R-T-B based permanent magnet

Abstract

An R-T-B based permanent containing main phase grains with a composition of (R.sub.1-xY.sub.x).sub.2T.sub.14B (R is rare earth element(s) composed of one or more elements selected from the group consisting of La, Ce, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb and Lu, T is one or more transition metal elements including Fe or a combination of Fe and Co as essential elements, and 0.2x0.7), wherein the residual magnetic flux density Br is 1.1 T or more, the coercivity HcJ is 400 kA/m or less, and the ratio Hex/HcJ of the external magnetic field Hex required for obtaining a residual magnetic flux density Br of 0 to the coercivity HcJ is 1.10 or less.

Claims

1. An R-T-B based permanent magnet comprising main phase grains with a composition of (R.sub.1-xY.sub.x).sub.2T.sub.14B with a residual magnetic flux density Br of 1.1 T or more and a coercivity HcJ of 400 kA/m or less, wherein: a ratio (Hex/HcJ) of the external magnetic field of Hex to the coercivity (HcJ) is 1.10 or less, where: the Hex is the external magnetic field required for obtaining a residual magnetic flux density Br of 0, R is rare earth element(s) composed of one or more elements selected from the group consisting of La, Ce, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb and Lu; T is one or more transition metal elements including Fe or a combination of Fe and Co; and 0.2x0.7.

2. A rotating machine comprising the R-T-B based permanent magnet of claim 1.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) FIG. 1 is a magnetization-magnetic field curve used to find the external magnetic field Hex when the residual magnetic flux density Br becomes 0 in Example 4 of the present invention.

DETAILED DESCRIPTION OF EMBODIMENTS

(2) Hereinafter, the preferable embodiments of the present invention will be described in detail. In addition, the embodiments do not limit the invention but are only examples, and all the features and the combinations thereof recited in the embodiments are not necessarily limited to the substantive contents of the invention.

(3) The R-T-B based permanent magnet of the present invention is characterized in that it contains main phase grains with a composition of (R.sub.1-xY.sub.x).sub.2T.sub.14B (R is rare earth element(s) composed of one or more elements selected from the group consisting of La, Ce, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb and Lu, T is one or more transition metal elements including Fe or a combination of Fe and Co as essential elements, and 0.2x0.7); the residual magnetic flux density Br is 1.1 T or more; the coercivity HcJ is 400 kA/m or less; and the ratio Hex/HcJ of the external magnetic field Hex which is required for obtaining a residual magnetic flux density Br of 0 to the coercivity HcJ is 1.10 or less.

(4) In the present embodiment, R is rare earth element(s) composed of one or more selected from the group consisting of La, Ce, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb and Lu.

(5) In the present embodiment, the amount x of Y in the composition of the main phase grains satisfies 0.25x0.7. As x increases, only the coercivity HcJ decreases while the residual magnetic flux density Br is approximately maintained. The inventors of the present invention suppose that the magneto crystalline anisotropy in the sample decreases as the amount of Y increases. However, if x exceeds 0.7, the squareness ratio Hk/HcJ will evidently reduce and the magnetic flux obtained when used as the magnet for the motor will decrease. Also, in the present embodiment, it is more preferably from the viewpoint of the stability in preparation that if the amount x of Y in the composition of the main phase grains satisfies 0.4x0.6 because the variation of the magnetic properties accompanying with the composition variation will be smoothed.

(6) In the present embodiment, part of B can be replaced by C. The amount of C to replace B is preferred to be 10 atomic % or less relative to B.

(7) In the present embodiment, T as the balance of the composition is one or more transition metal elements including Fe or Fe and Co as essential elements. The amount of Co is preferably 0 atomic % or more and 10 atomic % or less relative to the amount of T. With the increase of the amount of Co, the Curie temperature can be increased and the decrease of the coercivity relative to the increase of temperature can be inhibited to be small. Further, the corrosion resistance of the rare earth based permanent magnet can be improved by increasing the amount of Co.

(8) Hereinafter, the preferable examples of the preparation method for the present invention will be described.

(9) In the preparation of the R-T-B based permanent magnet in the present embodiment, the alloy raw material(s) will be prepared first with which an R-T-B based magnet with the desired composition can be obtained. The alloy raw material(s) can be prepared by the strip casting method or other well known melting methods under vacuum or at an inert atmosphere, preferably at Ar atmosphere. In the strip casting method, the molten metal obtained by melting the starting metal(s) at a non-oxidative atmosphere such as Ar atmosphere is sprayed to the surface of the rotating roll. The molten metal quenched on the roll will be condensed into a thin plate or a sheet (a scale-like shape). The quenched and condensed alloy is provided with a dendritic structure formed by the R.sub.2T.sub.14B crystals which are the main phase grains with a particle size of 1 to 50 m and the R-rich grain boundary phase grains. The method for preparing the alloy raw material is not limited to the strip casting method, and the alloy raw material can also be obtained by melting methods such as the high frequency induction melting method. Further, in order to prevent the segregation from happening after the melting process, for example, the molten metal can be poured on a water cooled copper plate so as to be solidified. Also, the alloy obtained by the reduction diffusion method can be used as the alloy raw material.

(10) In the case of obtaining the R-T-B based permanent magnet in the present invention, for the alloy raw materials, substantially, the so-called single-alloy method for manufacturing a magnet from alloy of one kind of metal may be used, but the so-called mixing method may also be used, which uses a main phase alloy and a alloy contributing to the formation of the grain boundary effectively. In the mixing method, the main phase alloy (low-R alloy) has the main phase grains (i.e., R.sub.2T.sub.14B crystals) as the main part while the alloy contributing to the formation of the grain boundary effectively (high-R alloy) contains more R than the low-R alloy.

(11) The alloy raw material is subjected to a hydrogen adsorbing process. The alloy raw material was embrittled via hydrogen adsorption and will be easily pulverized in the following pulverization process. On the other hand, in the alloy raw material composed of the main phase grains and the grain boundary phase grains, cracks will be generated due to the difference between the amounts of the adsorbed hydrogen (i.e., the difference of the specific volumetric dilatations) of the main phases and the grain boundary phases, and the alloy raw material will be easily pulverized in the following pulverization process. The lower the temperature is, the higher the amount of hydrogen can be absorbed by the alloy raw material is. Thus, it will be effective to perform the hydrogen adsorbing process at a lower temperature to make the pulverization process easier. However, there is a problem in the preparation that a long time is required if the hydrogen adsorption is performed at a low temperature, so the alloy raw material is usually heated and then kept at about 200 to 400 C. in the hydrogen adsorbing process. If the alloy raw material is heated and then kept at a temperature of 700 C. or higher, the amount of adsorbed hydrogen will sharply increase. This is due to the disproportionation reaction in which the main phase Nd.sub.2Fe.sub.14B is decomposed into three phases, i.e., NdH.sub.2, Fe.sub.2B and Fe. There is an HDDR (HydrogenerationDecompositionDesorptionRecombination) method which takes advantage of such a phenomena to micronize the crystal grains so as to provide a powder with high coercivity. In the present embodiment, the temperature at which the alloy raw material is heated and kept in the hydrogen adsorbing process will vary according to the composition of the alloy raw material but goes within the range of 600 to 800 C. The inventors of the present invention considered that if the temperature in the hydrogen adsorbing process is controlled within the range mentioned above, the disproportionation reaction occurs only in part of the alloy raw material, which is good for the low coercivity due to the heterogeneity of the structure and is also good for the pinning of the magnetization mechanism due to the generation of the pin phases.

(12) The alloy raw material after the hydrogen adsorption is subjected to a hydrogen-releasing process. The hydrogen-releasing process is performed under vacuum or at an inert atmosphere with a controlled pressure. The desorption and recombination processes following the hydrogenation and decomposition processes in the HDDR method are extremely important for a high coercivity. However, the present invention aims to provide a permanent magnet whose magnetization state can be controlled by a small external magnetic field, so the hydrogen-releasing process does not need to be strictly controlled as that in the HDDR method. In the present embodiment, the temperature at which the alloy raw material is heated and kept in the hydrogen-releasing process will vary depending on the composition of the alloy raw material but is within the range of 650 to 850 C. The desorption and recombination processes are performed and Nd.sub.2Fe.sub.14B is generated from the three phases of NdH.sub.2. Fe.sub.2B and Fe while the temperature in the hydrogen-releasing process is controlled to be within the range mentioned above and the partial pressure of hydrogen is reduced in the atmosphere. The inventors of the present invention think that during the generation of Nd.sub.2Fe.sub.14B via the desorption and recombination reactions, the incomplete reaction leads to the remain of heterogeneous phases or defects, which is good for the low coercivity and the pinning of the magnetization mechanism due to the generation of the pin phases. In another respect, it will be effective to make the following pulverization process easier by carrying out the hydrogen adsorption against the alloy raw material at a temperature where the hydrogenation and decomposition reactions will not initiate (especially at a low temperature which aims to increase the amount of the adsorbed hydrogen) after the Nd.sub.2Fe.sub.14B is generated via the desorption and recombination reactions in the hydrogen-releasing process. In this case, although the alloy raw material is subjected to the pulverization process when hydrogen has been adsorbed to it, there is no problem because the adsorbed hydrogen is released during the early stage of the sintering process when the temperature rises.

(13) The alloy raw material after the hydrogen-releasing process is subjected to a coarse pulverization process. The alloy raw material is pulverized to have a particle size of several hundreds of microns by a stamp mill, a jaw crusher, a braun mill or the like so as to provide a coarsely pulverized powder. Further, the coarse pulverization process is preferably carried out at an inert atmosphere. If the alloy raw material after the hydrogen adsorbing process and the hydrogen-releasing process almost has the desired particle size, the coarse pulverization process can be omitted.

(14) The coarsely pulverized powder is subjected to a fine pulverization process. The coarsely pulverized powder is pulverized to have an average particle size of 1 to 5 m by a jet mill, a wet pulverizer (a ball mill, an attritor) or the like so as to provide a finely pulverized powder. The jet mill ejects a gas with a high pressure via a narrow nozzle so as to provide a gas flow with a high speed by which the coarsely pulverized powder is accelerated and then hit each other to perform the pulverization. The pulverized powder can be prevented from oxidizing by using an inert gas as the working gas. The wet pulverizer provides the media in the dispersion medium and the pulverized powder with kinetic energies and then pulverizes the pulverized powder. The oxidation of the pulverized powder can be inhibited by selecting an appropriate dispersion medium.

(15) The finely pulverized powder is subjected to a molding process in a magnetic field. In the molding process in a magnetic field, the molding pressure may be set to be in a range of 0.3 to 3 ton/cm.sup.2 (30 to 300 MPa). The molding pressure can be constant or incremental or degressive from the start to the end of the molding process. Otherwise, the pressure can be randomly changed. The lower the molding pressure is, the better the orientation is. However, if the molding pressure is much too low, the strength of the molded body will be insufficient, which will cause problems in the handling. Thus, the molding pressure is to be selected within the range mentioned above. The molded body obtained in the molding process in a magnetic field will usually have a final relative density of 40 to 60%. The applied magnetic field can be made to be around 960 to 1600 kA/m (12 to 20 kOe). The applied magnetic field is not limited to be a static magnetic field. A pulsed magnetic field can also be used. Further, the static magnetic field and the pulsed magnetic field can be used in combination.

(16) During the fine pulverization, about 0.01 to 0.3 wt % of an fatty acid or an derivative of an fatty acid or an hydrocarbon may be added to improve the lubrication and the orientation in the molding process such as zinc stearate, calcium stearate, aluminum stearate, octadecanamide, oleamide, ethylene-bis-isostearic acid amide (all of which are stearic acid based or oleic acid based compounds), paraffin and naphthalene (which two are hydrocarbons) or the like.

(17) The molded body is subjected to a sintering process. The sintering process is performed under vacuum or at an inert atmosphere. The temperature and the duration for the sintering process need to be adjusted depending on various conditions such as the composition, the pulverization method, the average particle size, and the distribution of particle size or the like. Nevertheless, the process may be performed at a temperature of approximately 1000 to 1200 C. for 2 to 20 hours.

(18) It is well known that a permanent magnet with a high residual magnetic flux density and a low coercivity can be obtained by elevating the temperature and prolonging the duration in the sintering process. However, the decrease of coercivity in the sintered body occurred during the sintering process with a high temperature and a long time is due to the coarse crystal gains. Further, an external magnetic field with intensity several times that of the coercivity is required for the magnetization switching, so the magnetization state cannot be controlled by a small external magnetic field. In other words, the permanent magnet with a high residual magnetic flux density and a low coercivity obtained by long-lasting sintering process at a high temperature is not suitable to be used as the variable magnet for the variable magnetic flux motor.

(19) After sintered, the obtained sintered body is subjected to an aging treatment. The aging treatment is effective in adjusting the coercivity, but the coercivity which can be adjusted in the aging treatment is about 400 kA/m. Thus, it is difficult to decrease the coercivity of the NdFeB based permanent magnet (1000 kA/m or more) to a level suitable for the variable magnet used in the variable magnetic flux motor only via the aging treatment. That is, the major adjustment of the coercivity is entrusted to the composition (the adjustment of the Y amount) and the aging treatment process remains in a level of minor adjustment of the coercivity. In this way, the permanent magnet with a high residual magnetic flux density and a low coercivity which is suitably used as the variable magnet for the variable magnetic flux motor can be obtained by relatively easy preparation processes.

EXAMPLES

(20) Hereinafter, the present invention will be further described based on the examples and comparative examples. However, the present invention is not limited to the examples described below.

(21) Specified amounts of the metal Nd, the metal Y, the electrolytic iron and ferro-boron were weighed by which a composition of the main phase grains of (R.sub.1-xY.sub.x).sub.2T.sub.14B (R=Nd, T=Fe, x=0 to 1.0) can be obtain, and a sheet like alloy was obtained via a strip casting method. The alloy was subjected to a hydrogen adsorbing process which was performed at an atmosphere with the partial pressure of hydrogen P.sub.HD being 10 to 100 kPa at a temperature for hydrogen adsorbing T.sub.HD of 500 to 800 C. for 1 hour. After the hydrogen adsorbing process, a hydrogen-releasing process was performed under vacuum at a temperature for hydrogen-releasing T.sub.DR of 800 C. for 1 hour. Next, the alloy after the hydrogen-releasing process was subjected to a hydrogen adsorbing process again which was performed at an atmosphere with the partial pressure of hydrogen P.sub.AB controlled to be 1 MPa at a temperature for hydrogen adsorbing T.sub.AB of 50 C. for 3 hours. Further, as the alloy raw material after the hydrogen adsorbing process had been pulverized to have a particle size of several hundreds of microns, the coarse pulverization process was omitted here. Oleamide of 0.1 wt % was added as the lubricant, and then finely pulverized powder with an average particle size of 3 m was obtained by using a jet mill at an Ar atmosphere. The finely pulverized powder was filled into a mold (with an opening size of 20 mm18 mm), and subjected to uniaxial pressing molding with a pressure of 2.0 ton/cm.sup.2 under a magnetic field (2T) applied in a direction perpendicular to the pressing direction. The obtained molded body was heated to the sintering temperature T.sub.s of 1090 C. and was kept for 4 hours. Then, it was cooled down to room temperature. Thereafter, an aging treatment was provided in which a primary treatment lasted for 1 hour at a temperature T.sub.1 of 850 C. and a secondary treatment lasted for 1 hour at a temperature T.sub.2 of 530 C. were performed, so that a sintered body was obtained.

(22) The magnetic properties of the sintered body were measured by a BH tracer. Firstly, external magnetic fields sufficient to magnetically saturate the sintered body were applied in the positive direction and the negative direction so as to provide a magnetization-magnetic field curve (full loop). Then, after a specified magnetic field was applied in the negative direction, a magnetic field sufficient to magnetically saturate the sintered body was applied in the positive direction so as to provide another magnetization-magnetic field curve (minor loop). Repeated measurements were provided with the specified magnetic field applied in the negative direction increased gradually so as to find out the external magnetic field Hex when the residual magnetic flux density Br became 0. FIG. 1 exemplarily showed a magnetization-magnetic field curve for finding out the external magnetic field Hex when the residual magnetic flux density Br became 0 in Example 4 of present invention.

(23) The mainly generated phase in the sintered body was confirmed to be the tetragonal R.sub.2T.sub.14B structure via X-ray diffraction method. Then, the vicinity around the center of the main phase grains were analyzed by an energy dispersive spectroscopy (EDS) equipped on a scanning transmission electron microscope (STEM), and the composition of the main phase grains was quantified.

Examples 1 to 6 and Comparative Examples 1 to 7

(24) When the composition of the main phase grains was set as (R.sub.1-xY.sub.x).sub.2T.sub.14B (R=Nd, T=Fe, x=0.0 to 1.0), as the replacement amount x of Y relative to Nd increased, only the coercivity HcJ was reduced while the residual magnetic flux density Br was substantially maintained. Further, when x was 0.2 or more, a coercivity of 400 kA/m or less could be obtained which is suitable for the use of the variable magnet in the variable magnetic flux motor. However, if x exceeded 0.7, the squareness ratio Hk/HcJ decreased significantly and the magnetic flux obtained as the magnet for the motor also decreased. In other words, it could be seen that when x was in the range of 0.2x0.7, a permanent magnet with a high residual magnetic flux density and a low coercivity could be provided which was suitably used as the variable magnet for the variable magnetic flux motor. On the other hand, it could be seen that when x was within the range mentioned above, a permanent magnet suitably used as the variable magnet for the variable magnetic flux motor could be obtained, in which the ratio Hex/HcJ of the external magnetic field Hex required for obtaining a residual magnetic flux density Br of 0 to the coercivity HcJ was 1.10 or less, and the magnetization state could be controlled by a small external magnetic field.

Example 4 and Examples 7 to 8

(25) When the composition of the main phase grains was set as (R.sub.1-xY.sub.x).sub.2T.sub.14B (R=Nd or Pr, T=Fe, x=0.5), being independent of the percentages of Nd and Pr in R, an effectiveness of only decreasing the coercivity HcJ while substantially maintaining of the residual magnetic flux density Br as well as the squareness ratio Hk/HcJ can be obtained by the replacement of R with Y. Further, the ratio Hex/HcJ of the external magnetic field Hex required for obtaining a residual magnetic flux density Br of 0 to the coercivity HcJ was almost maintained constant independent of the percentages of Nd and Pr in R. It could be seen that in the R-T-B based permanent magnet with the composition of main phase grains being (R.sub.1-xY.sub.x).sub.2T.sub.14B (0.2x0.7), a permanent magnet suitably used as the variable magnet for the variable magnetic flux motor could be obtained, in which even if an element other than Nd was used as R, the magnetization state could be controlled by a small external magnetic field.

Example 4, Example 9 and Comparative Examples 8 to 9

(26) When the composition of the main phase grains was set as (R.sub.1-xY.sub.x).sub.2T.sub.14B (R=Nd, T=Fe, x=0.5) and the partial pressure of hydrogen P.sub.HD in the hydrogen adsorbing process was set to be 10 to 100 kPa, the residual magnetic flux density Br sharply decreased if the partial pressure of hydrogen P.sub.HD was much too high, and the ratio Hex/HcJ of the external magnetic field Hex required for obtaining a residual magnetic flux density Br of 0 to the coercivity HcJ sharply increased if the partial pressure of hydrogen P.sub.HD was much too low. In other words, if the partial pressure of hydrogen P.sub.HD was controlled within a proper range, a permanent magnet with a high residual magnetic flux density and a low coercivity could be obtained which was suitable for use in the variable magnet for the variable magnetic flux motor. Further, it could be seen that if the partial pressure of hydrogen P.sub.HD was controlled within a proper range, a permanent magnet suitably used as the variable magnet for the variable magnetic flux motor could be obtained, in which the ratio Hex/HcJ of the external magnetic field Hex required for obtaining a residual magnetic flux density Br of 0 to the coercivity HcJ was 1.10 or less, and the magnetization state could be controlled by a small external magnetic field.

Example 4, Example 10 and Comparative Examples 10 to 11

(27) When the composition of the main phase grains was set as (R.sub.1-xY.sub.x).sub.2T.sub.14B (R=Nd, T=Fe, x=0.5) and the temperature for hydrogen adsorbing T.sub.HD in the hydrogen adsorbing process was set to be 500 to 800 C., it could be seen that the residual magnetic flux density Br sharply decreased if the temperature for hydrogen adsorbing T.sub.HD was much too high, and the ratio Hex/HcJ of the external magnetic field Hex required for obtaining a residual magnetic flux density Br of 0 to the coercivity HcJ sharply increased if the temperature for hydrogen adsorbing T.sub.HD was much too low. In other words, if the temperature for hydrogen adsorbing T.sub.HD was controlled within a proper range, a permanent magnet with a high residual magnetic flux density and a low coercivity could be obtained which was suitable for use in the variable magnet for the variable magnetic flux motor. Further, it could be known that if the temperature for hydrogen adsorbing T.sub.HD was controlled within a proper range, a permanent magnet suitably used as the variable magnet for the variable magnetic flux motor could be obtained, in which the ratio Hex/HcJ of the external magnetic field Hex required for obtaining a residual magnetic flux density Br of 0 to the coercivity HcJ was 1.10 or less, and the magnetization state could be controlled by a small external magnetic field.

(28) TABLE-US-00001 TABLE 1 External magnetic Hydrogen adsorbing-hydrogen Sintering-aging field R Y releasing processes processes HeJ Hk/ Hex Hex/ at at P.sub.HD T.sub.HD T.sub.DR P.sub.AR T.sub.AB T.sub.1 T.sub.1 T.sub.2 Br kA/ HeJ kA/ HeJ Composition % % kPa C. C. MPa C. C. C. C. mT m % m Example 1 (Nd.sub.0.80Y.sub.0.20).sub.2Fe.sub.14B Nd 80 20 50 600 800 1 50 1090 850 530 1347 389 87 426 1.10 Example 2 (Nd.sub.0.70Y.sub.0.30).sub.2Fe.sub.14B Nd 70 30 50 600 800 1 50 1090 850 530 1329 291 85 316 1.09 Example 3 (Nd.sub.0.60Y.sub.0.40).sub.2Fe.sub.14B Nd 60 40 50 600 800 1 50 1090 850 530 1301 198 84 212 1.07 Example 4 (Nd.sub.0.50Y.sub.0.50).sub.2Fe.sub.14B Nd 50 50 50 600 800 1 50 1090 850 530 1280 144 83 157 1.09 Example 5 (Nd.sub.0.40Y.sub.0.60).sub.2Fe.sub.14B Nd 40 60 50 600 800 1 50 1090 850 530 1268 121 82 129 1.07 Example 6 (Nd.sub.0.30Y.sub.0.70).sub.2Fe.sub.14B Nd 30 70 50 600 800 1 50 1090 850 530 1249 107 81 112 1.05 Example 7 (Nd.sub.0.25Pr.sub.0.25Y.sub.0.50).sub.2- Nd, 50 50 50 600 800 1 50 1090 850 530 1264 166 82 181 1.09 Fe.sub.14B Pr Example 8 (Pr.sub.0.50Y.sub.0.50).sub.2Fe.sub.14B Nd 50 50 50 600 800 1 50 1090 850 530 1248 187 80 206 1.10 Example 9 (Nd.sub.0.50Y.sub.0.50).sub.2Fe.sub.14B Nd 50 50 20 600 800 1 50 1090 850 530 1284 218 81 239 1.10 Example 10 (Nd.sub.0.50Y.sub.0.50).sub.2Fe.sub.14B Nd 50 50 50 700 800 1 50 1090 850 530 1242 334 81 358 1.07 Comparative Nd.sub.2Fe.sub.14B Nd 100 0 50 600 800 1 50 1090 850 530 1379 927 88 2146 2.32 Example 1 Comparative (Nd.sub.0.90Y.sub.0.10).sub.2Fe.sub.14B Nd 90 10 50 600 800 1 50 1090 850 530 1362 640 86 1028 1.61 Example 2 Comparative (Nd.sub.0.85Y.sub.0.15).sub.2Fe.sub.14B Nd 85 15 50 600 800 1 50 1090 850 530 1355 487 87 587 1.21 Example 3 Comparative (Nd.sub.0.25Y.sub.0.75).sub.2Fe.sub.14B Nd 25 75 50 600 800 1 50 1090 850 530 1234 103 75 107 1.04 Example 4 Comparative (Nd.sub.0.20Y.sub.0.80).sub.2Fe.sub.14B Nd 20 80 50 600 800 1 50 1090 850 530 1221 99 62 102 1.03 Example 5 Comparative (Nd.sub.0.10Y.sub.0.90).sub.2Fe.sub.14B Nd 10 90 50 600 800 1 50 1090 850 530 1213 74 51 75 1.01 Example 6 Comparative Y.sub.2Fe.sub.14B Nd 0 100 50 600 800 1 50 1090 850 530 1202 71 43 72 1.01 Example 7 Comparative (Nd.sub.0.60Y.sub.0.50).sub.2Fe.sub.14B Nd 50 50 10 600 800 1 50 1090 850 530 1293 832 88 2455 2.95 Example 8 Comparative (Nd.sub.0.50Y.sub.0.50).sub.2Fe.sub.14B Nd 50 50 100 600 800 1 50 1090 850 530 973 539 63 593 1.10 Example 9 Comparative (Nd.sub.0.50Y.sub.0.50).sub.2Fe.sub.14B Nd 50 50 50 500 800 1 50 1090 850 530 1287 783 89 2201 2.81 Example 10 Comparative (Nd.sub.0.50Y.sub.0.50).sub.2Fe.sub.14B Nd 50 50 50 800 800 1 50 1090 850 530 1092 483 72 521 1.08 Example 11

(29) As described above, the R-T-B based permanent magnet of the present invention has a high residual magnetic flux density and its magnetic force can reversibly changed via a small external magnetic field. Thus, such a permanent magnet can be suitably used as a magnet with variable magnetic force for a variable magnetic flux motor which can provide a high efficiency in the operation of consumer, industries and transportation equipments where variable speed is needed.