Computational model of the magnetic and transport properties of interacting fine particles

A computational model is applied to the study of the hysteresis properties of a system of interacting single domain particles. The model is based on Monte Carlo techniques and takes into account both magnetostatic and exchange interactions. The results presented concentrate on a detailed study of the behavior of Co particles, with the interaction strength varied by variations in the packing density. It is found that the magnetic properties are strongly dependent on the parameter $\beta =KV/kT,$ with K the anisotropy constant and $V$ the mean particle volume. For small β—i.e., close to superparamagnetic systems—the microstructure is dominated by a tendency to flux closure. However, the interactions lead to an increase in the local energy barriers, resulting in an increase in $H_c$ with packing density ε. For large β the anisotropy and magnetostatic interaction fields become comparable and the competition leads to a decrease in the coercivity $H_c$ with ε. For intermediate values of β a maximum in the variation of $H_c$ with ε is predicted. The irreversible susceptibility is shown to have a complex dependence on interactions, especially in small fields where frustration effects arising from the competition between exchange and magnetostatic interactions are apparent. Exchange and magnetostatic interactions give rise to local magnetic order which is strongly dependent on the relative strength of the exchange interactions. The magnetic order has a strong bearing on the magnetic properties. A link is also made to the transport properties of the system, which are dependent on a spin-spin correlation function. It is shown that exchange interactions give rise to a significant deviation from the quadratic dependence of the giant magnetoresistance on $M^2.$