Tutorial: Fundamentals of light-driven ultrafast spin dynamics in magnetic materials

Time: Tuesday, October 23rd, 15:00
Speaker: Peter OPPENEER, Uppsala

Essential spin operations such as spin-polarized current-induced switching or all-optical helicity-dependent spin switching have already been shown to be realizable, however, the underlying fundamental mechanisms are still poorly understood. In this tutorial I aim to address theoretical advancements in three areas, 1) ultrafast laser-induced demagnetization, 2) all-optical helicity-dependent magnetization switching (AOS), and 3) spin-polarized current or electric-field induced spin switching. Ultrafast laser-induced demagnetization was discovered two decades ago [1], however, the underlying fundamental processes of the fast magnetization decay continue to be debated. I shall discuss various proposed microscopic mechanisms [2-4] as well as ab initiocalculations that might lead to identification of the responsible mechanism(s) and compare to recent experiments.
All-optical switching is a very different, interesting process in which the magnetization of a material is manipulated with ultrashort circularly polarized laser pulses [5,6]. Its origin is still not well understood. In order to progress in this technologically relevant area, ab initiomaterials’ specific theory is needed to know precisely how much spin and orbital magnetization can be induced by a circularly polarized pulse by the inverse Faraday effect in absorbing metals [7]. To understand the process of AOS, multiscale modeling is required, which combines the effects of thermal laser heating with laser-induced magnetization to reach a predictive description of AOS in real materials [8].
Switching the magnetization of a thin layer by a spin-polarized current is the underlying principle of STT-MRAM technology. This process has often been modeled in micromagnetics by adding ad hoca spin-transfer torque (STT) to the phenomenological Landau-Lifshitz-Gilbert (LLG) of magnetization dynamics. The LLG equation has been extended further in recent years, by adding other terms, such as spin-orbit torques (SOTs) [9,10]. It is our aim is to achieve a unified theoretical framework to describe the ultrafast spin dynamics in magnetic systems and quantify the spin dynamics’ quantities on the basis of materials’ specific abinitiocalculations. To this end I discuss recent theory that rigorously shows that the LLG equation can be derived from the fundamental Dirac equation [11], and in addition, leads to the nonrelativistic adiabatic and nonadiabatic STT terms as well as relativistic SOT terms.
The presented theory work has been done together with R. Mondal, M. Berritta, A.K. Nandy, L. Salemi, K. Carva, U. Ritzmann, P. Maldonado, and J. Hurst, and with D. Hinzke and U. Nowak.

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[6] C.-H. Lambert, S. Mangin et al, Science 345, 1337 (2014).
[7] M. Berritta, R. Mondal, K. Carva, and P.M. Oppeneer, Phys. Rev. Lett. 117, 137203 (2016).
[8] R. John et al, Sci. Rep. 7, 4114 (2017).
[9] I.M. Miron et al. Nature 476, 189 (2011)
[10] H. Kurebayashi et al, Nat. Nanotechn. 9, 211 (2014).
[11] R. Mondal, M. Berritta, and P.M. Oppeneer, Phys. Rev. B 94, 144419 (2016).