Highly efficient and tunable spin-to-charge conversion through Rashba coupling at oxide interfaces

Abstract

The spin-orbit interaction couples the electrons' motion to their spin. Accordingly, passing a current in a material with strong spin-orbit coupling generates a transverse spin current (spin Hall effect, SHE) and vice-versa (inverse spin Hall effect, ISHE) 1-3 . The emergence of SHE and ISHE as charge-to-spin interconversion mechanisms offers a variety of novel spintronics functionalities 4,5 and devices, some of which do not require any ferromagnetic material 6 . However, the interconversion efficiency of SHE and ISHE (spin Hall angle) is a bulk property that rarely exceeds ten percent, and does not take advantage of interfacial and low-dimensional effects otherwise ubiquitous in spintronics hetero-and mesostructures. Here, we make use of an interface-driven spin-orbit coupling mechanism  the Rashba effect 7  in the oxide two-dimensional electron system (2DES) LaAlO3/SrTiO3 to achieve spin-to-charge conversion with unprecedented efficiency. Through spin-pumping, we inject a spin current from a NiFe film into the oxide 2DES and detect the resulting charge current, which can be strongly modulated by a gate voltage. We discuss the amplitude of the effect and its gate dependence on the basis of the electronic structure of the 2DES. Perovskite oxide materials possess a broad range of functionalities, some of which can be very appealing for spintronics. This includes half-metallicity in mixed-valence manganites  that can be used to produce giant tunnel magnetoresistance 8  or multiferroicity  through which magnetization direction can be electrically controlled at low power 9 . The recent years have seen the emergence of novel spintronics effects based on the generation and control of pure spin currents through spin-orbit effects in semiconducting and metallic systems 1-3 . However, despite a renewal of interest for 4d and 5d transition metal perovksites 10 , spin-orbit effects remained largely unexplored in oxide spintronics.An emerging direction in oxide research aims at discovering novel electronic phases at interfaces between two oxide materials 11 . A well-known example is the LaAlO3/SrTiO3 system: while both LaAlO3 (LAO) and SrTiO3 (STO) are wide bandgap semiconductors, a high-mobility two-dimensional electron system (2DES) forms at their interface 12 if the LAO thickness is at least 4 unit-cells (uc). Interestingly, LAO/STO possesses several remarkable extra functionalities including a gate-tuneable Rashba effect 13,14 , which makes it particularly appealing for spintronics.The Rashba effect is a manifestation of the spin-orbit interaction (SOI) in solids, where spin degeneracy associated with the spatial inversion symmetry is lifted due to a symmetry-breaking electric field normal to an heterointerface 15 . In a Rashba 2DES, the flow of a charge current results in the creation of a nonzero spin accumulation 16,17 coming from uncompensated spin-textured Fermi surfaces. Recently, the converse effect so-called inverse Edelstein effect (IEE)  that is a spin-to-charge conversion through SOI  was discovered a...

Room-Temperature Giant Charge-to-Spin Conversion at the SrTiO3–LaAlO3 Oxide Interface

“…Therefore, the STO/LAO heterostructure can enable potential spintronic innovations and now is attracting rising research interests. [8][9][10][11][12][13] Experimentally, Narayanapillai et al 8 have reported an extremely strong charge current induced Rashba field in the 2DEG layer, which verifies the presence of strong Rashba SOC predicted at the STO/LAO interface. Most recently, the inverse Edelstein effect at the STO/LAO interface has been demonstrated by using the spin pumping technique, [10][11][12] which shows a notable spin-to-charge conversion.…”

“…As an in-plane radio-frequency (rf) current (IRF) is applied in the STO/LAO 2DEG layer, non-equilibrium spins are generated due to the Rashba-Edelstein effect. [10][11][12]23,24 Subsequently, these spins are absorbed by the CFB layer and exert oscillating damping-like torque (DL) and/or a field-like torque (FL) on the CFB local magnetization. In addition, the rf current induces Oersted field (HRF) torque (Oe).…”

Rashba and Dresselhaus Couplings in Halide Perovskites: Accomplishments and Opportunities for Spintronics and Spin–Orbitronics

“…It has recently been achieved using perovskite oxides. 121 Because, Rashba and Dresselhaus couplings are sensitive to external electric fields, they can be used to manipulate spins. It has been proposed to use the electric control of spins to build a Stern-Gerlach spin-filter without ferromagnetic materials.…”

Theory of current-induced spin polarization in an electron gas

“…Notice that spin-to-charge conversion in this context is sometimes referred to as inverse Rashba-Edelstein effect [15] -for a recent theoretical discussion see [16]. The SGE/IEE has also been observed at ferromagnet-topological insulator interfaces [17,18] and in LAO|STO systems [19]. In semiconducting structures the reciprocal ISGE/EE is measured via optical detection of the current-induced spin polarization [20][21][22][23][24][25].…”