A Weakly Anisotropic Ising Model in a Random Field

Abstract

The application of a uniform field to a random antiferromagnet produces a random staggered field [1], and so enables the effect of applying a random field to be studied experimentally in a convenient and controllable manner. Detailed experiments have been performed on the good Ising models Rb₂Co_xMg_1−x F₂ [2], Co_xZn_1−xF₂ [3], and Fe_xZn_1−xF₂ and as reviewed in detail elsewhere [4], the results reveal a number of unexpected features which are still only partially understood. In particular when the samples are cooled from the paramagnetic phase in an applied field, the system shows unusual behaviour, characterised by a Lorentzian squared profile in the quasi-elastic scattering cross-section, and a lack of long range order. In contrast if the samples are cooled in zero applied field, and the field applied at low temperatures, the long range antiferromagnetic state persists. On heating, the long range order persists, in the case of the three-dimensional systems, up to the phase transition temperature.