Novel Characterisation and Modelling Techniques for Spintronic Materials

Abstract

Spintronic materials have the potential to revolutionise certain aspects of information technology. An ideal material for spintronics is a zero-moment half-metal (ZMHM), which exhibits no stray magnetic field but simultaneously intrinsic spin-polarised conduction. The first ZMHM, Mn_2Ru_xGa from the Heusler alloy family of materials, was discovered in 2014 and has been studied intensively over the last decade. With many of the static properties such as magnetisation and spin-polarisation now relatively well-understood, the focus here has been to develop a model and physical understanding for the dynamic magnetic properties which are very important for most of the interesting and promising device applications. To this end, the work has been split between computer simulations and experimental material characterisation. The importance of the relationship between crystal structure and the material properties which control the magnetodynamics cannot be understated, this relationship is introduced and explained as a motivation for the work. Computational models for simulating magnetodynamics have been developed, ranging from a simple classical Heisenberg spin-dynamics model to a fully-fledged semi-classical Ehrenfest dynamics model which accounts for the conduction electrons, atomic spins and nu- clei. Some results demonstrating the capabilities of these models are presented and the future goals are outlined: we wish to investigate quasi-particle interactions such as plasmon-magnon and magnon-phonon scattering with the eventual goal of properly physically simulating a full route for externally introduced energy (via photons etc.) to transfer through the different subsystems of the material before dissipating into the surroundings. Characterising these materials, especially their crystal structure, is vital to elucidating the physical reason for observed effects. Having emphasised the strong dependence of magnetic properties on inter-atomic separation, different diffraction-based techniques are described for probing the crystal structure as a function of the deposition conditions. Vertical strain-profiles and tetragonal distortion of thin film samples are among the features observed and related to the material properties. Another technique for which an apparatus has been constructed is M?ssbauer spectroscopy. This technique, in the current iteration, only measures static properties of Fe-containing materials, but nonetheless provides crucial information about local order and magnetic properties in those materials. The development of this system allows for non- destructive probing of the local hyperfine parameters of Fe in a wide range of samples, from amorphous ?m-sized ribbons to a slab from a FeNi meteorite, with a fully quantum model used to fit obtained data. Finally, I conclude what has been accomplished in terms of developing a complete dynamics model for this class of spintronic materials and some complementary experimental techniques, with a view to the future work that stands to be taken on.