Spin transport in Ni₈₁Fe₁₉/AlO_x/SrTiO₃ heterostructure

The generation and manipulation of a spin current is fundamental for the spintronics devices. The conversion phenomena between spin and charge currents in solids have been intensely investigated in a wide variety of systems. One of the promising candidates is the two-dimensional electron gas (2DEG) in SrTiO₃-based structures with a strong Rashba spin-orbit coupling. In a Rashba system, an in-plane charge current generates a transverse spin density, which is known as the Edelstein effect (EE). An efficient spin-to-charge conversion through the inverse EE has been demonstrated in metal oxide/SrTiO₃ heterostructures with 2DEGs at the interfaces. However, evidence for the charge-to-spin conversion, a technologically more important process, has been lacking. Furthermore, the spin transport mechanism in this system has been unclear.

Here, we demonstrate the charge-to-spin conversion by the direct EE in a 2DEG at an AlO_x/SrTiO₃ interface. We conduct the spin torque ferromagnetic resonance measurements on Ni₈₁Fe₁₉ (4.2 nm)/AlO_x (2 nm)/SrTiO₃ devices, where a spin current generated by the direct EE in the 2DEG is injected into the Ni₈₁Fe₁₉ through the AlO_x and exerts a spin torque on the local magnetization of the Ni₈₁Fe₁₉. The effective charge-to-spin conversion efficiency of the 2DEG exceeds 10% at room temperature, which is comparable to that of the archetypal spin-to-charge converter, Pt. The effective charge-to-spin conversion efficiency is found to be suppressed by decreasing temperature. From the analysis regarding to the carrier density and band structure of the 2DEG, we confirm that the spin density generated by the EE does not decrease with decreasing temperature. Therefore, the decrease of the effective charge-to-spin conversion efficiency is ascribed to the decrease of the spin transmission. We demonstrate a crossover of the dominant spin transport mechanism from the inelastic tunneling to elastic tunneling induced by decreasing temperature.