Dipolar Interactions in Superparamagnetic Nano-Granular Magnetic Systems

In this paper, the effect of a symmetric distribution of dipolar fields acting along the applied field on the magnetization curve of a superparamagnetic nano-granuler system is examined. The calculations show that the magnetization curves are depressed when the spread of dipolar fields is increased. This reduction in magnetization is attributed to the nonlinear response of magnetization which weighs negative dipolar fields more strongly than the positive dipolar fields. Thus, the reduction in magnetization is not indicative of a predominant negative interaction field, since positive and negative fields are equally probable. Also, for the system examined, the nonlinear response of magnetization is responsible for the predicted negative average ordering temperature in the temperature variation of the initial susceptibility. Thus, in random systems, the interpretation of a negative ordering temperature as antiferromagnetic order, or the positive ordering temperature as a ferromagnetic order should be cautiously stated.