Random exchange and random field ising systems: Static and dynamic critical behavior

Abstract

Theoretical and experimental studies of random magnets have generated considerable interest and controversy during the last decade. This critical review attempts to clarify experimental aspects of the REIM and, particularly, the RFIM problems in dilute Ising antiferromagnets. It has been shown that REIM to RFIM crossover scaling governs both static and dynamic critical behavior in all d = 3 systems (Fe x Zn 1− x F 2, Mn x Zn 1− x F 2 and Fe x Mg 1− x Cl 2), as well as nonequilibrium behavior, with a universal exponent φ = 1.42 ± 0.03 which satisfies the Aharony inequality φ#62; γ, the REIm susceptibility exponent. Extreme critical slowing down is observed in the d = 3 RFIM case. The behavior of RFIM systems at the lower critical dimension d ℓ = 2 differs in almost every respect from that d = 3 #62; d ℓ, contrary to what was claimed in the earliest neutron experiments and is still popularly believed.