Magnetisation dynamics and tuneable GHz properties of unsaturated magnetic nanostructures

Abstract

In this Thesis, investigations on spin wave propagation in sufficiently thick, magnetic unsaturated nanostructures of various shapes have been carried out. Analytical, numerical and experimental techniques in the time and the frequency domains have been used throughout this thesis. Unsaturated magnetic states are of high interest, more specifically the magnetic vortex configuration since the remanent state is typically dominated and stabilised by the shape anisotropy of the structure, with no need of strong or any bias fields. Due to the diversity of magnetic inhomogeneities, a high degree of reconfigurability by applying external low bias fields can be obtained. On the other hand, the inhomogeneous magnetic landscape becomes, very often, difficult to model analytically and therefore, propagating spin waves can turn out to be complicate to control in practice. The aim of this work has been twofold. Firstly, to explore the most prominent magnetisation dynamics found in thick enough magnetic patches, or ‘2.5-dimensional’ nanostructures. The explored propagating modes are exchange-dominated spin waves in the range of GHz, which allows us to obtain very short wavelength spin waves that can propagate along different ‘paths’ in the unsaturated landscape. The ‘thickness and shape-induced’ enhancing of these spin waves in structures in a flux closure configuration, suggests their use as highly tuneable spin wave emitters. Secondly, analytical and mathematical models are proposed for controlling the spin wave propagation in multidomain structures of various shapes and in domain walls. Their magnetic configuration is dominated by the shape anisotropy of the patch, which allows us to design particularized shapes to control the spin wave wavenumber (or equivalently, wavelength) while it propagates. Also, the modes are shown to be sensitive to bias fields, which further enhances their tuneability and reconfigurability. All this previous work is joint in the last part of the Thesis, more focused on potential applications for Magnonics, where spin waves are not necessarily seen as undesirable energy loss mechanisms in magnetic structures but as information carriers or the base of novel computing paradigms. Single, or interconnected unsaturated elements of various shapes are proposed to be the base of interesting highly tuneable spin wave devices.