Ultrashort magnon pulses and phonon-magnon interactions

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Investigation of ultrafast dynamics in nanostructures is crucial to overcome the speed limits in modern data recording and telecommunication technologies. Whereas ultrashort laser and acoustic stimuli become rather common to modulate optical properties and magnetic order, possibility to excite ultrashort magnon pulses at the nano-scale remains unexplored. The aim of this project is to develop an experimental technique to generate ultrashort magnon pulses, which can be potentially applied to manipulate magnetization in future spintronic devices. In addition to that, this study is important to reveal mechanisms of phonon-magnon interactions.In order to shed light on ultrafast magnetoacoustics, I will perform optical pump-probe experiments on thick films of different ferromagnetic materials. Magnetization dynamics will be monitored by means of the time-resolved magneto-optical Kerr effect (MOKE). Due to essentially non-uniform light absorption profile in thick ferromagnetic films, optical excitation with ultrashort laser pulses results in the generation of picosecond acoustic strain pulses. At the same time the laser-induced heating leads to the spatially localized demagnetization and changes in the magnetocrystalline anisotropy thereby altering the direction of the effective magnetic field within the heat penetration depth. This effect drives coherent magnetization dynamics which can be represented as a packet of spin waves. In my study I want to investigate propagation of the spin wave packet and its dependence on the spin wave dispersion, systematically varying the direction and magnitude of the external magnetic field. Furthermore, using trains of several pump pulses with adjustable delay between them, it should be possible to excite shorter magnon pulses. Experiments on different ferromagnetic materials (Ni, Co, Fe and their alloys) will help to reveal the material-specific properties of the spin wave generation and propagation as well as the phonon-magnon interactions at ultrafast time scales.