Dresden 2017 – wissenschaftliches Programm

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MA: Fachverband Magnetismus

MA 23: Spin Dynamics and Transport: Ultrafast Effect

MA 23.6: Vortrag

Dienstag, 21. März 2017, 11:00–11:15, HSZ 403

Off-resonant excitation of Co films by intense single-cycle THz pulses: computer simulations and experiments — •Andreas Donges¹, Mostafa Shalaby², Carlo Vicario², Karel Carva³, Peter M. Oppeneer⁴, Christoph P. Hauri², and Ulrich Nowak¹ — ¹Universität Konstanz, DE-78457 Konstanz, Germany — ²SwissFEL, Paul Scherrer Institute, CH-5232 Villigen PSI, Switzerland — ³Charles University, CZ-12116 Praha 2, Czech Republic — ⁴Uppsala University, SE-75120 Uppsala, Sweden

It was recently demonstrated that THz radiation is an efficient tool to coherently excite ferro- [1] and antiferromagnets [2] on the sub-ps time scale. With the availability of more intense THz sources, there is experimental evidence that the excitation of the magnetization becomes incoherent–an effect that is not understood yet. We performed multiscale modeling simulations, starting from ab-initio calculations of the Heisenberg exchange constants for the spin Hamiltonian of Co. We further used the stochastic Landau-Lifshitz-Gilbert equation in combination with an extended two-temperature model, to compute the spin dynamics response to a THz bullet. The results are compared to our THz pump–NIR MOKE probe measurements, for fluences up to 90 mJ/cm². From our findings we conclude that the coherent oscillations are caused by a direct coupling of the spins to the Zeeman field of the THz pulse, whereas the incoherent dynamics is due to the electric field which affects the spins indirectly, via heating of the electrons.

[1] C. Vicario, et al., Nature Photonics, 7, 720-723 (2013).

[2] T. Kampfrath, et al., Nature Photonics, 5, 31-34 (2011).