Fermi-Level-Dependent Charge-to-Spin Conversion of the Two-Dimensional Electron Gas at the γ-${\mathrm{Al}}_2{\mathrm{O}}_3/{\mathrm{KTaO}}_3$ Interface

Charge-to-spin conversion is crucial for the application of emerging spintronic devices. A two-dimensional electron gas (2DEG) at a complex oxide interface usually possesses strong Rashba spin-orbit coupling, and spin-momentum locking offers a great possibility for efficient charge-to-spin conversion through the Rashba-Edelstein effect. Here, we report the fabrication of metallic 2DEGs in γ-${\mathrm{Al}}_2{\mathrm{O}}_3/{\mathrm{KTaO}}_3$ spinel/perovskite heterostructures and investigate the charge-to-spin conversion for Py/γ-${\mathrm{Al}}_2{\mathrm{O}}_3/{\mathrm{KTaO}}_3$ devices using the technique of spin-torque ferromagnetic resonance. The sizable spin splitting of the band structure results in a large current-induced spin-orbit torque efficiency with values up to around 3.6 at 5 K and about 1.1 at 300 K, which are more than an order of magnitude higher than those of heavy metals (0.07 for $\mathrm{Pt}$ at 300 K). Moreover, both theoretical and experimental results show that the charge-to-spin conversion is strongly dependent on the position of the Fermi level. These results demonstrate that optimizing the band filling of a ${\mathrm{KTaO}}_3$-based 2DEG is crucial for maximizing the conversion efficiency.

Figure 1

(a) Sketch of the γ-${\mathrm{Al}}_2{\mathrm{O}}_3$/KTO (001) heterostructure. (b) X-ray diffraction spectrum specifying the well-orientated γ-${\mathrm{Al}}_2{\mathrm{O}}_3$ film on KTO substrate. (c) Temperature dependence of the sheet resistance for samples with the γ-${\mathrm{Al}}_2{\mathrm{O}}_3$ thicknesses of 3, 4, 6, and 8 u.c. (d),(e) Corresponding carrier density and Hall mobility, respectively, as functions of temperature. Solid lines are guides to the eye.

Figure 2

(a) Schematic diagram of the structure of ST-FMR device with the SOT-induced magnetization dynamics and the ST-FMR experimental setup, where $J_C$ is the uniform charge current density flowing through the $\gamma \text{−}{\mathrm{Al}}_2{\mathrm{O}}_3$/KTO 2DEG layer, J_S represents the spin current density injected into the Py layer, and $\sigma$ denotes the unit vector of spin polarization. (b) Typical ST-FMR signal of Py(6 nm)/γ-${\mathrm{Al}}_2{\mathrm{O}}_3$(4 u.c.)/KTO device, measured under f = 7 GHz and φ = 45° at room temperature. Symbols are experimental data and solid line is the result of curve fitting, which can be separated into symmetric (blue dashed) and antisymmetric (green dashed) components. (c) Angular dependence of V_S and V_A and the corresponding fitting curves by using the relationship of sin2φ cosφ.

Figure 3

(a) Effective magnetization as a function of γ-${\mathrm{Al}}_2{\mathrm{O}}_3$ layer thickness. The inset is the relationship between the resonant frequency and the resonant field for t_γ-Al2O3= 4 u.c., and the red line is the result of fitting to the Kittel formula. (b) Dependence of Gilbert damping constant on γ-${\mathrm{Al}}_2{\mathrm{O}}_3$ layer thickness. The inset is the linewidth of the resonance peak versus the resonant frequency for $t_{\gamma \text{−}\mathrm{Al}2\mathrm{O}3}=4\text{u.c.}$, and the red line is the linear fitting. (c) Efficiency of charge-to-spin conversion for different $t_{\gamma \text{−}\mathrm{Al}2\mathrm{O}3}$ at room temperature. (d) Dependence of the torque ratio τ_S/τ_A on γ-${\mathrm{Al}}_2{\mathrm{O}}_3$ thickness for devices at a given frequency (f = 7 GHz).

Figure 4

(a) Charge-to-spin conversion efficiency measured at 5 K as a function of the Fermi energy, where the n_S values at 5 K in Fig. 1 are adopted for the calculation of E_F and the dashed line indicates the Lifshitz transition. Solid line is a guide to the eye. (b) Energy dispersion relation of the 2DEG at (001)-oriented KTO interface, calculated by a six-band tight-bonding model. Dashed lines mark the positions of typical Fermi levels. (c) Fermi surfaces and spin textures at various energies corresponding to the band filling states marked in (b), arranged in energetically ascending order from top to bottom. For clarity, only clockwise (anticlockwise) spin helicity is shown for the outside (inside) Fermi contours. (d) Dependence of the theoretically calculated Edelstein tensor κ_xy on the Fermi energy.