Electron energy loss spectroscopy determination of Ti oxidation state at the (001) LaAlO3/SrTiO3 interface as a function of LaAlO3 growth conditions

At the (001) interface between the two band-insulators LaAlO3 and SrTiO3, a high-mobility electron gas may appear, which has been the object of numerous works over the last four years. Its origin is a subject of debate between the interface polarity and unintended doping. Here we use electron energy loss 'spectrum images', recorded in cross-section in a scanning transmission electron microscope, to analyse the Ti3+ ratio, characteristic of extra electrons. We find an interface concentration of Ti3+ that depends on growth conditions.Comment: 6 page

Crossbar operation of BiFeO3/Ce–CaMnO3 ferroelectric tunnel junctions: From materials to integration

Ferroelectric Tunnel Junctions (FTJs) are a candidate for the hardware realization of synapses in artificial neural networks. The fabrication process for a 784 x 100 crossbar array of 500 nm large FTJs, exhibiting effective On/Off currents ratio in the range 50-100, is presented. First, the epitaxial 4 nm-BiFeO3/Ca0.96Ce0.04MnO3//YAlO3 is combined with Ni electrodes. The oxidation of Ni during the processing affects the polarity of the FTJ and the On/ Off ratio, which becomes comparable to that of CMOS-compatible HfZrO4 junctions. The latter have a wider coercive field distribution: consequently, in test crossbar arrays, BiFeO3 exhibits a smaller cross-talk than HfZrO4. Furthermore, the relatively larger threshold for ferroelectric switching in BiFeO3 allows the use application of half-programming schemes for supervised and unsupervised learning. Second, the heterostructure is combined with W and Pt electrodes. The design is optimized for the controlled collapse chip connection to neuromorphic circuits.ISSN:0884-2914ISSN:2044-532

Ultra-low damping insulating magnetic thin films get perpendicular

A magnetic material combining both low losses and strong perpendicular magnetic anisotropy (PMA) was so far missing in the field of magnon-spintronics. The authors here report on Bismuth doped YIG nanometer thick films showing both PMA and low magnetic losses for ultra-thin PMA materials

Polar Chirality in BiFeO 3 Emerging from A Peculiar Domain Wall Sequence

International audienceTopological 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 last decade, a new paradigm is emerging with the proposal of topologically-protected objects in ferroelectric materials. Recently, ferroelectric skyrmions and vortices were observed in PbTiO3/SrTiO3 superlattices, opening the path towards ultra-small topological objects with low-power electric-field control. Here we report the observation of chiral polar windings in a single epitaxial thin film, triggered by its self-organized stripe domain pattern arrangement. Combining resonant elastic X-ray scattering and scanning transmission electron microscopy, we show signatures of polar chirality in epitaxial BiFeO3 thin films corroborated with a complex ferroelectric domain wall structure. 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

Voltage-Controlled Reconfigurable Magnonic Crystal at the Sub-micrometer Scale

International audienceMultiferroics offer an elegant means to implement voltage-control and on the fly reconfigurability in microscopic, nanoscaled systems based on ferromagnetic materials. These properties are particularly interesting for the field of magnonics, where spin waves are used to perform advanced logical or analogue functions. Recently, the emergence of nano-magnonics is expected to eventually lead to the large-scale integration of magnonic devices. However, a compact voltage-controlled, on demand reconfigurable magnonic system has yet to be shown. Here, we introduce the combination of multiferroics with ferromagnets in a fully epitaxial heterostructure to achieve such voltagecontrolled and reconfigurable magnonic systems. Imprinting a remnant electrical polarization in thin multiferroic BiFeO 3 with a periodicity of 500 nm yields a modulation of the effective magnetic field in the micron-scale, ferromagnetic La 2/3 Sr 1/3 MnO 3 magnonic waveguide. We evidence the magneto-electrical coupling by characterizing the spin wave propagation spectrum in this artificial, voltage induced, magnonic crystal and demonstrate the occurrence of a robust magnonic bandgap with > 20 dB rejection

Interfacial Strain Gradients Control Nanoscale Domain Morphology in Epitaxial BiFeO3 Multiferroic Films

International audienceIn ferroelectric thin films, the domain structure defines ferroelectric switching pathways and thus influences device performance. In epitaxial bismuth ferrite (BiFeO3) films, fractal-like domains have been observed, but direct evidence of their origins has remained unclear. Here, we show that the nature of the ferroelectric domain structure-i.e. striped vs. fractal-like-in epitaxial BiFeO3 is defined by the strain profile across the film-substrate interface. In samples with fractal-like domains, X-ray diffraction analysis reveals strong strain gradients, while geometric phase analysis using atomic resolution scanning transmission electron microscopy reveals that within a few nanometers of the film-substrate interface, the out of plane strain shows an anomalous dip while the in-plane strain is constant. Electron energy-loss near edge structure at the oxygen K edge shows that in the vicinity of the interface, the oxygen coordination is locally modified; this combined with the anomalous strain behavior thus drives the formation of fractal-like domains. Conversely, if uniform strain is maintained across the interface, characteristic striped domains are formed. Interestingly, conversion from the fractal-like arrangement to striped domains is found possible by an ex-situ thermal treatment step. Critically, the antiferromagnetic state of the BiFeO3 is influenced by the domain structure, whereby the fractal-like domains disrupt the long-range spin cycloid. Finally, as a demonstration of the applicability of this concept, we show that a carefully engineered lower electrode with large strain gradient can be used to induce fractal domains

Interfacial Strain Gradients Control Nanoscale Domain Morphology in Epitaxial BiFeO3 Multiferroic Films

International audienceIn ferroelectric thin films, the domain structure defines ferroelectric switching pathways and thus influences device performance. In epitaxial bismuth ferrite (BiFeO3) films, fractal-like domains have been observed, but direct evidence of their origins has remained unclear. Here, we show that the nature of the ferroelectric domain structure-i.e. striped vs. fractal-like-in epitaxial BiFeO3 is defined by the strain profile across the film-substrate interface. In samples with fractal-like domains, X-ray diffraction analysis reveals strong strain gradients, while geometric phase analysis using atomic resolution scanning transmission electron microscopy reveals that within a few nanometers of the film-substrate interface, the out of plane strain shows an anomalous dip while the in-plane strain is constant. Electron energy-loss near edge structure at the oxygen K edge shows that in the vicinity of the interface, the oxygen coordination is locally modified; this combined with the anomalous strain behavior thus drives the formation of fractal-like domains. Conversely, if uniform strain is maintained across the interface, characteristic striped domains are formed. Interestingly, conversion from the fractal-like arrangement to striped domains is found possible by an ex-situ thermal treatment step. Critically, the antiferromagnetic state of the BiFeO3 is influenced by the domain structure, whereby the fractal-like domains disrupt the long-range spin cycloid. Finally, as a demonstration of the applicability of this concept, we show that a carefully engineered lower electrode with large strain gradient can be used to induce fractal domains

Learning through ferroelectric domain dynamics in solid-state synapses

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.ISSN:2041-172

Surface and bulk ferroelectric phase transition in super-tetragonal BiFeO 3 thin films

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Onset of multiferroicity in prototypical single spin cycloid BiFeO 3 thin films

In the room-temperature magnetoelectric multiferroic, BiFeO3, the non-collinear antiferromagnetic state is coupled to the ferroelectric order, opening applications for low-power electric-field-controlled magnetic devices. While several strategies have been explored to simplify the ferroelectric landscape, here we directly stabilize a single domain ferroelectric and spin cycloid state in epitaxial BiFeO3(111) thin films grown on orthorhombic DyScO3(011). Comparing with films grown on SrTiO3(111), we identify anisotropic in-plane strain as a powerful handle to tailor the single antiferromagnetic state. In this single domain multiferroic state, we establish the thickness limit of the coexisting electric and magnetic orders and directly visualize the suppression of the spin cycloid induced by the magnetoelectric interaction below the ultrathin limit of 1.4 nanometers. This as-grown single domain multiferroic configuration in BiFeO3 thin films opens an avenue both for fundamental investigations and for electrically-controlled non-collinear antiferromagnetic spintronics