Magnetic Phase Transitions in Rb_1−xK_xNiCl₃ System: A Mixture of Antiferromagnets on Deformed Triangular Lattice

Abstract

The Rb_1−xK_xNiCl₃ system is a mixture of S=1 Heisenberg antiferromagnets on deformed triangular lattice with the easy-axis and easy-plane types of anisotropy. The magnetic phase transitions in the present system were investigated by susceptibility and torque measurements. A phase diagram for temperature vs x was determined. With increasing x the anisotropy changes from the easy-axis type to the easy-plane one at x_c=0.38. For x<x_c one phase transition occurs, while for x_c<x<0.8 two transitions take place. The intermediate phase appears at x_c where the anisotropy changes its sign, and the temperature range of the intermediate phase is enhanced on increasing x. The magnetic properties for x_c<x<0.8 are similar to those of RbVBr₃ which has a deformed triangular lattice in the c-plane. The successive transition was not observed in the vicinity of x=1 where the symmetry of the crystal lattice is lower than that for x<0.8.