Quadrupolar and Magnetic Phase Diagram of UNiSn

Abstract

UNiSn undergoes a multiplex transition from a paramagnetic semiconductor (Phase I) to an antiferromagnetic metal with ferroquadrupolar order (Phase III) at T N ∼43 K in zero field. We have measured elastic moduli, thermal expansion, magnetization and electrical resistivity of UNiSn in magnetic field. Magnetic field raises the quadrupole-ordering temperature T Q , which was found to be identical to the semiconductor-metal transition temperature T SM , while T N is nearly field-independent. We observed a separation of T N and T Q in high magnetic field above ∼4 T, and we found a new phase (Phase II) between T N and T Q , which has been identified as a paramagnetic metal with ferroquadrupolar order. The coincidence of T Q and T SM strongly suggests that the ferroquadrupolar order is responsible for a change in electronic structure, leading to metallic conduction.