Magnetic-field- and pressure-induced quantum phase transition in CsFeCl$_3$ proved via magnetization measurement

We have performed magnetization measurements of the gapped quantum magnet CsFeCl$_3$ at temperatures ($T$) down to 0.5\,K at ambient pressure and down to 1.8\,K at hydrostatic pressures ($P$) of up to 1.5\,GPa. The lower-field ($H$) phase boundary of the field-induced ordered phase at ambient pressure is found to follow the power-law behavior expressed by the formula $H_{\rm N}(T)$\,$-$\,$H_{\rm c}$\,$\propto$\,$T_{\rm N}^{\phi}$. The application of pressure extends the phase boundary to both a lower field and higher temperature. Above the critical pressure $P_{\rm c}$\,$\sim$\,0.9\,GPa, the transition field $H_{\rm N}$ associated with the excitation gap becomes zero, and a signature of the magnetic phase transition is found in the $T$-dependence of magnetization in a very low applied field. This suggests that CsFeCl$_3$ exhibits a pressure-induced magnetic phase transition at $P_{\rm c}$.