Boosting the Edelstein Effect of Two-Dimensional Electron Gases by Ferromagnetic Exchange

Strontium titanate (SrTiO₃) two-dimensional electron gases (2DEGs) have broken spatial inversion symmetry and possess a finite Rashba spin-orbit coupling [1], which endows the electronic bands with unique spin textures and couples the spin of mobile electrons to their momentum. This enables the interconversion of charge and spin currents through the direct and inverse Edelstein effects (EE/IEE), with record efficiencies at low temperature [2,3].<br/>In this work, we show that making these 2DEGs ferromagnetic enhances the conversion efficiency by nearly one order of magnitude. Starting from the experimental band structure of non-magnetic SrTiO₃ 2DEGs [4], we mimic magnetic exchange coupling by introducing an out-of-plane Zeeman term in a tight-binding model. We then calculate the band structure and spin textures for increasing internal magnetic fields and compute the Edelstein effect using a semiclassical Boltzmann approach [5]. We find that the conversion efficiency first increases strongly with increasing magnetic field, then shows a maximum and finally decreases.<br/>This field dependence is caused by the competition of the exchange coupling with the effective Rashba interaction. While enhancing the splitting of band pairs amplifies the Edelstein effect, weakening the in-plane Rashba-type spin texture reduces it.<br/>Experimentally, we are studying the 2DEG at the interface between SrTiO₃ and the ferromagnetic oxide EuO, to imprint ferromagnetism in the gas.<br/><br/>[1] Y. Kim, R. M. Lutchyn, and C. Nayak, <i>Origin and Transport Signatures of Spin-Orbit Interactions in One- and Two-Dimensional SrTiO₃ -Based Heterostructures</i>, Phys. Rev. B <b>87</b>, 245121 (2013).<br/>[2] A. G. Aronov and Yu. B. Lyanda-Geller, <i>Nuclear Electric Resonance and Orientation of Carrier Spins by an Electric Field</i>, Sov. J. Exp. Theor. Phys. Lett. <b>50</b>, 431 (1989).<br/>[3] V. M. Edelstein, <i>Spin Polarization of Conduction Electrons Induced by Electric Current in Two-Dimensional Asymmetric Electron Systems</i>, Solid State Commun. <b>73</b>, 233 (1990).<br/>[4] D. C. Vaz et al., <i>Mapping Spin–Charge Conversion to the Band Structure in a Topological Oxide Two-Dimensional Electron Gas</i>, Nat. Mater. <b>18</b>, 11 (2019).<br/>[5] A. Johansson, B. Göbel, J. Henk, M. Bibes, and I. Mertig, <i>Spin and Orbital Edelstein Effects in a Two-Dimensional Electron Gas: Theory and Application to SrTiO₃ Interfaces</i>, Phys. Rev. Res. <b>3</b>, 013275 (2021).