Easy-axis antiferromagnets with intermediate anisotropy

Abstract

The easy-axis Heisenberg antiferromagnet in a longitudinal field is studied for the case of anisotropy strengths in the intermediate range. In the low-anisotropy limit the system is known to have a spin-flop phase and a bicritical point; for large anisotropy it may become an Ising-like metamagnet with a tricritical point. Mean-field predictions for the system are reviewed, and it is pointed out that a class of real magnetic materials, namely certain alkylammonium metal halides with weak antiferromagnetic coupling, may be candidates for intermediate behaviour, having both a spin-flop phase and a tricritical point. Next, the problem of bicritical to tricritical crossover is considered in a renormalisation-group context. For the cases of uniaxial and orthorhombic anisotropy, it is found that the biconical fixed point of Nelson, Kosterlitz and Fisher controls the behaviour of the system at the multicritical point that arises when the bi- and tricritical points coincide.