Same driving force for laser-induced ultrafast demagnetization and THz spin transport

We investigate the driving force of two phenomena that follow femtosecond laser excitation: ultrafast magnetization quenching and spin transport. Two model systems are considered here: (A) a single ferromagnetic (F) metal layer (e.g. Co) and (B) a F|N bilayer consisting of a ferromagnetic (F) and nonmagnetic (N) layer (e.g. Pt). First, laser-induced demagnetization of F is measured by detecting the magnetic-dipole radiation emitted by the dynamically quenched F magnetization [1]. Second, laser-induced spin transport in F|N leads to an in-plane charge current in N due to the inverse spin Hall effect. This current is measured by detecting the concomitantly emitted THz electric-dipole radiation [2]. We observe that, apart from a global scaling factor, the emitted THz pulse follows the same temporal dynamics for both the F and F|N system. This remarkable agreement arises because (i) optically triggered demagnetization in F layer and spin transport in F|N are driven by the same force; (ii) The spin current is only a small perturbation of the spin dynamics in the isolated F layer.
[1] Zhang, et al. Ultrafast terahertz magnetometry. Nat Commun 11, 4247 (2020)
[2] Seifert, et al. "Efficient metallic spintronic emitters of ultrabroadband terahertz radiation." Nature photonics 10.7 (2016)