HYPERFINE FIELDS IN Co-BASED HEUSLER ALLOYS

Abstract

From Mossbauer experiments with 19Sn, 121Sb and 178Hf we have determined the hyperfine fields at Sn, Sb and Hf, respectively, in Heusler alloys of the composition CozVSn, CozNbSn, CozHfSn, Co2-,FezTiSn (x = 0.2, 0.4), C O ~ ~ N ~ ~ T ~ S ~ (x = 0.1, 0.2) and Co2TiSn0.9Sbo.l. Moreover, the hyperfine field at Ta on the Hf site in CozHfSn alloy has been measured by the time differential perturbed angular correlation method. The results are discussed in terms of the electronic structure of these alloys. In particular, the observed anomalies in the tin hyperfine field are explained by the assumption that the polarization of the p-electrons of tin depends decisively on the nature of the neighbouring nonmagnetic transition metal atoms. Introduction. In the ferromagnetic Heusler alloys Co2YSn, where the magnetic Y atoms are either the group IVa elements Ti, Zr and Hf or the group Va elements V and Nb, both the Co and Sn hyperfine fields have rather anomalous values. Thus the NMR value for the Co hyperfine field in Co2TiSn is + 21 kOe [I], while a negative sign would be expected if the core polarization hypedine field were the dominant contribution, as it usually is. Mossbauer experiments, on the other hand, yield [l] large positive hyperline fields of+ 82, + 88 and + 106 kOe at Sn in Co,TiSn, Co,ZrSn and Co2HfSn, respectively. These positive values of the Sn fieIds are suprising because the only magnetic atoms in these systems are the Co atoms, the contribution of which to the Sn hyperfine field should be negative according to the