Monte Carlo studies of hysteresis curves in magnetic composites with fine magnetic particles

The hysteresis of composite systems consisting of nanosized ferromagnetic p articles embedded in a nonmagnetic metal matrix is studied using the Monte Carlo method. In the model, the nanosized particles are assumed to be singl e domain and to possess random uniaxial anisotropy. The particles are well dispersed so that the interactions between them are weak and can be neglect ed. These particles may be superparamagnetic (unblocked) or not (blocked) d epending on the temperature T, energy barrier DeltaE, and measurement time tau (m). It is found that the hysteresis loop, which is obtained even in th e absence of interparticle interactions, is mainly caused by the blocked pa rticles as a result of the irreversible transitions between states of minim um energy. The remanence and coercivity of such systems decrease as T incre ases and disappear at high T. The results are in agreement with recent expe rimental observations. (C) 2001 American Institute of Physics.