Topological Defects and Dipolar Magnetism in Magnetic Nanostructures
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In this work, magnetism in micro- and nanoscale magnetic structures has been explored. We focus on two different topics, topological defects and dipolar magnetism. As a model system for the study of interacting topological defects, we used a La0.7Sr0.3MnO3 (LSMO) square micromagnet with a Landau flux-closure ground state. A magnetic field of increasing magnitude was applied along the square diagonal, so as to drive the vortex core out of the structure and form a new stable ‘Z-domain’ state. The motivation to study this particular domain state transition originates from a previous study on patterned LaFeO3 (LFO)/LSMO bilayer structures. It was found that antiferromagnetic LFO imposes an exchange bias on the ferromagnetic LSMO, sufficiently large to drive formation of the Z-domain state. In the present work, we effectively simulate the presence of this bias by an external magnetic field. Combining micromagnetic simulations and magnetic force microscopy measurements, we show that this domain state transition occurs through creation and annihilation of topological (edge) defects. This finding is used to assess the stability of the Z-domain state, as well as to optimize switching of the magnetization direction in square micromagnets. Moreover, we numerically study the dynamic properties of the topological edge defects in this micromagnet. During demagnetization following removal of the external magnetic field, we find that the edge defects undergo exchange explosions, which lead to repeated bursts of short-wavelength spin waves. These exchange explosions occur upon annihilation of vortex/antivortex pairs close to the edge, injected by the edge defect. Such systems could prove useful for devices that convert Zeeman energy to short wavelength spin waves. When the dimensions of patterned thin film magnetic structures are reduced below a certain critical size, their magnetization will form a single domain state, producing a large magnetic stray field. In the second part of this thesis, we have investigated the emerging magnetism in ensembles of nanoscale single-domain disks coupled via their stray field, which is known to lead to dipolar magnetism. Antiferromagnetic ordering emerges in a nanodisk array with square lattice symmetry, whereas hexagonal lattice symmetry leads to ferromagnetic ordering. As such, dipolar magnetic systems are important candidates for metamaterials in which the magnetic order can be precisely tailored by adjusting the shape, size, and configuration of the individual nanomagnets that constitute the arrays. Studies on these metamaterial systems were performed both numerically and experimentally, using micromagnetic simulations and x-ray photoemission electron microscopy to find the magnetization in regular arrays of 100 nm diameter disks. In addition, we investigate finite size effects for such arrays and find that anisotropy arising from the array shape has little impact on the magnetization compared to anisotropies arising from deviation from perfect circularity (e.g., a minor disk ellipticity). In the dynamic case, these arrays can serve as magnonic crystals, where the dynamic properties, such as damping and magnonic bandgaps, can be tailored by tuning the size, the ordering symmetry, and the spacing between the nanodisks. We have studied the spin wave modes and damping properties in these arrays and found an enhanced damping for the spin wave modes known as edge modes.