Basal-plane anisotropy for Sm2Co17: The crucial role of J mixing

The sixth-order basal-plane anisotropy constant K-3' was deduced at differe nt temperatures for Sm2Co17. The values were derived from measurements of t he anisotropy field H-A performed along the two hard directions correspondi ng to the crystallographic a and b axes. In the frame of a single-ion model with exchange and crystal field, K-3' is proportional to the sixth-order c rystal-field term. However, the sixth-order term is zero for the Sm3+ ion w ithin the ground state multiplet. The experimental data were then compared with a curve K-3'(T) calculated considering the three lowest-lying J multip lets. Using perturbation theory it was shown that K-3' is directly proporti onal to the sixth-order crystal-field parameter B-66 even in the case of st rong J mixing due to the exchange field. The best fit of the experimental d ata was obtained by means of a set of parameters which are consistent with those given in the literature, and in particular an accurate determination of B-66 was possible. The results of the theoretical calculation were also compared with anisotropy field measurements on the isostructural compound E r2Co17, for which the J mixing is not expected to be significant.