Theory of the Spin Galvanic Effect at Oxide Interfaces

Abstract

The spin galvanic effect (SGE) describes the conversion of a non-equilibrium spin polarization into a transverse charge current. Recent experiments have demonstrated a large conversion efficiency for the two-dimensional electron gas formed at the interface between two insulating oxides, LaAlO3 and SrTiO3. Here we analyze the SGE for oxide interfaces within a three-band model for the Ti t2g orbitals which displays an interesting variety of effective spin-orbit couplings in the individual bands that contribute differently to the spin-charge conversion. Our analytical approach is supplemented by a numerical treatment where we also investigate the influence of disorder and temperature, which turns out to be crucial to provide an appropriate description of the experimental data.PACS numbers: 71.70.Ej, 72.20.Dp, The spin galvanic effect (SGE) is the generation of an electrical current by a non-equilibrium spin density. The latter may be obtained by optical or magnetic pumping. There exists also the inverse effect (ISGE) by which the spin of free carriers can be oriented by an applied electric field. The SGE effect was first predicted [1] and then observed for Te crystals [2]. About a decade later, the SGE was studied theoretically [3][4][5] in the two-dimensional electron gas (2DEG) in the presence of Rashba spin-orbit coupling (SOC) arising from the asymmetry of the quantum well [6]. The effect was later observed in quantum wells by absorption of polarized light [7][8][9]. Whereas at microscopic level the coupling between the spin polarization and the electrical current is provided by the SOC, the origin of the effect rests on the restricted symmetry conditions of gyrotropic media, where polar and axial vectors transform according to the same representation (c.f. the review by Ganichev et al., [10]). The conditions of sizable SOC and lack of inversion symmetry can be obtained also in other physical systems. Recently, indeed, the SGE has been observed at a silver-bismuth interface where the non-equilibrium spin polarization has been pumped by an adjacent ferromagnetic layer with a precessing magnetization [11]. Magnetic spin pumping was successively used to observe the SGE in a number of different interface systems as in ferromagnetictopological insulators [12,13] and ferromagnetic-oxide systems [14][15][16]. In the latter case, the oxide being a heterostructure of LaAlO 3 (LAO) and SrTiO 3 (STO), the complex band structure originating from the Ti orbitals may lead to a richer phenomenology [17,18], as compared to other systems where the standard continuum 2DEG model with spin-orbit split bands provides a good quantitative and qualitative understanding of the effect. Indeed, the spin-polarization induced transverse voltage V SGE can be modulated by gating the heterostructure which changes the chemical potential µ, i.e. the carrier density in the 2D interface layer. Quite dramatically, a sign change occurs [15] in V SGE at a particular value of µ, and has been attributed to a Lifshitz transition, where the d xz , d ...

Role of interface quality for the spin Hall magnetoresistance in nickel ferrite thin films with bulk-like magnetic properties

“…Last but not least, the bulk spin Hall effect is not the only cause for the conversion of a charge current into a pure spin current in a NM. It is quite possible that contributions from the spin-galvanic effect [26][27][28][29][30][31][32] arising at the NFO/NM interface also give rise to additional contributions to the SMR. As the spin-galvanic effect is caused by spin-orbit fields as a result of the broken inversion symmetry at the NFO/NM interface, it is quite possible that such a contribution depends on the charge current direction.…”

Anomalous Hall and Nernst effects in a two-dimensional electron gas with an anisotropic cubic Rashba spin-orbit interaction

“…The electronic energy spectrum describing STO surfaces and STO/LAO interfaces has been calculated recently within the tight-binding approach and DFT modeling [17,18,[29][30][31][32]. Based on these calculations the effective Hamiltonian describing the neighborhood of the Γ point in the Brillouin zone has been derived [17][18][19][20]. This energy spectrum is formed by three pairs of bands as presented in Fig.…”

“…1(b)-(e)). The highest, in energy, pair of bands is characterized by effective Hamiltonian with k-linear Dresselhaus-like form of spinorbit [20]. Since electronic transport characteristics in a system with conventional Rashba term is rather well described, within this article, we focus only on the transport properties of quasiparticles from the middle pair of bands.…”

“…The so-called cubic Rashba interaction is responsible, e.g., for spin and transport properties of * adyrdal@amu.edu.pl a two-dimensional hole gas formed at the interfaces of III-V semiconductor heterostructures [11][12][13][14][15][16]. Recently, the cubic character of Rashba interaction has also been found in a two-dimensional electron gas at the perovskite oxides surfaces and interfaces such as LaAlO 3 /SrTiO 3 (LAO/STO) [17][18][19][20].…”

Thermal generation of shift electric current

“…In structures of low spatial symmetry, direct currents can also emerge when the system is driven out of thermal equilibrium by an undirected force with zero average driving [1,2]. Well known examples are ratchet and photogalvanic effects [3][4][5][6][7][8][9][10][11] and the spin-galvanic effect [12][13][14], when non-equilibrium spin polarization drives an electric current in gyrotropic structures. Here, we study the effect of generating direct electric current from the kinetic energy of hot electrons.…”