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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 systems.sup.1 needs to be contained. These two factors require the introduction of non-traditional materials and state variables. As recently highlighted.sup.2, 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 currents.sup.3. However, magnetization reversal by spin transfer torques.sup.4 is a power-consuming process. This is driving research on multiferroics to achieve low-power electric-field control of magnetization.sup.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 ferroelectric-like state of strontium titanate (SrTiO.sub.3).sup.6-9 to manipulate the spin-orbit properties.sup.10 of a two-dimensional electron gas.sup.11, 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. The polarization direction of a ferroelectric-like state can be used to control the conversion of spin currents into charge currents at the surface of strontium titanate, a non-magnetic oxide., Author(s): Paul Noël [sup.1] [sup.4] , Felix Trier [sup.2] , Luis M. Vicente Arche [sup.2] , Julien Bréhin [sup.2] , Diogo C. Vaz [sup.2] [sup.5] , Vincent Garcia [sup.2] , [...]

Bismuth ferrite (BiFeO3) thin films were epitaxially grown on (110)- and (001)-oriented NdGaO3 single crystal orthorhombic substrates by pulsed laser deposition. The films grown on NdGaO3(110) are fully strained and show two ferroelectric variants that arrange in a stripe domain pattern with 71° domain walls, as revealed by piezoresponse force microscopy. We explored their antiferromagnetic textures using scanning nitrogen-vacancy magnetometry. Surprisingly given the large compressive strain state, the films still show a spin cycloid, resulting in a periodic zig-zag magnetic pattern due to the two ferroelastic variants. The films grown on NdGaO3(001) are also fully strained, but the (001) orthorhombic substrate imposes a strongly anisotropic in-plane strain. As a consequence, the ferroelectric polarization exhibits a uniaxial in-plane component, parallel to the b-axis of the substrate. The ferroelectric domain pattern consists of 109° charged domain walls between the two selected ferroelastic variants. This anisotropic strain impacts the magnetic state of BiFeO3 and leads to a simpler spin texture defined by a single propagation vector for the spin cycloid. In both cases, electric-field control of ferroelectric domains tends to favor a transition to a canted antiferromagnetic order. These results reveal that the cycloidal structure of BiFeO3 can undergo large compressive strain and open further electrical means to tune the magnetic state of this room-temperature multiferroic compound. [ABSTRACT FROM AUTHOR]

Topological states are currently gathering intensive investigation in condensed matter physics due to their potential as configurable electronic devices for the future era coined "topotronics." Beyond numerous breakthroughs in magnetism over the past decade, a new paradigm is emerging with the proposal of topologically protected objects in ferroelectric materials. Recently, ferroelectric skyrmions and vortices are observed in PbTiO3/SrTiO3 superlattices, opening the path toward ultra‐small topological objects with low‐power electric‐field control. Here, the observation of chiral polar windings in a single epitaxial thin film, triggered by its self‐organized stripe domain pattern arrangement, is reported. Combining resonant elastic X‐ray scattering and scanning transmission electron microscopy, signatures of polar chirality in epitaxial BiFeO3 thin films corroborated with a complex ferroelectric domain wall structure are shown. The net chirality suggests that domain walls induce a polar rotation through a small path alternating with an unexpected long path at every second domain wall. In addition, scanning probe microscopy reveals singularities associated to this peculiar domain wall structure. These results bring new insights into the unexpected complexity of standard striped‐domain BiFeO3 thin films and open questions as for the driving force of this polar chirality. [ABSTRACT FROM AUTHOR]

The ferroelectric control of a Mott transistor is a promising strategy for nonvolatile low‐power electronics. Understanding the fundamental limits of the ferroelectric‐field effect is challenging, as the relevant length scales are restricted to a few atomic planes within the interface. Here, the polarization switching process of a prototypical ferroelectric Mott transistor combining BiFeO3, a ferroelectric material with a large polarization, and (Ca,Ce)MnO3, a charge‐transfer insulator in which a few percent of Ce doping triggers a metal–insulator transition is investigated. While scanning probe microscopy indicates a complete switching of the ferroelectric gate, in‐depth atomic‐scale polarization mapping with scanning transmission electron microscopy reveals incomplete polarization reversal at the interface. Therefore, transport measurements show that the electronic properties of the Mott channel are virtually unchanged by the polarization direction. Nevertheless, in nanometer size areas where interfacial polarization switching occurs, dramatic changes of the electronic properties of (Ca,Ce)MnO3 are revealed. These results indicate how the performance of mesoscale Mott devices is hindered, and at the same time reveal the possibility of nanoscale energy‐efficient Mott transistors. [ABSTRACT FROM AUTHOR]

Domain switching pathways fundamentally control performance in ferroelectric thin film devices. In epitaxial bismuth ferrite (BiFeO3) films, the domain morphology is known to influence the multiferroic orders. While both striped and mosaic domains have been observed, the origins of the latter have remained unclear. Here, it is shown that domain morphology is defined by the strain profile across the film–substrate interface. In samples with mosaic domains, X‐ray diffraction analysis reveals strong strain gradients, while geometric phase analysis using scanning transmission electron microscopy finds that within 5 nm of the film–substrate interface, the out‐of‐plane strain shows an anomalous dip while the in‐plane strain is constant. Conversely, if uniform strain is maintained across the interface with zero strain gradient, striped domains are formed. Critically, an ex situ thermal treatment, which eliminates the interfacial strain gradient, converts the domains from mosaic to striped. The antiferromagnetic state of the BiFeO3 is also influenced by the domain structure, whereby the mosaic domains disrupt the long‐range spin cycloid. This work demonstrates that atomic scale tuning of interfacial strain gradients is a powerful route to manipulate the global multiferroic orders in epitaxial films. [ABSTRACT FROM AUTHOR]

The electric field control of functional properties is an important goal in oxide-based electronics. To endow devices with memory, ferroelectric gating is interesting, but usually weak compared to volatile electrolyte gating. Here, we report a very large ferroelectric field-effect in perovskite heterostructures combining the Mott insulator CaMnO3 and the ferroelectric BiFeO3 in its "supertetragonal" phase. Upon polarization reversal of the BiFeO3 gate, the CaMnO3 channel resistance shows a fourfold variation around room temperature, and a tenfold change at ∼200 K. This is accompanied by a carrier density modulation exceeding one order of magnitude. We have analyzed the results for various CaMnO3 thicknesses and explain them by the electrostatic doping of the CaMnO3 layer and the presence of a fixed dipole at the CaMnO3/BiFeO3 interface. Our results suggest the relevance of ferroelectric gates to control orbital- or spin-ordered phases, ubiquitous in Mott systems, and pave the way toward efficient Mott-tronics devices. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

Ferroic-order parameters are useful as state variables in non-volatile information storage media because they show a hysteretic dependence on their electric or magnetic field. Coupling ferroics with quantum-mechanical tunnelling allows a simple and fast readout of the stored information through the influence of ferroic orders on the tunnel current. For example, data in magnetic random-access memories are stored in the relative alignment of two ferromagnetic electrodes separated by a non-magnetic tunnel barrier, and data readout is accomplished by a tunnel current measurement. However, such devices based on tunnel magnetoresistance typically exhibit OFF/ON ratios of less than 4, and require high powers for write operations (>1 × 106 A cm?2). Here, we report non-volatile memories with OFF/ON ratios as high as 100 and write powers as low as ?1 × 104 A cm?2 at room temperature by storing data in the electric polarization direction of a ferroelectric tunnel barrier. The junctions show large, stable, reproducible and reliable tunnel electroresistance, with resistance switching occurring at the coercive voltage of ferroelectric switching. These ferroelectric devices emerge as an alternative to other resistive memories, and have the advantage of not being based on voltage-induced migration of matter at the nanoscale, but on a purely electronic mechanism. [ABSTRACT FROM AUTHOR]

Organic materials are attractive for building spintronics devices owing to their expected long spin lifetimes. Moreover, the ability to control their properties by changing their composition and molecular structure makes them easier to tailor to given tasks than inorganic materials. However, most studies of candidate organic spintronics materials focus on their bulk spin transport characteristics. Here we investigate the equally important process of spin injection and how it is influenced by interface coupling in the prototype organic semiconductor, Alq3. We fabricate nanometre-scale (La,Sr)MnO3/Alq3/Co magnetic tunnel junctions that exhibit a magnetoresistive response of up to 300%. Furthermore, we develop a spin transport model that describes the role of interfacial spin-dependent metal/molecule hybridization on the effective polarization allowing enhancement and even sign reversal of injected spins. We expect such insights to lead towards the molecular-level engineering of metal/organic interfaces to tailor spin injection and bring new electrical functionalities to spintronics devices. [ABSTRACT FROM AUTHOR]

A present challenge in the field of spintronics is the study of spin-dependent transport in nanodevices where all dimensions are reduced to only a few nanometres. The archetypical device structure used to explore spin transport at such a scale consists of a nanometre-sized object such as a molecule, quantum dot or nanotube connected by ferromagnetic contacts. New magneto-Coulomb effects are expected as a consequence of the interaction between charge and spin degrees of freedom. However, the focus has been mainly theoretical up to now with a lack of results on the experimental side. More importantly, in most of the studies the influence of the ferromagnetic electrodes has been largely overlooked. Here, we demonstrate that a significant magneto-Coulomb effect can be induced by conventional magnetic electrodes and mimic spin-valve magnetoresistance. Moreover, we show that the magnetic electrode can act as a gate leading to a ferromagnetic single-electron transistor with only two terminals. [ABSTRACT FROM AUTHOR]

The revival of multiferroics is motivated by their exciting physics and their ability to bring novel functionalities to a number of technological fields such as spintronics. The lack of room temperature ferroelectric ferromagnets is a problem and has driven most of the attention thus far to BIFeO3, a ferroelectric weak-ferromagnet with both transition temperatures superior to 300K. In this paper, we report on the properties of BiFeO3 heterostructures and focus on two types of approaches towards BiFeO3-based spintronics devices. One uses BiFeO3 as an exchange bias layer in spin-valve structures in which the magnetic configuration is potentially switchable by an electric field, via the magnetoelectric coupling existing in BiFeO3. The other consists in integrating BiFeO3 ultrathin films as tunnel barriers in magnetic tunnel junctions with the objective of exploiting their ferroelectric character along with their specific symmetry filtering properties. General perspectives for multiferroics in spintronics are given. [ABSTRACT FROM AUTHOR]

Multiferroics are singular materials that can exhibit simultaneously electric and magnetic orders. Some are ferroelectric and ferromagnetic and provide the opportunity to encode information in electric polarization and magnetization to obtain four logic states. However, such materials are rare and schemes allowing a simple electrical readout of these states have not been demonstrated in the same device. Here, we show that films of La0.1Bi0.9MnO3 (LBMO) are ferromagnetic and ferroelectric, and retain both ferroic properties down to a thickness of 2 nm. We have integrated such ultrathin multiferroic films as barriers in spin-filter-type tunnel junctions that exploit the magnetic and ferroelectric degrees of freedom of LBMO. Whereas ferromagnetism permits read operations reminiscent of magnetic random access memories (MRAM), the electrical switching evokes a ferroelectric RAM write operation. Significantly, our device does not require the destructive ferroelectric readout, and therefore represents an advance over the original four-state memory concept based on multiferroics. [ABSTRACT FROM AUTHOR]

In the brain, learning is achieved through the ability of synapses to reconfigure the strength by which they connect neurons (synaptic plasticity). In promising solid-state synapses called memristors, conductance can be finely tuned by voltage pulses and set to evolve according to a biological learning rule called spike-timing-dependent plasticity (STDP). Future neuromorphic architectures will comprise billions of such nanosynapses, which require a clear understanding of the physical mechanisms responsible for plasticity. Here we report on synapses based on ferroelectric tunnel junctions and show that STDP can be harnessed from inhomogeneous polarization switching. Through combined scanning probe imaging, electrical transport and atomic-scale molecular dynamics, we demonstrate that conductance variations can be modelled by the nucleation-dominated reversal of domains. Based on this physical model, our simulations show that arrays of ferroelectric nanosynapses can autonomously learn to recognize patterns in a predictable way, opening the path towards unsupervised learning in spiking neural networks. [ABSTRACT FROM AUTHOR]

Ultrashort light pulses induce rapid deformations of crystalline lattices. In ferroelectrics, lattice deformations couple directly to the polarization, which opens the perspective to modulate the electric polarization on an ultrafast time scale. Here, we report on the temporal and spatial tracking of strain and polar modulation in a single-domain BiFeO3 thin film by ultrashort light pulses. To map the light-induced deformation of the BiFeO3 unit cell, we perform time-resolved optical reflectivity and time-resolved x-ray diffraction. We show that an optical femtosecond laser pulse generates not only longitudinal but also shear strains. The longitudinal strain peaks at a large amplitude of 0.6%. The access of both the longitudinal and shear strains enables to quantitatively reconstruct the ultrafast deformation of the unit cell and to infer the corresponding reorientation of the ferroelectric polarization direction in space and time. Our findings open new perspectives for ultrafast manipulation of strain-coupled ferroic orders. [ABSTRACT FROM AUTHOR]

Using heterostructures that combine a large-polarization ferroelectric (BiFeO3) and a high-temperature superconductor (YBa2Cu3O7-δ), we demonstrate the modulation of the superconducting condensate at the nanoscale via ferroelectric field effects. Through this mechanism, a nanoscale pattern of normal regions that mimics the ferroelectric domain structure can be created in the superconductor. This yields an energy landscape for magnetic flux quanta and, in turn, couples the local ferroelectric polarization to the local magnetic induction. We show that this form of magnetoelectric coupling, together with the possibility to reversibly design the ferroelectric domain structure, allows the electrostatic manipulation of magnetic flux quanta. [ABSTRACT FROM AUTHOR]

A first-principles-based effective Hamiltonian technique is used to investigate the polarization switching mechanisms in two polymorphic phases of BiFeO3 having no defects. The switching mechanism is homogeneous for any switching field in the rhombohedral phase, while in the supertetragonal phase it changes from the classical nucleation and domain-wall motion to nucleation-limited switching with virtually no propagation, and then to homogeneous switching with increasing electric field. The first two inhomogeneous switching mechanisms of the supertetragonal phase of BiFeO3 are thus intrinsic in nature, and can be well described by the classical and nucleation-limited switching models, respectively. The reason behind their absence in the rhombohedral phase is also indicated. Moreover, the field-induced changes of switching mechanism within the supertetragonal phase are further elucidated from an energetic point of view. [ABSTRACT FROM AUTHOR]

Organic or molecular spintronics is a rising field of research at the frontier between condensed matter physics and chemistry. It aims to mix spin physics and the richness of chemistry towards designing new properties for spin electronics devices through engineering at the molecular scale. Beyond the expectation of a long spin lifetime, molecules can be also used to tailor the spin polarization of the injected current through the spin-dependent hybridization between molecules and ferromagnetic electrodes. In this Letter, we provide direct evidence of a hybrid interface spin polarization reversal due to the differing hybridization between phthalocyanine molecules and each cobalt electrode in Co/CoPc/Co magnetic tunnel junctions. Tunnel magnetoresistance and anisotropic tunnel magnetoresistance experiments show that interfacial hybridized electronic states have a unidirectional anisotropy that can be controlled by an electric field and that spin hybridization at the bottom and top interfaces differ, leading to an inverse tunnel magnetoresistance. [ABSTRACT FROM AUTHOR]