MAGNETIC PHASE-TRANSITIONS IN RB1-XKXNICL3 SYSTEM - A MIXTURE OF ANTIFERROMAGNETS ON DEFORMED TRIANGULAR LATTICE

The Rb1-xKxNiCl3 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.