RANDOM FIELD SYSTEMS

Spin models in weak random fields are good representations of a large number of impure compounds which undergo magnetic or structural transitions. From a theoretical point of view these systems exhibit frustration on a mesoscopic scale and take therefore an indermediate position between (spin) glasses and unfrustrated (diluted) disordered systems. A simple but rigorously proven domain argument shows that long range order is not destroyed by weak random fields in more than two dimensions if the order parameter has a discrete (e.g. Ising) symmetry. The characteristic interaction energy between the spins and the frozen-in defects increases as Lo with the length scale L so that the static random-field-induced fluctuations dominate over the dynamic thermal fluctuations. Consequently, the critical behaviour is described by a T = 0 fixed point of the RNG transformation. This leads to pronounced non-classical critical exponents, a modified hyperscaling v(d - @) = 2 - a and an activated critical dynamics. Domain wall roughening due to random fields produces metastability and hysteresis. The predicted logarithmic time dependence of the radius of the metastable microdomains sets in after a characteristic time which depends on the details of the pinning mechanism and may exceed the age of the universe. © 1990, Taylor & Francis Group, LLC. All rights reserved.