Method for manufacturing Halbach magnet array body

Abstract

Provided is a method for easily manufacturing a Halbach magnet array body having a large ratio of magnetic flux density between the surface and the back surface. A method for manufacturing a Halbach magnet array body, comprising steps (a) and (b) in sequence, wherein step (a) is a step of magnetizing at least two first magnetic sheets in a direction parallel to a first direction, and step (b) is a step of magnetizing at least one second magnetic sheet in a direction parallel to a second direction, wherein the second direction is perpendicular to the first direction. The first magnetic sheets and the second magnetic sheets are alternately arranged in the second direction, and each of the first magnetic sheets is bonded to an adjacent second magnetic sheet. The first magnetic sheet has an easy magnetization axis parallel to the first direction, and the second magnetic sheet has an easy magnetization axis parallel to the second direction. The magnetization is performed under the condition that the residual magnetic susceptibility r1 of the first magnetic sheet is higher than the residual magnetic susceptibility r2 of the second magnetic sheet.

Description

Technical Field

[0001] The invention relates to a method for manufacturing a Halbach magnet array body. Background Art

[0002] Patent Document 1 describes a Halbach magnetic circuit including a plurality of permanent magnets having a plurality of regions magnetized in mutually different directions.

[0003] Halbach magnetic circuit, generally Figure 1 As shown, it includes a plurality of permanent magnets arranged in one direction, and the magnetization directions of adjacent permanent magnets form a specified angle (for example, 90°). With such an arrangement, one side (surface) of the Halbach magnetic circuit has a high surface magnetic flux (surface magnetic flux density), and the opposite side (back side) has a low surface magnetic flux or ideally zero surface magnetic flux.

[0004] Prior Art Literature

[0005] Patent Literature

[0006] Patent Document 1: Japanese Patent Application Publication No. 2018-092988 Summary of the invention

[0007] When a Halbach magnetic circuit is manufactured by bonding multiple magnetized magnets, it is difficult to perform accurate position control due to the repulsion between the magnets, and a large external force is required. Therefore, such a manufacturing method is not suitable for mass production. On the other hand, when a Halbach magnetic circuit is manufactured by bonding multiple unmagnetized magnetic bodies and then magnetizing each magnetic body in a specified direction, and when a Halbach magnetic circuit is manufactured by forming multiple regions magnetized in different directions in one permanent magnet as in Patent Document 1, there is a tendency for the ratio of the magnetic flux density on the surface and back of the Halbach magnetic circuit to be small.

[0008] Therefore, a method is provided that can easily manufacture a Halbach magnet array body having a large ratio of the magnetic flux density between the front surface and the back surface.

[0009] According to one embodiment of the present invention, a method for manufacturing a Halbach magnet array is provided, the method comprising:

[0010] a) magnetizing at least two first magnetic sheets in a direction parallel to the first direction,

[0011] wherein the at least two first magnetic sheets and the at least one second magnetic sheet are alternately arranged in a second direction perpendicular to the first direction,

[0012] The at least two first magnetic sheets are each bonded to an adjacent second magnetic sheet.

[0013] The at least two first magnetic sheets have easy magnetization axes parallel to the first direction.

[0014] The at least one second magnetic sheet has an easy magnetization axis parallel to the second direction.

[0015] The magnetization is performed under the condition that the at least two first magnetic sheets and the at least one second magnetic sheet satisfy the following formula (1),

[0016] r1>r2...(1)

[0017] In formula (1), r1 is the residual magnetic susceptibility of the at least two first magnetic sheets, which is expressed by the following formula (2), and r2 is the residual magnetic susceptibility of the at least one second magnetic sheet, which is expressed by the following formula (3):

[0018] r1=Br1 / Brs1...(2)

[0019] In formula (2), Br1 represents the residual magnetization when an external magnetic field parallel to the easy magnetization axis is applied to the at least two first magnetic sheets, Brs1 represents the saturated residual magnetization of the at least two first magnetic sheets,

[0020] r2=Br2 / Brs2...(3)

[0021] In formula (3), Br2 represents the residual magnetization intensity when an external magnetic field parallel to the easy magnetization axis of the at least one second magnetic body piece is applied to the at least one second magnetic body piece, and Brs2 represents the saturated residual magnetization intensity of the at least one second magnetic body piece; and

[0022] b) a step of magnetizing the at least one second magnetic sheet in a direction parallel to the second direction.

[0023] According to the manufacturing method of the present invention, it is possible to easily manufacture a Halbach magnet array body having a large ratio of magnetic flux density between the front surface and the back surface. BRIEF DESCRIPTION OF THE DRAWINGS

[0024] Figure 1 This is a diagram schematically showing an example of a Halbach magnet array.

[0025] Figure 2 It is a flowchart of the manufacturing method involved in embodiment.

[0026] Figure 3 This is a diagram schematically showing an example of an array body provided in the step of magnetizing the first magnetic sheet.

[0027] Figure 4 This is a graph showing the magnetizing properties of the first magnetic sheet and the second magnetic sheet used in Examples.

[0028] Figure 5 This is a graph showing the magnetizing characteristics of the third magnetic sheet used in Examples.

[0029] Figure 6 The graph shows the ratio of the magnetic flux on the surface to the sum of the magnetic flux on the surface and the back surface and the ratio of the magnetic flux on the back surface to the sum of the magnetic flux on the surface and the back surface of the test bodies of Example 1 and Comparative Examples 1 and 2.

[0030] Figure 7 The graph shows the ratio of the magnetic flux on the surface to the sum of the magnetic flux on the surface and the back surface and the ratio of the magnetic flux on the back surface to the sum of the magnetic flux on the surface and the back surface of the test bodies of Example 2 and Comparative Examples 3 and 4.

[0031] Description of Reference Numerals

[0032] 1: first magnetic sheet; 2: second magnetic sheet; 10: array; 20: Halbach magnet array. DETAILED DESCRIPTION

[0033] Hereinafter, the embodiments will be described with reference to the accompanying drawings as appropriate. The present invention is not limited to the following embodiments, and various design changes can be made within the scope of the spirit of the present invention as described in the claims. Furthermore, in the drawings referred to in the following description, the same components or components having the same functions are marked with the same symbols, and repeated descriptions are sometimes omitted. In addition, for the sake of convenience of explanation, the dimensional ratios of the drawings are sometimes different from the actual ratios, part of the components are omitted in the drawings, and so on. In addition, in the present application, the numerical range represented by the symbol "~" includes the numerical values ​​recorded before and after the symbol "~" as the lower limit and upper limit, respectively.

[0034] The manufacturing method of the Halbach magnet array body is as follows: Figure 2 As shown, the method includes a step (S1) of magnetizing a first magnetic sheet and a step (S2) of magnetizing a second magnetic sheet.

[0035] a) Magnetization of the first magnetic sheet

[0036] First, prepare at least two unmagnetized first magnetic sheets and at least one unmagnetized second magnetic sheet. The first magnetic sheet and the second magnetic sheet contain permanent magnet materials. Examples of permanent magnet materials include Nd-Fe-B magnet materials, Sm-Co magnet materials, Sm-Fe-N magnet materials, ferrite magnet materials, and Al-Ni-Co magnet materials. The first magnetic sheet and the second magnetic sheet have magnetic anisotropy. That is, the first magnetic sheet and the second magnetic sheet have an easy magnetization axis and a hard magnetization axis, respectively. The first magnetic sheet and the second magnetic sheet can have any shape, for example, a roughly rectangular shape. The first magnetic sheet 1 and the second magnetic sheet 2 can be manufactured using a generally known manufacturing method. As the first magnetic sheet 1 and the second magnetic sheet 2, commercially available magnetic sheets can also be used.

[0037] like Figure 3 As shown in FIG. 1 , the first magnetic sheets 1 and the second magnetic sheets 2 are alternately arranged in a predetermined direction, and the adjacent first magnetic sheets 1 and second magnetic sheets 2 are bonded together to obtain an array body 10. Figure 3 In the embodiment, three first magnetic sheets 1 and two second magnetic sheets 2 are arranged alternately, but if the first magnetic sheets 1 and the second magnetic sheets 2 can be arranged alternately, at least two first magnetic sheets may include more or less than three first magnetic sheets 1, and at least one second magnetic sheet may include more or less than two second magnetic sheets 2. The first magnetic sheet 1 and the second magnetic sheet 2 may be bonded using any adhesive agent.

[0038] In the array body 10, the easy magnetization axis of the first magnetic sheet 1 (at Figure 3 , indicated by a blank arrow) and the first direction ( Figure 3 The easy magnetization axis of the second magnetic sheet 2 (in the Z direction) is parallel to Figure 3 , indicated by blank arrows) and the second direction ( Figure 3 Here, the first direction and the second direction are perpendicular to each other. In addition, the second direction is parallel to the arrangement direction of the first magnetic sheet 1 and the second magnetic sheet 2.

[0039] Next, the first magnetic sheet 1 of the array body 10 is magnetized in a direction parallel to the first direction. The magnetization directions of the first magnetic sheets 1 adjacent to each other with one second magnetic sheet 2 interposed therebetween are different by 180°.

[0040] The first magnetic sheet 1 can be magnetized using any magnetizer. For example, the first magnetic sheet 1 can be magnetized by placing the first magnetic sheet 1 in a magnetic field (external magnetic field) generated by a magnetizing yoke.

[0041] The intensity of the external magnetic field for magnetizing the first magnetic sheet 1, the temperature of the first magnetic sheet 1 and the second magnetic sheet 2 and other conditions are appropriately set according to the magnetic properties of the first magnetic sheet 1 and the second magnetic sheet 2, so that a sufficiently large residual magnetization intensity is generated in the first magnetic sheet 1 and a sufficiently small residual magnetization intensity is generated in the second magnetic sheet 2, or the residual magnetization intensity of the second magnetic sheet 2 becomes substantially zero.

[0042] Specifically, the first magnetic sheet 1 is magnetized under the condition that the first magnetic sheet 1 and the second magnetic sheet 2 satisfy the following formula (1).

[0043] r1>r2...(1)

[0044] In the formula (1), r1 represents the residual magnetic susceptibility of the first magnetic sheet 1 represented by the following formula (2).

[0045] r1=Br1 / Brs1...(2)

[0046] In the formula (2), Br1 represents the residual magnetization when an external magnetic field parallel to the easy magnetization axis is applied to the first magnetic sheet 1 , and Brs1 represents the saturated residual magnetization of the first magnetic sheet 1 .

[0047] In the formula (1), r2 represents the residual magnetic susceptibility of the second magnetic sheet 2 represented by the following formula (3).

[0048] r2=Br2 / Brs2...(3)

[0049] In the formula (3), Br2 represents the residual magnetization when an external magnetic field parallel to the easy magnetization axis is applied to the second magnetic sheet 2 , and Brs2 represents the saturated residual magnetization of the second magnetic sheet 2 .

[0050] An example satisfying the condition of the above formula (1) will be described later.

[0051] b) Magnetization of the second magnetic sheet

[0052] Next, the second magnetic sheet 2 of the array body 10 is magnetized in a direction parallel to the second direction. The magnetization directions of the second magnetic sheets 2 adjacent to each other with one first magnetic sheet 1 interposed therebetween are different by 180°.

[0053] The second magnetic sheet 2 can be magnetized using any magnetizer. For example, the second magnetic sheet 2 can be magnetized by placing the second magnetic sheet 2 in a magnetic field (external magnetic field) generated by a magnetizing yoke.

[0054] The intensity of the external magnetic field for magnetizing the second magnetic sheet 2 and the temperature of the second magnetic sheet 2 can be appropriately set according to the magnetizing characteristics of the second magnetic sheet 2 so that a sufficiently large residual magnetization intensity is generated in the second magnetic sheet 2 .

[0055] In this way, we can make Figure 1 The Halbach magnet array body 20 is shown.

[0056] The following describes the conditions for satisfying the above formula (1).

[0057] In one embodiment, the first magnetic sheet 1 is easier to magnetize than the second magnetic sheet 2. In the present application, "easier to magnetize" means that the magnetic flux density of the external magnetic field required to magnetize an unmagnetized magnetic body at a specified residual magnetic susceptibility under normal temperature conditions is smaller. Since the first magnetic sheet 1 is easier to magnetize than the second magnetic sheet 2, the above formula (1) can be satisfied when the first magnetic sheet 1 and the second magnetic sheet 2 are at normal temperature. Therefore, in this embodiment, the first magnetic sheet 1 can be magnetized at normal temperature.

[0058] In this embodiment, the difference between the magnetic flux density B2 of the external magnetic field required to magnetize the second magnetic sheet 2 at a residual magnetic susceptibility of 98% at room temperature and the magnetic flux density B1 of the external magnetic field required to magnetize the first magnetic sheet 1 at a residual magnetic susceptibility of 98% at room temperature may be greater than 0.2 T. That is, the equation: B2-B1>0.2 T may be satisfied. Furthermore, the equation: B2-B1>0.5 T may be satisfied, and in particular, the equation: B2-B1>1 T may be satisfied.

[0059] In this embodiment, the residual magnetic susceptibility r1 of the first magnetic sheet 1 under the condition of magnetizing the first magnetic sheet 1 can be, for example, greater than 95% and less than 100%, and the residual magnetic susceptibility r2 of the second magnetic sheet 2 under the same condition can be, for example, greater than 0% and less than 95%.

[0060] Generally, the ease of magnetization of a magnetic sheet depends on the ratio of the main phase (for example, in the case of Nd-Fe-B magnet material, Nd-Fe-B phase) of the magnetic sheet, the size of the crystal grain size, etc. The first magnetic sheet 1 may have a higher ratio of the main phase than the second magnetic sheet 2 and / or may have a larger average crystal grain size than the second magnetic sheet 2. Thus, the first magnetic sheet 1 can be magnetized more easily than the second magnetic sheet 2.

[0061] In addition, in this embodiment, the magnetic flux density Ba of the external magnetic field used in the step of magnetizing the first magnetic sheet 1 and the magnetic flux density Bb of the external magnetic field used in the step of magnetizing the second magnetic sheet 2 may also satisfy Ba<Bb. When the second magnetic sheet 2 is magnetized, the first magnetic sheet 1 has already been magnetized, so the influence of the external magnetic field used to magnetize the second magnetic sheet 2 on the magnetization direction of the first magnetic sheet 1 can be suppressed or reduced, and the second magnetic sheet 2 can be fully magnetized.

[0062] In another embodiment, the first magnetic sheet 1 is magnetized under the condition that the first magnetic sheet 1 has a higher temperature than the second magnetic sheet 2. Generally, the higher the temperature of the magnetic sheet during magnetization, the higher the residual magnetic susceptibility of the magnetic sheet. Therefore, under the condition that the first magnetic sheet 1 has a higher temperature than the second magnetic sheet 2, the above formula (1) can be satisfied. In this embodiment, the first magnetic sheet 1 and the second magnetic sheet 2 can be the same magnetic sheet. Furthermore, generally, the temperature dependence of the residual magnetic susceptibility of the magnetic sheet depends on the type of magnetic material contained as the main component in the magnetic sheet, the presence or absence of element substitution in the magnetic material and the type of the substitution element, and the structure of the magnetic sheet (e.g., crystal grain size).

[0063] In this embodiment, the first magnetic sheet 1 may be magnetized while being heated, or the second magnetic sheet 2 may be magnetized while being kept at room temperature or cooled to a temperature lower than room temperature. In this embodiment, the second magnetic sheet 2 may be magnetized while being heated, or the second magnetic sheet 2 may be magnetized while being kept at room temperature or cooled to a temperature lower than room temperature. The first magnetic sheet 1 and / or the second magnetic sheet 2 may be heated using any heating mechanism (e.g., a hot plate resistance heater and a rubber heater). The first magnetic sheet 1 and / or the second magnetic sheet 2 may be heated and magnetized using a magnetizing yoke with a heater. The first magnetic sheet 1 and / or the second magnetic sheet 2 may be cooled using any cooling mechanism (e.g., a water cooling block).

[0064] In this embodiment, the residual magnetic susceptibility r1 of the first magnetic sheet 1 under the condition of magnetizing the first magnetic sheet 1 can be, for example, greater than 90% and less than 100%, and the residual magnetic susceptibility r2 of the second magnetic sheet 2 under the same condition can be, for example, greater than 0% and less than 90%, or can be greater than 0% and less than 60%.

[0065] The embodiments of the present invention are described in detail above, but the present invention is not limited to the above embodiments, and various changes can be made within the scope of the spirit of the present invention described in the claims. The above embodiments can also be combined to provide further embodiments. For example, a first magnetic sheet and a second magnetic sheet having different magnetization faculties can be used, and the magnetization of the first magnetic sheet and / or the magnetization of the second magnetic sheet can be performed when the first magnetic sheet and the second magnetic sheet have different temperatures.

[0066] Example

[0067] Hereinafter, the present invention will be specifically described using examples, but the present invention is not limited to these examples.

[0068] Example 1

[0069] Three first magnetic sheets (neodymium magnet sintered bodies) and two second magnetic sheets (neodymium magnet sintered bodies) were prepared. The first magnetic sheets and the second magnetic sheets both had magnetic anisotropy and had Figure 4 The magnetic properties shown. Figure 4 This is a graph showing the residual magnetic susceptibility (i.e., the ratio of residual magnetization intensity to saturated residual magnetization intensity) when an external magnetic field parallel to the easy magnetization axis is applied to the first magnetic sheet and the second magnetic sheet relative to the magnetic flux density of the external magnetic field. The saturated residual magnetization intensity of the first magnetic sheet and the second magnetic sheet is obtained by magnetizing the first magnetic sheet and the second magnetic sheet using an external magnetic field with a magnetic flux density of 7 T and measuring the residual magnetization intensity. The magnetic flux density of the external magnetic field at which the residual magnetic susceptibility of the first magnetic sheet becomes 98% is about 0.6 T, and the magnetic flux density of the external magnetic field at which the residual magnetic susceptibility of the second magnetic sheet becomes 98% is about 1.6 T, and the difference is about 1 T.

[0070] The first magnetic sheet and the second magnetic sheet are alternately arranged in the second direction in such a manner that the easy magnetization axis of the first magnetic sheet is parallel to the first direction and the easy magnetization axis of the second magnetic sheet is parallel to the second direction perpendicular to the first direction, and the adjacent first magnetic sheet and second magnetic sheet are bonded using an adhesive.

[0071] The three first magnetic sheets are magnetized at room temperature using an external magnetic field having a magnetic flux density of about 0.5 T and parallel to the first direction (step a).

[0072] Next, the two second magnetic sheets are magnetized at room temperature using an external magnetic field having a magnetic flux density of about 1.4 T and parallel to the second direction (step b).

[0073] Thus, we have Figure 1 A test piece of the Halbach array as shown.

[0074] Comparative Example 1

[0075] A test body having a Halbach array was produced in the same manner as in Example 1 except that the first magnetic sheet was used instead of the second magnetic sheet and the magnetic flux density of the external magnetic field in step b was changed to 0.5 T.

[0076] Comparative Example 2

[0077] Five first magnetic sheets were prepared, and each first magnetic sheet was magnetized in the direction of its easy magnetization axis using an external magnetic field with a magnetic flux density of 0.5 T. Next, the first magnetic sheets were arranged and bonded together with an adhesive to produce a Figure 1 A test piece of the Halbach array as shown.

[0078] Example 2

[0079] A third magnetic sheet is used instead of the first magnetic sheet and the second magnetic sheet, and the sheets are arranged and bonded in the same manner as in Example 1. The third magnetic sheet has Figure 5 The magnetic properties shown. Figure 5 This is a graph showing the residual susceptibility (i.e., the ratio of the residual magnetization intensity to the saturation residual magnetization intensity) when an external magnetic field of a predetermined magnetic flux density parallel to the easy magnetization axis of the third magnetic body piece is applied to the third magnetic body piece relative to the temperature of the third magnetic body piece. The saturation residual magnetization intensity is obtained by magnetizing the third magnetic body piece using an external magnetic field of a magnetic flux density of 7 T and measuring the residual magnetization intensity.

[0080] While heating the three magnetic sheets whose easy magnetization axes are parallel to the first direction to 65°C and cooling the two magnetic sheets whose easy magnetization axes are parallel to the second direction to below room temperature, the three magnetic sheets whose easy magnetization axes are parallel to the first direction are magnetized using an external magnetic field with a magnetic flux density of about 0.2T and parallel to the first direction (step a).

[0081] While heating the two magnetic sheets whose easy magnetization axes are parallel to the second direction to above 65°C and cooling the three magnetic sheets whose easy magnetization axes are parallel to the first direction to below room temperature, the two magnetic sheets whose easy magnetization axes are parallel to the second direction are magnetized using an external magnetic field with a magnetic flux density of about 0.2T and parallel to the second direction (step b).

[0082] Thus, we obtain Figure 1 A test piece of the Halbach array as shown.

[0083] Comparative Example 3

[0084] A test body having a Halbach array was produced in the same manner as in Example 2 except that the magnetic sheet was not heated or cooled in steps a and b and the magnetic flux density of the external magnetic field was set to 0.4 T.

[0085] Comparative Example 4

[0086] Five third magnetic sheets were prepared, and each third magnetic sheet was magnetized in the direction of its easy magnetization axis using an external magnetic field with a magnetic flux density of 0.4 T. Next, the third magnetic sheets were arranged and bonded with an adhesive to produce a Figure 1 A test piece of the Halbach array as shown.

[0087] evaluate

[0088] The magnetic flux of each test body on two surfaces perpendicular to the first direction was measured using a flux meter. Of the two surfaces, the surface with a larger magnetic flux was the front surface, and the surface with a smaller magnetic flux was the back surface. The ratio of the magnetic flux of each surface to the sum of the magnetic flux of the front surface and the back surface was calculated. Figure 6 and Figure 7 The results are shown in .

[0089] like Figure 6 As shown in FIG. 1 , the test body of Example 1 has a larger surface magnetic flux ratio than the test body of Comparative Example 1. Figure 7 As shown, the test body of Example 2 has a larger surface magnetic flux ratio than the test body of Comparative Example 3.

[0090] Furthermore, the surface flux ratios of the test bodies of Examples 1 and 2 are smaller than those of Comparative Examples 2 and 4, respectively. However, the test bodies of Comparative Examples 2 and 4 are made by bonding magnetized magnetic sheets, and this production method is not suitable for mass production.

Claims

1. A method for manufacturing a Halbach magnet array, the method comprising steps a and b in sequence, The step a is a step of magnetizing at least two first magnetic sheets in a direction parallel to the first direction. in, The at least two first magnetic sheets and the at least one second magnetic sheet are alternately arranged in a second direction perpendicular to the first direction. The at least two first magnetic sheets are each bonded to an adjacent second magnetic sheet. The at least two first magnetic sheets have easy magnetization axes parallel to the first direction. The at least one second magnetic sheet has an easy magnetization axis parallel to the second direction. The magnetization is performed under the condition that the at least two first magnetic sheets and the at least one second magnetic sheet satisfy the following formula (1), r1＞r2…(1) In formula (1), r1 is the residual magnetic susceptibility of the at least two first magnetic sheets, which is expressed by the following formula (2), and r2 is the residual magnetic susceptibility of the at least one second magnetic sheet, which is expressed by the following formula (3): r1=Br1 / Brs1…(2) In formula (2), Br1 represents the residual magnetization when an external magnetic field parallel to the easy magnetization axis is applied to the at least two first magnetic sheets, and Brs1 represents the saturated residual magnetization of the at least two first magnetic sheets. r2=Br2 / Brs2…(3) In formula (3), Br2 represents the residual magnetization intensity when an external magnetic field parallel to the easy magnetization axis of the at least one second magnetic body piece is applied to the at least one second magnetic body piece, and Brs2 represents the saturated residual magnetization intensity of the at least one second magnetic body piece; The step b is a step of magnetizing the at least one second magnetic sheet in a direction parallel to the second direction.

2. The method according to claim 1, The at least two first magnetic sheets and the at least one second magnetic sheet satisfy the above formula (1) under room temperature conditions. In step a, the at least two first magnetic sheets are magnetized using an external magnetic field having a magnetic flux density of Ba. In step b, the at least one second magnetic sheet is magnetized using an external magnetic field having a magnetic flux density of Bb. in, Ba<Bb.

3. The method according to claim 1, The at least two first magnetic sheets and the at least one second magnetic sheet satisfy the following formula (4): B2-B1>0.2T…(4) In formula (4), B1 represents the magnetic flux density of the external magnetic field at which the residual magnetic susceptibility r1 of at least two first magnetic sheets becomes 98% at room temperature, and B2 represents the magnetic flux density of the external magnetic field at which the residual magnetic susceptibility r2 of at least one second magnetic sheet becomes 98% at room temperature.

4. The method according to claim 2, The at least two first magnetic sheets and the at least one second magnetic sheet satisfy the following formula (4): B2-B1>0.2T…(4) In formula (4), B1 represents the magnetic flux density of the external magnetic field at which the residual magnetic susceptibility r1 of at least two first magnetic sheets becomes 98% at room temperature, and B2 represents the magnetic flux density of the external magnetic field at which the residual magnetic susceptibility r2 of at least one second magnetic sheet becomes 98% at room temperature.

5. The method according to any one of claims 1 to 4, In step a, the at least two first magnetic sheets are magnetized in a state where the temperature of the at least two first magnetic sheets is higher than the temperature of the at least one second magnetic sheet, In step b, the at least one second magnetic sheet is magnetized in a state where the temperature of the at least two first magnetic sheets is lower than the temperature of the at least one second magnetic sheet.

6. The method according to claim 5, In step a, the at least two first magnetic sheets are heated, In step b, the at least one second magnetic sheet is heated.

7. The method according to claim 5, In step a, the at least one second magnetic sheet is cooled, and in step b, the at least two first magnetic sheets are cooled.

8. The method according to claim 6, In step a, the at least one second magnetic sheet is cooled, and in step b, the at least two first magnetic sheets are cooled.

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