Hysteresis and avalanches in disordered systems

Rate-independent hysteresis is studied in magnetic systems driven by an ext ernal field for which the influence of thermal fluctuations is negligible. In such systems, the hysteresis cycles are not continuous, but rather are c omposed of a sequence of magnetisation jumps or avalanches between metastab le states; the so-called Barkhausen noise. The study of the statistical dis tribution of such avalanches provides an alternative description to the mor e common procedure of measuring properties of the loop shape. We focus on f our different zero-temperature 3d lattice models: the random field Ising mo del, the random bond Ising model, the site-diluted Ising model and the rand om anisotropy Ising model. By defining appropriate local dynamics, we have studied the metastable evolution by numerical simulations. We analyse the a valanche size distribution as a function of the degree of quenched disorder in these systems. For specific amounts of disorder, the distributions exhi bit critical behaviour that can be characterised by universal exponents. (C ) 2000 Published by Elsevier Science B.V. All rights reserved.