The Core-Shell Separation of Ferromagnetic Nanoparticles with Strong Surface Anisotropy

A modified Monte Carlo Metropolis method is performed to simulate the low-temperature magnetization behavior of the ferromagnetic nanoparticles with different sizes and surface anisotropies, based on three-dimensional classical Heisenberg model. It is found that an insulating layer appears to cut off the interaction between the surface and the core as the surface anisotropy is strong enough. The results also indicate that the magnetizing state of nanoparticles may change from a collinear structure into a throttled, then into a hedgehog-like, and finally into a separation structure with the increase of surface anisotropy. However, the hedgehog-like structure does not appear in the bigger size nanoparticles. The energy competition between the surface and the core in the ferromagnetic nanoparticles may be the reason for producing these phenomena.