The reversible spin texture of ferroelectric GeTe for a tunable source of spin currents

Abstract

Spin-orbit coupling effects in materials with broken inversion symmetry are responsible for peculiar spin textures. Moreover, ferroelectrics could enable the non-volatile control of the spin degree of freedom through the electrical switching of the spin texture, achievable by acting on the spontaneous polarization [1]. Such functionality holds potential for technological applications exploiting spin effects controlled by voltage pulses. Within this framework, Germanium Telluride may represent a ground-breaking multifunctional material belonging to FErroelectric Rashba SemiConductors [1]. Its ferroelectricity provides the non-volatile state variable able to generate and drive a giant bulk Rashba-type spin-splitting of the electronic bands, while its semiconductivity and CMOS-compatibility allow for the realization of spin-based transistors. In this paper we present the idea of using GeTe has a switchable and tunable source of pure spin currents. Indeed, in a Rashba system, a charge current can generate a perpendicular pure spin current by intrinsic spin Hall effect (SHE) [2]. The ferroelectric control of the Rashba effect would then allow to tune the generation of the spin currents by intrinsic SHE. The ferroelectric control of the spin texture in GeTe has been experimentally proved by combined use of Piezoresponse Force Microscopy and Spin and Angular Resolved Photoemission Spectroscopy [3], [4]. Separately, spin-to-charge conversion was previously investigated by spin-pumping experiments on GeTe/Fe bilayers, providing the first evidence of sizable interconversion from spin to charge currents [5]. Here we investigate the efficiency of the charge-to-spin interconversion process due to spin Hall effect. To this aim, we used the concept of unidirectional spin Hall magnetoresistance (USMR), introduced by Gambardella et al. [6] and sketched in Figure 1a for Fe/GeTe heterostructures. A charge current flowing in the plane of a non-magnetic (NM) thin film generates a spin current perpendicular to the film itself. A ferromagnetic layer (FM) grown on top serves as spin “detector”. Spin accumulation and spin scattering at the NM/FM interface will depend on the relative orientation between magnetization and spins direction, resulting in a variation of the conductivity upon magnetization reversal. As shown in Ref. [6], the second harmonic resistance $( R_{2\omega })$ accounts for current-induced resistive terms, including both SHE and “spurious” thermoelectric effects [7]. Preliminary experiments on Fe/GeTe bilayers reveal the presence of sizable spin Hall effect in GeTe. We detected a non-negligible variation of the longitudinal resistance upon inversion of the in-plane iron magnetization at relatively low temperatures (120 K). The relative variation of the longitudinal resistance increases linearly with the current density J Figure 1b), as expected for spin Hall effect in GeTe and ruling out the role of “spurious” thermal effects [7]. Moreover, the slope of the effect versus J is reduced when the GeTe layer thickness increases from 5 to 15 nm, pointing out the interfacial origin of the resistance variation. A detailed investigation of USMR in GeTe versus temperature and ferroelectric polarization is ongoing to eventually demonstrate the ferroelectric control of spin transport with the non-volatile inversion of spin Hall effect in ferroelectric Rashba semiconductors. The research looks towards the realization of non-volatile, electrically-controlled sources of pure spin currents. This element would open unprecedented perspectives for spintronic applications, such as the electric control of magnetization in nanostructures, the realization of spin-based neuromorphic devices and reconfigurable logic elements, as well as the excitation of spin waves in magnonic devices.