Vortex and antivortex structures in stadium-shaped, magnetic particles

Magnetic force microscopy with applied magnetic fields has been used to develop an understanding of how two separate modifications influence the confined magnetic vortex structure in small magnetically soft particles. The first modification is the addition of an anisotropy energy aligned with the long axis in cylindrically shaped particles. The anisotropy energy induces domain patterns that are different than the usual structures observed in the confined vortex system. Micromagnetic simulations show that the structures observed in the measurements are higher energy states. The second modification is an elongation along the diameter of the cylindrical particles. These stadium shapes permit the nucleation of the multiple vortices and antivortices. Either a single or double vortex state can be nucleated based on the direction of the applied in-plane field. Once the particles are initialized into the double vortex state, the separation between the vortex pair may be controlled with an applied field. As the field is increased, the vortices move toward a critical separation. For larger applied fields the vortex-vortex separation increases and the pair rotate around each other until their eventual annihilation. For particles initialized into the single vortex state, a second nucleation event may occur in particles elongated further than those that show a single vortex a remanence. A vortex antivortex pair nucleates, and a structure is observed at remanence which include two vortices and an antivortex. Application of an applied field causes an annihilation between one of the vortices and the antivortex. At higher applied fields the single vortex annihilates. This system, with the antivortex, is a novel system for the study of cross-tie domain walls, and the nucleation of cross and circular Bloch lines. An explanation of the nucleation and annihilation of the vortex-antivortex pairs is given within this framework.