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

After 50 years of development, the technology of today's electronics is approaching its physical limits, with feature sizes smaller than 10 nanometres. It is also becoming clear that the ever-increasing power consumption of information and communication systems1 needs to be contained. These two factors require the introduction of non-traditional materials and state variables. As recently highlighted2, the remanence associated with collective switching in ferroic systems is an appealing way to reduce power consumption. A promising approach is spintronics, which relies on ferromagnets to provide non-volatility and to generate and detect spin currents3. However, magnetization reversal by spin transfer torques4 is a power-consuming process. This is driving research on multiferroics to achieve low-power electric-field control of magnetization5, 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 ferroelectric-like state of strontium titanate (SrTiO3)6-9 to manipulate the spin-orbit properties10 of a two-dimensional electron gas11, and efficiently convert spin currents into positive or negative charge currents, depending on the polarization 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.

Field-Free Switching of Perpendicular Magnetization in an Ultrathin Epitaxial Magnetic Insulator.

For energy-efficient magnetic memories, switching of perpendicular magnetization by spin-orbit torque (SOT) appears to be a promising solution. This SOT switching requires the assistance of an in-plane magnetic field to break the symmetry. Here, we demonstrate the field-free SOT switching of a perpendicularly magnetized thulium iron garnet (Tm3Fe5O12, TmIG). The polarity of the switching loops, clockwise or counterclockwise, is determined by the direction of the initial current pulses, in contrast with field-assisted switching where the polarity is controlled by the direction of the magnetic field. From Brillouin light scattering, we determined the Dzyaloshinskii-Moriya interaction (DMI) induced by the Pt-TmIG interface. We will discuss the possible origins of field-free switching and the roles of the interfacial DMI and cubic magnetic anisotropy of TmIG. This discussion is substantiated by magnetotransport, Kerr microscopy, and micromagnetic simulations. Our observation of field-free electrical switching of a magnetic insulator is an important milestone for low-power spintronic devices.

From Low-Field Sondheimer Oscillations to High-Field Very Large and Linear Magnetoresistance in a SrTiO3-Based Two-Dimensional Electron Gas.

Quantum materials harbor a cornucopia of exotic transport phenomena challenging our understanding of condensed matter. Among these, a giant, nonsaturating linear magnetoresistance (MR) has been reported in various systems, from Weyl semimetals to topological insulators. Its origin is often ascribed to unusual band structure effects, but it may also be caused by extrinsic sample disorder. Here, we report a very large linear MR in a SrTiO3 two-dimensional electron gas and, by combining transport measurements with electron spectromicroscopy, show that it is caused by nanoscale inhomogeneities that are self-organized during sample growth. Our data also reveal semiclassical Sondheimer oscillations arising from interferences between helicoidal electron trajectories, from which we determine the 2DEG thickness. Our results bring insight into the origin of linear MR in quantum materials, expand the range of functionalities of oxide 2DEGs, and suggest exciting routes to explore the interaction of linear MR with features like Rashba spin-orbit coupling.

Visualizing Giant Ferroelectric Gating Effects in Large-Scale WSe2/BiFeO3 Heterostructures.

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 BiFeO3, a room-temperature multiferroic oxide, and WSe2, a semiconducting two-dimensional material with potential for photovoltaics and photonics. WSe2 is grown by molecular beam epitaxy and transferred on a centimeter-scale onto BiFeO3 films. Using angle-resolved photoemission spectroscopy, we visualize the electronic structure of 1 to 3 monolayers of WSe2 and evidence a giant energy shift as large as 0.75 eV induced by the ferroelectric polarization direction in the underlying BiFeO3. Such a strong shift opens new perspectives in the efficient manipulation of TMD properties by proximity effects.

Nonreciprocal Transport in a Rashba Ferromagnet, Delafossite PdCoO2.

Rashba interfaces yield efficient spin-charge interconversion and give rise to nonreciprocal transport phenomena. Here, we report magnetotransport experiments in few-nanometer-thick films of PdCoO2, a delafossite oxide known to display a large Rashba splitting and surface ferromagnetism. By analyzing the angle dependence of the first- and second-harmonic longitudinal and transverse resistivities, we identify a Rashba-driven unidirectional magnetoresistance that competes with the anomalous Nernst effect below the Curie point. We estimate a Rashba coefficient of 0.75 ± 0.3 eV Å and argue that our results qualify delafossites as a new family of oxides for nanospintronics and spin-orbitronics, beyond perovskite materials.

Spatially Controlled Octahedral Rotations and Metal-Insulator Transitions in Nickelate Superlattices.

The properties of correlated oxides can be manipulated by forming short-period superlattices since the layer thicknesses are comparable with the typical length scales of the involved correlations and interface effects. Herein, we studied the metal-insulator transitions (MITs) in tetragonal NdNiO3/SrTiO3 superlattices by controlling the NdNiO3 layer thickness, n in the unit cell, spanning the length scale of the interfacial octahedral coupling. Scanning transmission electron microscopy reveals a crossover from a modulated octahedral superstructure at n = 8 to a uniform nontilt pattern at n = 4, accompanied by a drastically weakened insulating ground state. Upon further reducing n the predominant dimensionality effect continuously raises the MIT temperature, while leaving the antiferromagnetic transition temperature unaltered down to n = 2. Remarkably, the MIT can be enhanced by imposing a sufficiently large strain even with strongly suppressed octahedral rotations. Our results demonstrate the relevance for the control of oxide functionalities at reduced dimensions.

Metal-insulator-transition engineering by modulation tilt-control in perovskite nickelates for room temperature optical switching.

In transition metal perovskites ABO3, the physical properties are largely driven by the rotations of the BO6 octahedra, which can be tuned in thin films through strain and dimensionality control. However, both approaches have fundamental and practical limitations due to discrete and indirect variations in bond angles, bond lengths, and film symmetry by using commercially available substrates. Here, we introduce modulation tilt control as an approach to tune the ground state of perovskite oxide thin films by acting explicitly on the oxygen octahedra rotation modes-that is, directly on the bond angles. By intercalating the prototype SmNiO3 target material with a tilt-control layer, we cause the system to change the natural amplitude of a given rotation mode without affecting the interactions. In contrast to strain and dimensionality engineering, our method enables a continuous fine-tuning of the materials' properties. This is achieved through two independent adjustable parameters: the nature of the tilt-control material (through its symmetry, elastic constants, and oxygen rotation angles), and the relative thicknesses of the target and tilt-control materials. As a result, a magnetic and electronic phase diagram can be obtained, normally only accessible by A-site element substitution, within the single SmNiO3 compound. With this unique approach, we successfully adjusted the metal-insulator transition (MIT) to room temperature to fulfill the desired conditions for optical switching applications.

Electric-Field Control of Spin Current Generation and Detection in Ferromagnet-Free SrTiO3-Based Nanodevices.

Spintronics entails the generation, transport, manipulation and detection of spin currents, usually in hybrid architectures comprising interfaces whose impact on performance is detrimental. In addition, how spins are generated and detected is generally material specific and determined by the electronic structure. Here, we demonstrate spin current generation, transport and electrical detection, all within a single non-magnetic material system: a SrTiO3 two-dimensional electron gas (2DEG) with Rashba spin-orbit coupling. We show that the spin current is generated from a charge current by the 2D spin Hall effect, transported through a channel and reconverted into a charge current by the inverse 2D spin Hall effect. Furthermore, by adjusting the Fermi energy with a gate voltage we tune the generated and detected spin polarization and relate it to the complex multiorbital band structure of the 2DEG. We discuss the leading mechanisms of the spin-charge interconversion processes and argue for the potential of quantum oxide materials for future all-electrical low-power spin-based logic.

Mapping spin-charge conversion to the band structure in a topological oxide two-dimensional electron gas.

While spintronics has traditionally relied on ferromagnetic metals as spin generators and detectors, spin-orbitronics exploits the efficient spin-charge interconversion enabled by spin-orbit coupling in non-magnetic systems. Although the Rashba picture of split parabolic bands is often used to interpret such experiments, it fails to explain the largest conversion effects and their relationship with the electronic structure. Here, we demonstrate a very large spin-to-charge conversion effect in an interface-engineered, high-carrier-density SrTiO3 two-dimensional electron gas and map its gate dependence on the band structure. We show that the conversion process is amplified by enhanced Rashba-like splitting due to orbital mixing and in the vicinity of avoided band crossings with topologically non-trivial order. Our results indicate that oxide two-dimensional electron gases are strong candidates for spin-based information readout in new memory and transistor designs. Our results also emphasize the promise of topology as a new ingredient to expand the scope of complex oxides for spintronics.

Imaging and Harnessing Percolation at the Metal-Insulator Transition of NdNiO3 Nanogaps.

Competition between coexisting electronic phases in first-order phase transitions can lead to a sharp change in the resistivity as the material is subjected to small variations in the driving parameter, for example, the temperature. One example of this phenomenon is the metal-insulator transition (MIT) in perovskite rare-earth nickelates. In such systems, reducing the transport measurement area to dimensions comparable to the domain size of insulating and metallic phases around the MIT should strongly influence the shape of the resistance-temperature curve. Here we measure the temperature dependence of the local resistance and the nanoscale domain distribution of NdNiO3 areas between Au contacts gapped by 40-260 nm. We find that a sharp resistance drop appears below the bulk MIT temperature at ∼105 K, with an amplitude inversely scaling with the nanogap width. By using X-ray photoemission electron microscopy, we directly correlate the resistance drop to the emergence and distribution of individual metallic domains at the nanogap. Our observation provides useful insight into percolation at the MIT of rare-earth nickelates.

Electrically Switchable and Tunable Rashba-Type Spin Splitting in Covalent Perovskite Oxides.

In transition-metal perovskites (ABO_{3}) most physical properties are tunable by structural parameters such as the rotation of the BO_{6} octahedra. Examples include the Néel temperature of orthoferrites, the conductivity of mixed-valence manganites, or the band gap of rare-earth scandates. Since oxides often hold large internal electric dipoles and can accommodate heavy elements, they also emerge as prime candidates to display Rashba spin-orbit coupling, through which charge and spin currents may be efficiently interconverted. However, despite a few experimental reports in SrTiO_{3}-based interface systems, the Rashba interaction has been little studied in these materials, and its interplay with structural distortions remains unknown. In this Letter, we identify a bismuth-based perovskite with a large, electrically switchable Rashba interaction whose amplitude can be controlled by both the ferroelectric polarization and the breathing mode of oxygen octahedra. This particular structural parameter arises from the strongly covalent nature of the Bi-O bonds, reminiscent of the situation in perovskite nickelates. Our results not only provide novel strategies to craft agile spin-charge converters but also highlight the relevance of covalence as a powerful handle to design emerging properties in complex oxides.

Direct Mapping of Phase Separation across the Metal-Insulator Transition of NdNiO3.

Perovskite rare-earth nickelates RNiO3 are prototype correlated oxides displaying a metal-insulator transition (MIT) at a temperature tunable by the ionic radius of the rare-earth R. Although its precise origin remains a debated topic, the MIT can be exploited in various types of applications, notably for resistive switching and neuromorphic computation. So far, the MIT has been mostly studied by macroscopic techniques, and insights into its nanoscale mechanisms were only provided recently by X-ray photoemission electron microscopy through absorption line shifts, used as an indirect proxy to the resistive state. Here, we directly image the local resistance of NdNiO3 thin films across their first-order MIT using conductive-atomic force microscopy. Our resistance maps reveal the nucleation of ∼100-300 nm metallic domains in the insulating state that grow and percolate as temperature increases. We discuss the resistance contrast mechanism, analyze the microscopy and transport data within a percolation model, and propose experiments to harness this mesoscopic electronic texture in devices.

Depth profiling charge accumulation from a ferroelectric into a doped Mott insulator.

The electric field control of functional properties is a crucial goal in oxide-based electronics. Nonvolatile switching between different resistivity or magnetic states in an oxide channel can be achieved through charge accumulation or depletion from an adjacent ferroelectric. However, the way in which charge distributes near the interface between the ferroelectric and the oxide remains poorly known, which limits our understanding of such switching effects. Here, we use a first-of-a-kind combination of scanning transmission electron microscopy with electron energy loss spectroscopy, near-total-reflection hard X-ray photoemission spectroscopy, and ab initio theory to address this issue. We achieve a direct, quantitative, atomic-scale characterization of the polarization-induced charge density changes at the interface between the ferroelectric BiFeO3 and the doped Mott insulator Ca(1-x)Ce(x)MnO3, thus providing insight on how interface-engineering can enhance these switching effects.

Toward Reliable Synthesis of Superconducting Infinite Layer Nickelate Thin Films by Topochemical Reduction.

Infinite layer (IL) nickelates provide a new route beyond copper oxides to address outstanding questions in the field of unconventional superconductivity. However, their synthesis poses considerable challenges, largely hindering experimental research on this new class of oxide superconductors. That synthesis is achieved in a two-step process that yields the most thermodynamically stable perovskite phase first, then the IL phase by topotactic reduction, the quality of the starting phase playing a crucial role. Here, a reliable synthesis of superconducting IL  nickelate films is reported after successive topochemical reductions of a parent perovskite phase with nearly optimal stoichiometry. Careful analysis of the transport properties of the incompletely reduced films reveals an improvement in the strange metal behavior of their normal state resistivity over subsequent topochemical reductions, offering insight into the reduction process.

Voltage-based magnetization switching and reading in magnetoelectric spin-orbit nanodevices.

As CMOS technologies face challenges in dimensional and voltage scaling, the demand for novel logic devices has never been greater, with spin-based devices offering scaling potential, at the cost of significantly high switching energies. Alternatively, magnetoelectric materials are predicted to enable low-power magnetization control, a solution with limited device-level results. Here, we demonstrate voltage-based magnetization switching and reading in nanodevices at room temperature, enabled by exchange coupling between multiferroic BiFeO3 and ferromagnetic CoFe, for writing, and spin-to-charge current conversion between CoFe and Pt, for reading. We show that, upon the electrical switching of the BiFeO3, the magnetization of the CoFe can be reversed, giving rise to different voltage outputs. Through additional microscopy techniques, magnetization reversal is linked with the polarization state and antiferromagnetic cycloid propagation direction in the BiFeO3. This study constitutes the building block for magnetoelectric spin-orbit logic, opening a new avenue for low-power beyond-CMOS technologies.

A ferroelectric memristor.

Memristors are continuously tunable resistors that emulate biological synapses. Conceptualized in the 1970s, they traditionally operate by voltage-induced displacements of matter, although the details of the mechanism remain under debate. Purely electronic memristors based on well-established physical phenomena with albeit modest resistance changes have also emerged. Here we demonstrate that voltage-controlled domain configurations in ferroelectric tunnel barriers yield memristive behaviour with resistance variations exceeding two orders of magnitude and a 10 ns operation speed. Using models of ferroelectric-domain nucleation and growth, we explain the quasi-continuous resistance variations and derive a simple analytical expression for the memristive effect. Our results suggest new opportunities for ferroelectrics as the hardware basis of future neuromorphic computational architectures.

Room temperature electrical manipulation of giant magnetoresistance in spin valves exchange-biased with BiFeO3.

Magnetoelectric multiferroics are attractive materials for the development of low-power electrically controlled spintronic devices. Here we report the optimization of the exchange bias as well as the giant magnetoresistance effect (GMR) of spin valves deposited on top of BiFeO(3)-based heterostructures. We show that the exchange bias can be electrically controlled through a change in the relative proportion of 109° domain walls and propose solutions toward a reversible process.

Atomic and electronic structure of the BaTiO3/Fe interface in multiferroic tunnel junctions.

Artificial multiferroic tunnel junctions combining a ferroelectric tunnel barrier of BaTiO(3) with magnetic electrodes display a tunnel magnetoresistance whose intensity can be controlled by the ferroelectric polarization of the barrier. This effect, called tunnel electromagnetoresistance (TEMR), and the corollary magnetoelectric coupling mechanisms at the BaTiO(3)/Fe interface were recently reported through macroscopic techniques. Here, we use advanced spectromicroscopy techniques by means of aberration-corrected scanning transmission electron microscopy (STEM) and electron energy-loss spectroscopy (EELS) to probe locally the nanoscale structural and electronic modifications at the ferroelectric/ferromagnetic interface. Atomically resolved real-space spectroscopic techniques reveal the presence of a single FeO layer between BaTiO(3) and Fe. Based on this accurate description of the studied interface, we propose an atomistic model of the ferroelectric/ferromagnetic interface further validated by comparing experimental and simulated STEM images with atomic resolution. Density functional theory calculations allow us to interpret the electronic and magnetic properties of these interfaces and to understand better their key role in the physics of multiferroics nanostructures.

Non-volatile electric control of spin-orbit torques in an oxide two-dimensional electron gas.

Spin-orbit torques (SOTs) have opened a novel way to manipulate the magnetization using in-plane current, with a great potential for the development of fast and low power information technologies. It has been recently shown that two-dimensional electron gases (2DEGs) appearing at oxide interfaces provide a highly efficient spin-to-charge current interconversion. The ability to manipulate 2DEGs using gate voltages could offer a degree of freedom lacking in the classical ferromagnetic/spin Hall effect bilayers for spin-orbitronics, in which the sign and amplitude of SOTs at a given current are fixed by the stack structure. Here, we report the non-volatile electric-field control of SOTs in an oxide-based Rashba-Edelstein 2DEG. We demonstrate that the 2DEG is controlled using a back-gate electric-field, providing two remanent and switchable states, with a large resistance contrast of 1064%. The SOTs can then be controlled electrically in a non-volatile way, both in amplitude and in sign. This achievement in a 2DEG-CoFeB/MgO heterostructures with large perpendicular magnetization further validates the compatibility of oxide 2DEGs for magnetic tunnel junction integration, paving the way to the advent of electrically reconfigurable SOT MRAMS circuits, SOT oscillators, skyrmion and domain-wall-based devices, and magnonic circuits.