Evolution and propagation of magnetic vortices in chains of Permalloy nanospheres

Abstract

Magnetization reversal in a chain of Permalloy Fe0.2Ni0.8 spheres, whose diameters of 40–60nm are large enough to support vortex structures, is investigated using micromagnetic modeling based on the Landau-Lifshitz-Gilbert equations. The emphasis is on a chain of two spheres with fields applied along and perpendicular to the axis of rotational symmetry. Magnetization processes and critical fields are given for (1) inversion symmetry, with opposite senses of vortex rotation in the two spheres, and (2) a more uniform curling mode, with the sense of rotation the same in both spheres. Symmetry breaking perturbations are shown to be important in the nucleation of changes from one magnetic configuration to the next. As the geometry is approximated by cubic grid cells, whose centers lie within the boundaries of the ideally smooth surfaces, the critical fields are influenced by the grid size. The results for two spheres are generalized in the description of a chain of n spheres, in which at least 2n symmetry states can be selected by the application of inhomogeneous fields.