Birefringence Study of the Criticalityof the Magnetic Phase Transition in RbNiCl3

Abstract

Short-range spin ordering of RbNiCl 3 in the critical region of the magnetic phase transition is observed by the birefringence technique. The temperature derivative of the ac -plane birefringence, corresponding to specific heat, shows an almost divergent anomaly at 11.3 K, which is resulted from coalescing two singularities at the successive phase transitions inherent to an Ising-like Heisenberg triangular lattice antiferromagnet. The critical exponent \(\tilde{\beta}\) is determined as 0.76±0.03 from the data on the temperature dependence of the symmetry-breaking birefringence Δ n x y . The almost divergent anomaly of specific heat and the value of \(\tilde{\beta}\) mean that the criticality does not belong to the 3d- XY universality class, in which a cusp-like anomaly in specific heat and \(\tilde{\beta}\) of 0.85, are theoretically predicted for both of the successive phase transitions of Ising-like Heisenberg triangular lattice antiferromagnets. This can be explained by the crossover effect from the u...