Simulation of Ultrafast Spin-Currents in Optically-Excited Magnetic Multilayers

Optically excited spin-currents in magnetic heterostructures can induce magnetization dynamics even in non-excited layers [Alekhin et al. PRL 119, 017202 (2017)]. In this context, the hot carrier transport behavior is often characterized as superdiffusive [Battiato et al., PRB 86, 024404 (2012)]. We present a bottom-up approach to electron transport after ultrashort laser-excitation in magnetic multilayers, using the Particle-In-Cell method for the Boltzmann transport equation [Nenno et al., PRB 94, 115102 (2016)]. Simulating the induced dynamics in the whole slab, including transmission coefficients and ab-initio material data. From the calculations, we extract typical transport coefficients and clarify the contribution of secondary carrier generation in the transition from ballistic to diffusive transport behavior.

These optically excited spin currents have also been shown to play an important role in thin sandwich structures that act as spintronic terahertz emitters [Seifert et al., Nat. Photonics 10, 483 (2016)]. We combine our transport calculations with Maxwell's equations to calculate the emitted spectral intensity from both single spintronic emitters and periodic structures to find optimal broadband spintronic terahertz emitters.