Spin-flop transition in random antiferromagnets

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Abstract
The authors present a theory for treating randomly diluted antiferromagnets with uniaxial anisotropy in the presence of an arbitrary external field along the symmetry axis. In particular, they examine the behaviour of the interface between the antiferromagnetic and the spin-flop phases as a function of the dilution. It is found that the dependence of the critical field on magnetic concentration is steeper than indicated by virtual-crystal-mean-field arguments. Moreover, this dependence is found to be a strong function of the relative size of the anisotropy. The present theory uses a coherent potential ansatz for treating randomness in the exchange interactions but ignores dynamical effects occasioned by fluctuations in the local anisotropy field. Thus, its relevance would be limited to systems where the average anisotropy, D(2S-1), acting on a magnetic ion is substantially smaller than the average exchange, 2SzJ, experienced by the same.