Over the last decade, spin-orbit coupling effects became one of the central topics in spintronics. For example, spin Hall effect and Rashba-Edelstein effect enable electrical generation of spin current and density, respectively, which can be used to manipulate magnetic configuration. This has led to the notion of spin-orbit torque and spin-orbitronics. From fundamental physics aspect, spin-orbit torque is understood as angular momentum transfer from the lattice to the local moment, which is mediated by spin-orbit coupled electrons (Figure).  From this viewpoint, it is evident that not only spin but also orbital degrees of freedom of the electron plays an important role. However, orbital effect did not receive much attention due to the common notion that orbital is quenched in crystals.

In this seminar, I will demonstrate that orbital degree of freedom not only plays a crucial role in spin-orbit physics but also exhibits unique phenomena even when the spin-orbit coupling is negligible or absent. In the first part, I will explain how to electrically generate orbital angular momentum in solids. As an example, I will introduce “orbital Hall effect” as a precursor to spin Hall effect, i.e., orbital Hall effect arises regardless of the spin-orbit coupling and becomes correlated with spin Hall effect when the spin-orbit coupling is present [1]. In the second part, I will explain how to utilize orbital angular momentum for magnetization control in spintronic devices. Here, I will propose a concept of “orbital torque”, where orbital injection to a ferromagnet excites magnetization dynamics [2]. While orbital Hall effect and orbital torque are not yet experimentally verified, I will present theoretical prediction in real materials and discuss several ongoing experiments which seem to indicate orbital transport. Finally, I will explain our recently developed  theoretical formalism to track angular momentum transfer dynamics between various degrees of freedom in solids (Figure) and how it can be helpful in understanding microscopic mechanisms of spin-orbit torque [3].

 [1] D. Go, D. Jo, C. Kim, and H.-W. Lee, Phys. Rev. Lett. 121, 086602 (2018).
[2] D. Go and H.-W. Lee, Phys. Rev. Research 2, 013177 (2020).
[3] D. Go, F. Freimuth, P. M. Haney, Y. Mokrousov et al., arXiv:2004.05945.