Ultrafast demagnetisation dynamics in multi-component systems

Abstract : Excitation of multi-component materials by femtosecond laser pulses reveals interesting transient states, which could have an impact on the final physical configuration of the system. Using time-resolved magneto-optical Kerr effect in the pump-probe configuration, we evaluate the effect of fundamental parameters such as exchange interaction and spin currents on sub-picosecond dynamics of magnetisation in exemplary systems: (i) Copper-doped ferromagnetic alloys and, (ii) ferromagnetic bilayers separated by a conducting spacer layer.In Copper-doped NiFe, the observed anomalies in dynamics reveal that Ni moments are suppressed stronger than Fe moments. On the other hand, Cu-doped FePt exhibits a transient state which lasts for 150 ps with its magnetisation oriented opposite to an externally applied restoring magnetic field. This effect, seen as a negative Kerr signal, is also seen in the multilayer system of Ni and FePt films separated by Ru spacer. Such systems are potential candidates for all-optical switching of magnetisation. Additionally, we observe that the sub-picosecond demagnetisation of the Ni film is enhanced due to spin curents when the orientation of it’s magnetisation is non-collinear than when collinear with respect to the magnetisation of FePt. These results help to understand the nature of the interaction between magnetisation and spin currents in the scenario of ultra fast demonetization.