Transition from superparamagnetism to ferromagnetic single-domain in a Heisenberg model for nano-cluster magnetic system

By considering a magnetic system of an ensemble of nanometer clusters witho ut any external magnetic field, the Monte Carlo simulation of the Heisenber g model is used to investigate the transition from a superparamagnetism to a ferromagnetic single-domain state, which is caused by the growth of the c lusters. We studied the variation of the dynamic growth exponent P and the transition critical size of the cluster N-c against the reduced temperature t and the uniaxial anisotropy constant A. We found that the growth exponen t maximized at a certain temperature, other than being taken as a universal constant as some researchers suggested previously.