Open Access

Dennis Nissen

• The present thesis deals with the investigation of magnetic cap structures showing a vortex. For this, commercially available spheres (silica or polystyrene) have been ordered in a two-dimensional arrangement by self-assembly processes. By deposition of soft magnetic films on top of the spheres, we achieved large arrays of densely packed magnetic cap structures characterized by a magnetic vortex. The first part of this thesis focuses on the investigation of permalloy cap structures, with a diameter of about 330 nm. Here, mainly the magnetization reversal of the structures in a two-dimensional lattice was investigated. In particular, we present the dependence of the nucleation and annihilation fields as a function of permalloy thickness and temperature. We show that a critical thickness is required to stabilize a magnetic vortex in the cap structures. Furthermore, we could show that an increase of the film thickness is accompanied by an onset of exchange coupling of the capThe present thesis deals with the investigation of magnetic cap structures showing a vortex. For this, commercially available spheres (silica or polystyrene) have been ordered in a two-dimensional arrangement by self-assembly processes. By deposition of soft magnetic films on top of the spheres, we achieved large arrays of densely packed magnetic cap structures characterized by a magnetic vortex. The first part of this thesis focuses on the investigation of permalloy cap structures, with a diameter of about 330 nm. Here, mainly the magnetization reversal of the structures in a two-dimensional lattice was investigated. In particular, we present the dependence of the nucleation and annihilation fields as a function of permalloy thickness and temperature. We show that a critical thickness is required to stabilize a magnetic vortex in the cap structures. Furthermore, we could show that an increase of the film thickness is accompanied by an onset of exchange coupling of the cap structures due to emergence of bridges connecting them, caused by closely packing of the underlying particles. Surprisingly, for the coupled structures we observed a domain like nucleation process of vortex structures with domains showing the same circulation sense. Additionally, increasing film thickness results in spatially enlarged vortex cores. Furthermore, we also investigated the response of a vortex to an external field. In particular, we investigated the vortex core switching in a lattice of closely packed cap structures by a global out of-plane field. Furthermore, we could also show that individual cores can be switched by a local field pulse using a scanning magnetoresistive microscope. The second part of this work concentrates on the investigation of exchange biased magnetic vortices in cap structures. Therefore, in addition to a soft magnetic layer promoting the formation of a vortex, an antiferromagnetic layer has been deposited on the caps. In this study, we mainly investigated the magnetization reversal as a function of temperature and cooling field. Overall, we could show that zero field cooling results in the stabilization of the vortex structure due to imprinting of the vortex spin structure into the antiferromagnet. For samples with CoO as antiferromagnet, we found a strong athermal training effect after field cooling. Furthermore, the magnetization reversal occurs via formation of a distorted viscous vortex. We also investigated the effect of the blocking temperature. In particular, for IrMn as antiferromagnet, we could show that approaching the Néel temperature results in an increase in coercivity.…