Dongwook Go I ial Role of Long-Range Orbital Transport in Spintronic Phenomena 

 Dongwook Go, Peter Grünberg Institut and Institute for Advanced Simulation, Forschungszentrum Jülich

Within the paradigm of spintronics so far, most spintronic phenomena such as current-induced magnetic torques are interpreted in terms of spin-transfer physics and transport of angular momentum in the form of spin currents. At a fundamental level, however, the angular momentum of electrons is not only carried by the spin degree of freedom but also by the orbital degrees of freedom, but the possibility of transport and transfer of angular momentum via the orbital channel has been overlooked so far. In recent years, we have shown that even if the orbitals in the ground state is entirely quenched, OAM can be induced and transported via non-equilibrium states driven by external perturbations such as electric and magnetic fields [1,2]. The induced OAM can also be transferred to local moments to exert a torque, which offers a promising route for enhancing the efficiency of spin torque devices [3]. Despite many promising features of OAM for spintronics, one of the biggest challenges is to unambigiously quantify it, and in particular, disentangle the signals coming from the spin and orbital, whose phenomenological behaviors are nearly identical.

In this talk, I will show that nonequilibrium OAM exhibits long-range correlation in its response and transport, which are different from the behavior of the spin both quantitatively and qualitatively. I will show that injection of OAM in a ferromagnet exerts a torque on the local moment over unexpectedly long distance [4]. The injected OAM decays monotonically without any oscillation, which is distinct from the dephasing of spins in the mechanism of the spin-transfer torque. I will explain that this behavior originates from the momentum-space hotspots where Bloch states with different orbital characters are degenerate, which is imposed by the symmetry of the crystal field. This results in the nontrivial behavior of the current induced torque: the torque efficiency increases as the thickness of the ferromagnet grows. This prediction was verified by a recent experiment by Hayashi et al. [5], which found long (~ 20 nm ) orbital dephasing length in Ni by magnetic torque measurements. The experiment also revealed a strikingly large orbital relaxation length ~ 70 nm in W. The long-range transport of the OAM in W was also experimentally verified by Seifert et al. [6], where the OAM transport triggered by a THz pulse was directly observed in the time domain. I will discuss the physical origin of the robustness of the OAM transport and its exceptionally long relaxation length. I will also discuss outlook for utilizing long-range OAM for spintronic and orbitronic device applications.

References

1. [1]  D. Go, D. Jo, C. Kim, and H.-W. Lee, Phys. Rev. Lett. 121, 086602 (2018).
2. [2]  D. Go, D. Jo, H.-W. Lee, M. Kläui, and Y. Mokrousov, Europhys. Lett. 135, 37001 (2021).
3. [3]  D. Go and H.-W. Lee, Phys. Rev. Research 2, 013177 (2020).
4. [4]  D. Go, D. Jo, K.-W. Kim, S. Lee, M.-G. Kang, B.-G. Park, S. Blügel, H.-W. Lee, and Y. Mokrousov, arXiv: 2106.07928.
5. [5]  H. Hayashi, D. Jo, D. Go, T. Gao, S. Haku, Y. Mokrousov, H.-W. Lee, and K. Ando, Comm. Phys. 6, 32 (2023).
6. [6]  T. S. Seifert, D. Go, H. Hayashi, R. Rouzegar, F. Freimuth, K. Ando, Y. Mokrousov, and T. Kampfrath, arXiv:2301.00747.