Unusual magnetic and lattice transformation in UNiSn, a possible half-metallic ferromagnetic system

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Abstract
UNiSn, as well as certain other ternary actinide compounds, displays very anomalous behavior in the temperature dependence of the electrical resistivity, ρ(T). In UNiSn this unusual ρ(T) dependence has been attributed to half-metallic behavior with a semiconducting gap in the minority spin band and metallic behavior in the majority spin band with a ferromagnetic transition in the vicinity of 50 K. We have studied UNiSn using 119Sn Mössbauer spectroscopy and observed that annealing procedures influence the microscopic properties of the sample in a profound way. As-cast samples are nonmagnetic. Annealing in the neighbor of 800 °C results in the formation of a magnetic component. The conversion to a magnetic phase is incomplete even after annealing for 210 days at 800 °C. The magnetic component goes through two transformations: (1) a region below approximately 60 K where there is a coexistence and gradual transformation of paramagnetic to magnetically ordered material and (2) a region below 43 K where the magnetic component is completely ordered. In the region between 43 and 60 K, the hyperfine field shows a sequence of first-order transitions. The temperature dependence of the 119Sn hyperfine field for UNiSn is presented and discussed along with measurements of the temperature dependence of the specific heat, electrical resistivity, and the temperature and field dependence of the magnetization.