Micromagnetism in the ultrathin limit

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In many physical systems, competing interactions lead to a frustrated state where the system decays into homogeneous subunits of characteristic extent and geometry. In ultrathin magnetic films, the competition between weak long-ranged magnetostatic interactions and strong short-ranged exchange interactions causes the magnetization to break into domains. This work presents an experimental study of ultrathin Fe films on Cu (001), where the domain structure is imaged using a Scanning Electron Microscope with Polarization Analysis. The domain structure evolves through transformations as temperature varies, mediated by topological defects. Images taken in close succession reveal interactions of these defects, highlighting an inverse melting process and a dynamic transition akin to a glass transition just below the Curie temperature. The domain patterns exhibit varying degrees of orientational and positional order. Additionally, the study explores the domain structure in laterally confined ultrathin particles with perpendicular magnetization, observing a single- to multi-domain transition as particle dimensions change. Unique to low-dimensional systems, magnetically charged domain walls arise from magnetization divergence at domain boundaries, which would incur significant energy costs in three-dimensional systems. The internal structure of these charged domain walls in thin iron films on W(110) is experimentally determined, reveali