Pressure-Induced Successive Magnetic Phase Transitions in the Spin Gap   System TlCuCl$_3$
  Under the hydrostatic pressure $P=1.48$ GPa, polarized neutron elastic scattering experiments have been carried out on the coupled spin dimer system TlCuCl$_3$ with the gapped ground state at ambient pressure. Pressure-induced magnetic ordering occurs at the transition temperature $T_{\rm N}=16.9$ K. As in the field-induced and impurity-induced magnetic ordered phases, the ordered moments lie in the $a-c$ plane just below $T_{\rm N}$. An additional spin reorientation (SR) phase transition was observed at $T_{\rm SR}=10$ K, where the ordered moments start to incline toward the b-axis. The temperature variations of the direction and the magnitude of the ordered moments were also investigated. 
A. Oosawa, K. Kakurai, T. Osakabe, M. Nakamura, M. Takeda, H. Tanaka