Mn-Bi-BASED MAGNETIC POWDER, METHOD FOR PRODUCING SAME, COMPOUND FOR BOND MAGNET, BOND MAGNET, Mn-Bi-BASED METAL MAGNET AND METHOD FOR PRODUCING SAME

Abstract

A MnBi-based magnetic powder, which contains a hexagonal MnBi-based magnetic phase containing Sn and has a Sn content of 0.2 to 5 at % with respect to a sum of Mn, Bi and Sn, is provided. In addition, a bond magnet containing a kneaded product of this MnBi-based magnetic powder with a resin binder is provided. Furthermore, a MnBi-based metal magnet, which contains a hexagonal MnBi-based magnetic phase containing Sn and has a Sn content of 0.2 to 5 at % with respect to a sum of Mn, Bi and Sn, is provided.

Claims

1. A MnBi-based magnetic powder comprising a hexagonal MnBi-based magnetic phase containing Sn, wherein a content of Sn is 0.2 to 5 at % with respect to a sum of Mn, Bi and Sn.

2. The MnBi-based magnetic powder according to claim 1, wherein the hexagonal MnBi-based magnetic phase contains Zn, and a total content of Sn and Zn is 5 at % or less with respect to a sum of Mn, Bi, Sn and Zn.

3. A method for producing a MnBi-based magnetic powder, the method comprising a step of generating a hexagonal MnBi-based magnetic phase containing Sn by heating an alloy containing Mn, Bi and Sn and obtaining a MnBi-based magnetic powder having a Sn content of 0.2 to 5 at % with respect to a sum of Mn, Bi and Sn.

4. The method for producing a MnBi-based magnetic powder according to claim 3, wherein the alloy is heated in an inert atmosphere.

5. The method for producing a MnBi-based magnetic powder according to claim 3, wherein the alloy is an alloy powder and the MnBi-based magnetic powder having a total content of Sn and Zn of 5 at % or less with respect to a sum of Mn, Bi, Sn and Zn is obtained by mixing the alloy powder with a Zn powder, heating a mixed powder containing the alloy powder and the Zn powder, and generating a hexagonal MnBi-based magnetic phase containing Sn and Zn.

6. The method for producing a MnBi-based magnetic powder according to claim 5, wherein the mixed powder is heated in an inert atmosphere in a vacuum or under reduced pressure.

7. The method for producing a MnBi-based magnetic powder according to claim 3, wherein the alloy is an alloy powder obtained by pulverizing a quenched ribbon obtained by quenching a molten material containing Mn, Bi and Sn.

8. A compound for bond magnet comprising a kneaded product of the MnBi-based magnetic powder according to claim 1 with a resin binder.

9. A bond magnet comprising the MnBi-based magnetic powder according to claim 1 and a resin binder.

10. A method for producing a MnBi-based metal magnet, the method comprising a step of heating a molded body containing the MnBi-based magnetic powder according to claim 1.

11. A MnBi-based metal magnet comprising a hexagonal MnBi-based magnetic phase containing Sn, wherein a content of Sn is 0.2 to 5 at % with respect to a sum of Mn, Bi and Sn.

12. The MnBi-based metal magnet according to claim 11, wherein the hexagonal MnBi-based magnetic phase contains Zn, and a total content of Sn and Zn is 5 at % or less with respect to a sum of Mn, Bi, Sn and Zn.

Description

BRIEF DESCRIPTION OF DRAWINGS

[0024] FIG. 1(A) is an example of an electron micrograph illustrating a part of a cross section of a MnBi-based magnetic powder. FIG. 1(B) is a view illustrating the results on the linear analysis of the cross section in FIG. 1(A).

[0025] FIG. 2 is an enlarged view illustrating the analysis results of Sn in FIG. 1(B).

[0026] FIG. 3 is a flowchart illustrating an example of a method for producing a MnBi-based magnetic powder, a bond magnet, and a MnBi-based metal magnet.

[0027] FIG. 4 is a flowchart illustrating another example of a method for producing a MnBi-based magnetic powder, a bond magnet, and a MnBi-based metal magnet.

[0028] FIG. 5 is a flowchart illustrating still another example of a method for producing a MnBi-based magnetic powder, a bond magnet, and a MnBi-based metal magnet.

[0029] FIG. 6 is a perspective view illustrating an embodiment of a MnBi-based metal magnet.

[0030] FIG. 7(A) is an example of an electron micrograph illustrating a part of a cross section of a MnBi-based magnetic powder containing a hexagonal MnBi-based magnetic phase containing Zn. FIG. 7(B) is a view illustrating the results on the linear analysis of the cross section in FIG. 7(A).

DESCRIPTION OF EMBODIMENTS

[0031] An embodiment of the present invention will be described in detail with reference to the drawings according to circumstances. It should be noted that the following embodiments are examples for describing the present invention and are not intended to limit the present invention to the following contents.

[0032] The MnBi-based magnetic powder of the present embodiment contains a hexagonal MnBi-based magnetic phase as a main component. The MnBi-based magnetic powder may include at least either of a Bi phase or a Mn phase as an auxiliary component in addition to the hexagonal MnBi-based magnetic phase. The Mn phase is composed of, for example, Mn simple substance or Mn oxide. The Bi phase is composed of, for example, Bi simple substance.

[0033] The hexagonal MnBi-based magnetic phase contains Sn. The content of Sn in the MnBi-based magnetic powder is 0.2 to 5 at % with respect to the sum of Mn, Bi and Sn. It is possible to increase the oxidation resistance of the MnBi-based magnetic powder while maintaining the magnetic properties thereof high by setting the content of Sn to this range. The lower limit of the Sn content is preferably 0.3 at % and more preferably 0.5 at % from the viewpoint of further increasing the oxidation resistance. The upper limit of the Sn content is preferably 3 at % and more preferably 2 at % from the viewpoint of further increasing the magnetic properties.

[0034] The content of Mn in the MnBi-based magnetic powder is, for example, 35 to 65 at % with respect to the sum of Mn, Bi and Sn. The content of Mn may be 40 to 60 at % or 45 to 55 at % with respect to the sum of Mn, Bi and Sn from the viewpoint of increasing the proportion of the hexagonal MnBi-based magnetic phase and thus further increasing the magnetic properties. It is preferable that the content of Bi in the MnBi-based magnetic powder is lower than the content of Mn on the basis of atomic percentage. This is because Sn tends to substitute Bi of the hexagonal MnBi-based magnetic phase. In other words, it is possible to increase the proportion of the hexagonal MnBi-based magnetic phase contained in the MnBi-based magnetic powder by setting the content of Bi to be lower than the content of Mn on the basis of atomic percentage.

[0035] The content of Bi in the MnBi-based magnetic powder is, for example, 33 to 63 at % with respect to the sum of Mn, Bi and Sn. The content of Bi may be 38 to 58 at % or 43 to 53 at % with respect to the sum of Mn, Bi and Sn from the viewpoint of increasing the proportion of the hexagonal MnBi-based magnetic phase and thus further increasing the magnetic properties.

[0036] FIG. 1(A) is an example of an electron micrograph illustrating an enlarged part of a cross section obtained by embedding the MnBi-based magnetic powder in a resin and polishing this using a cross-section polisher. The broken line in the electron micrograph of FIG. 1(A) is drawn in order to clarify the boundary. FIG. 1(B) is a view illustrating the results on the linear analysis (EDX analysis) performed along the line segment by taking point a in the electron micrograph of FIG. 1(A) as a starting point.

[0037] FIG. 1(A) illustrates a hexagonal MnBi-based magnetic phase 10 contained in the MnBi-based magnetic powder. In FIG. 1(A), a pair of resin portions 30 are illustrated so as to sandwich the MnBi-based magnetic powder 10. FIG. 2 is an enlarged view illustrating the analysis results of Sn in FIG. 1(B). According to the linear analysis illustrated in FIG. 1(B) and FIG. 2, the hexagonal MnBi-based magnetic phase 10 contains Mn and Bi as main components and Sn as a trace component. It is presumed that Sn substitutes Bi in the crystal structure of the hexagonal MnBi-based magnetic phase 10.

[0038] In the MnBi-based magnetic powder, the oxidation resistance of the hexagonal MnBi-based magnetic phase 10 can be effectively improved as the hexagonal MnBi-based magnetic phase 10 contains Sn. Incidentally, Sn may be contained in phases other than the hexagonal MnBi-based magnetic phase 10. A hexagonal MnBi-based magnetic phase containing MnBi as a main component and Sn or Zn together with Sn as an auxiliary component is referred to as a hexagonal MnBi-based magnetic phase in the present specification. In the same manner, a MnBi-based magnetic powder containing MnBi as a main component and Sn or Zn together with Sn as an auxiliary component is referred to as a MnBi-based magnetic powder in the present specification.

[0039] In some embodiments, the MnBi-based magnetic powder may contain Zn in the hexagonal MnBi-based magnetic phase in addition to Sn. The oxidation resistance of the MnBi-based magnetic powder is further improved as the MnBi-based magnetic powder contains Zn. It is presumed that this is because the oxidation resistance of the hexagonal MnBi-based magnetic phase is improved as the hexagonal MnBi-based magnetic phase contains Zn. The total content of Zn and Sn in the MnBi-based magnetic powder is 0.2 to 5 at % with respect to the sum of Mn, Bi, Sn and Zn from the viewpoint of increasing the magnetic properties while sufficiently increasing the oxidation resistance in a high temperature and high humidity environment. The lower limit of the total content of Zn and Sn is preferably 0.3 at % and more preferably 0.5 at % from the viewpoint of further increasing the oxidation resistance of the MnBi-based magnetic powder. The upper limit of the total content of Zn and Sn is preferably 3 at % and more preferably 2 at % from the viewpoint of further increasing the magnetic properties.

[0040] Zn has a lower boiling point than Sn, and there is thus a case in which Zn volatilizes in the producing process when a raw material containing Zn is used. Hence, the atomic ratio of Zn to Sn may be, for example, 3 or less or 2 or less from the viewpoint of increasing the suitability for the producing process on an industrial scale. On the other hand, the atomic ratio of Zn to Sn may be 0.5 or more or 0.8 or more from the viewpoint of sufficiently increasing the oxidation resistance at a high temperature and a high humidity.

[0041] The MnBi-based magnetic powder may be a magnet powder subjected to a heat treatment in a magnetic field. In the MnBi-based magnetic powder of the present embodiment, the hexagonal MnBi-based magnetic phase exhibits high oxidation resistance. For this reason, a decrease in the saturation magnetization can be diminished even when the MnBi-based magnetic powder is stored for a long period of time in a high temperature and high humidity environment. A decrease in the saturation magnetization can be diminished even when a bond magnet and a MnBi-based metal magnet which are formed using such a MnBi-based magnetic powder are stored for a long period of time in a high temperature and high humidity environment.

[0042] Next, several embodiments of a method for producing a MnBi-based magnetic powder will be described below.

[0043] A method for producing a MnBi-based magnetic powder according to an embodiment includes a step of obtaining a MnBi-based alloy powder containing Mn, Bi and Sn and a step of generating a hexagonal MnBi-based magnetic phase by heating the MnBi-based alloy powder and obtaining a MnBi-based magnetic powder containing a hexagonal MnBi-based magnetic phase containing Sn. The content of Sn in this MnBi-based magnetic powder is 0.2 to 5 at % with respect to the sum of Mn, Bi and Sn.

[0044] Examples of the step of obtaining a MnBi-based alloy powder may include the following methods (i) to (iv).

[0045] (i) A powder metallurgy method in which Mn, Bi and Sn are pulverized, sintered, and then pulverized to obtain a MnBi-based alloy powder

[0046] (ii) An atomizing method in which a MnBi-based alloy (MnBiSn alloy) is melted and atomized to obtain a MnBi-based alloy powder

[0047] (iii) A melting method in which a lump of a MnBi-based alloy (MnBiSn alloy) obtained through melting in an arc furnace is pulverized to obtain a MnBi-based alloy powder

[0048] (iv) A liquid quenching method in which a melted MnBi-based alloy (MnBiSn alloy) is quenched by using a roll and the quenched ribbon thus obtained is pulverized to obtain a MnBi-based alloy powder

[0049] In each of the above methods, a chip, a shot, a powder, and the like can be appropriately selected and used as raw materials of MnBi and Sn.

[0050] Among the above methods (i) to (iv), (iv) the liquid quenching method is preferable. In (iv) the liquid quenching method, a MnBi-based magnetic powder having a sufficiently small particle diameter can be obtained by pulverizing a quenched ribbon obtained by quenching a molten alloy by using a roll. This makes it possible to further increase the saturation magnetization.

[0051] FIG. 3 is a flowchart illustrating an example of the method for producing a MnBi-based magnetic powder, a compound for bond magnet, a bond magnet, and a MnBi-based metal magnet according to the present embodiment. In this example, a MnBi-based alloy powder is prepared by (iv) the liquid quenching method described above. In other words, raw materials of Mn, Bi and Sn are first prepared, and a melting step S1 of melting (melting and casting) the MnBi-based alloy by, for example, arc melting is performed. Thereafter, a liquid quenching step S2 of obtaining a quenched ribbon by a liquid quenching method is performed. Subsequently, a pulverization step S3 of pulverizing the quenched ribbon thus obtained is performed to obtain a MnBi-based alloy powder.

[0052] Coarse pulverization may be performed in the pulverization step S3 and fine pulverization may be performed after a heat treatment step S4 to be described later, or coarse pulverization and fine pulverization may be performed in the pulverization step S3. In the case of performing fine pulverization as well, it is preferable to pulverize the quenched ribbon to have an average particle diameter of about 10 m at which the average particle diameter becomes a single domain level. An anisotropic MnBi-based magnet powder can be obtained without applying a magnetic field in the heat treatment step S4 as the quenched ribbon is pulverized to this degree.

[0053] In the heat treatment step S4, the MnBi-based alloy powder is heated at 250 C. to 380 C. for 30 to 200 minutes in an inert atmosphere to obtain a MnBi-based magnetic powder. Examples of the inert gas may include argon gas and nitrogen gas. A hexagonal MnBi-based magnetic phase containing Sn is generated by performing a heat treatment under such heating conditions.

[0054] When the heating temperature in the heating conditions described above is lower than 250 C., there is a tendency that the hexagonal MnBi-based magnetic phase is not sufficiently generated or the generation rate slows down and the heat treatment step S4 is prolonged. On the other hand, when the heating temperature is higher than 380 C., there is a tendency that a structural phase transition from a ferromagnetic low temperature phase of the hexagonal MnBi-based magnetic phase to a paramagnetic high temperature phase occurs and the ferromagnetism disappears. In addition, Bi dissolves and the Bi phase precipitates and there is thus a tendency that the proportion in the hexagonal MnBi-based magnetic phase decreases and the saturation magnetization value decreases. When the treatment time of the heating conditions is too short, there is a tendency that the hexagonal MnBi-based magnetic phase is not sufficiently generated and the value of the saturation magnetization decreases. On the other hand, when the treatment time is too long, there is a tendency that the hexagonal MnBi-based magnetic phase grows and the coercive force decreases. The MnBi-based magnetic powder obtained by the producing method described above contains a hexagonal MnBi-based magnetic phase exhibiting high oxidation resistance. For this reason, a decrease in the saturation magnetization can be diminished even when the MnBi-based magnetic powder is stored for a long period of time in a high temperature and high humidity environment.

[0055] The composition of the MnBi-based magnetic powder, namely, the molar ratio of Mn, Bi and Sn is substantially equal to the blending ratio of the respective raw materials of Mn, Bi and Sn to be used in the melting step S1. This is because the scattering of raw materials can be sufficiently suppressed in the producing process. Hence, the producing method described above is suitable for industrial production as the composition of the MnBi-based magnetic powder can be easily adjusted in the method.

[0056] In the example of FIG. 3, a molding step in a magnetic field S5 of molding the MnBi-based magnetic powder in a magnetic field to fabricate a molded body is performed. The magnetic field to be applied is, for example, 4000 to 15000 G, and the molding pressure is, for example, 1 to 10 t/cm.sup.2. Subsequently, a hot compression step S6 of applying a pressure to the molded body thus obtained while heating the molded body at a temperature of 280 C. to 380 C. in an inert atmosphere.

[0057] Through the above steps, it is possible to obtain a MnBi-based metal magnet, which can maintain a high saturation magnetization even when being used in a high temperature and high humidity environment.

[0058] In another example, in the heat treatment step S4, the heat treatment may be performed while applying a magnetic field of, for example, 4000 to 8000 G to the MnBi-based magnetic powder. It is possible to obtain an anisotropic MnBi-based magnet powder through such a heat treatment step in a magnetic field.

[0059] When the heating temperature in the heat treatment step in a magnetic field is too low, there is a tendency that the hexagonal MnBi-based magnetic phase is not sufficiently generated or the hexagonal MnBi-based magnetic phase generated cannot become sufficiently anisotropic. On the other hand, when the heating temperature is too high, there is a tendency that a structural phase transition from a ferromagnetic low temperature phase of the hexagonal MnBi-based magnetic phase to a paramagnetic high temperature phase occurs and the ferromagnetism disappears. When the heating time in the heat treatment step in a magnetic field is too short, there is a tendency that the hexagonal MnBi-based magnetic phase is not sufficiently generated or the hexagonal MnBi-based magnetic phase generated cannot become sufficiently anisotropic.

[0060] Another pulverization step may be performed after the heat treatment step S4 (which may be a heat treatment step in a magnetic field) to obtain an anisotropic MnBi-based magnet powder having a desired particle diameter. The MnBi-based magnet powder, which is one form of the MnBi-based magnetic powder, has the same composition as that of the MnBi-based magnetic powder described above.

[0061] A compound for bond magnet and a bond magnet may be fabricated using the MnBi-based magnetic powder. In this case, a kneading step S11 is performed in which a resin binder containing a thermoplastic resin, a coupling material, and a lubricant is added to the MnBi-based magnetic powder and the mixture is kneaded. A molding step S12 is performed in which a compound for bond magnet containing the kneaded product obtained by kneading is molded in a magnetic field of, for example, 4000 to 8000 G, whereby a bond magnet can be produced. The molding can be performed by a known method such as compression molding and injection molding at a molding pressure of 1 to 10 t/cm.sup.2.

[0062] The resin binder may be a thermosetting resin such as an epoxy resin. In this case, it is possible to produce a bond magnet by kneading the MnBi-based magnetic powder with a resin binder, molding the kneaded product by pressure molding or the like, and then performing a heat treatment of the molded body. It is possible to suppress a decrease in the saturation magnetization even when the bond magnet obtained by using the MnBi-based magnetic powder is used in a high temperature and high humidity environment. In some other embodiments, a compound for bond magnet and a bond magnet may be fabricated using the MnBi-based magnet powder as a MnBi-based magnetic powder.

[0063] Next, another embodiment of the method for producing a MnBi-based magnetic powder will be described. The method for producing a MnBi-based magnetic powder according to the present embodiment includes a step of obtaining a MnBi-based alloy containing Mn, Bi and Sn, a step of generating a hexagonal MnBi-based magnetic phase containing Sn by heating the MnBi-based alloy, and a step of obtaining a MnBi-based magnetic powder by pulverizing the MnBi-based alloy containing a hexagonal MnBi-based magnetic phase containing Sn. In the MnBi-based alloy, the content of Sn in this MnBi-based magnetic powder is 0.2 to 5 at % with respect to the sum of Mn, Bi and Sn.

[0064] FIG. 4 is a flowchart illustrating an example of the method for producing a MnBi-based magnetic powder, a compound for bond magnet, a bond magnet, and a MnBi-based metal magnet according to the present embodiment. In this example, the melting step S1 and the liquid quenching step S2 are performed in the same manner as in the example of FIG. 3. The MnBi-based alloy (quenched ribbon) obtained in the liquid quenching step S2 is subjected to a heat treatment. The conditions of the heat treatment here are the same as those of the heat treatment step S4 in FIG. 3. It is possible to sufficiently diminish deviation in the composition from the blending ratio of raw materials since the heat treatment is performed before pulverization in this manner.

[0065] In the pulverization step S7, coarse pulverization and fine pulverization are performed to pulverize the quenched ribbon until to have an average particle diameter of about 10 m at which the average particle diameter becomes a single domain level. A MnBi-based magnetic powder can be obtained in this manner. Incidentally, the heat treatment step S4 may be a heat treatment step in a magnetic field in which the MnBi-based alloy is heated while applying a magnetic field of 4000 to 8000 G to the MnBi-based alloy. In this case, an anisotropic ribbon is obtained. Coarse pulverization is performed in the pulverization step S7 after this anisotropic ribbon has been obtained, whereby an anisotropic MnBi-based magnet powder can be obtained. In this case, fine pulverization may not be performed. The molding step in a magnetic field 55, the hot compression step S6, the kneading step S11 and the molding step S12 are performed in the same manner as in the example of FIG. 3. This makes it possible to obtain a MnBi-based metal magnet, a compound for bond magnet and a bond magnet.

[0066] Next, still another embodiment of the method for producing a MnBi-based magnetic powder will be described. The method for producing a MnBi-based magnetic powder of the present embodiment includes a step of obtaining a MnBi-based alloy powder, a step of mixing the MnBi-based alloy powder with a Zn powder and obtaining a mixed powder, and a step of generating a hexagonal MnBi-based magnetic phase containing Sn and Zn by heating the mixed powder under reduced pressure and obtaining a MnBi-based magnetic powder containing a hexagonal MnBi-based magnetic phase. In this MnBi-based magnetic powder, the total content of Sn and Zn is 0.2 to 5 at % with respect to the sum of Mn, Bi, Sn and Zn.

[0067] The step of obtaining a MnBi-based alloy powder in the producing method of the present embodiment can be performed in the same manner as in the above embodiments. In the producing method of the present embodiment, volatilization of Zn can be sufficiently suppressed since a Zn powder is mixed after the MnBi-based alloy powder has been obtained. The Zn powder is blended so that the total ratio of Sn and Zn to the sum of Mn, Bi, Sn and Zn is, for example, 0.5 to 6 mol % with respect to the MnBi-based alloy powder.

[0068] FIG. 5 is a flowchart illustrating an example of the method for producing a MnBi-based magnetic powder, a compound for bond magnet, a bond magnet, and a MnBi-based metal magnet according to the present embodiment. In this example, a MnBi-based alloy powder is obtained by performing the same melting step S1, liquid quenching step S2, and pulverization step S3 as those in the producing method illustrated in the flowchart of FIG. 3. The producing method of this example is different from the producing method illustrated in the flowchart of FIG. 3 in that a Zn powder is blended into the MnBi-based alloy powder and the mixing step S8 is performed.

[0069] It is possible to decrease the amount of Zn scattered as compared with a case in which Zn is blended at the time at which the MnBi-based alloy is melted by preparing a MnBi-based alloy powder and then blending a Zn powder into the MnBi-based alloy powder as in this example. In the heat treatment step S9 of this example, the mixed powder containing the MnBi-based alloy powder and the Zn powder is heated at 250 C. to 380 C. for 30 to 200 minutes in an inert atmosphere in a vacuum or under reduced pressure (preferably 10 Pa or lower). Zn is efficiently evaporated and smoothly deposited on the surface of the MnBi-based alloy powder by heating the mixed powder in an inert atmosphere in a vacuum or under reduced pressure in this manner. In the heat treatment step S9, Zn deposited on the surface of the MnBi-based alloy powder can be diffused to the interior of the hexagonal MnBi-based magnetic phase 10.

[0070] In such a heat treatment step S9, a part of the evaporated Zn scatters in some cases. For this reason, it is preferable to blend the Zn powder in a more amount than the intended Zn content in the MnBi-based magnetic powder. After the heat treatment step S9, the quenched ribbon is pulverized to have an average particle diameter of about 10 m at which the average particle diameter becomes a single domain level, whereby a MnBi-based magnetic powder can be obtained. The heat treatment step S9 may be a heat treatment step in a magnetic field in which the mixed powder is heated while applying a magnetic field of, for example, 4000 to 8000 G to the mixed powder. In this case, a MnBi-based magnetic powder can be obtained without performing pulverization.

[0071] The MnBi-based magnetic powder obtained by such a producing method has a hexagonal MnBi-based magnetic phase containing Sn and Zn. For this reason, it is possible to diminish a decrease in the saturation magnetization even when the MnBi-based magnetic powder is stored for a long period of time in a high temperature and high humidity environment. A MnBi-based metal magnet can be obtained by performing the molding step in a magnetic field S5 and the hot compression step S6 using such a MnBi-based magnetic powder. In the same manner, a compound for bond magnet can be produced by performing the kneading step S11 and a bond magnet can be produced by further performing the molding step S12. The conditions of each step are the same as those in the producing example illustrated in the flowchart of FIG. 3.

[0072] FIG. 6 is a perspective view of a MnBi-based metal magnet 50. In an embodiment, as the composition of the MnBi-based metal magnet 50, a hexagonal MnBi-based magnetic phase containing Sn is contained and the content of Sn is 0.2 to 5 at % with respect to the sum of Mn, Bi and Sn. The contents of Mn, Bi and Sn may be the same as those described for the MnBi-based magnetic powder.

[0073] In another embodiment, as the composition of the MnBi-based metal magnet 50, a hexagonal MnBi-based magnetic phase containing Sn and Zn is contained and the total content of Sn and Zn is 0.2 to 5 at % with respect to the sum of Mn, Bi, Sn and Zn. In this case as well, the contents of Mn, Bi, Sn and Zn may be the same as those described for the MnBi-based magnetic powder.

[0074] The shape of the MnBi-based metal magnet is not limited to a cylindrical shape as illustrated in FIG. 6, and it may be, for example, a prismatic shape or a spherical shape. The bond magnet may also have a cylindrical shape as illustrated in FIG. 6, or it may have a prismatic shape or a spherical shape.

[0075] A MnBi-based magnetic powder containing Zn will be described. FIG. 7(A) is an example of an electron micrograph illustrating an enlarged part of a cross section obtained by embedding a MnBi-based magnetic powder containing Zn in a resin and polishing this using a cross-section polisher. The broken line in the electron micrograph of FIG. 7(A) is drawn in order to clarify the boundary. FIG. 7(B) is a view illustrating the results on the linear analysis (EDX analysis) performed along the line segment by taking point a in the electron micrograph of FIG. 7(A) as a starting point.

[0076] As illustrated in FIG. 7(A), the MnBi-based magnetic powder has a MnBi-based magnetic phase 100 and a Bi phase 200 so as to sandwich this hexagonal MnBi-based magnetic phase 100. As illustrated in the linear analysis results of FIG. 7(B), the hexagonal MnBi-based magnetic phase 100 contains Mn, Bi and Zn. On the other hand, the Bi phase 200 does not substantially contain Mn and Zn. By the heat treatment step S9 in FIG. 5 described above, Zn diffuses into the hexagonal MnBi-based magnetic phase 100 and is unevenly distributed in the hexagonal MnBi-based magnetic phase 100.

[0077] The oxidation resistance of the hexagonal MnBi-based magnetic phase 100 is improved as it contains Zn. However, Zn may be contained in phases other than the hexagonal MnBi-based magnetic phase 100 of the MnBi-based magnetic powder.

[0078] Several embodiments of the present invention have been described above, but the present invention is not limited to the above embodiments. For example, an arbitrary element may be contained in addition to the elements described above. The methods for producing a MnBi-based magnetic powder, a compound for bond magnet containing the MnBi-based magnetic powder, and a bond magnet, and a MnBi-based metal magnet are not limited to the embodiments described above, and these may be produced by other producing methods. For example, the MnBi-based magnetic powder and the like may be produced by blending Zn at the stage of the melting step S1 in addition to Mn, Bi and Sn as raw materials. In addition, the heat treatment step S4 may be performed before the pulverization step S3 in FIG. 3, for example. In this case, the MnBi-based magnetic powder can be obtained through the pulverization step S3.

EXAMPLES

[0079] Hereinafter, the contents of the present invention will be described in more detail with reference to Examples and Comparative Examples. However, the present invention is not limited to the following Examples.

Example 1

[0080] <Fabrication of MnBi-Based Magnetic Powder>

[0081] A MnBi-based alloy was prepared by melting Mn chips, Bi shots and Sn shots in an arc melting furnace. As presented in Table 4, the blending amounts of Mn, Bi and Sn were set to 9.56 g for Mn, 34.2 g for Bi and 1.24 g for Sn. The MnBi-based alloy obtained in the arc melting furnace was placed in a tapping nozzle of a transparent quartz tube having an orifice diameter of 0.45 mm at the bottom. After the interior of the chamber of the liquid quenching apparatus was evacuated, the MnBi-based alloy in the tapping nozzle was melted by high-frequency melting in an argon gas atmosphere at 50 kPa. Thereafter, a MnBi-based melt was injected onto a copper roll at a melt injection pressure of 10 kPa and a quenched ribbon was obtained. The rotational speed of the roll was set to 20 m/s and the distance between the roll and the tapping nozzle was set to 0.4 mm.

[0082] The quenched ribbon thus obtained was subjected to a heat treatment under the following conditions.

[0083] <Heat Treatment Conditions>

[0084] Atmosphere: nitrogen gas

[0085] Temperature: 300 C.

[0086] Time: 120 minutes

[0087] The quenched ribbon was subjected to a heat treatment under the heat treatment conditions described above and then pulverized, thereby obtaining a MnBi-based magnetic powder (MnBi magnet powder) having an average particle diameter of 10 m. The content of each element in this MnBi-based magnetic powder was measured according to the following procedure.

[0088] The content of each of Mn, Bi, Sn and Zn with respect to the sum of Mn, Bi, Sn and Zn in the MnBi-based magnetic powder was measured by using an ICP emission spectrophotometer (apparatus name: iCAP 6000 manufactured by Thenuo Fisher Scientific Inc.). The measurement results are as presented in Table 1.

[0089] The saturation magnetization (4I.sub.max), residual magnetic flux density (Br), and coercive force (Hcj) of the MnBi-based magnetic powder were measured by using a vibrating sample type magnetometer (VSM: VSM-5 Model manufactured by Toei Industry Co., Ltd.). The measurement results are as presented in the column for Before storage in Table 2.

[0090] The MnBi-based magnetic powder fabricated was stored in the air for 1 hour under the respective temperature conditions of 100 C., 150 C. and 200 C., and the magnetic properties thereof after storage were measured. The measurement results are as presented in Table 2. The relative humidity of air in the respective storage conditions was 50%. The decreasing rate of saturation magnetization after storage under each condition was calculated by the following formula (1) by taking the residual magnetic flux density before storage as the reference. The calculation results are as presented in Table 2.

Decreasing rate of saturation magnetization (%)=100[saturation magnetization after storage at predetermined temperature]/[saturation magnetization before storage]100(1)

Examples 2 to 4

[0091] MnBi-based magnetic powders were prepared in the same manner as in Example 1 except that the blending amounts of Mn chips, Bi shots and Sn shots were changed as presented in Table 4. Thereafter, the measurement and evaluation were performed in the same manner as in Example 1. The results are as presented in Tables 1 and 2.

Example 5

[0092] A quenched ribbon was obtained in the same manner as in Example 1 except that the blending amounts of Mn chips, Bi shots and Sn shots were changed as presented in Table 4. The quenched ribbon thus obtained was pulverized, thereby preparing a MnBi-based alloy powder having a particle diameter of 150 m or less. A mixed powder was obtained by blending and mixing 0.28 g of Zn powder with the MnBi-based alloy powder thus obtained. The mixed powder thus obtained was subjected to a heat treatment under the following conditions to diffuse Zn, thereby obtaining a MnBi-based magnetic powder containing a hexagonal MnBi-based magnetic phase containing Sn and Zn.

[0093] <Heat Treatment Conditions>

[0094] Atmosphere: nitrogen gas

[0095] Temperature: 300 C.

[0096] Time: 120 minutes

[0097] The mixed powder was subjected to a heat treatment under the heat treatment conditions described above and then pulverized, thereby obtaining a MnBi-based magnetic powder having an average particle diameter of 10 m. The content of each element in this MnBi-based magnetic powder was measured in the same manner as in Example 1. In addition, the magnetic properties of the MnBi-based magnetic powder were evaluated in the same manner as in Example 1. The results are as presented in Tables 1 and 2.

Example 6

[0098] A MnBi-based magnetic powder was prepared in the same manner as in Example 5 except that the blending amounts of Mn chips, Bi shots and Sn shots were changed as presented in Table 2 and the blending amount of Zn powder was changed as presented in Table 4. Thereafter, the measurement and evaluation were performed in the same manner as in Example 1. The results are as presented in Tables 1 and 2.

Comparative Example 1

[0099] A MnBi-based magnetic powder was prepared in the same manner as in Example 1 except that Sn shots were not added and the blending amounts of Mn chips and Bi shots were changed as presented in Table 4. Thereafter, the measurement and evaluation were performed in the same manner as in Example 1. The results are as presented in Tables 1 and 3.

[0100] (Comparative Examples 2 and 3)

[0101] MnBi-based magnetic powders were prepared in the same manner as in Example 1 except that the blending amounts of Mn chips, Bi shots and Sn shots were changed as presented in Table 4. Thereafter, the measurement and evaluation were performed in the same manner as in Example 1. The results are as presented in Tables 1 and 3.

Comparative Example 4

[0102] A MnBi-based magnetic powder was prepared in the same manner as in Example 1 except that the Sn shots were not used and the blending amounts of Mn chips and Bi shots were changed as presented in Table 4, and the blending amount of Zn powder was changed as presented in Table 4. Thereafter, the measurement and evaluation were performed in the same manner as in Example 1. The results are as presented in Tables 1 and 3.

Comparative Example 5

[0103] A MnBi-based magnetic powder was prepared in the same manner as in Example 1 except that Fe shots were used instead of Sn shots and the blending amounts of Mn chips, Bi shots and Fe shots were changed as presented in Table 4. Thereafter, the measurement and evaluation were performed in the same manner as in Example 1. The results are as presented in Tables 1 and 3.

TABLE-US-00001 TABLE 1 Composition (at %) Mn Bi Sn Zn Fe Example 1 50.0 47.0 3.0 0.0 0.0 Example 2 50.0 49.0 1.0 0.0 0.0 Example 3 50.0 49.5 0.5 0.0 0.0 Example 4 50.0 49.7 0.3 0.0 0.0 Example 5 49.5 49.5 0.5 0.5 0.0 Example 6 49.0 49.5 0.5 1.0 0.0 Comparative Example 1 50.0 50.0 0.0 0.0 0.0 Comparative Example 2 50.0 49.9 0.1 0.0 0.0 Comparative Example 3 50.0 42.0 8.0 0.0 0.0 Comparative Example 4 47.3 50.0 0.0 2.7 0.0 Comparative Example 5 42.5 55.0 0.0 0.0 2.5

TABLE-US-00002 TABLE 2 Magnetic properties Decreasing rate of Storage saturation temperature 4I.sub.max Br Hcj magnetization C. T T kA/m % Example 1 Before storage 0.52 0.47 852 100 0.51 0.47 844 0.0 150 0.45 0.42 828 10.6 200 0.37 0.33 763 29.8 Example 2 Before storage 0.64 0.58 672 100 0.59 0.55 666 5.2 150 0.50 0.47 682 19.0 200 0.43 0.39 724 32.8 Example 3 Before storage 0.70 0.63 767 100 0.64 0.60 802 4.8 150 0.49 0.45 742 28.6 200 0.29 0.25 510 60.3 Example 4 Before storage 0.69 0.62 1031 100 0.56 0.51 1039 17.7 150 0.46 0.38 966 38.7 200 0.28 0.20 661 67.7 Example 5 Before storage 0.66 0.61 672 100 0.61 0.57 690 6.6 150 0.55 0.51 662 16.4 200 0.46 0.41 529 32.8 Example 6 Before storage 0.64 0.58 723 100 0.62 0.57 727 1.7 150 0.53 0.48 734 17.2 200 0.45 0.40 744 31.0

TABLE-US-00003 TABLE 3 Magnetic properties Decreasing rate of Storage saturation temperature 4I.sub.max Br Hcj magnetization C. T T kA/m % Comparative Before storage 0.70 0.64 972 Example 1 100 0.49 0.43 1013 32.8 150 0.35 0.34 1032 46.9 200 0.12 0.06 459 90.6 Comparative Before storage 0.60 0.56 1008 Example 2 100 0.45 0.41 1011 26.8 150 0.33 0.30 992 46.4 200 0.06 0.05 385 91.1 Comparative Before storage 0.24 0.20 590 Example 3 Comparative Before storage 0.70 0.64 669 Example 4 100 0.68 0.63 766 1.6 150 0.65 0.61 762 4.7 200 0.53 0.50 806 21.9 Comparative Before storage 0.36 0.31 286 Example 5

TABLE-US-00004 TABLE 4 Blending amount (g) Mn Bi Sn Zn Fe Example 1 9.56 34.20 1.24 0.00 0.00 Example 2 9.43 35.16 0.41 0.00 0.00 Example 3 9.40 35.40 0.20 0.00 0.00 Example 4 9.39 35.49 0.12 0.00 0.00 Example 5 9.30 35.38 0.20 0.14 0.00 Example 6 9.20 35.37 0.20 0.28 0.00 Comparative Example 1 9.37 35.63 0.00 0.00 0.00 Comparative Example 2 9.37 35.59 0.04 0.00 0.00 Comparative Example 3 9.91 31.67 3.43 0.00 0.00 Comparative Example 4 8.84 35.56 0.00 0.75 0.00 Comparative Example 5 7.52 37.03 0.00 0.00 0.45

[0104] As presented in Tables 2 and 3, the MnBi-based magnetic powders of Examples 1 to 6 have lower decreasing rates of saturation magnetization than those of Comparative Examples 1 and 2. It is considered that this is because oxidative decline of the MnBi-based magnetic powders of Examples 1 to 6 is suppressed as the hexagonal MnBi-based magnetic phase contains Sn or Sn and Zn. On the other hand, it is considered that oxidation proceeds and the decreasing rate of saturation magnetization increases in Comparative Examples 1 and 2 since the hexagonal MnBi-based magnetic phase does not contain Sn or the Sn content in the hexagonal MnBi-based magnetic phase is low.

[0105] In Comparative Example 3, the saturation magnetization value is low since the content of nonmagnetic components is increased. The MnBi-based magnetic powder of Comparative Example 3 was not subjected to the evaluation after storage since the saturation magnetization thereof before storage was too low in this manner. The MnBi-based magnetic powder containing Fe of Comparative Example 5 was not also subjected to the evaluation after storage since the saturation magnetization thereof before storage was too low.

[0106] The blending ratio (mol %) calculated from the blending amount (g) of the raw material is also presented in Table 5. From the comparison between the composition of the MnBi-based magnetic powder presented in Table 1 and the blending ratio (mol %) in Table 5, the content of Zn is lower in the composition of the MnBi-based magnetic powder than in the blending ratio of the raw material in Examples 5 and 6 and Comparative Example 4 in which Zn is used. This indicates that Zn volatilizes in the producing process of the MnBi-based magnetic powder. On the other hand, in each of Examples in which Sn is used, the composition of the MnBi-based magnetic powder presented in Table 1 is substantially the same as the blending ratio (mol %) in Table 5. From this fact, a MnBi-based magnetic powder containing Sn is more suitable for production on an industrial scale than a MnBi-based magnetic powder containing Zn.

TABLE-US-00005 TABLE 5 Blending ratio (mol %) Mn Bi Sn Zn Fe Example 1 50.0% 47.0% 3.0% 0.0% 0.0% Example 2 50.0% 49.0% 1.0% 0.0% 0.0% Example 3 50.0% 49.5% 0.5% 0.0% 0.0% Example 4 50.0% 49.7% 0.3% 0.0% 0.0% Example 5 49.4% 49.4% 0.5% 0.6% 0.0% Example 6 48.9% 49.4% 0.5% 1.2% 0.0% Comparative Example 1 50.0% 50.0% 0.0% 0.0% 0.0% Comparative Example 2 50.0% 49.9% 0.1% 0.0% 0.0% Comparative Example 3 50.0% 42.0% 8.0% 0.0% 0.0% Comparative Example 4 47.0% 49.7% 0.0% 3.3% 0.0% Comparative Example 5 42.5% 55.0% 0.0% 0.0% 2.5%

INDUSTRIAL APPLICABILITY

[0107] According to the present disclosure, a MnBi-based magnetic powder, which can maintain a high saturation magnetization even when being stored for a long period of time in a high temperature and high humidity environment, and a method for producing the same are provided. In addition, a bond magnet, which can maintain a high saturation magnetization even when being stored for a long period of time in a high temperature and high humidity environment, and a compound for bond magnet, from which such a bond magnet can be produced, are provided. Furthermore, a MnBi-based metal magnet, which can maintain a high saturation magnetization even when being stored for a long period of time in a high temperature and high humidity environment, and a method for producing the same are provided.

REFERENCE SIGNS LIST

[0108] 10: hexagonal MnBi-based magnetic phase, 50: MnBi-based metal magnet.