Quadrupolar and magnetic phase diagram of UNiSn

UNiSn undergoes a multiplex transition from a paramagnetic semiconductor (Phase I) to an antiferromagnetic metal with ferroquadrupolar order (Phase III) at T-N similar to 43K 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 TN is nearly field-independent. We observed a separation of T-N and T-Q in high magnetic field above similar to 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.