Metropolis algorithm for simulating hysteresis in ferromagnetic nanoparticles

We report of Monte Carlo simulations by Metropolis algorithm of the magnetization cycles of ∼3 nm non-interacting ferromagnetic amorphous nanoparticles. Simulations based on a core-shell model, describing the nanoparticles as consisting of a ferromagnetically ordered core and a disordered surface shell, reproduce satisfactorily the significant features observed in experimental magnetization measurement and hysteresis cycles, i.e. HC(T)HC(T) and Mr(T)Mr(T) curves. The results provide a clear evidence of the major role played at low temperature by the surface anisotropy and surface–core exchange interaction in determining some anomalies of the hysteresis cycles and the large increase of the effective particle anisotropy at low temperature.