Spin-orbitronics with oxide-based two-dimensional electron gases - Manuel Bibes, Université Paris-Saclay

10.06.2021 09:45 – 10:30

Spin-orbitronics with oxide-based two-dimensional electron gases
Manuel Bibes
Unité Mixte de Physique CNRS/Thales, Université Paris-Saclay, 91767 Palaiseau, France

One particularly promising direction for future information processing is spintronics, a vibrant field of research that exploits the spin degree of freedom of electrons in addition to their charge. While classical spintronics has traditionally relied on ferromagnetic metals as spin generators and spin detectors, a new approach called spin-orbitronics [1] exploits the interplay between charge and spin currents enabled by the spin-orbit coupling (SOC) in non-magnetic systems. An important advantage of spin-orbitronics is that it allows the generation of pure spin currents from charge currents and vice versa. This spin-charge interconversion phenomenon is particularly efficient at interfaces where the so-called Rashba effect locks the spin direction of the electrons perpendicular to their momentum. In such structures, applying a charge current thus generates a transverse spin density, and vice-versa.
In oxide heterostructures, interfaces can be particularly well controlled and often possess unexpected properties. One example is that of two-dimensional electron gases (2DEGs) based on SrTiO3 (STO) [2] that display a sizeable Rashba spin-orbit coupling, as discovered in 2010 at the University of Geneva [3]. In this talk, I will show that this unusual electronic state can be harnessed to interconvert spin and charge currents with unprecedented efficiencies, opening an exciting physics playground as well as offering opportunities for oxide 2DEGs in information and communication technology [4].
In a first part, I will present experiments in which we used the spin-pumping technique to inject a spin current into the 2DEG where it is converted into a charge current. By applying a gate voltage, we observe a strong variation of the spin-charge conversion amplitude with even sign changes associated to tuning the position of the Fermi level through the complex multi-orbital band structure of STO [5,6]. Importantly, a finite conversion effect persists at room temperature, with a figure-of-merit competitive for new beyond-CMOS spin-based transistors recently proposed by Intel [7]. In a second part, I will show results of charge-spin conversion detected through simple magnetotransport experiments from which the strength of the Rashba coupling can be determined [8]. I will also present the first results of spin-charge interconversion with a new type of 2DEGs based on an oxide with higher SOC, KTaO3.
Finally, I will show how ferroelectricity can be introduced as an additional degree of freedom into spin-orbitronics, yielding a non-volatile electrical control of spin-charge conversion [9]. These findings open the way to a new generation of spin-based devices, in which non-volatility would be provided by ferroelectricity rather than by ferromagnetism.

[1] F. Trier, P. Noël, J.-V. Kim, J.-P. Attané, L. Vila, and M. Bibes, Oxide Spin-Orbitronics: Spin-Charge Interconversion and Topological Spin Textures, Nature Reviews Materials (in press). Available at ArXiv:2103.16271 [Cond-Mat] (2021).
[2] A. Ohtomo and H. Y. Hwang, A High-Mobility Electron Gas at the LaAlO3/SrTiO3 Heterointerface, Nature 427, 423 (2004).
[3] A. D. Caviglia, M. Gabay, S. Gariglio, N. Reyren, C. Cancellieri, and J.-M. Triscone, Tunable Rashba Spin-Orbit Interaction at Oxide Interfaces, Phys. Rev. Lett. 104, 126803 (2010).
[4] J. Varignon, L. Vila, A. Barthélémy, and M. Bibes, A New Spin for Oxide Interfaces, Nature Phys. 14, 322 (2018).
[5] E. Lesne, Y. Fu, S. Oyarzun, J. C. Rojas-Sánchez, D. C. Vaz, H. Naganuma, and G. Sicoli, Highly Efficient and Tunable Spin-to-Charge Conversion through Rashba Coupling at Oxide Interfaces, Nature Materials 15, 1261 (2016).
[6] D. C. Vaz, P. Noël, A. Johansson, B. Göbel, F. Y. Bruno, G. Singh, S. McKeown-Walker, F. Trier, L. M. Vicente-Arche, A. Sander, S. Valencia,
P. Bruneel, M. Vivek, M. Gabay, N. Bergeal, F. Baumberger, H. Okuno, A. Barthélémy, A. Fert, L. Vila, I. Mertig, J.-P. Attané, and M. Bibes, Mapping Spin–Charge Conversion to the Band Structure in a Topological Oxide Two-Dimensional Electron Gas, Nature Materials 18, 11 (2019).
[7] S. Manipatruni, D. E. Nikonov, C.-C. Lin, T. A. Gosavi, H. Liu, B. Prasad, Y.-L. Huang, E. Bonturim, R. Ramesh, and I. A. Young, Scalable Energy-Efficient Magnetoelectric Spin–Orbit Logic, Nature 565, 35 (2019).
[8] D. C. Vaz, F. Trier, A. Dyrdał, A. Johansson, K. Garcia, A. Barthélémy, I. Mertig, J. Barnaś, A. Fert, and M. Bibes, Determining the Rashba Parameter from the Bilinear Magnetoresistance Response in a Two-Dimensional Electron Gas, Phys. Rev. Materials 4, 071001 (2020).
[9] P. Noël, F. Trier, L. M. Vicente Arche, J. Bréhin, D. C. Vaz, V. Garcia, S. Fusil, A. Barthélémy, L. Vila, M. Bibes, and J.-P. Attané, Non-Volatile Electric Control of Spin–Charge Conversion in a SrTiO3 Rashba System, Nature 580, 483 (2020).

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