Rare earth iron-based permanent magnet

Abstract: A rare earth-based permanent magnet is formed of a ternary compound composed of 12-30% by weight of a rare earth element, 1-10% by weight of titanium and the balance of iron or a quater­ nary compound composed of 12-30% by weight of a rare earth ele­ ment, 1-10% by weight of titanium, up to 34% by weight of cobalt and the balance of iron. Specifically, the ternary and quaternary compounds have a crystalline structure belonging to the body-­ centered tetragonal system of the ThMn₁₂ type. Different from con­ ventional neodymium-boron-iron magnets, the inventive magnets are corrosion resistant and free from rusting and have a greatly improved Curie point as compared with rare earth-iron binary magnets. The incorporation of cobalt to the ternary compound has an effect of further increasing the Curie point.

Description: BACKGROUND OF THE INVENTION The present invention relates to a rare earth-based permanent magnet alloy or, more particularly, to a rare earth-based alloy for permanent magnet having excellent magnetic properties and useful as a component of various kinds of electric and electronic instruments. Various types of rare earth-based alloys are known and now under industrial production as a material of permanent magnets, of which samarium-cobalt alloys give high-performance permanent magnets useful in speakers, electric motors, measuring instruments and the like. One of the problems in these samarium-cobalt alloys is the economical disadvantage thereof as a consequence of the expensiveness of samarium and cobalt as the constituent metals. Accordingly, it is an important technical problem in the magnet industry to develop a permanent magnet having performance as high as that of the samarium-cobalt magnets by replacing a substantial portion of the expensive cobalt with less expensive iron. Despite the efforts for increasing the proportion of iron substitution for cobalt as far as possible, practically useful rare earth-based permanent magnets can hardly be obtained by replacing about 20% by moles or more of cobalt with iron in Sm₂Co₁₇-type magnets currently under mass production in the magnet industry. Ternary alloys of neodymium, boron and iron have been proposed in recent years as a material of permanent magnets having magnetic properties even higher than those of smarium-cobalt magnets (see, for example, Japanese Patent Kokai 59-46008) and greatly highlighted in respect of the abundance of the naturally occurring resources of neodymium and iron as compared with samarium and cobalt. Unfortunately, this ternary magnet alloy has a serious defect of high susceptibility to rusting so that no practically usable permanent magnet can be prepared from the alloy unless the permanent magnet is provided with a protective coating layer against rusting. No industrially applicable coating method for protective coating, however, has yet been developed and this problem is a bottleneck which prevents the permanent magnets of this type from prevalence. In addition, permanent magnets of the ternary alloy of neodymium, iron and boron have a relatively low Curie point T c of 310 °C and the residual magnetization thereof has a large temperature dependency of -0.12%/°C so that they can hardly be used in the field of applications in which stability of the magnetic properties is essential against variation of the temperature as in electric motors and measuring instruments. Extensive investigations are of course now under way to develop rare earth-based permanent magnet alloys other than the above mentioned one such as an alloy composed of a rare earth element R and a transition metal M of which the ratio of R:M is 10 or larger and ternary alloys other than R₂Fe₁₄B but no promising magnet alloys have yet been discovered. On the other hand, binary intermetallic compounds composed of a rare earth element R and iron are well known as a magnetic material including RFe₂, RFe₃ and R₂Fe₁₇. Tey are, however, not satisfactory as a material of permanent magnets because each of them has a disadvantageously low value of either one of the important magnetic properties such as the Curie point T c , saturation magnetization M S and magnetic anisotropy coefficient K u . Despite these problems, Croat et al. have reported in Appl. Phys. Lett., volume 37, page 1096 (1981) that a permanent magnet of a rare earth-iron binary system can be obtained by undertaking the method of thin-film quenching method in which a metastable phase is quenched and immobilized. Besides, Cadieu et al. have reported in J. Appl. Phys., volume 55, page 811 (1984) that permanent magnets in the form of a thin film can be prepared from SmFe₅ and (SmTi)Fe₅ by the method of sputtering. The above described binary intermetallic compounds are each in a metastable phase produced by the method of sputtering in the form of a thin film of which the crystalline structure is hexagonal according to the report of the authors. It is generally understood that these binary intermetallic compounds cannot provide a permanent magnet in a bulky form. Such a magnet is magnetically isotropic with consequently low magnetic properties and the stability thereof is also questionable as an attribute of the metastable phase forming the basic structure of the magnet. Accordingly, it is eagerly desired to develop a rare earth-based alloy for permanent magnets having high magnetic properties with stability and rustproofness from inexpensive materials. In J. Appl.Phys. Vol. 64, No. 10, 15/11/88, pages 5714-5716, New York, US, "Magnetic properties of Fe-rich rare-earth intermetallic compounds with a ThMn₁₂ structure" there is described a permanent magnet consisting of a ternary alloy of RM x Fe 12-x whereby R represents a rare earth element and M is Ti, Si, V, Cr and Mo. A typical example of such a ternary alloy is SmTiFe₁₁. These alloys crystallize in a tetragonal ThMn₁₂ structure. Also EP-A 0 253 428 teaches a magnetic material having a tetragonal crystal structure of the ThMn₁₂-type whereby said magnetic material has the formula Re(Me I 1-x Me II x )₁₂ wherein Re is one or more rare earth metals, Me I is Fe, Co or a mixture of Fe and Co, Me II is Ti, V, Cr, Si, W or Mo, and x represents 0,1 to 0,35. Furthermore, EP-A 0 106 948 teaches magnetic materials comprising Fe, B, R (rare earth elements) and Co having a major phase of Fe-Co-B-R intermetallic compound(s) of tetragonal system. There it is also taught that the substitution of Fe with Co generally causes complicated results which are almost unexpectable. As is mentioned above, no R-Fe binary intermetallic compounds were hitherto known which provide a permanent magnet in a bulky form having magnetic properties such as the Curie point sufficiently high as a practically useful permanent magnet. In view of the magnetic properties of the bulky permanent magnets based on the neodymium-iron-boron ternary alloy developed by Sagawa et al., the inventors have continued extensive investigations to develop a rare earth-based magnetic alloy composed of a rare earth element, iron and one or two elements other than boron and capable of giving a permanent magnet free from the problems in the neodymium-iron-boron ternary magnets. It is therefore an object of the present invention to provide a novel high-performance permanent magnet capable of exhibiting magnetic properties equivalent to or even higher than those of the samarium-cobalt permanent magnets and freed from the defects in the neodymium-iron-boron ternary magnets. Said object is solved by a permanent magnet on the basis of an alloy consisting essentially of (a) from 12 to 30 % by weight of at least one rare earth element selected from the group consisting of yttrium and the elements having an atomic number of 57 to 71, (b) from 1 to 10 % by weight of titanium (c) up to 34 % by weight of cobalt, excluding 0%, and (d) the balance of iron, the principal phase of the alloy having a crystalline structure belonging to the body-centered tetragonal system of the ThMn₁₂-type and the volume fraction of said phase being at least 50 %. The rare earth element denoted by R in the above and useful as an ingredient of the inventive magnet include yttrium and the elements having an atomic number of 57 to 71, i.e. lanthanum, cerium, praseodymium, neodymium, samarium, europium, gadolinium, terbium, dysprosium, holmium, erbium, thulium, ytterbium and lutetium. Preferably, the rare earth element is yttrium or a so-called light rare earth element having an atomic number of, for example, 57 to 64. Heavy rare earth elements are less preferable because of the possible decrease in the saturation magnetization M s of the magnet prepared therefrom. It is of course optional that two kinds or more of the rare earth elements are used in combination, if so desired. The balance of the rare earth element and titanium is iron and an unavoidable amount of impurities including carbon, oxygen and the like. The permanent magnet of the invention can be prepared by the well known powder metallurgical method. Namely, the rare earth element, titanium and iron each in the metallic from are melted together and cast in a mold and the powder of the compound obtained by pulverizing the ingot is molded in a magnetic field into a powder compact or green body which is sintered and aged according to a schedule of the heat treatment in such a manner that the crystalline grains in the resultant magnet have a particle diameter not ex-ceeding 25 µm or, preferably, in the range from 5 to 15 µm. It is a matter of possibility that the magnetic compound of the invention is further admixed with a transition element and a light element such as aluminum and silicon with an object to further improve the magnetic properties or, in particular, coercive force i H c although, in most cases, the saturation magnetization M s is more or less decreased thereby so that it is important when addition of such elements is intended to consider the balance of the coercive force and the saturation magnetization in the selection of the kind and amount of the additive elements. As is mentioned before, the most disadvantageous defect in the neodymium-iron-boron magnets is the high susceptibility of the ternary compound to oxidation and a great decrease is caused in the magnetic properties of the magnets prepared by the powder metallurgical method due to the rapid oxidation of the surface of the fine particles thereof in the course of the magnet preparation. A magnet prepared thereby is also susceptible to rusting and cannot be used in a practical application unless a protective surface coating is provided thereon. In contrast thereto, the rare earth-based permanent magnet of the invention is highly corrosion-resistant despite the high content of iron and can be used as such without a protective surface coating although the corrosion resistance can be further enhanced when the magnet is provided with a surface coating by spraying or electrodeposition of a resinous coating composition or by vapor-phase deposition, sputtering or ion plating of a highly corrosion-resistant metallic material. The quenching thin-film method is also applicable to the inventive permanent magnet to give a thin-film magnet having a high coercive force which is pulverized and processed into an isotropic plastic magnet according to a known procedure. It is of course possible that an anisotropically sintered magnet is pulverized and the powder is processed into a magnetically anisotropic plastic magnet. A ternary magnetic compound of a rare earth element, titanium and iron of the formula RTiFe₁₁ has an outstandingly high Curie point as compared with the R₂Fe₁₇-type binary magnetic compounds. For example, a magnet of SmTiFe₁₁ has a Curie point of about 310 °C. However, it is sometimes desired to obtain a permanent magnet having a still higher Curie point. In addition, the improvement obtained by the above described ternary magnetic compound of a rare earth element, titanium and iron is still insufficient in respect of the relatively large temperature dependency of the magnetic properties as in the neodymium-iron-boron magnets. In view of this problem, the inventors have performed investigations and arrived at a discovery that this problem can be solved when a part of the iron constituent is replaced with cobalt. Namely, the magnetic compound or alloy, respectively, of the inventive permanent magnet consists essentially of (a) from 12% to 30% by weight of a rare earth element or a combination of two kinds or more of rare earth elements, (b) from 1% to 10% by weight of titanium, (c) up to 34% by weight or, preferably, up to 27% by weight of cobalt and (d) the balance of iron, the principal crystalline phase of the compound belonging to the body-centered tetragonal system of the ThMn₁₂ type. Although no lower limit is given of the content of cobalt to be contained in the magnetic compound, the amount of cobalt should appropriately be selected depending on the desired degree of improvement in the Curie point. It should be noted, however, increase in the content of cobalt over the above mentioned upper limit has no particularly advantageous effect in further increasing the Curie point rather with disadvantageous influences on other magnetic properties if not to mention the increased cost due to the use of a large amount of the expensive cobalt metal. Generally speaking, a cobalt content of 10% by weight has an effect of increasing the Curie point of the magnet by about 90 °C or more along with a remarkable effect in decreasing the temperature dependency of the magnetic properties. In the following, examples are given to illustrate the present invention in more detail. Example 1. Four permanent magnets, referred to as No. 1, No. 2, No. 3 and No. 4 hereinbelow, were prepared each by using metals of neodymium, titanium, iron and cobalt each having a purity of 99.9% in the formulation shown in Table 1 below which also shows the Curie point of the respective magnets. Said metals were taken by weighing in the proportions shown in Table 2 below and they were melted together in a high-frequency induction furnace. The melt was cast in a steel-made mold to prepare three ingots of different compositions indicated as No. 1, No. 2 and No. 3 in the table. Each of the ingots was crushed and pulverized in a jet mill using nitrogen gas as the jet gas into a fine powder having an average particle diameter of 2 to 10 µm. The powder in a mold was magnetically oriented in a magnetic field of 1,19 x 10⁶ A/m (15 kOe) and shaped by press-molding in a hydraulic press under a pressure of 1.5 tons/cm² into a powder compact which was sintered for 1 hour in an atmosphere of argon gas at a temperature of 1000 to 1200 °C and subjected to an aging treatment for 1 hour at 400 to 900 °C followed by quenching. Example 4. Four permanent magnets, referred to as No. 5, No. 6, No. 7 and No. 8 hereinbelow, were prepared each in substantially the same manner as in Example 1 by using metals of samarium, titanium, iron and cobalt each having a purity of 99.9% in the formulation shown in Table 2 below which also shows the coercive force and Curie point of the respective magnets. Example 3. Four permanent magnets, referred to as No. 9, No. 10, No. 11 and No. 12 hereinbelow, were prepared each in substantially the same manner as in Example 1 by using metals of neodymium Nd, samarium Sm, dysprosium Dy, yttrium Y, titanium Ti, iron Fe and cobalt Co each having a purity of 99.9% in the formulation shown in Table 3 below which also shows the coercive force i H c and temperature dependency of saturation magnetization ΔM/ΔT of the respective magnets.

Claims: A permanent magnet on the basis of an alloy consisting essentially of (a) from 12 to 30 % by weight of at least one rare earth element selected from the group consisting of yttrium and the elements having an atomic number of 57 to 71, (b) from 1 to 10 % by weight of titanium (c) up to 34 % by weight of cobalt, excluding 0%, and (d) the balance of iron, the principal phase of the alloy having a crystalline structure belonging to the body-centered tetragonal system of the ThMn₁₂-type and the volume fraction of said phase being at least 50 %. The permanent magnet as claimed in claim 1 wherein the rare earth element is selected from the group consisting of yttrium and the elements having an atomic number of 57 to 64. The permanent magnet as claimed in claim 1 wherein the amount of cobalt as the component (c) does not exceed 27 % by weight.

Inventors: OHASHI KEN, OSUGI RYO, TAWARA YOSHIO, YOKOYAMA TOSHIKAZU, OHASHI, KEN, OSUGI, RYO, TAWARA, YOSHIO, YOKOYAMA, TOSHIKAZU
Applicants: SHINETSU CHEMICAL CO, SHIN-ETSU CHEMICAL CO., LTD.
Application year: 1989
Main classification: H01F1
IPC codes: H01F1