QUANTITATIVE-ANALYSIS OF THE COLLECTIVE BEHAVIOR IN A MICROMAGNETIC MODEL

The basic features of the collective demagnetization processes (avalan ches) faking place in magnetically ordered systems are investigated in terms of a micromagnetic model of a textured polycrystalline material . The model, considering anisotropy, exchange and Zeeman as well as ma gnetostatic contributions to the;internal energy of the system, is cha racterized by the possibility of the simultaneous nucleation of differ ent avalanches (at different regions of the system) and by the occurre nce of time-dependent effects (associated with thermally activated dem agnetization). We have considered a cyclically driven system in which we have quantified the probability p(L) of nucleation of an avalanche of size L (given by the number of grains which reverse their magnetiza tion through such an avalanche). That probability depends on the time for which the system is allowed to relax at every field value, on the total size of the system, and on the model parameters measuring the in trinsic properties of the material. Depending on the ratio, r of a typ ical structural length (the grain size, measured in number of moments per grain) to the correlation length characterizing the magnetic momen t structure, we were able of detect subcritical (r much greater than 1 ), supercritical (r much less than 1), and critical (r similar or equa l to 1) demagnetization regimes. When the system is tuned (for instanc e, by varying its grain size) to the critical state, the size distribu tion of the avalanches is characterized by the occurrence of scale inv ariance with respect to the total size of the system. We have also inv estigated the statistics of the demagnetization process developing whe n the system is kept under a constant demagnetizing field. In this par ticular case and in addition to p(L), we obtained the function p(T) gi ving the probability of nucleation of an avalanche which propagates du ring a time T. Our results evidenced the presence of very long tails ( of the logarithmic type) in both the p(L) and the p(T) distributions, which were clearly correlated to the logarithmic relaxation of the tot al magnetization of the system.