Hysteresis of cobalt nanoparticles organized in a two-dimensional network:dipolar interaction effects

The hysteresis curve at very low temperature of Co nanoparticles organized in a two-dimensional network on a graphite substrate is determined both exp erimentally and from a numerical calculation. The Co particles are modeled as spherical particles of uniaxial symmetry and the easy axes are randomly distributed. We focus on the effect of the dipolar interactions between par ticles on the hysteresis loop: the magnetization curve is thus compared to the case of non-interacting particles, which corresponds to particles dispe rsed in solution at vanishing concentration. The magnetization curve is cal culated for two orientations of the applied field: normal and parallel to t he substrate surface. The ratio between the remanence magnetizations obtain ed with the field, normal and parallel to the surface, appears to be a conv enient parameter to estimate the importance of the dipolar interactions. A satisfactory agreement with experiment is obtained for this ratio. (C) 2000 Elsevier Science B.V. All rights reserved.