Pressure-induced reentrant oblique antiferromagnetic phase in the spin-dimer system TlCuCl₃

Magnetization measurements under a hydrostatic pressure of P=1.4 GPa were performed on the coupled spin dimer system TlCuCl₃, which exhibits a pressure-induced quantum phase transition from a gapped singlet state to an antiferromagnetic state at P_c=0.042 GPa. Antiferromagnetic ordering with ordered moments parallel to the ac plane was observed at T_N=16.7 K at 1.4 GPa. The spin reorientation phase transition was observed at T _R≃9.2 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.