Study of the influence of the lattice structure and the surface anisotropy on the behaviour of a small magnetic cluster

We have simulated by Monte Carlo method the thermal behaviour of a small (N=13 particles) cluster described by a Heisenberg model including nearest neighbours ferromagnetic interactions and radial surface anisotropy, in an applied magnetic field. We have studied three different lattice structures: hexagonal close packed, face centered cubic and icosahedral. We show that the zero field thermal behaviour depends not only on the anisotropy constant value but on the lattice structure as well. Concerning the behaviour in an applied field, it depends in addition, on the different orientations of the field with respect to the crystalline axes. These facts could be helpful to understand the dispersion observed in the deflection profiles of the Stern-Gerlach experiments. According to the orientation of the applied field with respect to the crystalline axes, hysteresis cycles show step-like character. PACS. 75.75.+a Magnetic Properties of Nanostructures – 75.30.Gw Magnetic Anisotropy – 02.70.Uu Application of Monte Carlo Methods