STATIC AND DYNAMIC CRITICAL BEHAVIOR IN RANDOM MAGNETS

Abstract
Experiments in recent years of critical behavior in random magnets are reviewed. Attention is focused on the random exchange (REIM) and random field (RFIM) Ising model problems, as realized in dilute, insulator antiferromagnets. The static exponents of the d = 3 REIM universality class are now measured and agree with theory. REIM to RFIM crossover scaling has been shown to govern the static and dynamic critical behavior in all d = 3 systems (Fe_xZn-_1-xF₂, Mn_xZn_1-xF₂ and Fe_xMg_1-x Cl₂ studied, with a universal exponent φ = 1.42±0.03, which satisfies the Aharony inequality φ > γ, the REIM susceptibility exponent. So extreme is the slowing down of the critical fluctuations in d = 3 RFIM systems that it is at the roots of earlier misunderstandings of what was the lower critical dimension of the RFIM and may well limit the accuracy to which the static exponents of the d = 3 RFIM universality class may be obtained. Only qualitative models and limited Monte Carlo calculations exist with which to make comparison with experiment.