Monte carlo simulation of magnetization and coercivity of free magnetic clusters

Based on Monte Carlo method, the oscillatory behaviour of theaverage magnetic moment as a function of the cluster sizes and thetemperature dependences of magnetic moment with different sizes havebeen studied. It is found that the oscillations superimposed on thedecreasing moment are associated with not only the geometricalstructure effects but also the thermal fluctuation. The hysteresesand thermal coercivities for free clusters with zero and finiteuniaxial anisotropies have been calculated. The simulated thermaldependence of the coercivity is consistent with the experimentalresult, but does not fit the T^α law in the whole temperaturerange. It is evident that an easy magnetization direction and ananisotropy resulting from the spin configurations exist in the freeclusters with the pure exchange interaction, which is also proved bythe natural angle and energy distribution of clusters. A systematictheoretical analysis is also made to establish the relationshipbetween natural angle and coercivity.