2nd Users' Conference of IT4Innovations

Ultrafast demagnetization dynamics at M2,3 edges of hcp Co – first-principles calculations

7 Nov 2018, 09:30

30m

atrium (IT4Innovations)

User's talk Users' talks Keynote I

Speaker

Dr Dominik Legut (IT4I)

Description

More information on the evolution of the electronic band structure of a ferro-magnetic material during laser-induced demagnetization is needed to understand properly the underlying mechanisms of femtosecond demagnetization dynamics. These can be obtained by combining experiment and modeling. Using ab initio calculations we investigated the effect of a Stoner-like reduction of the exchange splitting, to simulate the previously proposed fast quenching of the exchange splitting, as well as the effect of Heisenberg-like magnon excitations in hcp Co. In our simulations we employed density functional theory calculations using the WIEN2k code [1]. The exchange splitting was reduced to different values employing the methods described in Ref. [2]. To simulate the effects of these mechanisms on the magneto-optical signal, we computed the quasi-static magneto-optical response at the M2,3 edges using the Kubo formula [3]. Influence of non-equilibrium electron populations was determined and computed data were compared to ultrafast high-harmonics measurements of time-, energy-, and angle-resolved magneto-optical effects spectra obtained at the M$_{2,3}$ edges of Co films after optical pumping to induce ultrafast demagnetization [4]. These Co films were on insulating substrates to exclude possible effects of superdiffusive spin currents. For the examined timescales the contribution of transversal spin excitations (i.e., magnon excitation) has been found to be appreciably higher than that of exchange splitting reduction that would originate from longitudinal spin flips [4]. Interestingly, the ratio between these two was not growing with time in favor of transversal excitations, as would be expected from models based on electron-magnon interactions. In recent paper [5] we identify that the changes in xy are uniform throughout the spectrum, to within our experimental precision. This result suggests that, in the regime of strong demagnetization, the ultrafast demagnetization response is primarily dominated by magnon generation. We estimate the contribution of exchange-splitting reduction to the ultrafast demagnetization process to be no more than 25%. This work was done with experimentalists of Refs. [4,5].

Summary

In summary we revealed with simulation and experimental studies of magneto-optic effects at M2,3 edges of hcp Co
that to the ultrafast demagnetization process at shorter times of 400(700)fs 22(33)% is contributed by transverse (exchange splitting)
and the rest by longitudional (magnon) excitations, whenever at on 3ps timescale the contributios are equal.

References

[1] P. Blaha, K. Schwarz, G. K. H. Madsen, D. Kvasnicka, and J. Luitz, WIEN2k: An augmented plane wave + local orbitals program for calculating crystal properties. Vienna, Austria: Vienna University of Technology, 2008.

[2] P. M. Oppeneer, V. N. Antonov, T. Kraft, H. Eschrig, A. N. Yaresko, and A. Y. Perlov, , J. Appl. Phys., 80, 1099 (1996).

[3] S. Valencia, A. Kleibert, A. Gaupp, J. Rusz, D. Legut, J. Bansmann, W. Gudat, and P. M. Oppeneer,, Phys. Rev. Lett. 104, 187401 (2010).

[4] E. Turgut, D. Zusin, D. Legut, K. Carva, R. Knut, J. Shaw, C. Chen, Z. Tao, H. Nembach, T. J. Silva, S. Mathias, M. Aeschlimann, P. M. Oppeneer, H. C. Kapteyn, M. M. Murnane, and P. Grychtol, Phys. Rev. B 94, 220408(R) (2016).

[5] D. Zusin, P. M. Tengdin, M. Gopalakrishnan, Ch. Gentry, A. Blonsky, M. Gerrity, D. Legut, J. M. Shaw, H. T. Nembach, T. J. Silva, P. M. Oppeneer, H. C. Kapteyn, and M. M. Murnane, , Phys. Rev. B 97**, 024433 (2018)

Presentation materials

Slides

Legut Dominik: Ultrafast demagnetization dynamics at M2,3 edges of hcp Co – first-principles