Magnetization reversal dynamics in clusters of single domain Ni nanoparticles

We present the magnetization reversal dynamics of clusters of single domain nickel nanoparticles. Experimental results of magnetization reversal of nickel nanoparticles are completely different from that of bulk nickel and single nickel nanoparticles in terms of the overall shape, coercive field, and the saturation magnetization. Simulations show that a cluster consists of minimum 5 x 5 x 3 nanoparticles with no physical overlap between the particles reproduces the loop shape and the coercive field but not the saturation magnetization. However, the same cluster with partially overlapped nanoparticles reasonably reproduces all features of the magnetization reversal due to the presence of both magnetostatic and exchange interactions between the particles. Simulated magnetization images show that the reversal of the cluster occurs through the formation of a C-like state, followed by the creation and annihilation of a vortex like structure, an inverse C-state, and finally, a fully saturated reversed magnetic state. Additional simulations on random clusters show lower coercive field and nearly bulklike saturation magnetization, which are much different than the experimental results. This suggests that the experimental clusters are not totally random or periodically arranged but have a certain amount of ordering. (C) 2010 American Institute of Physics. [doi:10.1063/1.3359422]