Crossover from impurity-induced ordered phase to uniform antiferromagnetic phase under hydrostatic pressure in the doped spin-gap system TlCu_(1-x)Mg_xCl_3

摘要

The magnetic phase transition under hydrostatic pressure in TlCu_(0.988)Mg_(0.012)Cl_3 was investigated by magnetization measurements. The parent compound TlCuCl_3 is a coupled-spin-dimer system, which undergoes a pressure-induced quantum phase transition from a gapped ground state to an antiferromagnetic state at P_c =0.42 kbar due to the shrinkage of the gap. At ambient pressure, the present doped system exhibits impurity-induced magnetic ordering at T_N=2.5 K. With increasing pressure, T_N increases. This is because the effective exchange interaction J_(eff) between unpaired spins is enhanced by the shrinkage of the gap. With a further increase in pressure, the present system undergoes a phase transition to a uniform antiferromagnetic phase due to the closing of the triplet gap in the intact dimers. The crossover from the impurity-induced ordered phase to the uniform antiferromagnetic phase occurs at P approx = 1.3 kbar.