Phase Transitions, Metastable States, and Random Fields

Random fields have a drastic effect on the development of long range order and phase transitions of materials. The most convenient experimental realisation of random fields is provided by the application of a uniform field to a random Ising antiferromagnet. Experiments have now been performed on a variety of two and three dimensional systems, here we review recent neutron scattering measurements from the three dimensional system, Mn_xZn_1-xF₂. The results show that at low fields there is a well defined metastability boundary in the (H, T) plane, which separates a high temperature paramagnetic phase from a low temperature phase in which there are many metastable states. The critical phenomena close to this boundary is discussed; the results are consistent with the theories suggesting that there is either a continuous transition inhibited by anomalously slow dynamics or a first order transition. The effect of random fields produced by defects on the structural phase transitions of BaMnF₄ and RbCaF₃ is briefly reviewed.