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

Abstract

Spintronics has traditionally relied on ferromagnetic materials to provide non-volatility and to generate and detect spin currents. However, magnetization reversal by spin transfer and spin orbit torques is a power-consuming process. This is driving research on multiferroics to achieve low-power electric-field control of the magnetization, but practical materials are scarce and magnetoelectric switching remains difficult to control. In this presentation we will focus on an alternative strategy: use ferroelectricity to obtain the remanent control of the spin-charge interconversion. At the interface between a ferroelectric and an ultrathin spin orbit coupling (SOC) systems -such as 2DEG- electrons are accumulated or depleted depending on the polarization direction. This modifies the electric field in the interface region, and in the ideal case changes its sign. If a Rashba state is present in the SOC system at the interface with the ferroelectric, reversing the sign of the local electric field reverses the chirality of the spin textures in both split Fermi contours as depicted in figure 1.This mechanism offers the possibility to design a wealth of devices such as bipolar memories and can also be the basis of logic devices akin to the magnetoelectric spin–orbit device proposed by Intel A highly efficient conversion was evidenced in the two-dimensional electron gas (2DEG) obtained at the interface between SrTiO3 (STO) and LaAlO3 (LAO). Moreover, thanks to the high dielectric constant of STO at cryogenic temperature the spin current generation and detection can be tuned using gate voltage [1,2]. A similar 2DEG can be obtained by depositing a thin layer of a reducing metal as Al on top of STO alleviating the need for the high-temperature growth of crystalline LAO and allowing the modulation of the carrier density through the adjustment of the Al thickness. In this system the gate tunability of the spin to charge conversion and its link to the bandstructure of STO was evidenced [3]. But despite the possibility to tune the conversion using gate voltage in STO based Rashba system, the remanent control of the 2DEG was still not achieved. While STO is not a ferroelectric it is possible to induce ferroelectricity in STO based structure either by doping it with Calcium [4] or by applying high electric field. In this presentation we will evidence that it is possible to control the 2DEG carrier density and the sign of the spin to charge conversion in a remanent way in ferroelectric-like STO based structures [5]. After applying a high enough electric field to induce a ferroelectric state in STO It is possible to modulate the sign of the conversation with positive and negative voltage pulses as depicted in figure 2.