06 September 2022

Seminar Axe 4 presented by Paul NOËL

Spin-charge interconversion in oxide based 2D electron gases : towards oxide spinorbitronics

speaker : Paul NOËL (ETH Zurich)

Abstract : Spinorbitronics has traditionally relied on heavy metal systems to provide efficient spin-charge interconversion. Among these materials, 5d transition metals such as Pt, Ta or W offer large spin-charge conversion efficiency for spin current detections and large spin orbit torques for efficient current induced magnetization switching in SOT-MRAM devices [1].
Recently spin-orbit effects in oxides became the focus of intense interest and were exploited to realize various exotic phenomena connected with spintronics applications [2]. Among the multiple systems of interest for oxide spinorbitronics, the two-dimensional electron gas (2DEG) obtained at the surface/interface of strontium titanate (SrTiO3) attracted great attention due to its efficient and tunable spin-charge conversion [3] and large unidirectional magnetoresistance [4] associated with the Rashba Edelstein effect. While SrTiO3 is insulating in the bulk it is possible to obtain a conductive 2DEG at its surface by depositing a thin layer of insulating LaAlO3 [5]. A similar 2DEG can be obtained by depositing a thin layer of a reducing metal such as Al, Ta or Y on top of SrTiO3 alleviating the need for the high-temperature growth of crystalline LaAlO3 [6,7] and allowing the modulation of the carrier density through the adjustment of the metallic layer thickness. In the Al/SrTiO3 system, the gate tunability of the spin to charge conversion was demonstrated and a connection between the band structure of SrTiO3 and the conversion was evidenced. In particular the conversion is enhanced at certain Fermi level position corresponding to avoided band crossings and topological features [8].
Another interest of SrTiO3 is the possibility to induce ferroelectricity at low temperature. While SrTiO3 is paraelectric, it is possible to induce ferroelectricity at cryogenic temperature in STO based structure either by doping with Calcium [9] or by applying high electric field [10]. When combining the ferroelectricity of SrTiO3 with a conductive 2DEG at its surface it is possible to obtain the persistent control of the 2DEG properties such as the conductivity and carrier density as well as the non-volatile control of the spin to charge current conversion [11,12]. In particular it is possible to control the sign of the spin to charge conversion in a persistent way in a ferroelectric-like STO based structures. This non-volatile effect opens the way to the electric-field control of spin currents and to ultralow-power spintronics in which non volatility would be provided by ferroelectricity rather than by ferromagnetism [13].

1 A. Manchon, J. Železný, I. M. Miron, T. Jungwirth, J. Sinova, A. Thiaville, K. Garello, and P. Gambardella, Current-Induced Spin-Orbit Torques in Ferromagnetic and Antiferromagnetic
Systems, Rev. Mod. Phys. 91, 035004 (2019).
2 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, Nat Rev Mater 7, 4 (2022).
3 E. Lesne et al., Highly Efficient and Tunable Spin-to-Charge Conversion through Rashba Coupling at Oxide Interfaces, Nature Mater 15, 12 (2016).
4 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).
5 A. Ohtomo and H. Y. Hwang, A High-Mobility Electron Gas at the LaAlO3/SrTiO3 Heterointerface, Nature 427, 6973 (2004).
6T. C. Rödel et al., Universal Fabrication of 2D Electron Systems in Functional Oxides, Advanced Materials 28, 1976 (2016).
7 L. M. Vicente-Arche, S. Mallik, M. Cosset-Cheneau, P. Noël et al., Metal/SrTiO3 Two-Dimensional Electron Gases for Spin-to-Charge Conversion, Phys. Rev. Materials 5, 064005 (2021).
8 D. C. Vaz, P. Noël et al., Mapping Spin–Charge Conversion to the Band Structure in a Topological Oxide Two-Dimensional Electron Gas, Nat. Mater. 18, 11 (2019).
9 J. G. Bednorz and K. A. Müller, Sr1-xCaxTiO3: An xy Quantum Ferroelectric with Transition to Randomness, Phys. Rev. Lett. 52, 2289 (1984).
10 J. Hemberger, P. Lunkenheimer, R. Viana, R. Böhmer, and A. Loidl, Electric-Field-Dependent Dielectric Constant and Nonlinear Susceptibility in SrTiO3Phys. Rev. B 52, 13159 (1995).
11 J. Bréhin, F. Trier, L. M. Vicente-Arche, P. Hemme, P. Noël et al., Switchable Two-Dimensional Electron Gas Based on Ferroelectric Ca-SrTiO3, Phys. Rev. Materials 4, 041002 (2020).
12 P. Noël et al., Non-Volatile Electric Control of Spin–Charge Conversion in a SrTiO3 Rashba System, Nature 580, 7804 (2020).
13 M. Bibes, L. Vila, J.-P. Attané, P. Noël, and D. C. Vaz, Electronic Device, Digital Port, Analog Component, and Method for Generating a Voltage, US20220076868A1 (10 March 2022).

Contact : Matthieu BAILLEUL : matthieu.bailleul@ipcms.unistra.fr

Zoom connexion :
https://cnrs.zoom.us/j/92405602824?pwd=NEpsWU1PbG4vMUFQT0dsQjJuRmc3UT09 ID de réunion : 924 0560 2824
Code secret : ZjB91d

06 September 2022, 11h0012h00
IPCMS' Auditorium

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