Terahertz spin-orbitronics: insights and applications

Tobias Kampfrath1,2

1Department of Physics, Freie Universität Berlin, Germany
2Fritz Haber Institute of the Max Planck Society, Berlin, Germany

Sub-picosecond electromagnetic pulses covering the range ~1-30 THz have been shown to be capable of probing and even controlling numerous low-energy excitations of condensed matter, for example, phonons, excitons and Cooper pairs. Recent work has shown that ultrabroadband terahertz (THz) radiation is also a very useful and versatile tool in the fields of spintronics and ultrafast magnetism. The insights obtained are relevant not only for a better understanding of fundamental processes involving electron spins, but also for applications in THz photonics.
As a first example, we consider THz spin transport. To launch it, we excite a ferromagnetic-metal thin film FM such as Fe or Co (magnetization M) by a femtosecond optical pulse (Fig. 1). As a result of its increased electron temperature, FM exhibits a transient excess of magnetization that is released by spin-angular-momentum transfer to the crystal lattice or by a spin current jS into an adjacent metal layer HM [1]. To measure j_S, we use heavy metals such as Pt (platinum) for HM because they exhibit a large inverse spin Hall effect and, thus, convert jS into an in-plane charge current jC (Fig. 1). The jC, in turn, acts as a source of an electromagnetic pulse with frequencies reaching the THz range [2]. Interesting applications such as terahertz spin-conductance spectroscopy [3], spin-charge-conversion spectroscopy [4] and the generation of ultrashort terahertz electromagnetic pulses [5] with fields exceeding 1 MV/cm [6] emerge.
This principle can be transferred from the spin S to the so far highly unexplored orbital angular momentum L of electrons. We obtain new insights into orbitronic phenomena on their natural time scales, for example, time-domain signatures of giant propagation lengths of orbital currents in W (tungsten) [7].

In the second example, THz pulses are not used as a probe but as a stimulus of ultrafast spin dynamics. This approach is reciprocal to the scheme of Fig. 1. By applying an intense THz electric field to the antiferromagnetic metal Mn2Au, we exert so-called Néel spin-orbit torques on the spins [8]. We observe spin deflections by as much as 30° relative to the equilibrium direction. Therefore, ultrafast switching of antiferromagnetic order by THz electric fields is in close reach.
With regard to THz photonics, the Zeeman torque of THz magnetic fields on spins can be used for THz-field detection with a straightforward response function. In a ferromagnetic Fe layer of 8 nm thickness, such Zeeman-torque sampling can be used to reliably detect THz pulses with a bandwidth 0.1-11 THz and peak fields >0.1 MV/cm. Static calibration even provides access to the absolute transient THz field strength [9].

Figure 1: Schematic of spintronic THz emission induced by excitation with a femtosecond pump pulse.

[1] Rouzegar et al., Physical Review B 106, 144427 (2022)
[2] Kampfrath et al., Nature Nanotechnology 8, 256 (2013)
[3] Rouzegar et al., arXiv:2305.09074 (2023)
[4] Gueckstock et al., Advanced Materials 33, 2006281 (2021)
[5] Seifert et al., Nature Photonics 10, 483 (2016)
[6] Rouzegar et al., Physical Review Applied 19, 034018 (2023)
[7] Seifert et al., Nature Nanotechnology 18, 1132-1138 (2023)
[8] Behovits et al., Nature Communications 14, 6038 (2023)
[9] Chekhov et al., Physical Review Applied 20, 034037 (2023)

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