Fermi-Level-Dependent Charge-to-Spin Conversion of the Two-Dimensional Electron Gas at the γ - Al2 O3/KTaO3 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-Al2O3/KTaO3 spinel/perovskite heterostructures and investigate the charge-to-spin conversion for Py/γ-Al2O3/KTaO3 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 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 KTaO3-based 2DEG is crucial for maximizing the conversion efficiency.