Ultrafast Dynamics in Ferromagnetic and Synthetic Ferrimagnet Materials

Abstract

In this thesis, investigations into the ultrafast dynamics of magnetic materials are presented. The first of these is a study of the magnetisation dynamics predicted by an extended form of the Heisenberg model of ferromagnetism. The extension to the model consists of a reservoir of harmonic oscillators coupled to each spin, following from techniques used within the study of open quantum systems. The equations of motion are derived and the fluctuation dissipation relation is shown to be satisfied regardless of the specific form of coupling between the spins and reservoir. For an ‘Ohmic’ coupling in the limit of high temperature, the Landau Lifshitz Gilbert equation of spin dynamics with white noise thermal fluctuations is recovered. A ‘Lorentzian’ coupling is introduced which also converges to the Landau Lifshitz Gilbert dynamics under specific conditions. More generally, this form of coupling leads to different behaviour including non-Markovian dynamics and coloured noise thermal fluctuations. The equations of motion are then made semi-classical and numerically integrated under different conditions for a single spin system, highlighting the changes to both the dynamics and equilibrium behaviour predicted by a more general and quantum mechanically motivated model of spin dynamics. The second investigation looks at the properties of [Ni/Pt]/Ir/Co transition metal synthetic ferrimagnets, and their capacity for all optical switching. These materials and their constituent layers are characterised using wide field Kerr microscopy, bringing to light their complex and highly temperature sensitive properties. A laser is introduced to this setup and used to establish the conditions necessary for all optical switching of the magnetisation state of the synthetic ferrimagnets. In addition, time resolved Kerr microscopy measurements are presented, showing the magnetisation dynamics on both precessional and ultrafast timescales, and offering information on the mechanism behind all optical switching in these systems. The third investigation is on the development of microcoil devices for use in ultrafast time resolved measurement setups of magnetic materials. The characterisation of these devices is presented, along with magneto-optical measurements which show their potential for the generation of magnetic field pulses of sufficient strength to reset (i.e. saturate) magnetic samples between pulse-probe measurements.