Atomistic Models of Magnetic Systems with Combined Ferromagnetic and Antiferromagnetic Order

There has long been an interest in the exploitation of novel magnetic behaviour for practical applications such as magnetic storage devices. Some of the most interesting dynamical behaviour occurs when a material contains both ferromagnetic (FM) and antiferromagnetic (AF) characteristics. Many such systems have forgone study due to practical difficulties of experimental observation of antiferromagnetic order. In this work several systems of current interest which contain both AF and FM order are studied. These materials and systems are used, or are candidates for, technological applications, especially magnetic storage devices. The forefront in this area is concerned with laser induce magnetisation reversal and there are many unexplained phenomena, especially in ferrimagnetic and metamagnetic materials. A combination of analytical and large scale numerical calculations are used, often with comparison to experimental data where available. The approach used is generally based around so-called atomistic spin dynamics, where the Landau-Lifshitz-Gilbert equation, augmented with a Langevin term, is solved for each atomic moment. This allows the description of magnetic materials at elevated temperatures and through phase transitions. Semi-analytic formalisms are studied, comparing with atomistic spin dynamics and micromagnetics, to inform multiscale modelling techniques. The excitation of a localised mode an antiferromagnetic layer which is coupled to a ferro- magnetic layer is studied. It is shown that this excitation leads to an enhanced damping of the ferromagnet, an important consideration for the design and optimisation of spin valves. The metamagnet FeRh which undergoes an antiferromagnetic-ferromagnetic phase transition is also investigated. There is much debate about the origin of the phase transition and a model is constructed in this work which demonstrates that an all magnetic origin is possible if effective four spin exchange terms are considered. This model is also capable of explaining the observed dynamics in femtosecond laser heating experiments. Finally, the spin wave dynamics of the prototypical amorphous ferrimagnet GdFeCo are considered. The thermally induced switching which has been discovered in this material is explained as the excitation of a two-magnon state.