Direct observation of spin accumulation and spin-orbit torque driven by Rashba-Edelstein effect in an InAs quantum-well layer

For semiconductor spintronics, efficient spin generation in semiconductor and spin transfer to ferromagnetic metal (FM) are essentially required. Two-dimensional electron gas (2DEG) of semiconductor quantum wells is a promising system for generating spin via the Rashba-Edelstein effect (REE) because of its strong inversion symmetry breaking. In this study, we investigate spin accumulation through REE and spin Hall effect (SHE) in the 2DEG of an InAs quantum well. We use spatial- and polarization-resolved measurements of spin, which reveals that REE dominates SHE in 2DEG. Furthermore, REE in 2DEG induces a spin-orbit torque on FM in a 2DEG/insulator/FM heterostructure. Using direction- and time-resolved measurements of FM magnetization, we determine a sizeable fieldlike torque, which is attributed to the phonon-mediated spin transport from 2DEG to FM.

Figure 1

Channel structure and electrical property of 2DEG. (a) A quantum well of InAs 2 nm is formed between InGaAs and InAlAs layers [30]. (b) Band alignment for the quantum well [30]. A large potential gradient ∇V is shown in the InAs quantum well (dotted circle). (c) The measured Rashba parameter ${\alpha }_{\mathrm{R}}$ in a temperature range 1.8–300 K.

Figure 2

REE-driven spin accumulation in 2DEG. (a) A schematic illustration of spin polarization by REE. When charge current flows in the $x$ direction (white arrow), spin polarization along the $y$ direction is formed at the entire channel (blue arrows). A probe light (red arrow) detects the $y$-spin polarization via a linear longitudinal MOKE component of oblique incidence. (b) A spatial scanning of the $y$-polarized spin along the channel width. Black and red (blue and green) colors are the Kerr ellipticity (rotation) of the complex Kerr angle. Black and green or red and blue colors are with a charge current of ±1 mA, and correspond to a current density of $50\mathrm{A}{\mathrm{m}}^{-1}$. (c) The Kerr ellipticity at the center of the channel as a function of charge current. Black/red colors are with an incident angle (${\phi }_0$) of light of ±30°. The $y$-axis coordinate on the right is the calculated spin density by using Eq. (1). The inset image is a microscopic image of the sample, and the red point indicates the measured spot.

Figure 3

SHE-driven spin accumulation in 2DEG. (a) A schematic illustration of spin polarization by SHE. When charge current flows in the $x$ direction (white arrow), spin polarization along the $z$ direction is formed at the channel edge (blue arrows). A probe light (red arrow) detects the $z$-spin polarization via a linear polar MOKE component of oblique incidence. (b) A spatial scanning of the $z$-polarized spin along the channel width. Black/red colors are with a charge current of ±1 mA. (c) The Kerr ellipticity at the edges of the channel as a function of charge current. Black/red colors are results at the upper/lower edge of the channel. The $y$-axis coordinate on the right is the calculated spin density from the quantitative comparison between the linear longitudinal and polar MOKE components. The inset image is a microscopic image of the sample and white/red points indicate the measured spots for the upper/lower edge.

Figure 4

Steady-state tilting of magnetization in 2DEG/insulator/FM by REE-driven SOT. (a) A schematic illustration of the experiment. When charge current (white arrow) and external magnetic field $H_{\mathrm{ex}}$ (grey arrow) are along the $x$ direction, REE-driven SOT induces a tilting of magnetization of FM (black arrow). A probe light (red arrow), whose polarization angle (${\varphi }_p$) is controlled, detects the tilting of magnetization via a quadratic MOKE. Panels (b) and (c) are results with the 2DEG/insulator/NiFe (80 nm), (d) is a result with the Pt (5 nm)/${\mathrm{AlO}}_x$ (20 nm)/NiFe (80 nm), and (e) is a result with the NiFe (80 nm). (b) The ${\varphi }_p$ dependence on the Kerr rotation with $H_{\mathrm{ex}}$ of ±100 and ±500 Oe. (c)–(e) The external magnetic field dependence on the Kerr rotation with ${\varphi }_p$ of 0°, 45°, and 90°. (f) A quantitative determination of the REE-driven SOT at insulator thicknesses of 17, 23, and 29 nm. All results are obtained with a fixed charge current of 1 mA.

Figure 5

Transient dynamics of magnetization in 2DEG/insulator/FM by REE-driven SOT. (a) A schematic illustration of the experiment. A pump light generates a current pulse in the Auston switch. A current pulse induces transient dynamics of FM in the 2DEG/insulator/NiFe (10 nm) by REE-driven SOT. A probe light detects the $z$ component of magnetization dynamics via a linear polar MOKE. The time-resolved Kerr rotation in the (b) 2DEG/insulator/NiFe (10 nm) and (c) Pt (4 nm)/Co (3 nm) samples. The LLG fitting results with Eq. (6) for the (d) 2DEG/insulator/NiFe and (e) Pt/Co samples. Black circles are normalized data of (b) and (c). Red/blue lines are solutions of Eq. (6) with fieldlike/dampinglike torque. For fitting parameters of Eq. (6), we use ${\mu }_0{M}_{\mathrm{eff}}$ of 0.55 T for (d) and 0.45 T for (e), and the same ${\mu }_0{H}_{\mathrm{ex}}$ of 0.05 T and α of 0.05.

Figure 6

Paths of a probe beam.

Figure 7

A schematic of MOKE measurement.