Departament d'Estructura i Constituents de la Matèria.
University of Barcelona
Spain
toni@ecm.ub.es
First-order phase transitions are expected to occur abruptly at given values of external control parameters such as temperature, pressure or applied field (stress, magnetic, electric, ...). In ferroic materials, however, they rarely show such an ideal behaviour. Commonly, the expected sharp change of the order parameter is smoothed out and thus the transition spreads over a certain range of the external control parameter. I will establish that disorder, in the form of randomly quenched defects, deviations from stoichiometry, impurities, etc..., is at the origin of this behaviour. The transitions to be considered are thus associated with the existence of high energy barriers that must be overcome for the transition to proceed. Indeed, in the coexistence region the actual state of the system is metastable and history dependent. In general, thermal fluctuations play only a minor role, which means that the time scale of thermal activation processes (which depends on temperature and on the effective height of the energy barriers) is typically very large compared to the time scale associated with the driving field. The transition kinetics shows therefore, an athermal character. Moreover, within a broad range of driving rates, the response of the system to the driving parameter shows avalanche behaviour. That is, the evolution of the transition is characterized by burst-like events separated by inactivity periods. Such events correspond to jumps from one metastable state to another involving dissipation of energy. I will show that in a number of cases the properties of the avalanches reveal a lack of characteristic scales within a broad range of durations, sizes and energies which suggests the existence of disorder-induced criticality in the hysteresis loops. I will illustrate these ideas with experimental results for systems undergoing magnetic and martensitic transitions.