Pressure effects on phase transitions in several hexagonal antiferromagnets of ABX3 type

Magnetic measurements under high pressures have been performed on the hexagonal ABX(3) type antiferromagnets, CSFeCl3, CsNiCl3 and RbNiCl3 to investigate the pressure effect on the magnetic phase transitions. CsFeCl3 has the singlet ground state due to the large single-ion anisotropy of the easy-plane type at ambient pressure. Above the critical pressure P-c similar or equal to 0.9 GPa, CsFeCl3 exhibits the long range magnetic ordering at zero field as the result of the collapse of the energy gap between the singlet state and the lowest excited doublet. CsNiCl3 and RbNiCl3 having the easy-axis type magnetic anisotropy undergo the successive phase transitions in the temperature variations and exhibit the spin-flop phase transition in the field parallel to the c-axis. With increasing pressure, the temperature range of the collinear intermediate phase and the spin-flop field H-SF are enhanced. This fact indicates that the easy-axis anisotropies of CsNiCl3 and RbNiC13 are enhanced by the pressure as compared with the increase of the intra- and inter-chain interactions.