Magnetic and Thermal Phenomena in Topologically Constrained Systems Explored by Advanced Scanning Probe Microscopy Techniques

Abstract

As “big-data” becomes the norm the demand for improved, energy-efficient storage and computation is valuable and never satisfied. Magnets and their properties are highly favourable for this problem and it has driven research into novel architectures and the resulting effects. Scanning-probe microscopy (SPM) is ideal for visualising physical properties of nanoscale systems as SPM is cheap, modular, and possess high spatial resolution (~ 20 nm). Instrument modifications and image processing means complex architectures can be investigated. This thesis uses these techniques to explore nanomagnetic systems with promise in novel computation.First, a patterned magnetic probe was designed for customised magnetic force microscopy (MFM) measurements and evaluated against commercial equivalents. The novel probes revealed advantageous properties for imaging heterogeneous samples. Subsequently, a novel SPM method was developed for characterising spin textures in a nanowire with high magnetic susceptibility by inducing spin caloritronic effects with a joule-heated SPM probe. Both methods give precedentfor characterising challenging magnetic samples for applications including magnetic storage. Following this, local behaviours in artificial spin ice (ASI) are investigated by advanced MFM and supplementary techniques. ASI are 2D systems that exhibit collective dynamics and emergent monopoles, which have application to reconfigurable magnonics and probabilistic computation. Here, the formation of non-Ising states in a novel ASI lattice were shown to form and randomly distribute across the array deterministically and their frequency was highly tunable. The strayfield of many frustrated vertices was examined by quantitative-MFM and electron holography, which revealed that even periodic configurations were energetically disparate and influences the non-Ising state properties. Finally, artificial defects in an ASI lattice were shown to influenceneighbouring vertices by injecting monopoles into the lattice. Their characteristics are compared to those that form independently by in situ MFM and Lorentz microscopy. The described effects have implications for devices aimed towards energy-efficient storage and computation