Pressure-induced stabilization of an intermediate phase in the triangular lattice antiferromagnet CsNiCl₃

Pressure effects on the magnetic properties of the triangular lattice Heisenberg antiferromagnet CsNiCl₃ have been studied through heat capacity measurements up to the pressure of 6.6 kbar. With increasing pressure, two successive phase transitions at T_N1 = 4.74 K and T_N2 = 4.24 K at 0 kbar, which result from a small Ising-type anisotropy of S = 1 moments on Ni²⁺ ions, shift to higher temperatures in proportion to the pressure. Moreover, the pressure dependence of the entropy indicates that the intermediate phase AF₁ (T_N2 < T < T_N1) is stabilized. These results suggest that the pressurization enhances the Ising-type anisotropy in the spin moments.