Pressure-induced reentrant oblique antiferromagnetic phase in the spin-dimer systemTlCuCl3

Abstract

Magnetization measurements under a hydrostatic pressure of $P=1.4\text{ }\text{GPa}$ were performed on the coupled spin dimer system ${\text{TlCuCl}}_{3}$, which exhibits a pressure-induced quantum phase transition from a gapped singlet state to an antiferromagnetic state at ${P}_{c}=0.042\text{ }\text{GPa}$. Antiferromagnetic ordering with ordered moments parallel to the $ac$ plane was observed at ${T}_{N}=16.7\text{ }\text{K}$ at 1.4 GPa. The spin reorientation phase transition was observed at ${T}_{R}\ensuremath{\simeq}9.2\text{ }\text{K}$ for zero magnetic field, at which the ordered moments start to incline toward the $b$ axis. With increasing external field parallel to the $b$ axis, a second-order phase transition from the oblique antiferromagnetic (OAF) phase to the spin-flop (SF) phase occurs below ${T}_{R}$. We argue that the OAF phase arises from the competition between the anisotropic energies of the conventional second order and the fourth order, which increases with increasing pressure. We discuss the OAF-SF transition within the framework of the mean-field approximation.