Surface anisotropy in maghemite nanoparticles

In this work we address the effect of surface anisotropy upon the magnetic structure of ferrimagnetic maghemite gamma-Fe2O3 nanoparticles in the limit of low temperatures. Our model is based on a three-dimensional classical Heisenberg-Hamiltonian involving Fe3+ nearest-neighbor interactions, surface and core anisotropies, and a Monte Carlo-Metropolis approach via simulated annealing for energy minimization. Results reveal a marked decrease of the Curie temperature of the considered nanoparticle with that obtained of a bulk maghemite, as well as severe variations with respect to a single domain phenomenology as surface anisotropy increases. (c) 2006 Elsevier B.V. All rights reserved.