Production method for rare earth permanent magnet

Abstract

A production method for a rare earth permanent magnet, wherein: a sintered magnet body comprising an R.sup.1FeB composition (R.sup.1 represents one or more elements selected from among rare earth elements, including Y and Sc) is immersed in an electrodeposition liquid obtained by dispersing a powder containing an R.sup.2 oxyfluoride and/or an R.sup.3 hydride (R.sup.2 and R.sup.3 represent one or more elements selected from among rare earth elements, including Y and Sc) in a solvent; an electrodeposition process is used to coat the powder onto the surface of the sintered magnet body; and, in the state in which the powder is present on the surface of the magnet body, the magnet body and the powder are subjected to a heat treatment in a vacuum or an inert gas at a temperature equal to or less than the sintering temperature of the magnet.

Claims

1. A method for preparing a rare earth permanent magnet, comprising the steps of: immersing a sintered magnet body having a R.sup.1FeB base composition wherein R.sup.1 is at least one element selected from rare earth elements inclusive of Y and Sc, in an electrodepositing bath of a powder dispersed in water, said powder comprising an oxyfluoride of R.sup.2 and/or a hydride of R.sup.3 wherein R.sup.2 and R.sup.3 each are at least one element selected from rare earth elements inclusive of Y and Sc, said electrodepositing bath containing the powder in a weight fraction of 20% to 70%, effecting electrodeposition for letting the powder deposit on the surface of the magnet body in an area density of at least 10 g/mm.sup.2, by applying a DC voltage of 1 to 300 volts between the magnet body and a counter electrode for 1 to 60 seconds, and heat treating the magnet body with the powder deposited on its surface at a temperature equal to or less than a sintering temperature of the magnet body in vacuum or in an inert gas.

2. The method of claim 1 wherein the powder comprising an oxyfluoride of R.sup.2 and/or a hydride of R.sup.3 has an average particle size of up to 100 m.

3. The method of claim 1 wherein the powder comprising an oxyfluoride of R.sup.2 and/or a hydride of R.sup.3 is deposited on the magnet body surface in an area density of at least 60 g/mm.sup.2.

4. The method of claim 1 wherein in the oxyfluoride of R.sup.2 and hydride of R.sup.3, R.sup.2 and R.sup.3 each contain at least 10 atom % of Dy and/or Tb.

5. The method of claim 4 wherein in the powder comprising the oxyfluoride of R.sup.2 and/or hydride of R.sup.3, R.sup.2 and R.sup.3 each contain at least 10 atom % of Dy and/or Tb, and the total concentration of Nd and Pr in R.sup.2 and R.sup.3 is lower than the total concentration of Nd and Pr in R.sup.1.

6. The method of claim 1, after the heat treatment, further comprising aging treatment at a lower temperature than the heat treatment.

7. The method of claim 1, further comprising cleaning the sintered magnet body with at least one of an alkali, acid and organic solvent, prior to the immersion step.

8. The method of claim 1, further comprising shot blasting the sintered magnet body to remove a surface layer thereof, prior to the immersion step.

9. The method of claim 1, further comprising final treatment after the heat treatment, said final treatment being selected from the group consisting of cleaning with at least one selected from a group consisting of an alkali, acid and organic solvent, grinding, plating and coating.

10. The method of claim 1, wherein the temperature of the heat treatment is in the range of 350 C. to (Ts-10) C. wherein Ts is the sintering temperature of the magnet body.

11. A method for preparing a rare earth permanent magnet, comprising the steps of: immersing a sintered magnet body having a R.sup.1FeB base composition wherein R.sup.1 is at least one element selected from rare earth elements inclusive of Y and Sc, in an electrodepositing bath of a powder dispersed in an organic solvent, said powder comprising a hydride of R.sup.3 wherein R.sup.3 is at least one element selected from rare earth elements inclusive of Y and Sc, said electrodepositing bath containing the powder in a weight fraction of 20% to 70%, effecting electrodeposition for letting the powder deposit on the surface of the magnet body in an area density of at least 10 g/mm.sup.2, by applying a DC voltage of 1 to 300 volts between the magnet body and a counter electrode for 1 to 60 seconds, and heat treating the magnet body with the powder deposited on its surface at a temperature equal to or less than a sintering temperature of the magnet body in vacuum or in an inert gas.

12. The method of claim 11, wherein the powder has an average particle size of up to 100 m.

13. The method of claim 11, wherein the powder comprising a hydride of R.sup.3 is deposited on the magnet body surface in an area density of at least 60 g/mm.sup.2.

14. The method of claim 11, wherein in the hydride of R.sup.3, R.sup.3 contains at least 10 atom % of Dy and/or Tb.

15. The method of claim 14, wherein in the powder comprising the hydride of R.sup.3, R.sup.3 contain at least 10 atom % of Dy and/or Tb, and the total concentration of Nd and Pr in R.sup.3 is lower than the total concentration of Nd and Pr in R.sup.1.

16. The method of claim 11, after the heat treatment, further comprising aging treatment at a lower temperature than the heat treatment.

17. The method of claim 11, further comprising cleaning the sintered magnet body with at least one of an alkali, acid and organic solvent, prior to the immersion step.

18. The method of claim 11, further comprising shot blasting the sintered magnet body to remove a surface layer thereof, prior to the immersion step.

19. The method of claim 11, further comprising final treatment after the heat treatment, said final treatment being cleaning with at least one selected from the group consisting of an alkali, acid and organic solvent, grinding, plating and coating.

Description

BRIEF DESCRIPTION OF DRAWINGS

(1) FIG. 1 The only FIGURE, FIG. 1 schematically illustrates how particles are deposited during the electrodeposition step in the method of the invention.

DESCRIPTION OF EMBODIMENTS

(2) Briefly stated, the method for preparing a rare earth permanent magnet according to the invention involves feeding a particulate oxyfluoride and/or hydride of rare earth elements R.sup.2 and R.sup.3 onto the surface of a sintered magnet body having a R.sup.1FeB base composition and heat treating the particle-coated magnet body.

(3) The R.sup.1FeB base sintered magnet body may be obtained from a mother alloy by a standard procedure including coarse pulverization, fine pulverization, compacting, and sintering.

(4) As used herein, R and R.sup.1 each are selected from among rare earth elements inclusive of yttrium (Y) and scandium (Sc). R is mainly used for the magnet obtained while R.sup.1 is mainly used for the starting material.

(5) The mother alloy contains R.sup.1, iron (Fe), and boron (B). R.sup.1 represents one or more elements selected from among rare earth elements inclusive of Y and Sc, examples of which include Y, Sc, La, Ce, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Yb, and Lu. Preferably R.sup.1 is mainly composed of Nd, Pr, and Dy. The rare earth elements inclusive of Y and Sc should preferably account for 10 to 15 atom %, especially 12 to 15 atom % of the entire alloy. More preferably, R.sup.1 should contain either one or both of Nd and Pr in an amount of at least 10 atom %, especially at least 50 atom %. Boron (B) should preferably account for 3 to 15 atom %, especially 4 to 8 atom % of the entire alloy. The alloy may further contain 0 to 11 atom %, especially 0.1 to 5 atom % of one or more elements selected from among Al, Cu, Zn, In, Si, P, S, Ti, V, Cr, Mn, Ni, Ga, Ge, Zr, Nb, Mo, Pd, Ag, Cd, Sn, Sb, Hf, Ta, and W. The balance consists of Fe and incidental impurities such as C, N and O. Iron (Fe) should preferably account for at least 50 atom %, especially at least 65 atom % of the entire alloy. It is acceptable that Co substitutes for part of Fe, for example, 0 to 40 atom %, especially 0 to 15 atom % of Fe.

(6) The mother alloy is obtained by melting the starting metals or alloys in vacuum or in an inert gas, preferably Ar atmosphere, and then pouring in a flat mold or book mold, or casting as by strip casting. An alternative method, called two-alloy method, is also applicable wherein an alloy whose composition is approximate to the R.sub.2Fe.sub.14B compound, the primary phase of the present alloy and an R-rich alloy serving as a liquid phase aid at the sintering temperature are separately prepared, crushed, weighed and admixed together. It is noted that since the alloy whose composition is approximate to the primary phase composition is likely to leave Fe phase depending on the cooling rate during the casting or the alloy composition, it is subjected to homogenizing treatment, if desired for the purpose of increasing the amount of R.sub.2Fe.sub.14B compound phase. The homogenization is achievable by heat treatment in vacuum or in an Ar atmosphere at 700 to 1,200 C. for at least 1 hour.

(7) The alloy approximate to the primary phase composition may be prepared by strip casting. For the R-rich alloy serving as a liquid phase aid, not only the casting technique described above, but also the so-called melt quenching and strip casting techniques are applicable.

(8) Furthermore, in the pulverizing step to be described below, at least one compound selected from a carbide, nitride, oxide and hydroxide of R.sup.1 or a mixture or composite thereof can be admixed with the alloy powder in an amount of 0.005 to 5% by weight.

(9) The alloy is generally coarsely pulverized to a size of 0.05 to 3 mm, especially 0.05 to 1.5 mm. For the coarse pulverizing step, a Brown mill or hydrogen decrepitation (HD) is used, with the HD being preferred for the alloy as strip cast. The coarse powder is then finely pulverized to a size of 0.2 to 30 m, especially 0.5 to 20 m, for example, on a jet mill using high pressure nitrogen. The fine powder is compacted in a magnetic field by a compression molding machine and introduced into a sintering furnace. The sintering is carried out in vacuum or an inert gas atmosphere, typically at 900 to 1,250 C., especially 1,000 to 1,100 C.

(10) The sintered magnet thus obtained contains 60 to 99% by volume, preferably 80 to 98% by volume of the tetragonal R.sub.2Fe.sub.14B compound as the primary phase, with the balance being 0.5 to 20% by volume of an R-rich phase, 0 to 10% by volume of a B-rich phase, and at least one of carbides, nitrides, oxides and hydroxides resulting from incidental impurities or additives or a mixture or composite thereof.

(11) The sintered block is then machined into a preselected shape. The dimensions of the shape are not particularly limited. In the invention, the amount of R.sup.2 or R.sup.3 absorbed into the magnet body from the R.sup.2 oxyfluoride and/or R.sup.3 hydride-containing powder deposited on the magnet body surface increases as the specific surface area of the magnet body is larger, i.e., the size thereof is smaller. For this reason, the shape includes a maximum side having a dimension of up to 100 mm, preferably up to 50 mm, and more preferably up to 20 mm, and has a dimension of up to 10 mm, preferably up to 5 mm, and more preferably up to 2 mm in the direction of magnetic anisotropy. Most preferably, the dimension in the magnetic anisotropy direction is up to 1 mm. It is noted that the invention allows for effective treatment to take place over a larger area and within a short time since the powder is deposited by the electrodeposition technique (to be described later). Effective treatment is possible even when the block is a large one shaped so as to include a maximum side with a dimension in excess of 100 mm and have a dimension in excess of 10 mm in the magnetic anisotropy direction. With respect to the dimension of the maximum side and the dimension in the magnetic anisotropy direction, no particular lower limit is imposed. Preferably, the dimension of the maximum side is at least 0.1 mm and the dimension in the magnetic anisotropy direction is at least 0.05 mm.

(12) On the surface of a sintered magnet body as machined, a powder containing an oxyfluoride of R.sup.2 and/or hydride of R.sup.3 is attached by the electrodeposition technique. As defined above, each of R.sup.2 and R.sup.3 is at least one element selected from rare earth elements inclusive of Y and Sc, and should preferably contain at least 10 atom %, more preferably at least 20 atom %, and even more preferably at least 40 atom % of Dy and/or Tb. In a preferred embodiment, R.sup.2 and R.sup.3 each contain at least 10 atom % of Dy and/or Tb, and the total concentration of Nd and Pr in R.sup.2 and/or R.sup.3 is lower than the total concentration of Nd and Pr in R.sup.1.

(13) For the reason that a more amount of R.sup.2 or R.sup.3 is absorbed as the coating weight of the powder on the magnet surface is greater, the coating weight should preferably fall in a sufficient range to achieve the benefits of the invention. The coating weight is represented by an area density which is preferably at least 10 g/mm.sup.2, more preferably at least 60 g/mm.sup.2.

(14) The particle size of the powder affects the reactivity when the R.sup.2 or R.sup.3 in the powder is absorbed in the magnet body. Smaller particles offer a larger contact area available for the reaction. In order for the invention to attain its effects, the powder disposed on the magnet should desirably have an average particle size equal to or less than 100 m. No particular lower limit is imposed on the particle size although a particle size of at least 1 nm is preferred. It is noted that the average particle size is determined as a weight average diameter D.sub.50 (particle diameter at 50% by weight cumulative, or median diameter) using, for example, a particle size distribution measuring instrument relying on laser diffractometry or the like.

(15) The oxyfluoride of R.sup.2 and hydride of R.sup.3 used herein are preferably R.sup.2OF and R.sup.3H.sub.3, respectively, although they generally refer to oxyfluorides containing R.sup.2, oxygen and fluorine, and hydrides containing R.sup.3 and hydrogen, for example, R.sup.2O.sub.mF.sub.n and R.sup.3H.sub.n wherein m and n are arbitrary positive numbers, and modified forms in which part of R.sup.2 or R.sup.3 is substituted or stabilized with another metal element as long as they can achieve the benefits of the invention.

(16) The powder disposed on the magnet body surface contains the oxyfluoride of R.sup.2, hydride of R.sup.3, or a mixture thereof, and may additionally contain at least one compound selected from among oxides, fluorides, carbides, nitrides, and hydroxides of R.sup.4, or a mixture or composite thereof wherein R.sup.4 is at least one element selected from rare earth elements inclusive of Y and Sc. Further, the powder may contain fines of boron, boron nitride, silicon, carbon or the like, or an organic compound such as stearic acid in order to promote the dispersion or chemical/physical adsorption of particles. In order for the invention to attain its effect efficiently, the powder should preferably contain at least 10% by weight, more preferably at least 20% by weight (based on the entire powder) of the oxyfluoride of R.sup.2, hydride of R.sup.3, or a mixture thereof. In particular, it is recommended that the powder contain at least 50% by weight, more preferably at least 70% by weight, and even more preferably at least 90% by weight of the oxyfluoride of R.sup.2, hydride of R.sup.3, or a mixture thereof.

(17) The invention is characterized in that the means for disposing the powder on the magnet body surface is an electrodeposition technique involving immersing the sintered magnet body in an electrodepositing bath of the powder dispersed in a solvent, and effecting electrodeposition (or electrolytic deposition) for letting the powder (or particles) deposit on the magnet body surface. The solvent in which the powder is dispersed may be either water or an organic solvent. Although the organic solvent is not particularly limited, ethanol is most preferred.

(18) The concentration of the powder in the electrodepositing bath is not particularly limited. A slurry containing the powder in a weight fraction of at least 1%, more preferably at least 10%, and even more preferably at least 20% is preferred for effective deposition. Since too high a concentration is inconvenient in that the resultant dispersion is no longer uniform, the slurry should preferably contain the powder in a weight fraction of up to 70%, more preferably up to 60%, and even more preferably up to 50%.

(19) The step of depositing the powder on the magnet body surface via electrodeposition may be performed by the standard technique. For example, as shown in FIG. 1, a tank is filled with an electrodepositing bath 1 having the powder dispersed therein. A sintered magnet body 2 is immersed in the bath 1, and one or more counter electrodes 3 are placed in the tank. A power source is connected to the magnet body 2 and the counter electrodes 3 to construct a DC electric circuit, with the magnet body 2 made a cathode or anode and the counter electrodes 3 made an anode or cathode. With this setup, electrodeposition takes place when a predetermined DC voltage is applied. In FIG. 1, the magnet body 2 is made a cathode and the counter electrode 3 made an anode. Since the polarity of electrodepositing particles changes with a particular surfactant, the polarity of the magnet body 2 and the counter electrode 3 may be accordingly set.

(20) The material of which the counter electrode is made may be selected from well-known materials. Typically a stainless steel plate is used. Also electric conduction conditions may be determined as appropriate. Typically, a voltage of 1 to 300 volts, especially 5 to 50 volts is applied between the magnet body 2 and the counter electrode 3 for 1 to 300 seconds, especially 5 to 60 seconds. Also the temperature of the electrodepositing bath is not particularly limited. Typically the bath is set at 10 to 40 C.

(21) After the powder comprising the oxyfluoride of R.sup.2, hydride of R.sup.3, or a mixture thereof is disposed on the magnet body surface via electrodeposition as described above, the magnet body and the powder are heat treated in vacuum or in an atmosphere of an inert gas such as argon (Ar) or helium (He). This heat treatment is referred to as absorption treatment. The absorption treatment temperature is equal to or below the sintering temperature of the sintered magnet body.

(22) If heat treatment is effected above the sintering temperature (designated Ts in C.), there arise problems that (1) the structure of the sintered magnet can be altered to degrade magnetic properties, (2) the machined dimensions cannot be maintained due to thermal deformation, and (3) R can diffuse not only at grain boundaries, but also into the interior of the magnet body, detracting from remanence. For this reason, the temperature of heat treatment is equal to or below the sintering temperature of the sintered magnet body, and preferably equal to or below (Ts10) C. The lower limit of temperature may be selected as appropriate though it is typically at least 350 C. The time of absorption treatment is typically from 1 minute to 100 hours. Within less than 1 minute, the absorption treatment may not be complete. If the time exceeds 100 hours, the structure of the sintered magnet can be altered and oxidation or evaporation of components inevitably occurs to degrade magnetic properties. The preferred time of absorption treatment is from 5 minutes to 8 hours, and more preferably from 10 minutes to 6 hours.

(23) Through the absorption treatment, R.sup.2 and/or R.sup.3 contained in the powder deposited on the magnet surface is concentrated in the rare earth-rich grain boundary component within the magnet so that R.sup.2 or R.sup.3 is incorporated in a substituted manner near a surface layer of R.sub.2Fe.sub.14B primary phase grains. Where the powder contains the oxyfluoride of R.sup.2, part of the fluorine in the powder is absorbed in the magnet along with R.sup.2 to promote a supply of R.sup.2 from the powder and the diffusion thereof along grain boundaries in the magnet.

(24) The rare earth element contained in the oxyfluoride of R.sup.2 or hydride of R.sup.3 is one or more elements selected from rare earth elements inclusive of Y and Sc. Since the elements which are particularly effective for enhancing magnetocrystalline anisotropy when concentrated in a surface layer are Dy and Tb, it is preferred that a total of Dy and Tb account for at least 10 atom % and more preferably at least 20 atom % of the rare earth elements in the powder. Also preferably, the total concentration of Nd and Pr in R.sup.2 and R.sup.3 is lower than the total concentration of Nd and Pr in R.sup.1.

(25) The absorption treatment effectively increases the coercive force of the RFeB sintered magnet without substantial sacrifice of remanence.

(26) According to the invention, the absorption treatment may be carried out by effecting electrodeposition on the sintered magnet body in a slurry of R.sup.2 oxyfluoride-containing and/or R.sup.3 hydride-containing powder, for letting the powder deposit on the magnet body surface, and heat treating the magnet body having the powder deposited on its surface. Since a plurality of magnet bodies each covered with the powder are spaced apart from each other during the absorption treatment, it is avoided that the magnet bodies are fused together after the absorption treatment which is a heat treatment at a high temperature. In addition, the powder is not fused to the magnet bodies after the absorption treatment. It is then possible to place a multiplicity of magnet bodies in a heat treating container where they are treated simultaneously. The preparing method of the invention is highly productive.

(27) Since the powder is deposited on the magnet body surface via electrodeposition according to the invention, the coating weight of the powder on the surface can be readily controlled by adjusting the applied voltage and time. This ensures that a necessary amount of the powder is fed to the magnet body surface without waste. It is also ensured that a coating of the powder having minimal variation of thickness, an increased density, and mitigated deposition unevenness forms on the magnet body surface. Thus absorption treatment can be carried out with a minimum necessary amount of the powder until the increase of coercive force reaches saturation. In addition to the advantages of efficiency and economy, the electrodeposition step is successful in forming a coating of the powder on the magnet body, even having a large area, in a short time. Further, the coating of powder formed by electrodeposition is more tightly bonded to the magnet body than those coatings of powder formed by immersion and spray coating, ensuring to carry out ensuing absorption treatment in an effective manner. The overall process is thus highly efficient.

(28) The absorption treatment is preferably followed by aging treatment although the aging treatment is not essential. The aging treatment is desirably at a temperature which is below the absorption treatment temperature, preferably from 200 C. to a temperature lower than the absorption treatment temperature by 10 C., more preferably from 350 C. to a temperature lower than the absorption treatment temperature by 10 C. The atmosphere is preferably vacuum or an inert gas such as Ar or He. The time of aging treatment is preferably from 1 minute to 10 hours, more preferably from 10 minutes to 5 hours, and even more preferably from 30 minutes to 2 hours.

(29) Notably, when a sintered magnet block is machined prior to the coverage thereof with the powder by electrodeposition, the machining tool may use an aqueous cooling fluid or the machined surface may be exposed to a high temperature. If so, there is a likelihood that the machined surface (or a surface layer of the sintered magnet body) is oxidized to form an oxide layer thereon. This oxide layer sometimes inhibits the absorption reaction of R.sup.2 or R.sup.3 from the powder into the magnet body. In such a case, the magnet body as machined is cleaned with at least one agent selected from alkalis, acids and organic solvents or shot blasted for removing the oxide layer. Then the magnet body is ready for absorption treatment.

(30) Suitable alkalis which can be used herein include potassium pyrophosphate, sodium pyrophosphate, potassium citrate, sodium citrate, potassium acetate, sodium acetate, potassium oxalate, sodium oxalate, etc. Suitable acids include hydrochloric acid, nitric acid, sulfuric acid, acetic acid, citric acid, tartaric acid, etc. Suitable organic solvents include acetone, methanol, ethanol, isopropyl alcohol, etc. In the cleaning step, the alkali or acid may be used as an aqueous solution with a suitable concentration not attacking the magnet body. Alternatively, the oxide surface layer may be removed from the sintered magnet body by shot blasting before the powder is deposited thereon.

(31) Also, after the absorption treatment or after the subsequent aging treatment, the magnet body may be cleaned with at least one agent selected from alkalis, acids and organic solvents, or machined again into a practical shape. Alternatively, plating or paint coating may be carried out after the absorption treatment, after the aging treatment, after the cleaning step, or after the last machining step.

EXAMPLES

(32) Examples are given below for further illustrating the invention although the invention is not limited thereto. In Examples, the area density of terbium oxyfluoride or terbium hydride deposited on the magnet body surface is computed from a weight gain of the magnet body after powder deposition and the surface area.

Example 1

(33) An alloy in thin plate form was prepared by a strip casting technique, specifically by weighing Nd, Al, Fe and Cu metals having a purity of at least 99% by weight, Si having a purity of 99.99% by weight, and ferroboron, high-frequency heating in an argon atmosphere for melting, and casting the alloy melt on a copper single roll. The alloy consisted of 14.5 atom % of Nd, 0.2 atom % of Cu, 6.2 atom % of B, 1.0 atom % of Al, 1.0 atom % of Si, and the balance of Fe. Hydrogen decrepitation was carried out by exposing the alloy to 0.11 MPa of hydrogen at room temperature to occlude hydrogen and then heating at 500 C. for partial dehydriding while evacuating to vacuum. The decrepitated alloy was cooled and sieved, yielding a coarse powder under 50 mesh.

(34) Subsequently, the coarse powder was finely pulverized on a jet mill using high-pressure nitrogen gas into a fine powder having a mass median particle diameter of 5 m. The fine powder was compacted in a nitrogen atmosphere under a pressure of about 1 ton/cm.sup.2 while being oriented in a magnetic field of 15 kOe. The green compact was then placed in a sintering furnace with an argon atmosphere where it was sintered at 1,060 C. for 2 hours, obtaining a sintered magnet block. Using a diamond cutter, the magnet block was machined on all the surfaces into a magnet body having dimensions of 17 mm17 mm2 mm (magnetic anisotropy direction). It was cleaned in sequence with alkaline solution, deionized water, nitric acid and deionized water, and dried.

(35) Subsequently, terbium oxyfluoride (TbOF) having an average particle size of 0.2 m was thoroughly mixed with water at a weight fraction of 40% to form a slurry having terbium oxyfluoride particles dispersed therein. The slurry served as an electrodepositing bath.

(36) With the setup shown in FIG. 1, the magnet body 2 was immersed in the slurry 1. A pair of stainless steel plates (SUS304) were immersed as counter electrodes 3 while they were spaced 20 mm apart from the magnet body 2. A power supply was connected to construct an electric circuit, with the magnet body 2 made a cathode and the counter electrodes 3 made anodes. A DC voltage of 10 volts was applied for 10 seconds to effect electrodeposition. The magnet body was pulled out of the slurry and immediately dried in hot air. It was found that a thin coating of terbium oxyfluoride had deposited on the magnet body surface. The area density of terbium oxyfluoride deposited was 100 g/mm.sup.2 on the magnet body surface.

(37) The magnet body having a thin coating of terbium oxyfluoride particles tightly deposited thereon was subjected to absorption treatment in an argon atmosphere at 900 C. for 5 hours. It was then subjected to aging treatment at 500 C. for one hour, and quenched, obtaining a magnet body. The absorption treatment increased the coercive force by 720 kA/m.

Example 2

(38) As in Example 1, a magnet body having dimensions of 17 mm17 mm2 mm (magnetic anisotropy direction) was prepared. Also, terbium oxyfluoride (TbOF) having an average particle size of 0.2 m was thoroughly mixed with ethanol at a weight fraction of 40% to form a slurry having terbium oxyfluoride particles dispersed therein. The slurry served as an electrodepositing bath.

(39) As in Example 1, the magnet body and the counter electrodes were immersed in the slurry. A power supply was connected to construct an electric circuit, with the magnet body made a cathode and the counter electrodes made anodes. A DC voltage of 10 volts was applied for 10 seconds to effect electrodeposition. The magnet body was pulled out of the slurry and immediately dried in hot air. It was found that a thin coating of terbium oxyfluoride had deposited on the magnet body surface. The area density of terbium oxyfluoride deposited was 100 g/mm.sup.2 on the magnet body surface.

(40) The magnet body having a thin coating of terbium oxyfluoride particles deposited thereon was subjected to absorption treatment in an argon atmosphere at 900 C. for 5 hours. It was then subjected to aging treatment at 500 C. for one hour, and quenched, obtaining a magnet body. The absorption treatment increased the coercive force by 720 kA/m.

Example 3

(41) As in Example 1, a magnet body having dimensions of 17 mm17 mm2 mm (magnetic anisotropy direction) was prepared. Also, terbium hydride (TbH.sub.2) having an average particle size of 0.2 m was mixed with water at a weight fraction of 40% to form a slurry having terbium hydride particles dispersed therein. The slurry served as an electrodepositing bath.

(42) As in Example 1, the magnet body and the counter electrodes were immersed in the slurry. A power supply was connected to construct an electric circuit, with the magnet body made a cathode and the counter electrodes made anodes. A DC voltage of 10 volts was applied for 10 seconds to effect electrodeposition. The magnet body was pulled out of the slurry and immediately dried in hot air. It was found that a thin coating of terbium hydride had deposited on the magnet body surface. The area density of terbium hydride deposited was 100 g/mm.sup.2 on the magnet body surface.

(43) The magnet body having a thin coating of terbium hydride particles deposited thereon was subjected to absorption treatment in an argon atmosphere at 900 C. for 5 hours. It was then subjected to aging treatment at 500 C. for one hour, and quenched, obtaining a magnet body. The absorption treatment increased the coercive force by 720 kA/m.

Example 4

(44) As in Example 1, a magnet body having dimensions of 17 mm17 mm2 mm (magnetic anisotropy direction) was prepared. Also, terbium hydride (TbH.sub.2) having an average particle size of 0.2 m was thoroughly mixed with ethanol at a weight fraction of 40%, forming a slurry having terbium hydride particles dispersed therein. The slurry served as an electrodepositing bath.

(45) As in Example 1, the magnet body and the counter electrodes were immersed in the slurry. A power supply was connected to construct an electric circuit, with the magnet body made a cathode and the counter electrodes made anodes. A DC voltage of 10 volts was applied for 10 seconds to effect electrodeposition. The magnet body was pulled out of the slurry and immediately dried in hot air. It was found that a thin coating of terbium hydride had deposited on the magnet body surface. The area density of terbium hydride deposited was 100 g/mm.sup.2 on the magnet body surface.

(46) The magnet body having a thin coating of terbium hydride particles deposited thereon was subjected to absorption treatment in an argon atmosphere at 900 C. for 5 hours. It was then subjected to aging treatment at 500 C. for one hour, and quenched, obtaining a magnet body. The absorption treatment increased the coercive force by 720 kA/m.

Comparative Example 1

(47) As in Example 1, a magnet body having dimensions of 17 mm17 mm2 mm (magnetic anisotropy direction) was prepared. Also, terbium oxyfluoride (TbOF) having an average particle size of 0.2 m was thoroughly mixed with water at a weight fraction of 40%, forming a slurry having terbium oxyfluoride particles dispersed therein.

(48) The magnet body was immersed in the slurry for 7 seconds, pulled out of the slurry and immediately dried in hot air. It was found that a thin coating of terbium oxyfluoride had deposited on the magnet body surface. The area density of terbium oxyfluoride deposited was 20 g/mm.sup.2 on the magnet body surface.

(49) The magnet body having a thin coating of terbium oxyfluoride particles disposed thereon was subjected to absorption treatment in an argon atmosphere at 900 C. for 5 hours. It was then subjected to aging treatment at 500 C. for one hour, and quenched, obtaining a magnet body. The absorption treatment increased the coercive force by 360 kA/m.

Comparative Example 2

(50) As in Example 1, a magnet body having dimensions of 17 mm17 mm2 mm (magnetic anisotropy direction) was prepared. Also, terbium hydride (TbH.sub.3) having an average particle size of 0.2 m was thoroughly mixed with ethanol at a weight fraction of 40%, forming a slurry having terbium hydride particles dispersed therein.

(51) The magnet body was immersed in the slurry for 7 seconds, pulled out of the slurry and immediately dried in hot air. It was found that a thin coating of terbium hydride had deposited on the magnet body surface. The area density of terbium hydride deposited was 20 g/mm.sup.2 on the magnet body surface.

(52) The magnet body having a thin coating of terbium hydride particles disposed thereon was subjected to absorption treatment in an argon atmosphere at 900 C. for 5 hours. It was then subjected to aging treatment at 500 C. for one hour, and quenched, obtaining a magnet body. The absorption treatment increased the coercive force by 360 kA/m.

(53) As is evident from Examples 1 to 4 and Comparative Examples 1 and 2, the electrodeposition technique achieves a greater coercive force increase than the conventional immersion technique, both conducted once.