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International audience; Multilayers based on quantum materials (complex oxides, topological insulators, transition-metal dichalcogenides, etc.) have enabled the design of devices that could revolutionize microelectronics and optoelectronics. However, heterostructures incorporating quantum materials from different families remain scarce, while they would immensely broaden the range of possible applications. Here we demonstrate the large-scale integration of compounds from two highly multifunctional families: perovskite oxides and transition-metal dichalcogenides (TMDs). We couple BiFeO 3 , a room-temperature multiferroic oxide, and WSe 2 , a semiconducting two-dimensional material with potential for photovoltaics and photonics. WSe 2 is grown by molecular beam epitaxy and transferred on a centimeter-scale onto BiFeO 3 films. Using angle-resolved photoemission spectroscopy, we visualize the electronic structure of 1 to 3 monolayers of WSe 2 and evidence a giant energy shift as large as 0.75 eV induced by the ferroelectric polarization direction in the underlying BiFeO 3. Such a strong shift opens new perspectives in the efficient manipulation of TMD properties by proximity effects.

The development of spintronic devices has been limited by the poor compatibility between semiconductors and ferromagnetic sources of spin. The broken inversion symmetry of some semiconductors may allow for spin–charge interconversion, but its control by electric fields is volatile. This has led to interest in ferroelectric Rashba semiconductors, which combine semiconductivity, large spin–orbit coupling and non-volatility. Here we report room-temperature, non-volatile ferroelectric control of spin-to-charge conversion in epitaxial germanium telluride films. We show that ferroelectric switching by electrical gating is possible in germanium telluride, despite its high carrier density. We also show that spin-to-charge conversion has a similar magnitude to what is observed with platinum, but the charge current sign is controlled by the orientation of ferroelectric polarization. Comparison between theoretical and experimental data suggests that the inverse spin Hall effect plays a major role in switchable conversion. The ferroelectric polarization of epitaxial thin films of germanium telluride can be switched by electrical gating and used to control spin-to-charge conversion.

Flexible magneto-resistive heterostructures have received a great deal of attention over the past few years as they allow for new product paradigms that are not possible with conventional rigid substrates. While the progress and development of systems with longitudinal magnetic anisotropy on non-planar substrates has been remarkable, flexible magneto-resistive heterostructures with perpendicular magnetic anisotropy (PMA) have never been studied despite the possibility to obtain additional functionality and improved performance. To fill this gap, flexible PMA Co/Pd-based giant magneto-resistive (GMR) spin-valve stacks were prepared by using an innovative transfer-and-bonding strategy exploiting the low adhesion of a gold underlayer to SiOx/Si(100) substrates. The approach allows overcoming the limits of the direct deposition on commonly used polymer substrates, whose high surface roughness and low melting temperature could hinder the growth of complex heterostructures with perpendicular magnetic anisotropy. The obtained PMA flexible spin-valves show a sizeable GMR ratio (∼1.5%), which is not affected by the transfer process, and a high robustness against bending as indicated by the slight change of the magneto-resistive properties upon bending, thus allowing for their integration on curved surfaces and the development of a novel class of advanced devices based on flexible magneto-resistive structures with perpendicular magnetic anisotropy. Besides endowing the family of flexible electronics with PMA magneto-resistive heterostructures, the exploitation of the results might apply to high temperature growth processes and to the fabrication of other functional and flexible multilayer materials engineered at the nanoscale.

International audience; Oxide interfaces exhibit a broad range of physical effects stemming from broken inversion symmetry. In particular, they can display non-reciprocal phenomena when time reversal symmetry is also broken, e.g., by the application of a magnetic field. Examples include the direct and inverse Edelstein effects (DEE, IEE) that allow the interconversion between spin currents and charge currents. The DEE and IEE have been investigated in interfaces based on the perovskite SrTiO$_3$ (STO), albeit in separate studies focusing on one or the other. The demonstration of these effects remains mostly elusive in other oxide interface systems despite their blossoming in the last decade. Here, we report the observation of both the DEE and IEE in a new interfacial two-dimensional electron gas (2DEG) based on the perovskite oxide KTaO$_3$. We generate 2DEGs by the simple deposition of Al metal onto KTaO$_3$ single crystals, characterize them by angle-resolved photoemission spectroscopy and magnetotransport, and demonstrate the DEE through unidirectional magnetoresistance and the IEE by spin-pumping experiments. We compare the spin-charge interconversion efficiency with that of STO-based interfaces, relate it to the 2DEG electronic structure, and give perspectives for the implementation of KTaO$_3$ 2DEGs into spin-orbitronic devices.

Since the 1980s, the generation and detection of spin currents has relied on ferromagnets. Their switching today relies on spin-orbit torque from heavy metals. Nevertheless, spin injection in semiconductors has rather low efficiency. Ferroelectric Rashba semiconductors (FERSC) [1,2] may constitute a new paradigm for semiconductor spintronics, thanks to the combination of semiconductivity, large spin-orbit interaction, and the non-volatility provided by ferroelectricity. Here we report the room-temperature ferroelectric switching of spin-to-charge conversion in epitaxial GeTe films. We first show that ferroelectricity in GeTe can be reversed by electrical gating despite its high carrier density. Then, we reveal a spin-to-charge conversion as effective as in Pt, but whose sign switches with the ferroelectric polarization. These results open a route towards devices combining spin-logic and memory integrated into a silicon-compatible material.

The link between magnetization and Spin Hall Effect (SHE) has remained mostly unclear for now. In a first part of this contribution, we study oh the presence of the magnetization affect the SHE, by performing in the weak ferromagnet NiCu Spin Pumping-FMR measurements across the ferromagnetic / paramagnetic critical temperature. We show that the high spin Hall effects which can be obtained in 3d ferromagnets seems to be independent of the magnetic phase. In a second part, we show that the spin absorption process in a ferromagnetic material depends on the spin orientation relative to the magnetization. Using a ferromagnet to absorb the pure spin current created within a lateral spin valve, we evidence and quantify a sizable orientation dependence of the spin absorption in Co, CoFe, and NiFe. These experiments allow us to determine the spin-mixing conductance, an elusive but fundamental parameter of the spin-dependent transport.

Ethylene glycol-based solutions containing metal precursor chloride salts were investigated for the electrodeposition of cobalt thin films and nanowires. The electrochemical behavior of 0.5 M Co(II) chloride solution at 70 °C was studied by means of cyclic voltammetry (CV) on a Pt substrate. The reduction process was shown to be irreversible, with high faradaic efficiencies (85–90 %). A diffusion coefficient (D0) of 2.29 × 10−6 cm2 s−1 for the Co species was estimated from the electrochemical behavior at different scan rates (from 25 to 125 mV s−1). The electrodeposition process was also studied on a copper substrate at different cathodic potentials (from −0.75 V vs Pt to −0.95 V vs Pt). Field-emission scanning electron microscopy (FE-SEM) and electron dispersive spectroscopy (EDS) revealed high-purity, compact films. Template-assisted electrodeposition resulted in ~16–18 μm long cobalt nanowires with an aspect ratio L/D > 100. X-ray diffraction (XRD) analysis of Co thin films showed a preferential orientation along the HCP [100] direction, which was even more marked for the nanowires. Vibrating sample magnetometry (VSM) highlighted that the fact that Co thin films were magnetized in-plane, while in nanowires a competition between shape and magnetocrystalline anisotropy led to similar magnetic behavior for the in-plane and out-of-plane directions. Keywords: Organic solvent, Magnetization, Non-aqueous, Electroplating, Alumina template

Oxide interfaces exhibit a broad range of physical effects stemming from broken inversion symmetry. In particular, they can display non-reciprocal phenomena when time reversal symmetry is also broken, e.g., by the application of a magnetic field. Examples include the direct and inverse Edelstein effects (DEE, IEE) that allow the interconversion between spin currents and charge currents. The DEE and IEE have been investigated in interfaces based on the perovskite SrTiO

Large spin Hall angles have been observed in 3d ferromagnets, but their origin, and especially their link with the ferromagnetic order, remain unclear. Here, we investigate the inverse spin Hall effect of Ni60Cu40 and Ni50Cu50 across their Curie temperature using spin pumping experiments. We evidence that the inverse spin Hall effect in these samples is comparable to that of platinum, and that it is insensitive to the magnetic order. These results points towards a Heisenberg localized model of the transition, and suggest that the large spin Hall effects in 3d ferromagnets can be independent of the magnetic phase., 3 figures

By performing density functional theory and Green's functions calculations, complemented by x-ray photoemission spectroscopy, we investigate the electronic structure of Fe/GeTe(111), a prototypical ferromagnetic/Rashba-ferroelectric interface. We reveal that such a system exhibits several intriguing properties resulting from the complex interplay of exchange interaction, electric polarization, and spin-orbit coupling. Despite a rather strong interfacial hybridization between Fe and GeTe bands, resulting in a complete suppression of the surface states of the latter, the bulk Rashba bands are hardly altered by the ferromagnetic overlayer. This could have a deep impact on spin-dependent phenomena observed at this interface, such as spin-to-charge interconversion, which are likely to involve bulk rather than surface Rashba states.

Domain switching in polycrystalline BaTiO3 is exploited to realize self-holding phase actuators in Si-photonics. A non-volatile change of the BaTiO3 refractive-index is achieved and poly-BaTiO3-coated silicon photonic circuits are demonstrated.

The electric and nonvolatile control of the spin texture in semiconductors would represent a fundamental step toward novel electronic devices combining memory and computing functionalities. Recently, GeTe has been theoretically proposed as the father compound of a new class of materials, namely ferroelectric Rashba semiconductors. They display bulk bands with giant Rashba-like splitting due to the inversion symmetry breaking arising from the ferroelectric polarization, thus allowing for the ferroelectric control of the spin. Here, we provide the experimental demonstration of the correlation between ferroelectricity and spin texture. A surface-engineering strategy is used to set two opposite predefined uniform ferroelectric polarizations, inward and outward, as monitored by piezoresponse force microscopy. Spin and angular resolved photoemission experiments show that these GeTe(111) surfaces display opposite sense of circulation of spin in bulk Rashba bands. Furthermore, we demonstrate the crafting of nonvolatile ferroelectric patterns in GeTe films at the nanoscale by using the conductive tip of an atomic force microscope. Based on the intimate link between ferroelectric polarization and spin in GeTe, ferroelectric patterning paves the way to the investigation of devices with engineered spin configurations.

Spin-orbit coupling effects in materials with broken inversion symmetry are responsible for peculiar spin textures. Among them, ferroelectric materials allow for non-volatile control of the spin degree of freedom through the non-volatile electrical inversion of the spin texture, through to their reversible spontaneous polarization. Such functionality holds potential for technological applications exploiting spin effects controlled by electric fields. The ferroelectric Rashba semiconductor Germanium Telluride stands out as material for Spin-Orbitronics: its ferroelectricity provides a nonvolatile state variable able to generate and drive a giant bulk Rashba-type spin splitting of the electronic bands, while its semiconductivity would allow for the realization of spin-based transistors. The ferroelectric control of the bands topology and of the spin texture is expected to reflect in the tunability of the spin transport properties. Here we exploit the unidirectional spin Hall magnetoresistance of Fe/GeTe heterostructures to characterize charge-to-spin conversion in GeTe. Our preliminary results indicate a sizable conversion efficiency at low temperature (120 K), which promotes ferroelectric Rashba semiconductors as promising candidates for the implementation of non-volatile electrically reconfigurable computing devices based on spin transport in semiconductors.

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.