The Landau-Lifshitz (LL) equation, originally proposed at the macrospin level, is increasingly used in Atomistic Spin Dynamic (ASD) models. The models are based on a spin Hamiltonian featuring atomic spins of fixed length, with the exchange introduce
d using the Heisenberg formalism. ASD models are proving a powerful approach to the fundamental understanding of ultrafast magnetisation dynamics, including the prediction of the thermally induced magnetisation switching phenomenon in which the magnetisation is reversed using an ultrafast laser pulse in the absence of an externally applied field. The paper outlines the ASD model approach and considers the role and limitations of the LL equation in this context.
There has been much interest recently in the discovery of thermally induced magnetisation switching, where a ferrimagnetic system can be switched deterministically without and applied magnetic field. Experimental results suggest that the reversal occ
urs due to intrinsic material properties, but so far the microscopic mechanism responsible for reversal has not been identified. Using computational and analytic methods we show that the switching is caused by the excitation of two magnon bound states, the properties of which are dependent on material factors. This discovery allows us to accurately predict the switching behaviour and the identification of this mechanism will allow new classes of materials to be identified or designed to use this switching in memory devices in the THz regime.
