THz-driven Ultrafast Spin-Lattice Scattering in Metallic Ferromagnets
The interaction between magnetism and light is receiving considerable interest in recent years, after the groundbreaking experiments that showed that ultrashort (~100 fs) infrared light pulses can be used to demagnetize  or even switch  the magnetization of thin film ferromagnets. This has sparked excitement towards the possibility of realizing ultrafast magnetic data storage controlled by light. However, to date no clear and commonly accepted understanding of the fundamental physical processes governing the ultrafast magnetization has been reached, partly because accurate modelling of the infrared fs laser-induced highly non-equilibrium state remains a key obstacle.
We present recent experiments where we used strong THz fields, rather than infrared pulses, to excite ultrafast magnetization dynamics in thin film ferromagnets, and probed it with the time-resolved magneto-optical Kerr effect. In one case , we compared the response from two films with remarkably different lattice structure: a crystalline Fe film and an amorphous CoFeB film. We observe Landau- Lifshitz-torque magnetization dynamics of comparable magnitude in both systems, but only the amorphous sample shows ultrafast demagnetization caused by the spin-lattice depolarization of the THz-induced ultrafast spin current. Quantitative modelling shows that such spin-lattice scattering events occur on similar time scales than the conventional spin conserving electronic scattering (~30 fs). This is significantly faster that optical laser-induced demagnetization, and THz conductivity measurements point towards the influence of lattice disorder in amorphous CoFeB as the driving force for enhanced spin-lattice scattering. In another experiment, we used the multicycle THz pulses produced at the High-Field High-Repetition-Rate Terahertz facility @ ELBE (TELBE)  to drive magnetization dynamics in the amorphous CoFeB sample. Our results show that demagnetization is strongly dependent on the frequency of the multicycle THz pulses, possibly illustrating the relation between charge- and spin-dependent scattering of conduction electrons. Further measurements and modeling are ongoing.
Finally, we will give an overview over the next challenges and opportunities that the use of THz radiation offers towards a deeper understanding of ultrafast magnetism, also in combination with the use of coherent x-ray radiation now available at free electron lasers.
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