3d-anisotropy behaviour in R2Fe13MB compounds (RY, Gd; MAl, Ga, Si)

Journal of Magnetism and Magnetic Materials 104-107 (1992) 1431-1432 North-Holland

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3d-anisotropy behaviour in R 2Fe13 MB compounds (R = Y, Gd; M = A1, Ga, Si) G. Wiesinger a, R. Gr6ssinger a and X.C. Kou a Institute for Experimental Physics, TU Vienna, Vienna, Austria b Institute of Metal Research, Acad. Sinica, Shenyang, China

In order to study the 3d anisotropy contribution, the systems R2Fez3MB (R =Y, Gd) with M=AI, Ga, and Si as substituents were investigated by anisotropy and 57Fe M6ssbauer measurements. A correlation is proposed between the magnetic properties of a sample with a given composition and the preference of the substituent M for entering a certain Fe lattice site. Several kinds of experiments have already been performed, in order to study substituted compounds of the type R2Fe13MB (see, for example, refs. [1-7]. Thereby, a non-random occupation of the substituent M on the different Fe sites was observed, which significantly influences the 3d contribution to the anisotropy. Since, certainly at room temperature, the 3d anisotropy is of substantial importance for permanent magnet purposes, the systems Y(Gd)zFe13MB (M = Al, Ga, Si) were studied by means of anisotropy measurements and M6ssbauer spectroscopy. The samples were prepared from 99.95% pure starting materials in an arc furnace in a purified argon atmosphere. Subsequently they were annealed in high vacuum at 900°C for 3 weeks. The anisotropy field was determined between 4.2 K and Tc by applying the SPD (singular point detection) technique [8] in a pulsed field system. The 57Fe M6ssbauer spectra were recorded at room temperature and at 4.2 K. In the least squares fitting procedure, the relative intensities of the subspectra were not constrained, in order to yield information about a possible non-random Fe substitution. The temperature dependence of the 3d-anisotropy field in the present compounds shows an unusual increase with rising temperature, leading to a maximum below the Curie temperature (figs. la,b). The low temperature behaviour of the anisotropy field /X0HA(T) in R = Y is remarkably different from that in R = Gd. In the Y-case, it rises on the substitution of Ga and particularly of Si. For R = Gd, however, the addition of Ga leads to a substantial loss in /x0HA at low temperatures. This peculiar temperature behaviour can either be attributed to the specific influence of the inequivalent Fe sites which may even lead to opposite signs of the corresponding anisotropy contribution as well as to varying temperature dependences. Alternatively, it could be caused by magnetoelastic effects, because the rise of the 3d-anisotropy field with increasing temperature is very reminiscent to the volume

magnetostriction of ReFe;4B in the ferromagnetic state. For the ternary compounds ReFeaaB , the occupation numbers on the various Fe sites is readily observed as intensity of the corresponding subsextets in a Mfssbauer spectrum. Despite a certain complexity in the overall hyperfine patterns of the substituted compounds, a deviation from a random Fe-site occupation could be recognized. Compared to the ternary corn3.0 F 1

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0312-8853/92/$05.00 © 1992 - Elsevier Science Publishers B.V. All rights reserved

G. Wiesinger et al. / 3d-anisotropy behat;iour in R2Fe13MB

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Table 1 Fe occupation numbers on the k- and the j-lattice sites

R =Y R = Gd

M Ga Si Ga Si

kl 16 16 16 16

k2 12 13 14 12

Jl 8 7 8 8

J2 8 8 6 8

pounds, the average Fe hyperfine field values were found to be reduced in any case, the reduction for X = Si being roughly 25% larger than that for X = Ga. Contrary to the pure ternaries, a further weak ( < 10% of the total intensity) sextet was observed in addition to the six characteristic magnetic hyperfine patterns, which, in agreement with X-ray data, is attributed binary (Fe, M) precipitations in the sample. In the case of M = Ga, Si, the Fe occupation numbers derived from the intensity of the specific subspectra are displayed in table 1, where the c and e site values were omitted due to their larger uncertainty. The results obtained from M6ssbauer spectroscopy can be interpreted as follows. The k 1 sites on their own exhibit a nearest B neighbour at a distance of only 2.096 A (Nd2Fe14B). This is supposed to prevent large atoms from entering this particular site. In fact, the comparatively large atoms AI, Si and Ga preferably occupy the k 2 sites, the deviation from random distribution being largest in the case of Ga. On the other hand, Co and Ni, being distinctly smaller, also prefer the k 2 sites [6]. From all the elements under investigation only AI furthermore prefers to occupy the J2 sites, apparently because they exhibit the largest internal void size of all the sites. Ni, Si and Ga frequently prefer the Jl sites. This indicates that atomic size considerations on their own are incomplete to explain the complex crystallization mechanism in the compounds under investigation. Additionally, energy effects, i.e., the relative bond strengths between Fe, M and Nd neighbours have to be taken into account. The latter can be estimated from the corresponding heats of solution,

which certainly represents a crucial parameter for the procedure of Fe substitution [2]. Thus, the influence of the non-random distribution of the substituents M on the Fe sublattice upon the magnetic anisotropy can, at least qualitatively, be understood: a preference of the substituent for the k sites (corresponding to a preference of Fe for the j sites, particularly for J2) seems to favour an increase in anisotropy. Consequently, this site is considered to be the main reason for the large 3d-anisotropy. From this, we conclude that, for gaining high H A values, this peculiar site has to be occupied by Fe as much as possible. Obviously, this is not the casc for GdEF%3GaB, where the large Ga occupation on the J2 site is supposed to be responsible for the reduced anisotropy. Finally we want to point the importance of metallurgical effects for gaining high performancc permanent magnets. As mentioned before, the formation of binary precipitations is favoured, when Fe is substituted by another element. This brings about the creation of pinning centers and has to be considered as additional contribution to the coercivity of a multi-elemental compound, even in cases, when the substituent is a nonmagnetic element. References

[1] X.C. Kou, G. Wiesinger and R. Gr6ssinger, in: Proc. llth Int. Workshop on Rare-Earth Magnets, ed. S.K. Sankar, Carnegie Mellon University, Vol. II (1990) 262. [2] H.M. van Noort and K.H.J. Buschow, J. Less-Commun Met. 113 (1985) L9. [3] J.F. Herbst and W.B. Yelon, J. Appl. Phys. 60 (1986) 4224. [4] L. Pareti, M. Solzi, F. Bolzoni, O. Moze and R. Panizzieri, Solid State Commun. 61 (1987) 761. [5] Y. Yang, F. Xing, L. Kong, J. Yang, Y. Ding, B. Zhang, C. Ye, L. Jin and H. Zhou, ]. Phys. (Paris) 49 (1988) C8-597. [6] G. Wiesinger, R. Gr6ssinger, R. Krewenka, X.C. Kou, X.K. Sun and Y.C. Chuang, Hyper. Interact 50 (1989) 693. [7] O.A. Pringle, G.J. Long, G.K. Marasinghe, W.]. James, A.T. Pedziwiatr, W,E. Wallace and F. Grandjean, IEEE Trans. Magn. 25 (1989) 3440. [8] G. Asti, S. Rinaldi, J. Appl. Phys. 45 (1974) 3600.