Rare-earth magnet and method for producing the same

Abstract

Provided is a rare-earth magnet containing no heavy rare-earth metals such as Dy or Tb in a grain boundary phase, has a modifying alloy for increasing coercivity (in particular, coercivity under a high-temperature atmosphere) infiltrated thereinto at lower temperature than in the conventional rare-earth magnets, has high coercivity, and has relatively high magnetizability, and a production method therefor. The rare-earth magnet RM includes a RE-FeB-based main phase MP with a nanocrystalline structure (where RE is at least one of Nd or Pr) and a grain boundary phase BP around the main phase, the grain boundary phase containing a RE-X alloy (where X is a metallic element other than heavy rare-earth elements). Crystal grains of the main phase MP are oriented along the anisotropy axis, and each crystal grain of the main phase, when viewed from a direction perpendicular to the anisotropy axis, has a plane that is quadrilateral in shape or has a close shape thereto.

Claims

1. A rare-earth bulk magnet comprising: a RE-FeB-based main phase with a nanocrystalline structure, the main phase having crystal grain size in a range of 50 nm to 300 nm, where RE is at least one of Nd or Pr; and; and a grain boundary phase around the main phase, the grain boundary phase containing a RE-X alloy, where X is a metallic element other than heavy rare-earth elements, wherein crystal grains of the main phase are oriented along an anisotropy axis, each crystal grain of the main phase, when viewed from a direction perpendicular to the anisotropy axis, has a plane that is quadrilateral in shape or has a close shape thereto, a solid shape of the crystal grain of the main phase has a (001) plane as a plane that is perpendicular to the anisotropy axis, and has (110), (100), or a close low-index plane thereto as a side plane, and a coercivity of the rare-earth bulk magnet satisfies the following formula (1):
Hc=HaNMs(1), wherein, in formula (1), Hc denotes coercivity, denotes a factor attributable to a separation property between nanocrystalline grains of the main phase, Ha denotes magnetocrystalline anisotropy, which is specific to a material of the main phase, N denotes a factor attributable to a grain size of the main phase, and Ms denotes saturation magnetization, which is specific to the material of the main phase, and is in a range of 0.42 to 0.52, and N is in a range of 0.68 to 0.90 and wherein the rare-earth bulk magnet is obtained by: Step 1: sintering a powder, obtained through liquid quenching of a melt of a RE-FeB-based metal, at a temperature of 500 to 700 C., a pressure of 50 to 500 Mpa, and a time of 10 to 600 seconds to obtain a bulk sintered body having an isotropic crystalline structure; and Step 2: applying hot plastic processing to the bulk sintered body obtained in Step 1 at a temperature of 700 to 800 C., a predetermined plastic strain rate, a predetermine pressure and a predetermined processing time to obtain a molded body with magnetic anisotropy imparted thereto along the anisotropy axis, the molded body having the RE-FeB-based main phase and the grain boundary phase around the main phase; and Step 3: melting a RE-Z modifying alloy, where Z is a metallic element other than heavy rare-earth elements, for increasing coercivity of the molded body obtained in Step 3, together with the grain boundary phase, to cause liquid-phase infiltration of a melt of the RE-Z modifying alloy from a surface of the molded body, thereby obtaining the rare-earth bulk magnet, and wherein pressure is applied in Step 1 and Step 2 using a punch, and after the bulk sintered body is obtained in Step 1, an end face of the bulk sintered body is made to abut the punch so as to impart the anisotropy to the bulk sintered body during Step 2.

2. The rare-earth bulk magnet according to claim 1, wherein the RE-Z modifying alloy is a NdCu alloy.

3. The rare-earth bulk magnet according to claim 1, wherein the RE-Z modifying alloy is a NdAl alloy.

4. The rare-earth bulk magnet according to claim 1, wherein X is at least one element selected from the group consisting of Co, Fe and Ga, and Z is an element selected from the group consisting of Cu and Al.

5. The rare-earth bulk magnet according to claim 1, wherein the RE-Z modifying alloy is a NdCu alloy, and Step 3 includes melting the NdCu alloy together with the grain boundary phase at a temperature of 520 to 600 C. to cause the liquid-phase infiltration of a melt of the NdCu alloy.

6. The rare-earth bulk magnet according to claim 1, wherein the RE-Z modifying alloy is a NdAl alloy, and Step 3 includes melting the NdAl alloy together with the grain boundary phase at a temperature of 640 to 650 C. to cause the liquid-phase infiltration of a melt of the NdAl alloy.

Description

BRIEF DESCRIPTION OF DRAWINGS

(1) FIGS. 1(a), (b), and (c) are schematic views sequentially illustrating a first step of a production method of the present invention for producing a rare-earth magnet of the present invention;

(2) FIG. 2(a) is a view illustrating the micro-structure of a sintered body shown in FIG. 1(b), and FIG. 2(b) is a view showing the micro-structure of a molded body in FIG. 1(c);

(3) FIG. 3(a) is a view illustrating a second step of the production method, FIG. 3(b) is a view illustrating the micro-structure of a rare-earth magnet whose structure is being modified by a modifying alloy, and FIG. 3(c) is a view illustrating the micro-structure of the rare-earth magnet whose structure has been modified by the modifying alloy (i.e., the rare-earth magnet of the present invention);

(4) FIG. 4 shows the experimental results of the measurement of coercivity when a NdCu alloy is used for a modifying alloy and the amount of the modifying alloy added to the base magnet (i.e., the molded body before the modifying alloy is infiltrated thereinto) and the temperature in the second step are changed;

(5) FIG. 5 is a graph in which the coercivities of specimens of rare-earth magnets are arranged using the Kronmuller formula;

(6) FIG. 6 shows the experimental results of the measurement of coercivity and magnetization for when a NdCu alloy is used for a modifying alloy and the amount of the modifying alloy added to the base magnet and the temperature in the second step are changed; and

(7) FIGS. 7 are TEM image photographs of the structure of a rare-earth magnet in the production process; specifically, FIG. 7(a) is a photograph of a molded body, FIG. 7(b) is a photograph of after 10 minutes have elapsed after modification by a modifying alloy, and FIG. 7(c) is a photograph of after 30 minutes have elapsed after modification by a modifying alloy.

DESCRIPTION OF EMBODIMENTS

(8) Hereinafter, embodiments of a rare-earth magnet of the present invention and a method for producing the same will be described with reference to the drawings.

(9) (Method for Producing Rare-Earth Magnet)

(10) FIGS. 1(a), (b), and (c) are schematic views sequentially illustrating a first step of a method for producing a rare-earth magnet of the present invention, and FIG. 3(a) is a view illustrating a second step of the production method. In addition, FIG. 2(a) is a view illustrating the micro-structure of a sintered body shown in FIG. 1(b), and FIG. 2(b) is a view illustrating the micro-structure of the molded body in FIG. 1(c). Further, 3(b) is a view illustrating the micro-structure of a rare-earth magnet whose structure is being modified by a modifying alloy, and FIG. 3(c) is a view illustrating the micro-structure of the rare-earth magnet whose structure has been modified by the modifying alloy (i.e., the rare-earth magnet of the present invention).

(11) As shown in FIG. 1(a), an alloy ingot is melted at high frequency through single-roll melt-spinning in a furnace (not shown) with an Ar gas atmosphere whose pressure has been reduced to 50 kPa or less, and then a molten metal with a composition that provides a rare-earth magnet is sprayed at a copper roll R to produce a quenched thin strip B (i.e., a quenched ribbon). Then, the quenched thin strip B is coarsely ground.

(12) A cavity that is defined by a carbide die D and a carbide punch P, which slides in the hollow space in the carbide die D, is filled with the quenched thin strip B that has been coarsely ground as shown in FIG. 1(b), and pressure is applied thereto (in the X direction) with the carbide punch P, and further, electric current is caused to flow therethrough in the pressure application direction for heating purposes, whereby a sintered body S is produced that includes a NdFeB-based main phase with a nanocrystalline structure (i.e., a crystal grain size of about 50 to 200 nm) and a grain boundary phase around the main phase, the grain boundary phase containing a NdX alloy (where X is a metallic element).

(13) Herein, the NdX alloy that forms the grain boundary phase is an alloy of Nd and at least one of Nd, Co, Fe, or Ga, and contains, for example, one of NdCo, NdFe, NdGa, NdCoFe, or NdCoFeGa, or a mixture of two or more of them, and thus is in the Nd-rich state.

(14) As shown in FIG. 2(a), the sintered body S exhibits an isotropic crystalline structure in which the grain boundary phase BP fills the gaps between the nanocrystalline grains MP (i.e., the main phase). Herein, the carbide punch P is made to abut the end face of the sintered body S in the longitudinal direction (in FIG. 1(b), the horizontal direction corresponds to the longitudinal direction) as shown in FIG. 1(c) to impart anisotropy to the sintered body S, and then hot plastic processing is performed with pressure applied (in the X direction) with the carbide punch P, whereby a molded body C with a crystalline structure having anisotropic nanocrystalline grains MP is produced as shown in FIG. 2(b) (hereinabove, the first step).

(15) It should be noted that when the processing degree (i.e., compressibility) of the hot plastic processing is high, for example, when the compressibility is greater than or equal to about 10%, such processing may be called hot high-strength processing or be simply called high-strength processing.

(16) In the crystalline structure of the molded body C shown in FIG. 2(b), the nanocrystalline grains MP have flat shapes, and interfaces that are substantially parallel with the anisotropy axis are curved or bent, and thus are not formed by a specific planes.

(17) Next, as shown in FIG. 3(a), the produced molded body C is stored in a high-temperature furnace H with a built-in heater, and a modifying alloy M that contains no heavy rare-earth elements such as Tb (i.e., a Nd-Z alloy (where Z is a metallic element other than heavy rare-earth elements)) is brought into contact with the molded body C, and then, the furnace is set to a high-temperature atmosphere.

(18) Herein, for the Nd-Z alloy, one of a NdCu alloy or a NdAl alloy is used.

(19) The melting point of the grain boundary phase containing NdCo, NdFe, NdGa, NdCoFe, or NdCoFeGa, or a mixture thereof varies depending on the components or the ratio of the components, but is approximately around 600 C. (i.e., in the range of about 550 C. to 650 C. for which such variations are taken into consideration).

(20) When a NdCu alloy is used as a modifying alloy, the eutectic point thereof is about 520 C. that is substantially equal to the melting point of the grain boundary phase BP. Thus, when the high-temperature furnace H is set to a temperature atmosphere of 520 to 600 C., the grain boundary phase BP will melt and the NdCu alloy, which is the modifying alloy, will also melt.

(21) The melt of the molten NdCu alloy is caused to liquid-phase infiltrate into the grain boundary phase BP in the molten state. Thus, the grain boundary phase, which contains NdCo, NdFe, NdGa, NdCoFe, or NdCoFeGa, or a mixture thereof, that is partially or entirely modified by the NdCu alloy is formed.

(22) As described above, as a melt of a modifying alloy is caused to liquid-phase infiltrate into the grain boundary phase BP in the molten state, the diffusion efficiency and the diffusion speed are significantly superior to when DyCu alloys and the like are solid-phase diffused into the grain boundary phase, and thus diffusion of a modifying alloy in a shorter time is possible.

(23) When a NdAl alloy is used as a modifying alloy, the melting point thereof is 640 to 650 C. (and the eutectic point thereof is 640 C.), which is slightly higher than the melting point of the grain boundary phase BP. Thus, when the temperature atmosphere is set to 640 to 650 C., it is possible to melt the grain boundary phase BP and also melt the NdAl alloy, and thus cause a melt of the NdAl alloy to liquid-phase infiltrate into the grain boundary phase, whereby the grain boundary phase, which contains NdCo, NdFe, NdGa, NdCoFe, or NdCoFeGa, or a mixture thereof, that is partially or entirely modified by the NdAl alloy is formed.

(24) When the melt of the modifying alloy has been caused to liquid-phase infiltrate into the grain boundary phase, and a given period of time has elapsed, the crystalline structure of the molded body C shown in FIG. 2(b) will change, so that, as shown in FIG. 3(b), interfaces between the crystal grains MP become clearer and magnetization separation between the crystal grains MP and MP progresses, and thus the coercivity improves. However, in the mid stage of the modification of the structure by the modifying alloy shown in FIG. 3(b), interfaces that are substantially parallel with the anisotropy axis are not formed (i.e., are not formed by specific planes).

(25) At the stage where the modification by the modifying alloy has sufficiently progressed, interfaces (i.e., specific planes) that are substantially parallel with the anisotropy axis are formed as shown in FIG. 3(c), whereby a rare-earth magnet RM whose crystal grains MP, when seen from a direction perpendicular to the anisotropy axis (i.e., a direction from which FIG. 3(c) is seen), are rectangular in shape or have a close shape thereto is formed.

(26) As described above, the rare-earth magnet RM of the present invention obtained with the production method of the present invention is considered to have improved coercivity because, as a molded body that has been obtained by applying hot plastic processing to a sintered body to impart anisotropy thereto is used, and a melt of a NdCu alloy or a NdAl alloy, which is a modifying alloy containing no heavy rare-earth elements, is caused to liquid-phase infiltrate into the grain boundary phase in the molten state, the residual strains that have been produced by the hot plastic processing will come into contact with the melt of the modifying alloy and thus are removed, and further, a reduction in the crystal grain size as well as magnetization separation between the crystal grains progresses.

(27) In addition, since a modifying alloy that contains no heavy rare-earth elements such as Tb and has a melting point that is about equal to the melting point of the grain boundary phase is used, when both the grain boundary phase and the modifying alloy are melted at a relatively low temperature of about 600 C., coarsening of the nanocrystalline grains is suppressed, and this also contributes to an improvement in the coercivity. Further, since heavy rare-earth elements such as Tb are not used, the material cost can be significant low, which in turn leads to a significant reduction in the production cost of the rare-earth magnet.

(28) Experiments of Measuring Coercivity by Varying the Amount of a Modifying Alloy Added to a Base Magnet, Results Thereof, and Arrangement of the Coercivities of Rare-Earth Magnets Using the Kronmuller Formula

(29) The inventors conducted experiments to identify the optimal range of the amounts of infiltration by preparing specimens of rare-earth magnets made of nanocrystalline magnets by using a NdCu alloy as a modifying alloy and variously changing the temperature at the time of melting and the amount of infiltration of the modifying alloy.

(30) Further, the inventors also made an attempt to arrange improvements in the coercivities of the rare-earth magnets using the Kronmuller formula.

(31) It has been confirmed from a TEM image photograph that the specimen has a crystal grain size in the range of 50 to 200 nm. The sintered body was produced with a pressure of 300 MPa applied thereto for five minutes in a temperature atmosphere of 600 C. under a vacuum atmosphere. Such a sintered body was subjected to hot plastic processing at 780 C. at a strain rate of 1/s, whereby a molded body was produced.

(32) The amount of the NdCu alloy added to the obtained molded body was changed within the range of about 0 to 33 mass %, and the melting temperature in the second step was changed in four patterns that are 575 C., 600 C., 625 C., and 650 C. to fabricate a number of specimens, and then a graph was created on the basis of the test result of each specimen (i.e., the amount of the NdCu alloy added and the coercivity measured with a pulse-excited magnetic property measurement device) for each melting temperature. FIG. 4 shows the test results and an approximated curve Z created from the test results of the four patterns.

(33) FIG. 4 can confirm a tendency that coercivity increases depending on the amount of infiltration of the NdCu alloy, which is a modifying alloy, in each case, and demonstrates that the coercivity curve has an inflection point when the modifying alloy is added by (about) 5 mass % with respect to the mass of the molded body before the infiltration, and further that the coercivity curve is saturated to substantially the maximum coercivity when the modifying alloy is added by (about) 15 mass %.

(34) It has been identified by the inventors that, based on the general tendency that the higher the coercivity, the lower the magnetization, the amount of the modifying alloy is preferably (about) 10 mass % or less from the perspective of the maximum energy product BHmax. Thus, (about) 15 mass % for when the coercivity performance is prioritized can be defined as the upper limit value of the amount of the modifying alloy added (the amount of infiltration), and (about) 5 mass % for when both the adequate coercivity performance and the maximum magnetic energy product BHmax are prioritized can be defined as the lower limit value of the amount of the modifying alloy added.

(35) It should be noted that even when a NdAl alloy is used as a modifying alloy, similar experimental results are considered to be obtained. Thus, a similar optimum range of the amount of the modifying alloy to be added can be defined.

(36) Herein, the Kronmuller formula that is commonly known is shown below, and the coercivities of the rare-earth magnets that are based on the experimental results are arranged using the formula.
Hc=HaNMs,[Formula 1]

(37) where Hc denotes coercivity, denotes a factor attributable to the separation property (between the nanocrystalline grains) of the main phase, Ha denotes magnetocrystalline anisotropy (which is specific to the main-phase material), N denotes a factor attributable to the grain size of the main phase, and Ms denotes saturation magnetization (which is specific to the main-phase material).

(38) FIG. 5 shows the coercivities of the experimental results of the aforementioned specimens that are arranged using the above formula.

(39) The coordinate system shown in FIG. 5 is a coordinate system having the vertical axis N and the horizontal axis , and the value of each specimen is plotted. It can be seen that with a reduction in the crystal grain size and an improvement in the magnetic separation property, a rare-earth magnet that is produced through liquid-phase infiltration of a melt of a NdCu alloy tends to shift from the state of the molded body in the upper left region of the coordinates toward the lower right region of the coordinates.

(40) More specifically, it can be understood from the graph that as the amount of infiltration of the modifying alloy increases, the value N decreases, and then, coercivity increases along with an increase with the value (shifts in a stepwise manner in the lower right direction as indicated by a line Q in FIG. 5).

(41) It has also been identified that as the value is higher and the value N is lower, the heat resistance of the rare-earth magnet will improve.

(42) In the graph, the crystal grain size of the rare-earth magnet will never be larger than that of the raw material powder. Thus, 0.68 can be defined as the lower limit of the value N (i.e., a lower limit graph L1). It should be noted that the raw material powder (i.e., a ribbon of the nanocrystalline grain structure) has a small factor N attributable to the grain size, and also has a small separation property a between the crystals.

(43) There is no possibility that the separation property between the crystal grains will be lower than that of the molded body. Thus, 0.42 can be defined as the upper limit of the value (i.e., a lower limit graph L3).

(44) In addition, as the crystal grain size becomes smaller than that of the molded body, 0.9, which is the lower limit value of the crystal grain size of the molded body can be defined as the upper limit of the value N (i.e., an upper limit graph L2) of the rare-earth magnet.

(45) Further, the value : 0.52, which indicates the most excellent separation property in the present experiment, can be defined as the upper limit of the value (i.e., an upper limit graph L4).

(46) It should be noted that as shown in FIG. 5, although a sintered magnet has a high separation property between the grains (i.e., is large), the factor N attributable to the grain size is large, and the grain size of the sintered magnet does not change during the formation process. Thus, although the separation property between the grains will improve, an improvement in the grain size factor cannot be expected (N remains to be 1.4).

(47) FIG. 5 also shows that when the molded body obtained through hot plastic processing is left as is, the ranges of a and N will remain: <0.42 and N>0.9.

(48) As described above, when a NdCu alloy or a NdAl alloy is used and the amount of infiltration thereof is adjusted appropriately, it is possible to adjust the balance between magnetization and coercivity. Thus, when a rare-earth magnet with high coercivity is pursued or when a rare-earth magnet with excellent coercivity and magnetization and with high maximum energy product is pursued, for example, it is possible to design a rare-earth magnet with optimum performance in accordance with the required performance.

(49) Experiments of Measuring Coercivity and Magnetization by Changing the Amount of a Modifying Alloy Added to a Base Magnet, and Results Thereof

(50) The inventors have further conducted measurements of magnetization in addition to coercivity in the aforementioned experiments, and plotted the experimental results on the coercivity-magnetization coordinate system, and then verified the correlation of the optimal values between the amount of the modifying metal (i.e., a NdCu alloy) added and the temperature conditions in the second step. FIG. 6 shows the coercivity-magnetization coordinate system showing the experimental results.

(51) FIG. 6 can confirm a general tendency that as the amount of the NdCu alloy added changes from 5 mass % to 20 mass %, magnetization decreases and coercivity improves. It should be noted that a curve Y1 represents a line that connects the plotted value for each amount of addition when the melting temperature in the second step is 600 C., and a curve Y2 represents a line that connects the plotted value for each amount of addition when the melting temperature is 650 C.

(52) When the amount of the alloy added is 5 mass %, in the four cases where the melting temperature in the second step is 575 C., 600 C., 625 C., and 650 C., there is a general tendency that coercivity will decrease as the temperature is higher, and additionally, an improvement in magnetization cannot be confirmed (i.e., magnetization is at about the same level in all of the four cases).

(53) In contrast, in the other cases where the amount of the alloy added is 10, 15, and 20 mass %, it can be confirmed that both magnetization and coercivity are the highest when the temperature is 600 C. (to be exact, magnetization of when the amount of the alloy added is 10 mass % is slightly higher than when the temperature is 625 C.).

(54) Accordingly, when a NdCu alloy is used as a modifying alloy, it is considered that the melting temperature in the second step is desirably set to 600 C. (which is a temperature greater than or equal to the eutectic point of the NdCu alloy).

(55) From the foregoing results, it is estimated that when a NdAl alloy is used as a modifying alloy, the melting temperature in the second step is desirably set to a temperature of 640 to 650 C. that is the melting temperature of the NdAl alloy.

(56) Results of Observation of the Crystalline Structure of a Rare-Earth Magnet Obtained Through Sufficient Liquid-Phase Infiltration of a Melt of a Modifying Alloy into a Grain Boundary Phase in a Molten State

(57) The inventors captured TEM images of the structures of a molded body that has been produced through hot plastic processing, a rare-earth magnet that is being produced and in which a melt of a modifying alloy is caused to liquid-phase infiltrate into a grain boundary phase in a molten state for a given period of time, and further a rare-earth magnet that has been produced through sufficient liquid-phase infiltration of a melt of a modifying alloy into a grain boundary phase in a molten state. Then, the inventors observed changes in the shapes of the nanocrystalline grains.

(58) Herein, a sintered body was produced by grinding a quenched thin strip (i.e., a RE-TM-B-M alloy, where RE is NdPr, TM is FeCo, and M is Ga), which has been produced through a liquid quenching method, so that the central grain size becomes about 1000 m, and filling a cavity defined by a carbide die and a carbide punch with the ground quenched thin strip B, and then performing baking while applying pressure under the conditions of a temperature of 500 to 700 C. and a pressure of 50 to 500 MPa for a time of 10 to 600 seconds. Then, the sintered body was subjected to hot plastic processing under the conditions of a temperature of 600 to 800 C. and a strain rate of 100/s, whereby a molded body with magnetic anisotropy imparted thereto was produced.

(59) Such a molded body was stored in a high-temperature furnace, and a 10 to 20 mass % NdCu alloy (i.e., Nd70Cu30) as a modifying alloy was brought into contact with respect to the mass of the molded body, and then the temperature in the furnace was set to about 600 C., so that a melt of the modifying alloy was caused to liquid-phase infiltrate into the grain boundary phase in the molten state. Then, a TEM image of the molded body was captured and the coercivity thereof was also measured. A TEM image of each rare-earth magnet after 10 minutes have elapsed, and further, after 30 minutes have elapsed from the liquid-phase infiltration was captured. FIGS. 7(a), 7(b), and 7(c) show the respective TEM images.

(60) The molded body in FIG. 7(a) has a coercivity of 16 kOe (i.e., 1274 kA/m). It can be confirmed that the crystal grains have a structure in which the grains are perpendicular with respect to the orientation direction and are flat, while grain boundaries that are substantially parallel with the anisotropy axis are curved or bent, and thus are not formed by specific planes.

(61) In contrast, the rare-earth magnet shown in FIG. 7(b) that is being modified has an improved coercivity of 20 kOe (i.e., 1592 kA/m), and it can be confirmed that interfaces between the crystal grains are clearer than in FIG. 7(a), and magnetization separation between the crystal grains has progressed. However, interfaces that are substantially parallel with the anisotropy axis are not formed (i.e., the grain boundaries are not formed by specific planes).

(62) A rare-earth magnet shown in FIG. 7(c) that has been sufficiently modified by a modifying alloy has an improved coercivity of 25 kOe (i.e., 1990 kA/m). As shown in FIG. 7(c), it can be confirmed that interfaces (specific planes) that are substantially parallel with the anisotropy axis are formed, each crystal grain, when seen from a direction perpendicular to the anisotropy axis (i.e., a direction from which FIG. 7(c) is seen), exhibits a rectangular shape or a close shape thereto.

(63) The surface of each nanocrystalline grain is polyhedral (i.e., a hexahedron or an octahedron, or further, a close solid thereto) that is surrounded by low-index planes. For example, it has been confirmed that when the surface of the nanocrystalline grain is hexahedral, the orientation axis is formed along the (001) plane, and side planes are formed by (110), (100), or Miller indices that are close thereto.

(64) The observation results show that when a rare-earth magnet is produced with the aforementioned production method, it is possible to obtain a rare-earth magnet with a metal structure having nanocrystalline grains whose surfaces are polyhedral such as hexahedrons or octahedrons that are surrounded by low-index planes, and obtain a rare-earth magnet with excellent coercivity performance, in particular, excellent coercivity performance at high temperatures, and a high maximum energy product since a reduction in the crystal grain size as well as magnetic separation between the crystal grains is sufficiently achieved.

(65) Although the embodiments of the present invention have been described in detail with reference to the drawings, specific structures are not limited thereto. The present invention includes design changes and the like that may occur within the scope and spirit of the present invention.

REFERENCE SIGNS LIST

(66) R Copper roll B Quenched thin strip (quenched ribbon) D Carbide die P Carbide punch S Sintered body C Molded body H High-temperature furnace M Modifying alloy MP Main phase (nanocrystalline grains, crystal grains) BP Grain boundary phase RM Rare-earth magnet