Magnetic Field–Temperature Phase Diagram of the Ferro-quadrupolar State and Crystal Electric Field Effect in UCu₂Sn

Abstract

The ternary uranium compound UCu 2 Sn shows a ferro-quadrupolar transition at T Q ¼ 16 K originating from the ordering of the non-Kramers doublet À 5 ground state. To investigate the magnetic field-temperature phase diagram of the ferro-quadrupolar state in UCu 2 Sn, we measured elastic modulus C 66 at magnetic fields of up to 14 T. We found an anisotropic field dependence of T Q ðHÞ in H k ½ " 12 " 10, [0001], and ½01 " 10. We optimized the crystal electric field level scheme using C 66 under fields, magnetic susceptibility, and electronic entropy. The quadrupolar phase diagram is well reproduced by the obtained level scheme.