Evanescent Orbital Pumping by Magnetization Dynamics Free of Spin-Orbit Coupling

Abstract

Converting magnetization spin to orbital current often relies on strong spin-orbit interaction that may cause additional angular momentum dissipation. We report that coherent magnetization dynamics in magnetic nanostructures can evanescently pump an orbital current into adjacent semiconductors due to the coupling between its stray electromagnetic field and electron orbitals without relying on spin-orbit coupling. The underlying photonic spin of the electromagnetic field governs the orbital polarization that flows along the gradient of the driven field. Due to the joint effect of the electric and magnetic fields, the orbital Hall current that flows perpendicularly to the gradient of the time-varying field is also generated and does not suffer from the orbital torque. These findings extend the paradigm of orbital pumping to include photonic angular momentum and pave the way for developing low-dissipation orbitronic devices.

Figures (3)

• Figure 1: Pumping of longitudinal and Hall OAM currents in semiconductors by the AC magnetic field generated by, e.g., the electromagnetic radiation of magnetic nanostructures. The origin of the coordinate is located at the interface below the center of the nanowire. Under a local AC electromagnetic field (red dashed curve) generated by the magnetization precessing in a biased nanowire, a longitudinal OAM current (front blue arrows) with $\hat{\bf y}$-direction orbital polarization (front red arrows) is radiated from the local source in the $x$-$z$ plane. A Hall orbital current (top blue arrows) with $\hat{\bf x}/\hat{\bf z}$-direction orbital polarizations (top red arrows) is pumped along the wire $y$-axis.
• Figure 2: Spatial distribution of longitudinal OAM current density of holes when pumped by the electromagnet field emitted by the FMR of a magnetic nanowire. (a) and (b) compare the flow along the $-\hat{\bf z}$- and $\hat{\bf x}$-directions at $x=0$ nm and $z=-1$ nm, respectively. (c) plots the orbital torque density polarized along the magnetization $\hat{\bf y}$-direction. (d) compares the contribution of electric and magnetic fields in the orbital pumping with different chemical potentials at position $(x,z)=(0,-200)$ nm.
• Figure 3: Spatial distribution of orbital Hall current density flowing along the wire $\hat{\bf y}$-direction with orbital polarization along $\hat{\bf x}$ (a) and $\hat{\bf z}$ (b). (c) and (d) compare the orbital Hall current polarized along $\hat{\bf x}$ and $\hat{\bf z}$ at two sides of the nanomagnet. (e) plots the orbital torque density polarized normal to the magnetization $\hat{\bf y}$-direction. (f) Calculated DC OAM density $\overline{\bf L}_d(x,z,q_y)$ in the mixed spatial $(x,z)$ and wave-vector $q_y$ space at position $(x=0~{\rm nm},z=-100~{\rm nm})$.