Time: Tuesday, October 23rd, 9:20
Speaker: Bert KOOPMANS, Eindhoven
Novel schemes for controlling the ferromagnetic state at femtosecond time scales by pulsed laser excitation have received great interest. Driving systems into the strongly non-equilibrium regime, it has been shown possible not only to quench magnetic order, but also to switch the magnetization by single laser pulses – so-called all-optical switching (AOS). More recently, it has been proposed that pulsed laser excitation can also induce spin currents over several to tens of nanometers, which can act as an additional source of sub-picosecond magnetization dynamics. Thereby, an interesting link between the fields of ‘femtomagnetism’ and spintronic transport physics has emerged. In this presentation, further integration of all-optical magnetic control and spintronic will be explored in an attempt to develop hybrid spintronic-photonic devices.
After a general introduction into laser-induced magnetization dynamics, different processes that give rise to laser-induced spin currents will be distinguished. In particular I will address recent experiments that have demonstrated laser-induced spin transfer torqueon a free magnetic layer, using a collinear multilayer configuration consisting of a free in-plane layer on top of a PMA injection layer and separated by a nonmagnetic spacer [1]. As it will be shown, these non-collinear fs spin currents are absorbed within a few nanometers, and thereby provide ideal conditions for exciting THz spin waves. This allowed us to map out the dispersion of the frequency, , and the Gilbert damping, , of thin Co(B) layers [2,3].
In the second part, AOS in spintronically relevant structureswill be discussed. In particular, focus will be on highly efficient AOS and current-induced domain wall motion in the very same system: Pt/Co/Gd trilayers, displaying strong spin-orbit torques and Dzyaloshinskii-Moriya interaction. It will be shown that the magnetization of this synthetic ferrimagnetic thin film system can be reversed fully deterministically using single fs pulses. Threshold fluences are determined as a function of Co thickness and record-low efficiencies corresponding to below 50 fJ needed to switch a 50x50 nm2are found [3]. Simple modelling using a multi-layer version of the microscopic three-temperature model will be discussed. Moreover we quantitatively determined spin orbit torques and analyzed the coherent current-driven motion of opposite (up-down and down-up) domain walls. Finally, optically writing magnetic information ‘on-the-fly’ will be demonstrated. In these experiments, AOS is established while a constant electrical current is driving the magnetic information along a magnetic racetrack [4]. Such a scenario is envisioned to provide a route towards on-chip spintronic-photonic memories.
- J. Schellekens, K.C. Kuiper, R.R.J.C. de Wit, and B. Koopmans, Nature Comm. 5, 4333 (2014).
- L.M. Lalieu, P.L.J. Helgers, and B. Koopmans, Phys. Rev. B 96, 014417 (2017).
- L.M. Lalieu et al., to be published.
- L.M. Lalieu, M.J.G. Peeters, S.R.R. Haenen, R. Lavrijsen, and B. Koopmans, Phys. Rev. B 96, 220411(R) (2017).
- L.M. Lalieu et al., to be published.