IE901359A1 - Rare-earth based magnetic materials, production process and use - Google Patents

Abstract

A new magnetic material of the general formula: RxFeyX min aZb is derived from an intermetallic compound of rhomohedral or hexagonal crystal structure wherein R is one or more rare earth elements, X min is an element of groups IIIA, IIIB, IVA or IVB of the periodic table, Z is one or more elements of group VA of the periodic table, x is a value from 1 to 2, y is a value from 11 to 19, a is a value from 0 to 3, b is a value from 0.5 to 3 and Fe is unsubstituted or partly, substitued with the proviso that if the component Z is antimony or bismuth the element X min is not boron. These new materials exhibit increased Curie temperatures, magnetic strength and easy uniaxial anisotropy and are therefore suitable for fabricating into permanent magnets. Processes for preparing materials RxFeyX min aZb are also described.

Description

IMPROVED MAGNETIC MATERIALS The invention relates to new magnetic materials having improved magnetic properties, to processes for their production and to the use of the new materials to make permanent magnets.

Magnets have many applications in engineering and science as components of apparatus such as electric motors, electric generators, focussing elements, lifting mechanisms, locks, levitation devices, anti-friction mounts and so on. In order for a magnetic material to be useful for making a permanent magnet three intrinsic properties are of critical importance. These are the Curie temperature (Tc) i.e. the temperature at which a permanent magnet loses its magnetism, the spontaneous magnetic moment per unit volume (Ms) and the easy uniaxial anisotropy conventionally represented by an anisotropy field Ba. The Curie temperature is of particular significance because it dictates the temperature below which apparatus containing the magnet must be operated.

During this century much research has been directed to developing magnetic materials which combine high Curie temperatures and improved magnetic moments with strong uniaxial anisotropy. For many years magnetic materials of the AlNiCo type were used in permanent magnets for practical applications. In the late 1960’s it was discovered that alloys of the rare earth elements, particularly samarium when alloyed with cobalt, had magnetic properties which made them superior as permanent magnets to the AlNiCo type. Compounds of samarium and cobalt provided magnets which were particularly successful in many demanding practical applications requiring a magnet with a high energy product. However the high cost of cobalt as a raw material led investigators in the early 1980’s to consider the possibility of combining the cheaper and more abundant iron with the magnetically superior rare earth elements to produce 5 permanent magnets with improved magnetic properties.

A major breakthrough came in 1983 when the Sumitomo Special Metals Company and General Motors of America independently developed a magnetic material which combined a rare earth element and iron and incorporated a third element, boron, into the crystal lattice to give an intermetallic compound, Nd2Fei4B which can be used to produce magnets with an excellent energy product, but a lower Curie temperature than the Sm-Co materials. These Nd-Fe-B magnetic materials can have a Curie temperature of up to 320°C and are particularly described in three European applications, EP-A-0101552, EP-A-0106948 and EP-A-0108474. Derivatives of these boride materials represent the state of the art to date in magnet technology. However they are somewhat unstable in air and change chemically, gradually losing their magnetic properties so that despite Curie temperatures in excess of 300°C in practice they are not suitable for operating at temperatures greater than 150°C.

The fact that the incorporation of boron into the crystal lattice of intermetallic materials containing a rare earth element and iron serves to improve magnetic properties has encouraged investigators to search for new compounds of elements other than boron in combination with rare earth elements and iron.

In 1987 Higano et al (IEEE Transactions on Magnetics, vol, Mag-23. No. 5 Sept 1987) reported an attempt to carry out a nitriding reaction by exposure of powders of Sn^Fe^ alloy to gaseous nitrogen at temperatures of 500 and 1100°C. The experiment was intended to produce a compound of the formula Sm2Fej7~N which it was hoped would have improved magnetic properties. However Higano et al found no evidence that such a material was produced by this process but instead found that the nitriding process simply decomposed the rare-earth iron alloy starting material to produce iron and nitrides of the rare earth elements.

The present inventors have now produced a new magnetic material of improved properties which includes at least a rare earth element, iron and a group VA element with optionally one or more other elements. The successful production of these materials is unexpected having regard to the teaching of Higano et al.

In accordance with one aspect of the invention there is provided a magnetic material of the general formula: RxFeyX * aZj3 which is dervied from an intermetallic compound of rhombohedral, hexagonal or tetragonal crystal structure wherein R is one or more rare earth elements, X’ is an element of groups IIIA, IIIB, IVA or IVB of the periodic table, Z is one or more elements of group VA of the periodic table, x is a value from 0.5 to 2, y is a value from 9 to 19, a is a value from 0 to 3, b is a value from 0.3 to 3 and wherein when the magnetic material of said general formula is derived from an intermetallic compound of rhombohedral or hexagonal crystal structure Fe is unsubstituted or partially substituted by another element and when the magnetic material of said general formula is derived from an intermetallic compound of tetragonal crystal structure Fe is partially substituted by any element of group IIIA or IVA of the periodic table or a transition metal from another group with the further proviso that in the case of materials derived from said rhombohedral or hexagonal crystal structures the element X' is not boron when the component Z is antimony or bismuth.

It is to be understood that herein the term rare earth element includes the elements yttrium and thorium, and that the groups IIIA, IIIB, IVA, IVB and V of the periodic table are those defined by the CAS version of that table. By hexagonal, rhombohedral and tetragonal crystal structure is meant intermetallic compounds having a crystal structure analogous to ΤΙ^Νΐιγ, ΊΊ^Ζηιγ and ThMn^ respectively.

In the case where the material is derived from an intermetallic compound of hexagonal or rhombohedral crystal structure the element R may be samarium alone or a combination of samarium with one or more other rare earth elements selected from lanthanum, cerium, neodymium, praseodymium, erbium, thulium, yttrium, and mischmetal. R may also be yttrium, cerium, neodymium, gadolinium, terbium, dysprosium, holmium, erbium, thulium, ytterbium, lutetium, or a mixture of two or more thereof. In the case where the material is derived from an intermetallic compound of tetragonal crystal structure, R may be any rare earth element but preferred elements for R are cerium, praseodymium, neodymium, terbium, dysprosium, holmium or a mixture of two or more thereof. Particularly preferred are neodymium or praseodymium alone or in combination with other elements.

In the case of the hexagonal or rhombohedral materials as aforementioned the iron may be •E 901359 - 5 substituted by up to 50%, most preferably up to 33% with another element or elements. The element is preferably a magnetic transition metal, most preferably cobalt.

In the case of the tetragonal materials as aforementioned the iron is substituted with any element of group IIIA or IVA of the periodic table or with a transition metal not already included in those groups. Preferred substituents are silicon or aluminium or any of the transition metals titanium, vandium, molydenum or chromium.

Where an element X' is included in the materials it is preferably carbon, boron, silicon or zirconium and the value of a may be as low as 0.1 with a maximum of 3. Preferably the value of a+b is < 3.

The component Z may be nitrogen, phosphorus arsenic, antimony or bismuth or mixtures thereof and of these the particularly preferred element is nitrogen. For example, three materials in accordance with this aspect of the invention which demonstrate the requisite improved magnetic properties are Sm2Fej7N2.3, ^^2^^17^1.1^1.1 arid NdFenTiNo.s25 The magnetic materials of the invention display considerably improved magnetic properties over materials hitherto known. Firstly they have Curie temperatures in excess of 400°C. Secondly, they have improved easy uniaxial anisotropy as demonstrated by X-ray diffraction patterns of the material after a magnetic field has been applied. Thirdly, the magnetic moment is increased and finally the magnetic moment is subject to little variation with time or temperature around ambient temperature.

These increased intrinsic magnetic properties are all very favourable for permanent magnet applications. - 6 In accordance with a second aspect of the invention there is provided a process for modifying the magnetic properties of an intermetallic compound comprising at least one or more rare earth elements and the element iron in which the iron is optionally substituted with another element which process comprises heating said intermetallic compound with a gas containing at least one group VA element Z in the substantial absence of oxygen to incorporate the said at le^st one element Z interstltially into the crystal lattice of'the intermetallic compound by a gas-solid reaction. Where the intermetallic compound is of tetragonal crystal structure the iron may be substituted by an element of group IIIA or group IVA of thi periodic table or by a transition metal not already included in those groups.

: The sample material ie preferably placed in a sealed container from which the oxygen can be pumped and ttye reactive gas added and heated from the outside. Its temperature ie raised to a maximum not exceeding about 600°C. Optionally the intermetallic starting material is ground iron, for example, an ingot to a particle else of from 0.5 to 50 micions diameter before heating in a suitable gae. the preferred range is 0.5 to 20 microns.

Specific additives such as niobium or vanadium may be added to reduce or eliminate free .Iron preslent in the ingot facilitating the development of coeroivlty in .the resulting modified metallic material.

Alternatively, the starting material may be prepared into thin flakes or ribbons by melt spinning or into powder by mechanical alloying or epray casting. The heating] may proceed for a period not exceeding 8 hours, but the exact time will depend upon the gae and the I solid geometry of the starting material. The precise heating time for any starting material is therefore readily calculable.

Suitable gases to be used in the process include Ithose which produce radicals containing single atoms of a group VA element on contact with - 7 hot surfaces such as metal or quartz or by exposure to high frequency radiation, for example gaseous hydrides of the group VA elements.

The preferred magnetic materials of the 5 invention, in which the group VA component Z interstitially inserted into the crystal lattice is nitrogen, may be made from the appropriate intermetallic starting material using gaseous nitrogen, ammonia or hydrazine. When an intermetallic compound of the formula R2^e17' or RFe^xTi for example, is heated with nitrogen and the gas pressure monitored, a decrease in pressure occurs which begins at about 350°C and continues until the temperature reaches 650°C. The initial decrease in pressure is attributed to the reaction of nitrogen with the exemplified intermetallic compound and its incorporation into the R2Fe^7 or R(FeTi)i2 crystal lattice. That a new compound incorporating R, Fe and N has been formed is borne out by the fact that after the heating process the sample has increased weight and there is an increase in the crystal lattice parameters i.e. unit cell volume, as shown by X-ray diffraction.

The process of producing the new preferred materials may also be carried out using ammonia instead of nitrogen. In this case there is a rise in pressure starting at approximately 350°C. The rise in pressure is explained by the fact that at 350°C the ammonia decomposes to nitrogen and hydrogen. The nitrogen is taken up by the intermetallic sample as evidenced by a weight gain and increased crystal lattice parameters. It appears that once the temperature exceeds about 650°C the newly formed material decomposes to alpha-Fe and nitrides of the rare earth element or elements.

In accordance with a third aspect of the - 8 invention the new magnetic material produced as described herein is used for fabricating permanent magnets.

A preferred process by which this may be 5 achieved comprises milling the magnetic material with a metal such as aluminium, copper or zinc or a solder or an organic powder or resin, magnetically aligning the material by applying a magnetic field and then heating to a temperature not sufficient to decompose the material. Preferably the magnetic material is milled with zinc. This process of forming a magnet serves to increase the coercivity essential for forming magnets.

The following figures and tables give data relating to the magnetic properties of certain preferred intermetallic compounds of the invention and by way of example demonstrate the improvement in magnetic properties over known magnetic materials.

Table 1 The data in this table demonstrate the effect of incorporating nitrogen interstitially into the crystal lattice of compounds of the formula R2Fe17 with respect to crystal lattice parameters, Curie temperature (Tc) and spontaneous magnetization per unit mass (6S). These nitrogen-containing compounds were prepared by the process of heating in nitrogen gas in accordance with the invention.

The lattice parameters are determined by X-ray diffraction. R is represented by 12 different rare earth elements.

The spontaneous magnetization per unit mass 35 (6s) is converted to spontaneous magnetization per unit volume (Ms). by multiplying the value 6S by IE 901^59 the density of the magnetic material. Structure a c Tc G* 5 Compound type (nm) (nm) (°C) (JT-1Kg_1) Ce2Fe17 Th2Zn17 0.847 1.232 -32 0Ce2Fe17N2.8 VI 0.873 1.265 440 160 Pr2Fe17 1« 0.857 1.242 17 82 10 Pr2Fe17N2.5 II 0.877 1.264 455 167 Nd2Fe^7 II 0.856 1.244 57 77 Nd2Fei7N2.3 II 0.876 1.263 459 178 Sm2Fe17 It 0.854 1.243 116 100 Sm2Fe17N2.3 II 0.873 1.264 476 159* 15 Gd2Fe17 II 0.851 1.243 204 46 Gd2Fe17N2.4 It 0.869 1.266 485 115 Tb2Fe17 II 0.845 1.241 131 51 Tb2Fe17N2.3 II 0.866 1.266 460 96 DY2Fe17 Th2Ni17 0.845 0.830 94 50 20DY2Fe17N2.8 II 0.864 0.845 452 115Ho2Fe17 tl 0.844 0.828 54 49 Ho2Fe17N3.0 tl 0.862 0.845 436 115Er2Fe17 tl 0.842 0.827 23 32Er2Fe17N2.7 II 0.861 0.846 424 134 25 Tm2Fe17 It 0.840 0.828 -13 0 Tm2Fe17N2.7 11 0.858 0.847 417 137 Lu2Fe17 It 0.839 0.826 -18 0 Lu2Fe17N2.7 It 0.857 0.848 405 147Y2Fe17 It 0.848 0.826 52 92 30Y2Fe17N2.6 It 0.865 0.844 421 164 ♦Extrapolated value The data presented in Table 1 demonstrate that the interstitial nitride phase Ι^ΡβχγΝ),, where b is about 2.6, exists across the entire rare-earth series from Ce to Lu. The unit cell volume of the crystal lattice increases by 5 to 9% on forming the nitride and the Curie temperature To and spontaneous magnetization 6e are greatly increased. Pata further indicate that substitutions exist between nitrides of different rare earths so that.properties 10 such as magnetization or anisotropy field may be optimised-far. PartigU-lar^appllcations having regard to the cost of the particular fare earth component, TablJ 2 The data in the table demonstrate the effect on crystal lattice parameters, curie temperature (To) and tl^e spontaneous magnetic moment per unit volume (Me) cjf incorporating nitrogen into the crystal lattice of compounds of the formula: *2Fel7c1.0 and βη&ΡβιγΟχ,χ. Again the novel compound# were prepared by heating in a nitrogen-containing gas in accordance with the Invention. 1 1 Compound Structure type a (nm) q («») Tc (°C) pQM *2Fe17cl.O Th2Ni17 0.855 0.833 239 1.25Y2re17cli.0M1.4 H 0.867 0.851 428 1.40 Sm2Fex7Cji. x** ThaZni7 0.858 1.244 207 1.11s"2Fe17Cjl.lNl.l H 0.873 1*270 471 1.53 *(T) * at 18PC ** valutas are sensitive to conditions of heat treatment after melting the alloys.

The data again demonstrate the improvement in 35 magnetic properties, Tc, magnetisation and unit cell volume,jby interstitial incorporation of nitrogen into the crystal lattice of compounds of the general I formula RxFeyX·*. - 11 Table 3 The data presented in this table demonstrate the improved easy uniaxial anisotropy with, as an example, compounds where R is samarium. The value for easy uniaxial anisotropy represented by the anisotropy field Ba, in Tesla was obtained by aligning the rhombohedral c-axis in the direction of an applied magnetic field. From magnetization curves on oriented powders with the field applied parallel and perpendicular to the alignment direction the values for Ba shown in this table were obtained.

Compound Ba(T) Sm2Fe^7 < 1.0 Sir^Fe^^. 3 >12.0 Sn^Fej^C^.l 4.0 Sm2Fe17C1.iN1#o >8.0 Table 4 The data in this table presented give deduced values for iron-iron and iron-rare earth exchange interactions based on the variation in Curie temperature for the different heavy rare earths.

Compound nR-Fe(po) nFe-Fe(po) R2Fe^7 225 R2Fe^7Ny 208 181 515 It is deduced that the iron-iron interactions are enhanced by a factor of 2.5 in the new nitride compounds while iron-rare earth interactions are only slightly decreased. - 12 Table 5 The data presented in the table demonstrate the 5 effect of incorporating nitrogen interstitially into the crystal lattice of compounds of the formula RFej^Ti with respect to crystal lattice parameters, (a and c), Curie temperature (Tc), average hypefine field Bftf, In Tesla, and anisotropy. The starting materials were prepared by heating in a nitrogen-containing gas in accordance with the process of the invention. The particular process conditions in each case are given in the table.

Compound a(nm)c(nm) Tc(°C) anisotropy processing Nd(FenTi) 0.856 0.478 270 21.5 c-axis NdtFe^TONoj 0.879 0.487 475 28.0 c-axis 40’ at 450°C in N2 20 SmCFejjTi) 0.855 0.479 311 25.5 c-axis Sm(FeilTi)N08 0.864 0.484 496 29.1 c-plane 30’ at 480°C in N2 Sm(Fcj jTi)Ng 9 0.865 0.486 490 heat to 550°C @ 10°/min in NH 25 Dy(Fej jTi) 0.850 0.478 257 24.4 c-axis Dy(FeHTi)N0.6 0.867 0.480 473 28.2 c-axis 60' at 450°C in N2 Tb(FenTi) 0.852 0.479 281 24.2 c-axis 30 Tb(FenTi)N05 0.864 0.482 477 28.5 c-axis 40' at 450°C in N2 Y(FenTi) 0.851 0.479 251 23.5 c-axis Y(FcHTi)N0.8 0.862 0.481 460 28.8 c-axis 60' at 48O°C in N2 - 13 The interstitial incorporation of an element of group VA of the periodic table, for which the example is nitrogen, into selected intermetallic compounds of the formula R2Fei7 or R2Fel7x'a or R(FeM)i2 or R2(FeM)l7 where M is a substituent element as hereinbefore defined and the improved magnetic properties achieved thereby is further demonstrated by data presented in the figures in which:Figure 1 is a thermopiezic curve for absorption of nitrogen gas by Y2Fe^7 showing the drop in pressure of gas in the chamber as nitrogen is taken up by the sample. The pressure values on cooling demonstrate that the nitrogen remains absorbed by the Y2Fe17 saropl®,’ Figure 2 shows the isothermal reaction of nitrogen with Y2Fei7 powder, having an average grain size of approximately 2 microns diameter at 400°C, 450°C and 500°C, the value y being the number of moles of nitrogen atoms incorporated into a mole of the sample. The data indicate that the optimum temperature range for the operations of the process of the invention is between about 450°C and 600°C; Figure 3 is a thermopiezic curve for absorption of ammonia gas by Y2Fe17 an atmosphere of approximately 1 bar. The curves of heating demonstrate an increase in pressure due to uptake of nitrogen from the ammonia. There is an increase in weight after heating the sample to 550°C which is attributed to nitrogen absorption.

Figure 4 shows 57Fe Mossbauer spectra at room temperature of Y2Fei7 before (a) and after (b) heating to 500°C in 1 bar ammonia. The changes in Curie temperature and magnetic moment are reflected - 14 in the 57Fe Moesbauer spectra in which the average hyperfine field at 20°C, increases from 10 Tesla for Y2Fei7 to 30 Tesla for Ϊ2Γβχ7Ν2<6? Figure 5 shows the X-ray diffraction patterns of Y2Ρ·ΐ7 powder heated in a thermopiazic analyser in nitrogen at 10°C/minute up to the temperatures of 500°C, 550°C, 600®C, 700°C and 950°C. Powders of the formula R2Fe 17 where R is another rare earth element behave similarly. 1 The figure shows the appearance of a phase with expanded lattice parameters which co-exists with ths unexpanded phase after treatment up to 55p°c. The Y2Fel7N2.6 phase forms clearly at 600°C and on heating up to 700°C or above the alloy decomposes to YM and oFe; ; Figure 6 shows X-ray diffraction patterns of Y2pel7 Powder after heating in nitrogen gas ^ieothermally at 500®c for two hours. The extended 20 heat treatment produces the ^2^017^2,5 compound at a lower temperature than shown in the previous figure but further heat treatment to 850°C results in decomposition to YN and ®Fe.

~ Figure 7 ie a thermopiezio curve for 25 X2re17ic1.0 heated from room temperature in an atmosphere of approximately 1 bar ammonia., Again an increase in pressure at about 370°C is observed; Figure 8 shows the dependence of the Curie temperature (a) Tc(°C) and the unit cell volume of the lattice (b) V(X3) on the maximum heating temperature Tm for Y2F«i7C. For the sample treated at 450°C and 500°C there co-exiat two R2Fei7ftype phases one with the larger unit cell volume!and higher Curie temperature and the other with the smaller unit cell volume and lower Curie temperature. The more the crystal lattice is expanded the higher the Curie temperature. There is - 15 also a substantial increase in spontaneous magnetic moment (moMs) to 1.46 Tesla (see Table 2); Figure 9 shows Mossbauer spectra at room temperature of Y2Fe17c1.0 before (a) and after (b) heating in 1 bar ammonia at 550°C. The average hyperfine field at 18°C increases from 25.3 Tesla to 30.8 Tesla after the ammonia treatment; Figure 10 is a thermopiezic curve for Sm2Fe17Ci Analysis of the sample after heating to 600°C reveals that the material retains the rhombohedral (Tl^Zn^-type) structure with increased lattice parameters. From the increase in mass the nitrogen content is estimated to be 1.1 nitrogen atoms per Sm2Fe17Ci>l formula unit; Figure 11 is an X-ray diffraction pattern of Sm2Fe17C1e1 powder before (a) and after (b) orientation in an applied field of 1.2 Tesla for one hour. The figure demonstrates the strong uniaxial anisotropy possessed in particular where R is samarium; Figure 12 shows magnetization curves at 18°C of oriented samples of Sn^Fe^Ci.i before (a) and after (b) treatment in 1 bar ammonia up to 600°C. Curves are shown for the field applied parallel (J| ) and perpendicular (JL) to the axis of orientation. From these magnetization curves the values for poMs shown in Table 2 and Ba shown in Table 3 are obtained; Figure 13 shows the X-ray diffraction patterns of a) Siri2Fei7 powder with an average particle size of l^m and b) the same powder heated at 500°C in nitrogen gas for two hours to form Sm2Fei7N2.4; - IS Figures 14a and b show the radial distribution functions deduced from extended X-ray absorption fine structure data on the same samples as Figure 13. The peak appearing at 2.5 X shows the presence of approximately three nitrogen atoms at a distance 2.5 X from a samarium atom in the nitride; ! Figures 15a and b show the crystal structure of the xjhombohedral and hexagonal 2:17 structure, indicating the sites occupied by nitrogen; Figure 15a ijs the rhombohedral crystal structure and Figure 15b i'a the hexagonal crystal structure. Large circles represent rare earths, small shaded circles represent iron and small black circles represent nitrogen sites 9e or 6h. j Figure 16 is a histogram of the particle size distribution of a typical SmaFe^ powder ysed for nitrogen absorption; Figure 17 shows the variation of the diffusion coefficient for nitrogen in the 8»2Γ·ΐ7 powder as a function of inverse temperature. ί Figure 18 shows magnetization curves at 18°C for art oriented sample of Sm2?*17N2.3 after treatment with ammonia. Again curves are shown for the field applied parallel (}| ) and perpendicular φ to the axis of orientation. From these thfe values of the anisotropy field BA are obtained as shown in Table )3. The value of Ba for Sn^Fe^^.j is given ias >12.0 Tesla but in fact the curves shown in the figure indicate it may be as high a$ 20 Tesla; Figure 19(a) is a thermopiezic curve for e powderjmade from a cast ingot of 8m2Fex7 heated in nitrogen. Figure 19(b) is a thermopieziio curve for a powder made from an ingot and annealed for 100 hours at 950°C and heated in nitrogen. The differences in the two sets of curves clearly - 17 demonstrate that the treatment temperature required to form the R2Fei7Nb_Phas· depending on the metallurgical composition of the ingot used to I make ί the powder; : Figure 20 shows X-ray diffraction patterns of the Compounds NdjFe^Nj.j# SmjFe^Nj^ and B^2Fe17N2.7 after an applied field of 1.2 Teslal. In the case of Sn^Fe^Nj^ the c-axis is aligned parallel to the applied field indicating strong uniaxial anisotropy. However in the case where! R ie Nd or Er there is a tendency for the c-axi# to be aligned perpendicular to the direction of this applied magnetic field; Figure 21 shows the crystal structure of the tetragonal 1:12 compound showing sites occupied by nitrogen. The coding of the circles ie ae described for Figures 15a and 15b; : Figure 22 shows a thermopiezic trace for absorption of nitrogen gas by Sm(FenTi). The material was heated at a rate of 10°C/minuts at approximately 1 bar nitrogen. The figure demonstrates that the optimum temperature range for operation of the process is similar to that of the R2Fe17| compounds; ! Figure 23 shows room temperature 57 Mdssbauer spectra of SmfFenTi) before (a) and after (b) heating in a nitrogen containing gas in accordance with the invention. The average hypefine field increases from 25.5 Tesla in (a) to 29.1 Tesla in (b), reflecting the changes in Curie temperature and iron magnetic moment.

Figure 24 shows x-ray diffraction patterns of powder# of Sm(FenTi) (a) and Sm(FenTi)NQ.b (b) oriented in a magnetic field of 1.2 Tesla. The strong {uniaxial anisotropy of Sm(FenTi) ie transformed to easy-plane anisotropy in the interstitial nitride SmFejjTiNQ.g demonstrating a - 19 change in sign of the second-order crystal field coefficient A2o froin negative to positive. Hence the strong uniaxial anisotropy observed for interstitially-modified It 12 structure compounds of rare-aarths with a negative Stevens coefficient o 7(Nd,jEr,Tm), neodymium in particular; Figure 25 ie an illustration of interstitial i nitrogen atoms around the rare earth in the rhombohedral or hexagonal 2.17 structure (a) and in the tetragonal 1.12 structure (b). The electric field gradient experienced by the rare-eatth, quantified in the parameter Ajo, is mainly produced by surrounding interstitial atoms in the materials of the invention. It is negative for the configuration 15 of the 2.17 compounds and positlye for configuration of th<| 1.12 compounds; 'Figure 26 illustrates some of the effects of cobaltisubstitution for iron in materials of the invention having the rhombohedral or hexagonal crystal structure.

Figure 26(a) indicates the nitrogen content achieved by treating finely-ground powders of the R2 fFel7-cCoc)Nb typ® formula where o is the number of cobalt atoms in nitrogen gas at temperatures ranging from 400-600°C.

Figure 26(b) illustrates a broad maximum in magnetization with a transition metal substitute where R is ¥ and ο·0.2.

Figure 26(c) shows that the transition metal 30 substituents make a positive contribution to the anisotropy when o is >0.1; figure 27 is an illustration of the development of hysteresis in a powder of Sm2Fei7Nj,3 comprising first and second quadrant demagnetizing curves pt samples aligned and magnetized in a pulsed field of 6 Tesla. The data represented are as followsea) Powder Of SrnaFexyNj.s dispersed in - 19 •poxy reein b) Powder of S»2Fei7N2,3 milled with Zn powder (25 wt I) c) Powder of SmjFe^^.s milled with Zn powder (15 wt I) and heat treated at 400°C for two hours.

Figure 27 indicates the magnetic proparties of permanent magnets produced from the magnetic materials of the invention and methods by which the coercivity and hysteresis may be developed. For •xample in 21(c) the material is milled with 15 wt % Zn and heated to 400°C to produce a magnet having a coercivity of 0.5 Tesla and a maximum energy product of 86XJm~3.

The data shown in Figure 27 establishes conclusively that SmaFe^Na.a and the related compounds of the invention can be effectively processed to make magnets.

Further, thin films of materials of the invention may be exploited for magnetic or magneto-optic recording.

Claims (31)

CLAIMS:

1. A magnetic material of the general formula 5 R x FeyX 1 aZk which is dervied from an intermetallic compound of rhombohedral, hexagonal or tetragonal crystal 10 structure wherein R is one or more rare earth elements, X' is an element of groups IIIA, IIIB, IVA or IVB of the periodic table, Z is one or more elements of group VA of the periodic table, x is a value from 0.5 to

2. , y is a value from 9 to 19, a is 15 a value from 0 to 3, b is a value from 0.3 to 3 and wherein when the magnetic material of said general formula is derived from an intermetallic compound of rhombohedral or hexagonal crystal structure Fe is unsubstituted or partially substituted by another 20 element and when the magnetic material of said general formula is derived from an intermetallic compound of tetragonal crystal structure Fe is partially substituted by any element of group IIIA or IVA of the periodic table or a transition metal from 25 another group with the further proviso that in the case of materials derived from said rhombohedral or hexagonal crystal structures the element X· is not boron when the component Z is antimony or bismuth. 30 2. A magnetic material as claimed in claim 1 wherein when the material of said general formula is derived from an intermetallic compound of tetragonal crystal structure a=0.

3. A magnetic material as claimed in claim 1 or 2 wherein R is samarium or neodymium. - 21

4. A magnetic material as claimed in claim 1 or claim 3 wherein when the material of said general formula is derived from an intermetallic compound of 5. Hexagonal or rhombohedral crystal structure R is samarium.

5. A magnetic material as claimed in claim 1 wherein when the material of said general formula is 10 derived from an intermetallic compound of hexagonal or rhombohedral crystal structure R is samarium in combination with one or more rare earth elements selected from yttrium, lanthanum, cerium, neodymium, erbium, thulium and mischmetal.

6. A magnetic material as claimed in claim 1 wherein when the material of said general formula is derived from an intermetallic compound of hexagonal or rhombohedral crystal structure R is yttrium, 20 lanthanum, cerium, praseodymium, neodymium, gadolinium, terbium, dysprosium, erbium, thulium, ytterbium or lutetium or a mixture of two or more thereof. 25

7. A magnetic material as claimed in any of claims 1,2 or 3 wherein when the material is derived from an intermetallic compound of tetragonal crystal structure R is yttrium, thorium, cerium, praseodymium, neodymium, terbium, dysprosium or 30 holmium or a mixture of two or more thereof.

8. A magnetic material as claimed in any preceding claim wherein the element Fe is up to 33% substituted with a transition metal.

9. A magnetic material as claimed in claim 8 - 22 wherein when the material of said general formula is derived from an intermetallic compound of hexagonal or rhombohedral crystal structure the transition metal is cobalt.

10. A magnetic material as claimed in any one of claims 1,2,3 or 7 wherein when the material of said general formula is derived from an intermetallic compound of tetragonal crystal structure the element 10 Fe is partially substituted by titanium, vanadium, molydenum or chromium.

11. A magnetic material as claimed in any one of claims 1,2,3 or 7 wherein the material of said 15 general formula is derived from an intermetallic compound of tetragonal crystal structure the iron is partially substituted by aluminium or silicon.

12. A magnetic material as claimed in any of 20 claims 1,3,4,5 or 6 wherein when the material is derived from an intermetallic compound of rhombohedral or hexagonal crystal structure X’ is carbon, boron, silicon or zirconium and a is a value from 0.1 to 3.

13. A magnetic material as claimed in claim 12 wherein a+b £ 3.

14. A magnetic material as claimed in any 30 preceding claim wherein Z is nitrogen.

15. A magnetic material as claimed in any one of claims 1 to 13 wherein component Z is a combination of nitrogen with one or more other group 35 VA elements. - 23

16. A magnetic material as claimed in any one of claims 1 to 13 wherein component Z is one or more of P, As, Sb and Bi. 5

17. A magnetic material as claimed in claim 14 which has the formula Sm2Fei7N 2 .3 or Sn^Fe^C^ e . i or NdFe^^TiNg, q .

18. A process for modifying the magnetic 10 properties of an intermetallic compound comprising at least one or more rare earth elements and the element iron in which the iron is optionally substituted with another element which process comprises heating said intermetallic compound with a 15 gas containing at least one group VA element Z in the substantial absence of oxygen to incorporate the said at least one element Z interstitially into the crystal lattice of the intermetallic compound by a gas-solid reaction.

19. A process as claimed in claim 18 wherein when the intermetallic compound is of tetragonal crystal structure the iron is substituted by any element of group IIIA or group IVA of the periodic 25 table or by a transition metal from another group.

20. A process as claimed in claim 18 or claim 19 wherein the gas is one which produces radicals containing single atoms of the group VA element Z on 30 contact with hot surfaces such as metal or quartz or by exposure to high frequency radiation.

21. A process as claimed in claim 20 wherein the gas is a gaseous hydride of the group VA element 35 Z. - 24

22. A process as claimed in any one of claims 18 to 21 which produces a magnetic material as defined in any one of claims 1 to 17. 5

23. A process as claimed in claim 20 wherein the group VA element Z is nitrogen and the gas is nitrogen, ammonia or hydrazine.

24. A process as claimed in any one of claims 10 18 to 23 wherein said intermetallic compound is heated to a temperature not exceeding 650°C.

25. A process as claimed in any one of claims 18 to 24 wherein said intermetallic compound is 15 ground to a particle size of 1 to 50 microns diameter.

26. A process as claimed in claim 25 wherein the said ground compound is heated for up to 8 hours. 20

27. Use of the magnetic material as claimed in any one of claims 1 to 17 for fabricating a permanent magnet.

28. The use as claimed in claim 27, wherein a 25 magnet is formed by a process comprising the steps of: a) milling said magnetic material with a metal such as aluminium, copper or zinc or an organic powder or resin b) generating magnetic alignment in the said material by applying a magnetic field and c) heating the milled product to a temperature 35 sufficiently low to prevent decomposition of the magnetic material. - 25

29. The use as claimed in claim 28 wherein in the magnet fabricating process the magnetic material is milled with from 5 to 20 wt % zinc.

30. A permanent magnet comprising a magnetic material as claimed in any one or claims 1 to 17.

31. A permanent magnet comprising a magnetic 10 material which is produced by the process of any one of claims 18 to 26.