MÖSSBAUER AND MAGNETIC PROPERTIES OF HEUSLER ALLOYS

Abstract

Theoretical models that are currently used to describe the magnetic coupling between Mn ions and the magnetic hyperfine interactions at non-magnetic s, p ions in X2MnZ Heusler alloys are discussed. Experimental data on the spatial dependence of the Mn-Mn exchange interaction in Pd2MnSn is shown to be consistent with the long-range oscillatory mechanism that arises from the potential scattering of an s-like conduction electron by two Mn moments - the double resonance coupling. No definite evidence is found for any other significant coupling mechanism. Present models that involve local ionic effects in the production of the magnetic hyperfine fields at non-magnetic Z site ions are reviewed. Attention is concentrated on two models that describe the fields in terms of the local electronic structure of the ion, and reasons for this are given. Emphasis is placed on the approximate nature of these models and on the consequent inadvisability of using them for detailed comparisons with experiment. The need for more detailed theoretical work aimed at providing a rigorous basis for application and subsequent refining of these models is stressed. 119Sn and 121Sb Mossbauer measurements have been made on solid solutions of the Heusler alloys Pd2MnIn, Pd2MnSn and Pd2MnSb. Comparison of Mossbauer data with neutron diffraction data for alloys with fewer electrons than Pd2MnSn shows that the different magnetic structures that are observed depend very sensitively on the average number of electrons at a Z site ion. The double resonance magnetic coupling mechanism is seen to be at least qualitatively consistent with these observations. For alloys with electron densities approaching that of Pd2MnSb, rapid changes of both the 119Sn and 121Sb hyperfine fields with composition are observed. These are discussed in terms of the hyperfine field models previously described and, along with changes in this composition range in other quantities such as the Curie temperature, cannot be explained using the normal free electron-like description for the conduction band. Similar conclusions are drawn from 121Sb Mossbauer measurements on solid solutions of PdMnSb and Pd2MnSb.