Random-field effects in Fe1−xMgxCl2

Abstract

We analyze in detail how random fields are generated in a site‐random uniaxial antiferromagnet when a uniform magnet field is applied. Explicit expressions are given for the mean‐square random field. We also report on a systematic specific heat study of two three‐dimensional Ising systems, pure FeCl₂ and site‐random Fe_0.682Mg_0.318Cl₂ (MG318), in constant applied fields. The contrasting behavior of the two provide a vivid demonstration of random‐field effects. The unusual changes in the shape of the specific heat anomaly observed in MG318 are explained as crossover behavior and not a manifestation of new critical behavior. The crossover exponent φ associated with the random fields is determined from the shape of the magnetic phase boundary. We find φ=1.25±0.10, in excellent agreement with theoretical prediction.