- Karel Carva: Ultrafast magnetization and lattice dynamics: non-thermal effects
- 25. 4. 2018, 14:30
- lecture room F2, first floor Ke Karlovu 5
- more information
Abstract:
Femtosecond lasers allow to observe magnetization dynamics on unprecedently short timescale [1]. This dynamics has commonly been described employing the three temperature model, without verifying its validity. However, its limitations should be more thoroughly examined.
Here we study magnetization dynamics with a special emphasis to non-thermal effects. These include the deviation of the phonon population from the thermalized one. We have calculated electron-phonon scattering rates for systems with high electronic temperature, and phonon lifetimes due to phonon-phonon scattering. From these we obtain phonon populations that differ sharply from the thermal ones within picoseconds after the pump [2]. This allows us to understand recent experimental observations and disproves the applicability of the model based on one lattice temperature here.
Another important consequence of ultrafast demagnetization is the presence of laser induced spin currents. This is also a non-thermal effect since electrons with energy significantly above the Fermi level play the most important role there [3]. These spin currents arise and decay on timescales unprecedent in spintronics. We have extended the original model to study spin currents induced by a fs laser pulse in spin valves consisting of two perpendicularly oriented magnetic layers, FM1 and FM2, separated by a nonmagnetic one, NM. We have found that a femtosecond laser pulse focused on FM1 can lead to spin transfer torque causing magnetization precessions in FM2, in agreement with experiment [4]. We have also found an optimal thickness of the polarizing (FM1) layer that optimizes torque in the spin valve [5].
[1] E. Beaurepaire, J.-C. Merle, A. Daunois J.-Y. Bigot, Phys. Rev. Lett. 76, 4250 (1996); K. Carva, P. Baláž, and I. Radu, Laser-Induced Ultrafast Magnetic Phenomena, In: Brück, E (Ed.) Handbook of Magnetic Materials 26, 291 (2017).
[2] P. Maldonado, K. Carva, M. Flammer, P. M. Oppeneer, Phys. Rev. B, 2017, 96, 174439
[3] M. Battiato, K. Carva, P. Oppeneer Phys. Rev. Lett. 105, 027203 (2010);
[4] I. Razdolski et al., Nat. Comm. 8, 15007 (2017)
[5] P. Baláž, M. Žonda, K. Carva, P. Maldonado, P. M. Oppeneer, J. Phys.: Cond. Matter 30, 115801 (2018)