Seminarium wydziałowe

Dnia 2019-10-08 o godzinie 14:15 w sali 2011 Wydziału Fizyki UwB odbędzie się wykład, na którym dr Jakob Walowski z University of Greifswald, wygłosi wykład pt:

„Magnetization processes from the attosecond to the picosecond timescale„

Serdecznie zapraszamy

Andrzej Maziewski

 

After the seminal experiment showing ultrafast demagnetization on the femtosecond timescale in the transition metal ferromagnet nickel, by Beaurepaire et al. in 1996 [1], a huge research field evolved, exploring the underlying mechanisms triggering the magnetization dynamics, after the photons of an ultrashort laser pulse are absorbed by a ferromagnets electron system.
The access to ever shorter laser pulses in the attosecond range further pushed the research to study coherent electronic excitations during the first <10 fs. The experiments show that within this time scale, demagnetization is possible through optically induced spin transfer (OISTR) within inter- or intra-band excitations. I will discuss this mechanism based on two sample systems. A pure nickel layer does not show a change in magnetization, while in a nickel layer sandwiched between platinum layers, the depletion of majority spins into the platinum leads to a reduction of magnetic moment. These results have been shown using attosecond magnetic circular dichroism MCD and are supported by time resolved DFT ab-initio calculations [2].
Despite the advances in technology enabling the access on timescales below in the femtosecond regime, the traces of coherent electronic excitations can also be observed in time-resolved magneto-optic Kerr-effect (TRMOKE) in more complicated materials, where the magnetic moment can be transferred between sublattices e.g. half Heusler systems [3].
While the coherent excitations are still under investigation, processes on timescales in the 100-femtosecond range and beyond yield first applications [4]. Spin polarized currents emerging from the excitation by ultrashort laser pulses can be used and converted using the inverse spin-Hall effect (ISHE) into picosecond electric current pulses generating THz radiation, which is among others an interesting tool for investigations in the interdisciplinary field of life sciences. Further, during the timespan of reduced magnetic moment within the first ~200 fs after excitation, the momentum of circular polarized photons can be used for magnetization reversal and thus all-optical switching of magnetic storage media [5].

References:
[1] Beaurepaire, Merle, Daunois, Bigot. Ultrafast spin dynamics in ferromagnetic nickel. Phys Rev Lett. 1996;76:4250–3.
[2] Siegrist F, Gessner JA, Ossiander M, Denker C, Chang Y-P, Schröder MC, Walowski J, et al. Light-wave dynamic control of magnetism. Nature. 2019;571:240–4. doi:10.1038/s41586-019-1333-x.
[3] Steil D, Walowski J, Gerhard F, Kiessling T, Ebke D, Thomas A, et al. Eciency of ultrafast optically-induced spin transfer in Heusler compounds. unpublished. 2019.
[4] Walowski J, Münzenberg M. Perspective: Ultrafast magnetism and THz spintronics. J. Appl. Phys. 2016;120:140901. doi:10.1063/1.4958846.
[5] John R, Berritta M, Hinzke D, Müller C, Santos T, Ulrichs H, Walowski J, et al. Magnetisation switching of FePt nanoparticle recording medium by femtosecond laser pulses. Sci Rep;7:1–8. doi:10.1038/s41598-017-04167-w.