Effects of surface anisotropy and vacancies in ferromagnetic nanoparticles

The effect of surface anisotropy and vacancies upon the ground magnetic state of ferromagnetic nanoparticles is discussed. Our study is based on a random site-diluted classical Heisenberg Hamiltonian with nearest-neighbor interactions, surface and core anisotropies, and a Monte Carlo-Metropolis approach with simulated annealing for energy minimization. Results reveal severe variations with respect to a single domain phenomenology and wider ranges of metastability resembling a multi-valley energy landscape as vacancies are considered and surface anisotropy increases