assembly.

[0030] In step S110, a demagnetization coil is placed close to a local area A of the permanent magnet 200. Figure 2 As shown, the demagnetization coil may include an upper coil 310 and a lower coil 320, wherein the upper coil 310 is located above the local area A and the lower coil 320 is located below the local area A. In the present application, the permanent magnet 200 may be in the shape of a strip, a ring, or any other shape required by the design.

[0031] In step S120, current is passed through the demagnetization coil to generate a demagnetization field that is at a predetermined angle to the magnetization direction of the local area A. Figure 2As shown, 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 predetermined angle may be within the range of 90°±8°, preferably within the range of 90°±3°. Figure 2 Before demagnetization is performed using method 100, the magnetization direction of the local area A of the permanent magnet 200 is substantially perpendicular to the paper surface. According to an embodiment of the present application, the demagnetization magnetic field generated by the demagnetization coil can be a DC pulse magnetic field or an AC decaying magnetic field, preferably an AC decaying magnetic field.

[0032] Therefore, in order to obtain a magnet or magnet assembly whose magnetization intensity in a certain local area is much lower than that in other areas, the entire magnet or magnet assembly can be fully magnetized first (that is, the specific local area is also magnetized), and then the specific local area is demagnetized. Moreover, the direction of the magnetic field for demagnetization is basically perpendicular to the magnetization direction of the local area to be demagnetized. On the one hand, this can achieve the demagnetization effect for the local area and avoid affecting the magnetization intensity of the surrounding area; on the other hand, the demagnetization method of the present application can also avoid the problem in the prior art that the intensity of the reverse magnetic field is high and difficult to accurately control when demagnetizing using a magnetic field that is completely opposite to the magnetization intensity of the magnet.

[0033] According to one embodiment of the present application, the maximum magnetic field intensity of the demagnetization field generated by the demagnetization coil acting together in the local area A satisfies the following formula:

[0034] 2.1×HcJ / μ r ≤H≤3.5×HcJ / μ r

[0035] Where H is the magnetic field strength of the demagnetizing field, HcJ is the intrinsic coercive force of the local area A of the permanent magnet 200, μ r is the relative magnetic permeability of the local area A.

[0036] Therefore, when setting the magnitude of the demagnetization field generated by the demagnetization coil, the present application can ensure that the generated field strength is large enough (≥2.1×H cJ / μ r ), which is sufficient to achieve a good demagnetization effect in the local area A. On the other hand, if the demagnetization field strength is too large, the demagnetization area will increase, and the transition area between the demagnetization area and the non-demagnetization area will increase, thereby weakening the precise control of the demagnetization area. Therefore, in this application, H≤3.5×HcJ / μ r This can effectively avoid this situation.

[0037] Figure 3 FIG. 1 is a schematic diagram showing the gap setting between the upper coil and the lower coil according to an embodiment of the present application. Figure 3As shown, there is a gap D between the upper coil 310 and the lower coil 320, and the gap D is used to accommodate the local area A to be demagnetized. The gap D (in units of mm) satisfies the following formula:

[0038] D≤10 / μ r

[0039] where μ r is the relative magnetic permeability of the local area A of the permanent magnet 200 .

[0040] Therefore, the gap between the upper coil and the lower coil is set as small as possible to prevent the magnet from vibrating during the demagnetization field application and / or deflecting in the demagnetization field. In addition, if the gap between the upper coil and the lower coil is too large, the demagnetization field at the edge of the coil is also likely to affect the magnetization intensity of the non-demagnetization area other than the local area A to be demagnetized, for example, it may partially demagnetize it.

[0041] According to one embodiment of the present application, the maximum attenuation slope of the magnetic field strength of the demagnetization field generated by the demagnetization coil in the edge area of ​​the demagnetization coil is greater than or equal to 0.5T / mm, preferably, greater than or equal to 0.7T / mm. The demagnetization field generated by the demagnetization coil in the area outside the projection area of ​​the magnet or magnet assembly surface is relatively small. In order to minimize the influence of the demagnetization field on the area outside the local area A to be demagnetized, the attenuation slope of the demagnetization field needs to be larger as the distance from the edge of the projection area increases. In other words, the magnetic field strength generated by the demagnetization coil at its edge will drop sharply as it moves away from the local area to be demagnetized.

[0042] For example, taking a sintered NdFeB magnet as an example of a permanent magnet, the inventors found that after the sintered NdFeB magnet is saturated and magnetized, a demagnetization field H is applied perpendicular to the magnetization direction. When the field strength of the magnetic field H is ≥2.1×HcJ, a better demagnetization effect will be achieved, that is, after demagnetizing the local area A of the magnet, the demagnetization effect will reach M / M max <30% effect (M is the magnetic moment of local area A after demagnetization, M max is the saturation magnetic moment of local area A). However, since the magnetic circuit is open, the demagnetization magnetic field will also act on the non-demagnetization area B around the local area A to be demagnetized. When the demagnetization magnetic field strength acting on the surface of the non-demagnetization area B reaches 1.5×HcJ, it will affect the surface magnetization intensity of the magnetized non-demagnetization area B to decrease, which is an undesirable situation. That is, the demagnetization magnetic field needs to be concentrated in the local area A, and the non-demagnetization area B is affected as little as possible by the demagnetization magnetic field. Therefore, it is necessary to control the attenuation slope of the demagnetization magnetic field with the edge distance of the demagnetization coil (that is, the edge distance of the local area) to ensure that the area affected by the demagnetization magnetic field is limited to the local area A to be demagnetized.

[0043] Figure 4A flow chart showing a method for demagnetizing a local area of ​​a permanent magnet according to another embodiment of the present application; Figure 5 FIG. 4 shows a schematic diagram of a demagnetization coil according to this embodiment. Figure 4 As shown, in addition to steps S110 and S120, the method 100' for demagnetizing a local area of ​​a permanent magnet may further include step S130. Figure 4 In the method 100' shown in FIG. 1 , the operations of steps S110 and S120 are similar to those of steps S110 and S120. Figure 1 Similarly, for the sake of brevity, it will not be repeated here.

[0044] In step S130, a core rod 330 is disposed in the upper coil 310 and the lower coil 320. The core rod 330 is away from the local area A ( Figure 4 Part I of the core rod 330 (not shown) is made of a magnetic conductive material, and part II of the core rod 330 adjacent to the local area A is made of a non-magnetic conductive material. Thus, the magnetic conductive material part I can strengthen the magnetic field generated by the demagnetization coil, and the non-magnetic conductive material part II can prevent the local area A from being adsorbed on the demagnetization coil during or after the demagnetization operation, making it inconvenient to remove.

[0045] According to another embodiment of the present application, a device for demagnetizing a local area of ​​a permanent magnet is also provided. Figure 2 and Figure 3 , the device 300 may include an upper coil 310 and a lower coil 320. The upper coil 310 is used to be arranged above the local area A to be demagnetized of the permanent magnet, and the lower coil 320 is used to be arranged below the local area A to be demagnetized and opposite to the upper coil 310. According to the demagnetization requirements of the local area to be demagnetized, the magnetic fields generated by the upper coil 310 and the lower coil 320 have the same direction and are at a predetermined angle to the magnetization direction of the local area A.

[0046] See also Figure 5 , the device 300 may further include a core rod 330. The core rod 330 includes an upper core rod and a lower core rod, which are respectively arranged in the upper coil 310 and the lower coil 320. The portion of the core rod 330 away from the local area A to be demagnetized is made of a magnetic 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 local area A to be demagnetized is made of a non-magnetic conductive material to prevent the local area 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.

[0047] Embodiment 1:

[0048] Select a long strip of NdFeB permanent magnet sample (such as Figure 2 As shown in the figure, it is assembled into a magnet assembly by splicing multiple magnets. The dimensions of a single magnet are: thickness 2.0 mm, width 4.5 mm (magnetization direction), length 8.3 mm, and the μ of each single magnet isr =1.226, HcJ=12kOe, the magnetization direction of the magnet assembly is perpendicular to the paper surface. The sample is magnetized in the following manner, and then the local area A of the sample is demagnetized and tested:

[0049] Step 1: First, the magnet assembly is magnetized with a 2.5T magnetizing magnetic field, where the saturation magnetic moment value of each single magnet in the 2.5T magnetizing magnetic field is M max .

[0050] Step 2: Place the demagnetization coil close to the local area A of the permanent magnet. The demagnetization coil includes an upper coil and a lower coil. The upper coil is close to the top of the local area A, and the lower coil is close to the bottom of the local area A. The upper coil and the lower coil can respectively generate an AC attenuation magnetic field with the same magnetic field direction. The gap D between the upper coil and the lower coil is 3.0 mm, and the magnetic field direction is 90° to the magnetization direction of a single magnet. The maximum magnetic field intensity H of the demagnetization field in the demagnetization area is designed to be 2.7 T.

[0051] Step 3: A current is supplied to the demagnetization coil to generate a demagnetization field which is 90° to the magnetization direction of the local area A of the magnetized magnet assembly. Fig. 6A The demagnetization magnetic field simulation curve diagram of the demagnetization coil in the local area A of the magnet assembly and the adjacent area is shown, the horizontal axis is the lateral position L of the magnet assembly, the unit is mm, the vertical axis is the demagnetization magnetic field intensity generated by the demagnetization coil, the unit is T, where TR represents the edge area of ​​the demagnetization coil; Figure 6B The demagnetization magnetic field decay slope at different lateral positions is shown ( )'s simulation curve diagram, which | | max =0.8T / mm.

[0052] Step 4: After partial demagnetization is completed, remove the individual magnets A, B, and C in the magnet assembly (see Figure 2 ) respectively measure the magnetic moment M. The following table 1 shows the measurement results M / M max ratio.

[0053] Table 1

[0054]

[0055] Embodiment 2:

[0056] Select a circular NdFeB permanent magnet sample (such as Figure 7 As shown in the figure, it is assembled by splicing multiple magnets into a magnet assembly. The outer diameter of the circular magnetic ring assembly is D1 = 54mm, the inner diameter is D2 = 46mm, the center angle of each single magnet is 18°, the thickness is 1.0mm, and the μ of each single magnet is r= 1.022, HcJ=25kOe, the magnetization direction of the magnet assembly is toward the radial direction of the ring. The sample is magnetized in the following manner, and then the local area A of the sample is demagnetized and tested:

[0057] Step 1: First, use a 2.5T magnetizing magnetic field to magnetize the circular magnetic ring assembly as a whole, where the saturation magnetic moment value of each single magnet in the 2.5T magnetizing magnetic field is M max .

[0058] Step 2: Place the demagnetization coil close to the local area A of the circular magnetic ring. The demagnetization coil includes an upper coil and a lower coil. The upper coil is close to the top of the local area A, and the lower coil is close to the bottom of the local area A. The upper coil and the lower coil can generate a DC pulse magnetic field with the same magnetic field direction. The gap D between the upper coil and the lower coil is 2.0mm, and the magnetic field direction is 90° to the magnetization direction of a single magnet. The maximum magnetic field intensity H of the demagnetization field in the demagnetization area is designed to be 5.8T.

[0059] Step 3: A current is passed through the demagnetization coil to generate a demagnetization field that is 90° to the magnetization direction of the local area A of the magnetized circular magnetic ring. Fig. 8A The demagnetization magnetic field simulation curve diagram of the demagnetization coil in the local area A of the circular magnetic ring and the adjacent area is shown, the horizontal axis is the circumferential angle Angle of the circular magnetic ring along the circumference, the unit is degree, the vertical axis is the demagnetization magnetic field intensity generated by the demagnetization coil, the unit is T, where TR represents the edge area of ​​the demagnetization coil; Figure 8B The demagnetization magnetic field attenuation slope at different circumferential positions is shown ( )'s simulation curve diagram, which | | max =0.7T / degree, converted to length unit is | | max =1.5T / mm.

[0060] Step 4: After the partial demagnetization is completed, remove the individual magnets A, B, and C in the magnet assembly and measure the magnetic moment M respectively, where magnet A is the individual magnet to be demagnetized, magnet B is the magnet next to magnet A, and magnet C is the magnet next to magnet B. The following table 2 shows the measurement results M / M max ratio.

[0061] Table 2

[0062]

[0063] Based on the above experimental results, it can be seen that after the magnet is demagnetized using the various embodiments of the present application, the demagnetization effect of the target demagnetization local area is better, while the magnetization intensity of the area outside the target demagnetization local area is less affected by the demagnetization operation. max, preferably ≤30%M max ; The magnetic moment of the magnet in areas other than the target demagnetization area is ≥90%M max , preferably ≥98%M max .

[0064] In the above embodiments, the description of each embodiment has its own emphasis. For the part not described in detail in a certain embodiment, please refer to the relevant description of other embodiments. The technical features of the above embodiments can be combined arbitrarily. In order to make the description concise, all possible combinations of the technical features in the above embodiments are not described. However, as long as there is no contradiction in the combination of these technical features, they should be considered to be within the scope of this specification.

[0065] The embodiments of the present application are described in detail above. Specific examples are used herein to illustrate the principles and implementation methods of the present application. The description of the above embodiments is only used to help understand the method and its core idea of ​​the present application. At the same time, changes or deformations made by those skilled in the art based on the ideas of the present application, the specific implementation methods and the scope of application of the present application, all belong to the scope of protection of the present application. In summary, the content of this specification should not be construed as a limitation on the present application.

Claims

1. A method for demagnetizing a local area of ​​a permanent magnet, comprising: placing a demagnetization coil near the local area of ​​the permanent magnet, wherein the demagnetization coil comprises an upper coil and a lower coil, the upper coil is near the upper part of the local area, and the lower coil is near the lower part of the local area; and A current is passed through the demagnetization coil to generate a demagnetization field that is at a predetermined angle to the magnetization direction of the local area, wherein the magnetic fields generated by the upper coil and the lower coil have the same direction, The angle between the direction of the demagnetization magnetic field generated by the demagnetization coil and the magnetization direction of the local area is within the range of 90°±8°. The maximum magnetic field intensity of the demagnetization field generated by the demagnetization coil acting together in the local area satisfies the following formula: 2.1×HcJ / m r ≤H≤3.5×HcJ / μ r Where H is the magnetic field strength of the demagnetizing field, HcJ is the intrinsic coercivity of the local area, μ r is the relative magnetic permeability of the local area.

2. The method according to claim 1, wherein the gap between the upper coil and the lower coil satisfies the following formula: D≤10 / μ r Where D is the gap between the upper coil and the lower coil, expressed in mm, μ r is the relative magnetic permeability of the local area. 3 . The method according to claim 1 , wherein the maximum attenuation slope of the magnetic field intensity of the demagnetization magnetic field generated by the demagnetization coil in the edge region of the demagnetization coil is greater than or equal to 0.5 T / mm.

4. The method of claim 1, further comprising: A core rod is arranged in the upper coil and the lower coil, wherein a portion of the core rod away from the local area is made of a magnetic conductive material to strengthen the magnetic field generated by the demagnetization coil, and a portion of the core rod adjacent to the local area is made of a non-magnetic conductive material to prevent the local area from being adsorbed on the demagnetization coil.

5. The method of claim 1, further comprising: Before demagnetization, the permanent magnet is magnetized as a whole.

6. The method according to claim 1, wherein the demagnetization field generated by the demagnetization coil is a DC pulse magnetic field or an AC decay magnetic field.

7. A device for demagnetizing a local area of ​​a permanent magnet, comprising: An upper coil, used for being arranged above and close to the local area; as well as 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 predetermined angle with the magnetization direction of the local area, The angle between the direction of the magnetic field generated by the upper coil and the lower coil and the magnetization direction of the local area is within the range of 90°±8°. The maximum magnetic field intensity of the magnetic field generated by the upper coil and the lower coil acting together in the local area satisfies the following formula: 2.1×HcJ / m r ≤H≤3.5×HcJ / μ r Where H is the magnetic field strength, HcJ is the intrinsic coercivity of the local area, μ r is the relative magnetic permeability of the local area.

8. The device of claim 7, wherein the gap between the upper coil and the lower coil satisfies the following formula: D≤10 / μ r Where D is the gap between the upper coil and the lower coil, expressed in mm, μ r is the relative magnetic permeability of the local area.

9. The device of claim 7, wherein the magnetic field strength of the magnetic field generated by the upper coil and the lower coil has a maximum attenuation slope greater than or equal to 0.5 T / mm in the edge region of the upper coil and the lower coil.

10. The apparatus of claim 7, further comprising: The core rod is respectively arranged in the upper coil and the lower coil, the portion of the core rod away from the local area is made of magnetic conductive material to strengthen the magnetic field generated by the upper coil and the lower coil, and the portion of the core rod adjacent to the local area is made of non-magnetic conductive material to avoid the local area from being adsorbed on the upper coil and / or the lower coil.

11. The device as claimed in claim 7, wherein the magnetic field generated by the upper coil and the lower coil is a DC pulse magnetic field or an AC attenuated magnetic field.

12. A magnet obtained by the method according to any one of claims 1 to 6 or the device according to any one of claims 7 to 11, wherein: The magnitude of the residual magnetic moment of the local area of ​​the magnet is ≤40%M max The magnetic moment of the magnet in the area other than the local area is ≥90%M max , where M max is the saturation magnetic moment of the local area.

13. The magnet according to claim 12, wherein The magnitude of the residual magnetic moment in the local area of ​​the magnet is ≤30%M max The magnetic moment of the magnet in the area other than the local area is ≥98%M max .

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