Monte Carlo simulation of magnetic ordering of quasi-one dimensional Ising antiferromagnets CsCoBr3 and CsCoCl3

We make a realistic Monte Carlo simulation of the magnetic phase transition of quasi-one dimensional hexagonal antiferromagnets CsCoBr3 and CsCoCl3 using a cluster heat bath method. Results reproduce experimental observations of anomalous behaviors of magnetic quantities around two temperatures T-N1 and T-N2 (<T-N1). In particular, the rapid increases of the magnetic structure factors F(1/3, 1/3, 1) and F(1, 1, 1) around T-N2 without any anomaly of the specific heat are demonstrated. In addition, the mysterious increase of F(1, 1, 1) just below TN1 is reproduced. We suggest that the long-range order phase appearing below T-N1 is a Single phase described by three sublattices, and the change in the magnetic structure around T-N2 is a. crossover between a partially disordered antiferromagnet-like phase at higher temperatures and a ferrimagnet-like phase at lower temperatures.