Monte Carlo study of the magnetic behavior of self-assembled nanoparticles

Summary form only given. We study numerically the magnetic properties of self-assembled Co nanoparticles that form hexagonal arrays on a planar substrate and investigate the role of inter-particle dipolar interactions. A perfect two-dimensional hexagonal array and arrays with structural defects, such as voids and an unfinished top layer are modeled. The nanoparticles are treated as Stoner-Wolfarth particles with a random anisotropy axis and coupled via dipolar forces. The magnetic configuration is obtained by a Monte Carlo simulation technique. The zero-field cooled and field-cooled magnetization curves of the array are obtained and the blocking temperature of the system is extracted. Dipolar interactions in a perfect two-dimensional hexagonal array lead to a ferromagnetic ground state and to an increase of the blocking temperature. However, the blocking temperature has a nonmonotonous dependence on the coverage of the second layer which is attributed to the coupling of the ferromagnetic bottom layer to the incomplete top layer. In addition, the remanence and coercivity of the array at temperatures below the blocking are studied and a similar non-monotonous dependence on the coverage is found. The effects of the unfinished top layer are maximized around 50% coverage, which corresponds to the percolation threshold for a hexagonal lattice.