THE TEMPERATURE DEPENDENCE OF THE ANISOTROPY FIELD IN R2Fe14B COMPOUNDS (R = Y, La, Ce, Pr, Nd, Gd, Ho, Lu)

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1 THE TEMPERATURE DEPENDENCE OF THE ANISOTROPY FIELD IN R2Fe14B COMPOUNDS (R = Y, La, Ce, Pr, Nd, Gd, Ho, Lu) R. Grössinger, X. Sun, R. Eibler, K. Buschow, H. Kirchmayr To cite this version: R. Grössinger, X. Sun, R. Eibler, K. Buschow, H. Kirchmayr. THE TEMPERATURE DE- PENDENCE OF THE ANISOTROPY FIELD IN R2Fe14B COMPOUNDS (R = Y, La, Ce, Pr, Nd, Gd, Ho, Lu). Journal de Physique Colloques, 1985, 46 (C6), pp.c6-221-c < /jphyscol: >. <jpa > HAL Id: jpa Submitted on 1 Jan 1985 HAL is a multi-disciplinary open access archive for the deposit and dissemination of scientific research documents, whether they are published or not. The documents may come from teaching and research institutions in France or abroad, or from public or private research centers. L archive ouverte pluridisciplinaire HAL, est destinée au dépôt et à la diffusion de documents scientifiques de niveau recherche, publiés ou non, émanant des établissements d enseignement et de recherche français ou étrangers, des laboratoires publics ou privés.

2 JOURNAL DE PHYSIQUE Colloque C6, supplément au n 9, Tome 46, septembre 1985 page C6-221 THE-TEMPERATURE DEPENDENCE OF THE ANISOTROPY FIELD IN R 2 Fe 14 B COMPOUNDS (R = Y, La, Ce, Pr, Nd, Gd, Ho, Lu) R. Grossinger, X.K. Sun, R. Eibler, K.H.J. Buschow and H.R. Kirchmayr Institute for Experimental Physios, Technical University of Vienna, Austria ^Philips Research Laboratories, Eindhoven, The Netherlands ' Résumé - La variation thermique du champ d'anisotropie a été déterminée dans les composés du type (^Fej^B (R = Th, Y, La, Ce, Pr, Nd, Gd, Ho, Lu) à des températures, entre 4.2 K et la température de Curie. Dans les composés Nd2Fe^B et P^Fe^B on a observé une réorientation du spin. Abstract - The temperature dependence of the anisotropy field was measured in various compounds of the type I^Fej^B (R = Th, Y, La, Ce, Pr, Nd, Gd, Ho, Lu) at temperatures between 4.2 K and the Curie temperature. In Nd2Fej4B and ^r2' re 14^ a s Pi- n reorientation was found to occur below room temperature. I - INTRODUCTION New permanent magnet materials based on Nd-Fe-B were recently developed which have distinct advantages compared to permanent magnet materials based on Sm-Co /!/. Preliminary measurements of Nd-Fe-B at room temperature /2/ had characterized this material as having uniaxial magnetic anisotropy, the anisotropy field (H/\ = 75 kg) being of the same order of magnitude as in Sn^Coj^. More detailed measurements made at various temperatures with a Nd2Fe24B single crystal soon revealed that the temperature dependence of the anisotropy in these types of materials is far from simple, as is witnessed by the change in the easy magnetization direction from parallel to the c-axis above 135 K to a direction deviating by 30 degr. from the c axis at 4.2 K /3/. The anisotropy fields in the various R2Fe^4B compounds at 4.2 K were determined from high field measurements on aligned powder samples by Sinnema et al. /4/. In this paper we report on the temperature dependence of H/\ in various R2Fe^4B compounds as well as in several materials of a non-stolchiometric composition. II - EXPERIMENTAL The anisotropy field was measured on arc melted and vacuum annealed samples using the SPD (Singular Point Detection) technique /5/. The theory of this method predicts a singularity in the second derivative of the magnetization (d^m/dh^) at the anisotropy field Hft if the external field is applied parallel to the hard plane (K^ > 0). From the fact that this method samples the properties of the correctly oriented grains only (H ex t parallel to the hard axis), it follows that H/\ can be measured on aligned powder samples but also on polycrystalline samples. This method is well suited for uniaxial materials. If the anisotropy of a material having an easy plane magnetization has to be measured, the theory predicts that the third derivative (d^m/dh^) will show a singularity at H=H/^. Unfortunately, owing to experimental difficulties, measurement of d-'m/dh-' is nearly impossible. Therefore our anisotropy studies will be restricted to R2Fe^4B compounds of uniaxial magnetic anisotropy. III - RESULTS AND DISCUSSION The temperature dependences of the anisotropy field H/\ of various R2Fe^4B compounds are compared in Fig. 1. In general one expects the anisotropy field to decrease with increasing temperature. This behaviour is only observed for those R2Fe^4B compounds in which the R component has an orbital moment and where one may assume that the ani- Article published online by EDP Sciences and available at

3 C6-222 JOURNAL DE PHYSIQUE sotropy originates from crystal field effects. In the R2Fel4B compounds in which R is non-magnetic (R = Y,La,Lu,Ce or Th) the anisotropy is due to the 3d sublattice magnetization. It is interesting to note that the temperature dependence of HA in these materials shows the opposite behaviour to those referred to above, i.e. the anisotropy field slightly increases with increasing temperature (at least at temperatures well below the Curie temperature). These results suggest that the Fe atoms at the different crystallographic Fe sites contribute differently to HA and also that the temperature dependence of these contributions is not the same. A similar situation was assumed to exist in Y2Co17 /6/. Attempts were made to describe the 3d sublattice anisotropy in uniaxial intermetallic compounds in terms of the second order crystal field parameter B20 /7/. In this model a correlation between the anisotropy energy and the factor [l-3/8 (~/a)~l would be expected, where a and c are the lattice constants. The value$ of HA obtained in the course of the present investigation are compared with values of the factor [l-3/8 (~/a)~] in Table 1, using data for a and c reported elsewhere /4/. When interpreting the data shown in the table one has to take account of the fact that the situation in compounds where R is trivalent may be slightly different from that in compounds where R is tetravalent. In general the correlation appears to be quite satisfactory. A detailed inspection of the shape of the various d2~/d~2 versus H curves, measured at different temperatures to obtain the temperature dependence of HA, makes it clear that the compounds Nd2Fe14B and Pr2Fel4B behave somewhat anomalously at low temperatures. It is well known that there is a change in magnetization direction from an easz axis to an easy cone in Nd2Fe14B at 135 K /3/. An anisotropy anomaly in the d2w/d~ versus H curves was observed for the first time by Grgssinger et al. /2/. For temperatures above about 180 K the shape of these curves is close to that expected on the basis of the SPD theory, the anisotropy field being determined by the singularity appearing as a sharp cusp at the field HA (see Fig. 2a). However, there is a distinct change in the shape of these curves at lower temperatures (Fig. 2b). Closer Fig. 1 - Temperature dependence of the anisotropy field yoha in various R2Fe14B compounds (R = Y(x), La(.), Ce(A), Nd(O), Pr(+), Gd(o), Lu(v), Ho(@)).

4 analysis of the data shows that the cusp-like singularity is present here in the curves obtained by plotting the first derivative dm/dh versus field strength (Fig. 2c). These results can be interpreted as meaning that Nd2Fe14B gives rise to a first order magnetization proces (FOMP) close to the temperature where the change in easy direction occurs. A similar change in the shape of the SPD curves has also been observed in Pr2Fel4B below 230 K, albeit a less pronounced change than in Nd2Fe14B. No such changes have been found R2Fe14B compounds in which R is non-magnetic, suggesting that the FOMP transition originates from the rare earth sublattice magnetization. The occurrence of an easy c-axis magnetization at room temperature as well as at 4.2 K was proposed for compounds R2Fel4B in which R is one of the heavy rare earth metals Gd,Tb,Ho or Dy /1,4/. There is at most a small contribution to the anisotropy by Gd, which can be deduced from the fact that HA in Gd2Fel4B is about the same as in compounds where R is non-magnetic. In the remaining compounds with heavy rare earth elements the HA values expected for temperatures below 300 K are too high to be detected by our pulsed field equipment. This is probably the reason that attempts to measure the high temperature behaviour of HA(T) in R2Fel4B with R = Dy or Tb were unsuccessful even at temperatures close to the Curie temperature. Results obtained for Ho2Fel4B above room temperature are included in Fig. 1. The composition of materials used in permanent magnet fabrication is commonly chosen to be somewhat different (R15Fe77B8) from the stoichiometric composition (R2Fe14B= R12Feg2B6). In order to check whether such compositional changes have any bearing on the anisotropy and its temperature dependence we included in our investigation materials with non-stoichiometric compositions. The temperature dependence of HA in R15Fe77B8 (R = Nd,Ce,Y) is compared with that in the corresponding stoichiometric materials in Fig. 3. The data show that moderate changes in composition do not lead to a different anisotropy behaviour. The SPD technique preferentially samples the magnetic properties of magnetically hard grains present in the material, the simultaneous presence of one or more impurity phases with weak magnetic properties being unimportant. Apparently the anisotropic properties of these hard magnetic grains are similar for both compositions. Since the anisotropy can be expected to depend strongly on the local environment of the atoms (of the rare earth atoms in particular) these results indicate that the composition of the magnetic main phase is not changed when changing the overall composition of the material. Fig. 2 - Field dependence of d2~/dt2 in Nd2Fe14B at T = 300 K (a) and T = 135 K (b), and field dependence of dm/dt at T = 135 K (c). For H increasing linearly with time the time derivatives of M can be taken to represent field derivatives.

5 C6-224 JOURNAL DE PHYSIQUE Fig. 3 - Temperature dependence of the anisotropy field u 0 H a in various R-Fe-B alloys Nd 15 Fe7 7 B 8 (+), Nd 2 Fe 14 B (o); Y 15 Fe 7 7B 8 (x), Y 2 Fe 14 B (&); Ce 15 Fe 77 B 8 ( ), Ce 2 Fe 14 B (o) Table 1 : Comparison between the values of l-3/8 (c/a)^] derived from the lattice constants a and c and the values of the anisotropy field H/\ in various R 2 Fei 4 B compounds. Compound [1-3/8 (c/a)2j MA < T ) R valency Lu 2 Fe 14 B Y 2 Fe 14 B Gd 2 Fei 4 B Ce 2 Fej.4B Th 2 Fe 14 B La 2 Fe 14 B REFERENCES /l/ Sagawa, M., Fujimura, S., Yamamoto, H., Matsuura, Y. and Hiraga, K., IEEE Trans. on Magn. MAG-20 (1984) /2/ Grossinger, R., Obitsch, P., Sun, X.K., Eibler, R., Kircbmayr, H.R., Rothwarf,F. and Sassik, H., Mat. Letters 2 (1984) 539. /3/ Givord, D., Li, H.S., Moreau, J.M., Perrier de la BSthie, R. and Du Trgmolet de Lacheisserie, E., Proc. of REACIM (1984) St. Polten (Austria). /4/ Sinnema, S., Radwanski, R.H., Franse, J.J.M., De Mooij, D.B. and Buschow, K.H.J., J. Magn. Magn. Mater. 44 (1984) 333. /5/ Asti, G. and Rinaldi, S., J. Appl. Phys. 45 (1974) /(,/ Inomata, K., Jap. J. Appl. Phys. 15_ (1976T"821. /!/ Szpunar, B. and Lindgard, P.A., J. Phys. F (1979) L 55.