Non-volatile electric control of spin–charge conversion in a SrTiO3 Rashba system

Abstract

After 50 years of exponential increase in computing efficiency, the technology of today's electronics is approaching its physical limits, with feature sizes smaller than 10 nm. New schemes must be devised to contain the ever-increasing power consumption of information and communication systems 1 , which requires the introduction of non-traditional materials and new state variables. As recently highlighted 2 , the remanence associated with collective switching in ferroic systems is appealing to reduce power consumption. A particularly promising approach is spintronics, which relies on ferromagnets to provide non-volatility and to generate and detect spin currents 3. However, magnetization reversal by spin transfer torques 4 is a power consuming process. This is driving research on multiferroics to achieve a low-power electricfield control of magnetization 5 , but practical materials are scarce and magnetoelectric switching remains difficult to control. Here, we demonstrate an alternative strategy to achieve low-power spin detection, in a non-magnetic system. We harness the electric-field-induced ferroelectriclike state of SrTiO3 6-9 to manipulate the spin-orbit properties 10 of a two-dimensional electron gas 11 , and efficiently convert spin currents into positive or negative charge currents, depending on the polarisation direction. 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.

Prospect of Spin-Orbitronic Devices and Their Applications

“…Typically, the voltage-controlled magnetic anisotropy (VCMA) in the FM ( Lee et al., 2016 ; Baek et al., 2018b ; Wang, 2018 ; Grimaldi et al., 2020 ) (i.e., Figure 5 B), as well as the gate-voltage tunable spin-orbit current from either a metal oxide ( Mishra et al., 2019 ) as illustrated in Figure 5 C, semiconductor ( Chen et al., 2018b ), a van der Waals crystal ( Benitez et al., 2020 ), or a topological insulator ( Fan et al., 2016 ), provides a class of powerful electrical manipulation methods for programmable spin-orbit logics. Besides, introducing other multiferroic behaviors beyond the ferromagnetism, such as the ferroelectric/HM/FM heterostructure ( Cai et al., 2017 ; Belopolski et al., 2019 ; Filianina et al., 2020 ; Noël et al., 2020 ; Fang et al., 2020 ) (see Figure 5 D) and the novel proposal of magnetoelectric spin-orbit logics (MESO, see Figure 5 E), can also bring additional nonvolatile freedoms for realizing all electrically programmable functionalities ( Manipatruni et al., 2019 ). Most recently, the programmable spin-orbit domain-wall logics have been demonstrated and drawing attention by controlling either the initial domain states ( Lee et al., 2018 ) or the chirality of the domain walls ( Luo et al., 2019a , 2020 ), as shown in Figure 5 F. To look forward, more diverse methods might be proposed for the programmable spin-orbitronics, whose overall energy efficiency, scalability, and industrial feasibility should be compared and assessed before any potential practical applications.…”

“…To date, the SOT-induced manipulation of skyrmions motion ( Jiang et al., 2015 ; Buttner et al., 2017 ; Yu et al., 2016 ) (the topological spin textures stabilized by Dzyaloshinskii-Moriya interactions, DMI, see Figure 6 B) and magnetization switching in magnetic insulators ( Avci et al., 2017 ; Shao et al., 2018 ), ferromagnetic topological insulators ( Fan et al., 2014b , 2016 ), antiferromagnetic Weyl semimetals ( Tsai et al., 2020 ), and 2D ferromagnets ( Wang et al., 2019a ; Alghamdi et al., 2019 ; Ostwal et al., 2020 ) (see Figure 6 C) have already been studied intensively, and it is believed that extensive investigations of SOTs in other exotic magnetic materials, such as ferromagnetic Weyl semimetals ( Liu et al., 2019b ; Morali et al., 2019 ; Belopolski et al., 2019 ) and antiferromagnetic topological insulator ( Guin et al., 2019 ; Ghosh and Manchon, 2017 ; Otrokov et al., 2019 ) could attract intriguing interests to enrich the understanding of fundamental SOC physics and corresponding potential applications. Note that the giant amplitude of inverse and direct Rashba-Edelstein effect in oxide heterostructures of SrTiO 3 and LaAlO 3 /SrTiO 3 formed quasi 2D electron gas (2DEG) system also provide significant charge-spin interconversions ( Noël et al., 2020 ), holding the promise to pave the way from oxide spin-orbitronics prospect toward low-power electrical control of magnetizations. In addition, the frontier researches on spin-orbitronics could inspire innovations in other spintronic devices, e.g., the newly reported optical spin-orbit torque (OSOT) devices with an optical means for magnetization manipulation in FM layer ( Choi et al., 2020 ), and the magnonic devices where significant discoveries in the detection and the manipulation (see Figure 6 D) of magnetization via spin waves have been recently presented ( Han et al., 2019 ; Wang et al., 2019b ; Liu et al., 2019a ).…”

Rashba Effect in Functional Spintronic Devices

“…[8][9][10] Recently, nonvolatile control of spin-orbit effect was demonstrated using ferroelectric materials, which lead to low power operation of spin devices. [25] In another vein, electrical control over the magnetization direction of magnets is a critical feature for spintronics device applications. An asymmetric structure such as a heavy metal/ ferromagnet (FM)/oxide multilayer induces strong Rashba SOC at the interface, [2,11,12] thereby resulting in a spin torque called the Rashba spin-orbit torque (SOT).…”

“…[ 8–10 ] Recently, nonvolatile control of spin–orbit effect was demonstrated using ferroelectric materials, which lead to low power operation of spin devices. [ 25 ]…”

Spectral weight reduction of two-dimensional electron gases at oxide surfaces across the ferroelectric transition

“…electric field 19 , 20 and UV irradiation doses 14 – 16 ) can vary the 2DEG electron density, suggesting all-oxide-device applications and fabrication methods. There were also theoretical predictions that 2DEG states, which are formed at the interface between a ferroelectric oxide and , can be controlled via ferroelectric polarisation 21 , 22 ; experimentally, the control of 2DEG conductivity by using ferroelectric polarisation was observed in the modified structure of ferroelectric Pb( ) / / 13 and / 23 and the modified surface of 24 , 25 .…”