Hysteresis, Barkhausen noise, and disorder induced critical behavior

Hysteresis loops are often observed in experiments at first order phase tra nsformations when the system goes out of equilibrium. They may have a macro scopic jump, roughly as seen in the supercooling of liquids, or they may be smoothly varying, as seen in most magnets. The nonequilibrium zero-tempera ture random-field Ising-model can be used to model hysteretic behavior at f irst order phase transformations: as the disorder is decreased, one finds a transition from smooth hysteresis loops to loops with a sharp jump in magn etization (corresponding to an infinite avalanche). In a large region near the transition point the model exhibits power law distributions of noise (a valanches), universal behavior and a diverging length scale, Universal prop erties of this critical point are reported that were obtained using renorma lization group methods and numerical simulations, Connections to experiment al systems such as athermal martensitic phase transitions (with and without "bursts") and the Barkhausen effect in magnetic systems are discussed.