PNRB salle TD1
Hysteresis is a complex phenomenon determined by the lag that can be observed between the input parameter, i.e. the applied magnetic field in magnetism and the output parameter, i.e. the magnetic moment of the sample. A distinction has to be made between the rate dependent hystereses, which appear only as an effect of lag vanishes for low-frequency measurements and rate independent hystereses which do not change if the field rate in the measurements is modified in a wide range of values. The rate independent hysteresis can be linked with the existence in the system of entities with metastable states. Each such entity has its individual hysteresis loop that depends on the particle’s shape, anisotropy, volume, etc. It is characterized by a free energy function that has for a definite domain of the input parameter two minima separated by a maximum. The behavior of an ensemble of particles will also display hysteresis that will be controlled not only by the hysteretic properties of each isolated particle but also by the interactions between particles.
The First Order Reversal Curves (FORC) method is a general, model-independent technique which provides a sensitive characterization of the interactions and domain behavior in materials with hysteresis with applications in physics, geology and technology. The FORCs are a specific class of minor hysteresis loops, for which the sweeping process of the input parameter is reversed once from one of the branches of the major hysteresis loop. This method allows a direct determination of a two-dimensional distribution (FORC diagram), usually of individual hysteresis width and interactions between domains.
A special area of interest in recent years was the application of and FORC technique as a tool to understand the hysteretic behavior spin crossover molecular magnets, materials that do show a complex nonlinear behavior. The spin crossover materials display in the two stable states (low spin and high spin) different magnetic properties (diamagnetic and paramagnetic), but do not illustrate a classical magnetic hysteresis. Nevertheless, due to elastic interactions, some of these compounds show a complex nonlinear behavior including temperature, pressure, and light-induced thermal hysteresis. We have applied the FORC diagram method for the thermal hysteresis of spin crossover materials and have shown that the diagram can be interpreted in terms of distributions of physical parameters such as the energy gap between the states, or interactions between like-spin domains. The FORC method applied on diluted spin crossover materials has suggested that distributions of internal stresses and domain size increase with dilution. In addition, we present experimental FORC data for rate dependent light induced hysteresis and for the pressure hysteresis. Finally, we discuss the models for all hysteresis and their correlations with experiments.