DISORDER-DRIVEN 1ST-ORDER PHASE-TRANSFORMATIONS - A MODEL FOR HYSTERESIS

Hysteresis loops in some magnetic systems are composed of small avalan ches (manifesting themselves as Barkhausen pulses). Hysteresis loops i n other first-order phase transitions (including some magnetic systems ) often occur via one large avalanche. The transition between these tw o limiting cases is studied, by varying the disorder in the zero-tempe rature random-field Ising model. Sweeping the external field through z ero at weak disorder, we get one large avalanche with small precursors and aftershocks. At strong disorder, we get a distribution of small a valanches (small Barkhausen effect). At the critical value of disorder where a macroscopic jump in the magnetization first occurs, universal power-law behavior of the magnetization and of the distribution of (B arkhausen) avalanches is found. This transition is studied by mean-fie ld theory, perturbative expansions, and numerical simulation in three dimensions.