Two magnon bound state causes ultrafast thermally induced magnetisation switching

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 occurs 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.