Critical behavior in random anisotropic antiferromagnets

Abstract

The role of random uniaxial anisotropy on critical behavior has been studied in the antiferromagnetic system FexMn1−xF2 using linear magnetic birefringence. In the pure FeF2 and MnF2 crystals the deduced magnetic specific heat near TN shows no rounding for reduced temperature t as small as 10−4. Except for the effects of concentration gradients, no rounding is observed in the mixed crystals. In fact, for the Fe.75 Mn.25F2 sample, where such gradient effects are minimal, the transition is as sharp as in pure FeF2. Since MnF2 and FeF2 differ primarily in the size of the single ion anisotropy, it is clear that random anisotropy, by itself, does not broaden the transition. If anisotropy were not considered, MnF2 and FeF2 would be expected to have nearly identical TN. Although MnF2 is itself asymptotically Ising-like as T→TN because of its small but finite anisotropy, the larger TN in the more anisotropic FeF2 and the drastic increase in TN with x near x = 0 is attributable to the effects of crossover from Heisenberg to Ising critical behavior.