The Oxitronics team is part of the CNRS/Thales laboratory located within Thales Research and Technology in Palaiseau, France. Our main scientific interests are in oxide electronics, spintronics, multiferroics, ferroelectrics and oxide interfaces. This website contains information on our past and current research, our group members and our publications. See also our events page for information on workshops, schools, etc.
On July 15, 2020, Raphaël defended his PhD “Magnetic and transport properties of rare-earth titanate thin films and heterostructures”. A long and difficuly journey with exciting results on the way ! Also the first all on-line PhD defence in our group. Thanks a lot to all the Committee members and the Doctorate School of University Paris-Saclay. And of course congrats to Raphaël who is now a doctor !
A promising approach for beyond CMOS electronics is spintronics, which relies on ferromagnets to provide non-volatility and to generate and detect spin currents. Despite its numerous advantages, spintronics still requires the application of intense electrical currents for magnetization switching. This has been driving research on multiferroics for over a decade, with a view to achieve low-power electric-field control of magnetization. However, practical materials are scarce and magnetoelectric switching remains difficult to control. In a recent paper published in Nature, we demonstrate an alternative strategy to achieve low-power spin detection, in a non-magnetic system. We harnessed the electric-field-induced ferroelectric-like state of SrTiO3 to manipulate the spin–orbit properties of a two-dimensional electron gas – 2DEG (formed by depositing a few Å of Al onto SrTiO3). This led to the efficient conversion of spin currents into positive or negative charge currents, depending on the ferroelectric 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.
In another article just out in Phys. Rev. Materials, we also showed that a 2DEG could be formed by depositing Al onto Ca-substituted SrTiO3 (here Ca stabilizes the material into a ferroelectric ground state). The 2DEG showed signatures of a ferroelectric transition in its transport properties and its resistance shows a hysteretic dependence on electric field as a consequence of ferroelectricity.
With these two papers, we propose a new approach to bridge ferroelectricity and spintronics beyond magnetoelectricity and multiferroics, as envisioned in the ERC Advanced Grant FRESCO funding this research.
The work published in Nature was performed in collaboration with Spintec and that published in Phys. Rev. Mater. in collaboration with Spintec, the MPQ lab from Université de Paris and the SPMS lab of Centrale/Supélec.
Non-volatile electric control of spin–charge conversion in a SrTiO3 Rashba system
Paul Noël, Felix Trier, Luis M. Vicente Arche, Julien Bréhin, Diogo C. Vaz, Vincent Garcia, Stéphane Fusil, Agnès Barthélémy, Laurent Vila, Manuel Bibes & Jean-Philippe Attané
Nature, 580, 483 (2020)
Switchable two-dimensional electron gas based on ferroelectric Ca:SrTiO3
Julien Bréhin, Felix Trier, Luis M. Vicente-Arche, Pierre Hemme, Paul Noël, Maxen Cosset-Chéneau, Jean-Philippe Attané, Laurent Vila, Anke Sander, Yann Gallais, Alain Sacuto, Brahim Dkhil, Vincent Garcia, Stéphane Fusil, Agnès Barthélémy, Maximilien Cazayous, and Manuel Bibes
Phys. Rev. Mater. 4, 041002(R) (2020)
See also the Synopsis in Physics.
In a paper recently published in Nature Electronics, we show that a quantum point contact can be formed in a LaAlO3/SrTiO3 interface through electrostatic confinement of the 2-DEG using a split gate. Our device exhibits a quantized conductance due to ballistic transport in a controllable number of one-dimensional conducting channels. Under a magnetic field, the direct observation of the Zeeman splitting between spin-polarized bands allows the determination of the Landé g-factor, whose value differs strongly from that of the free electrons. Through source–drain voltage measurements, we also performed a spectroscopic investigation of the 3d energy levels inside the quantum point contact. The LaAlO3/SrTiO3 quantum point contact could potentially be used as a spectrometer
to probe Majorana states in an oxide 2-DEG.
This work was performed in collaboration with the LPEM lab of ESPCI in Paris, the C2N in Paris-Saclay and the CNR-SPIN lab in Naples.
Quantized conductance in a one-dimensional ballistic oxide nanodevice
A. Jouan, G. Singh , E. Lesne, D. C. Vaz, M. Bibes , A. Barthélémy, C. Ulysse,
D. Stornaiuolo, M. Salluzzo, S. Hurand , J. Lesueur, C. Feuillet-Palma and N. Bergeal
Nature Electron. https://doi.org/10.1038/s41928-020-0383-2 (2020)
Ferroelectric materials are made of domains in which electric dipoles are all aligned in the same direction. The manipulation of these domains by an electric field ensures low energy consumption in electronic devices based on ferroelectrics. In our recent article published in Nature, we evidence the existence of an inverse transition, because it is apparently against fundamental thermodynamic principles, in ferroelectric thin films. Under the progressive increase of temperature, a labyrinthine phase of high symmetry transforms into the less-symmetric parallel stripe domain structure. Using first-principles based effective Hamiltonian computational modeling, we find that this counter-intuitive inverse phase sequence is ascribed to an enhanced entropic contribution of domain walls, and that domain straightening and coarsening is predominantly driven by the relaxation and diffusion of topological defects. These numerical calculations are performed in thin films of ferroelectric materials (Pb(Zr0.4Ti0.6)O3 and BiFeO3) that are intensively investigated for applications. This inverse transition is also observed experimentally in BiFeO3 thin films, suggesting the universality of the phenomenon in ferroelectric oxides. Furthermore, conductivity mappings at the nanometer scale reveal that the topological defects of the labyrinthine phase are characterized by an enhanced conduction that can be up to fifty times larger than the conduction at straight segments of domain walls. Thus, the inverse transition associated to the diffusion of topological defects can be electrically sensed, opening the path to possible applications. In the figure above, the upper panel shows the inverse transition from a labyrinthine structure of ferroelectric domains to a stripe domain structure of lower symmetry as a function of temperature. The lower panel presents the experimental confirmation of these numerical simulations in BiFeO3 thin films, indicating that the topological defects trapped in the labyrinthine structure show an enhanced conductivity.
This work was performed in collaboration with the Physics Department and Institute for Nanoscience and Engineering, University of Arkansas at Fayetteville and with the SPMS lab at Centrale/Supélec.
Inverse transition of labyrinthine domain patterns in ferroelectric thin films
Y. Nahas, S. Prokhorenko, J. Fischer, B. Xu, C. Carrétéro, S. Prosandeev, M. Bibes, S. Fusil, B. Dkhil, V. Garcia, L. Bellaiche
Nature 577, 47-51 (2020)
Planar spin valves are mesoscale spintronics architectures in which spin currents are injected, transported, manipulated and detected. Traditionally, spin injection and detection are achieved with metallic ferromagnetic contacts while spins are transported in a high mobility channel based on e.g. graphene or semiconductors. This unavoidably requires interfaces between different materials, with a detrimental impact on device performance. In addition, how spins are generated and detected is inherently fixed by the electronic structure of the materials. In our paper just out in Nano Letters, we demonstrate the electric-field control of spin current generation and detection in planar nanodevices free from ferromagnets and only based on a SrTiO3 two-dimensional electron gas (2DEG) with Rashba spin-orbit coupling. The spin current is generated by the direct 2D spin Hall effect from a charge current running in the 2DEG, transported through the device and reconverted into a charge current by the inverse 2D spin Hall effect. By adjusting the Fermi level position with a gate voltage we tune the generated and detected spin polarization and relate it to the complex multiorbital band structure of the 2DEG. These findings highlight the potential of quantum oxide materials for future all-electric spin-based logic.
This work was performed in collaboration with Spintec in Grenoble and LPS in Orsay.
Electric-Field Control of Spin Current Generation and Detection in Ferromagnet-Free SrTiO3-Based Nanodevices
Felix Trier, Diogo C. Vaz, Pierre Bruneel, Paul Noël, Albert Fert, Laurent Vila, Jean-Philippe Attané, Agnès Barthélémy, Marc Gabay, Henri Jaffrès, Manuel Bibes
Nano Lett. 20, 395-401 (2020)
Materials displaying a metal-insulator transition (MIT) have been puzzling the condensed matter community for decades, as well as offering device opportunities as electronic switches and for neuromorphic computation. One family of MIT materials are perovskite rare-earth nickelates (RNiO3) and in the case of PrNiO3 and NdNiO3 the transition is first-order, i.e. it displays a thermal hysteresis within which metallic and insulating regions coexist (phase-separation). In a recent study just published in Nano Letters, we harness this phase-separated state to achieve ultrasharp resistance switching. 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 with the emergence and distribution of individual metallic domains at the nanogap. Our observation provides a useful insight into percolation at the MIT of rare-earth nickelates.
This work was performed in collaboration with the Helmholtz-Zentrum Berlin.
Imaging and Harnessing Percolation at the Metal–Insulator Transition of NdNiO3 Nanogaps
Jin Hong Lee, Felix Trier, Tom Cornelissen, Daniele Preziosi, Karim Bouzehouane, Stéphane Fusil, Sergio Valencia and Manuel Bibes
Nano Lett. 19, 7801 (2019)
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. This is providing new opportunities for devices, such as the MESO transistor proposed by Intel, that relies on writing of magnetic information through magnetoelectric coupling, and reading it by spin-charge conversion. For the latter, oxide 2DEGs are promising as their spin-charge conversion efficiency is large (see our earlier work with LaAlO3/SrTiO3 2DEGs). In a new paper just published in Nature Materials, we demonstrate a very large spin to-charge conversion effect in an high carrier-density SrTiO3 2DEG generated by the sputter-deposition of Al at room temperature, and map the dependence of this effect to the band structure (as measured by ARPES). 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 2DEGs are strong candidates for spin-based information readout in novel memory and transistor designs, and emphasize the promise of topology as a new ingredient to expand the scope of complex oxides for spintronics.
This work was performed in collaboration with Spintec and CEA-Inacin Grenoble, the Martin-Luther-Universität Halle-Wittenberg, the LPEM at ESPCI Paris, the Laboratoire de Physique des Solides in Orsay, the University of Geneva and the Helmholtz-Zentrum Berlin.
Mapping spin-charge conversion to the band structure in a topological oxide two-dimensional electron gas
Diogo C. Vaz, Paul Noël, Annika Johansson, Börge Göbel, Flavio Bruno, Gyanendra Singh, Siobhan McKeown-Walker, Felix Trier, Luis M. Vicente-Arche, Anke Sander, Sergio Valencia, Pierre Bruneel, Manali Vivek, Marc Gabay, Nicolas Bergeal, Felix Baumberger, Hanako Okuno, Agnès Barthélémy, Albert Fert, Laurent Vila, Ingrid Mertig, Jean-Philippe Attané and Manuel Bibes
Nature Mater. 18, 1187 (2019)
See also the News item on Eureka Alert.
On May 28, Lorenzo Vistoli successfully defended his PhD thesis entitled “Topological and electronic properties of electron-doped manganites thin films”. A lot of work and some very nice results ! Lorenzo passed with highest honours. Congratulations to him !
After several years of effort, the Roadmap of Oxide Electronics is now on line. This document was put together within the COST TO-BE project coordinated by Fabio Miletto Granozio from CNR-SPIN in Naples, and supervised by Mariona Coll and Josep Fontcuberta from ICMAB in Barcelona and Nini Pryds from Denmark Technical University. It covers virtually all areas of oxide research, from nanoelectronics, power electronics, spintronics to photonics, and gives a critical view on the future development of the field. We are glad to be a part of it !
Towards Oxide Electronics: a Roadmap
M. Coll et al, Appl. Surf. Sci. 482, 1 (2019)
Our PhD student Lorenzo Vistoli (second from the left) won the “Invention/innovation award” at the Italian embassy during the “Journée de la recherche italienne” organized by the RéCIF (Réseau des Chercheurs Italiens en France), in honor of the 500th anniversary of Leonardo da Vinci’s birth. He won the first place in a competition for young Italian researchers, based on scientific merit and a poster presentation on potential innovative applications of their research.
Congratulations Lorenzo !
Transition metal perovskite oxides ABO3 have a very rich array of properties, and many compounds show an insulating behavior, despite the presence of a finite number of d electrons. This insulating character is often ascribed to dynamical electron correlations, but perovskites also possess structural distorsions that break symmetries, lift electronic degeneracies, and may thus also open band gaps. In a paper just published in Nature Communications, we show that if one allows symmetry-breaking energy-lowering crystal symmetry reductions and electronic instabilities within Density Functional Theory (DFT), one successfully and systematically recovers the trends in the observed band gaps, magnetic moments, type of magnetic and crystallographic ground state, bond disproportionation and ligand hole effects, Mott vs. charge transfer insulator behaviors, and the amplitude of structural deformation modes including Jahn-Teller in low temperature spin-ordered and high temperature disordered paramagnetic phases. Since DFT does not include dynamic correlations, our work suggests that they do not play a major role in determining the metallic or insulating nature of these oxides. In other words, ABO3 may be complicated, but they are not necessarily strongly correlated.
This work was performed in collaboration with the University of Colorado at Boulder.
Origin of band gaps in 3d perovskite oxides
Julien Varignon, Manuel Bibes & Alex Zunger, Nature Commun. 10, 1658 (2019)
Since April 1st, we have a new group member, Srijani Mallik. Srijani is from Kolkota, India and in our group she is a postdoctoral researcher within a FEINMAN project sponsored by Intel Corporation. The project will focus on the study of oxide interfaces as efficient spin-charge converters for new non-volatile spin-based logic.
Welcome Srijani !
Congratulations to our group member Manuel Bibes was is one of the 222 laureates of an Advanced Grant from the European Research Council (ERC). His project, FRESCO, will focus on spin-charge interconversion effects in spin-orbitronics architectures based on ferroelectric materials. Stay tuned for future results!
It is a great pleasure to welcome two new members in our group, Julien Brehin and Pauline Dufour, who are both doing their Master internship with us. Julien will work on ferroelectric heterostructures and Pauline on antiferroelectrics. Welcome !
On December 10, Diogo succesfully defended his PhD thesis on “Spin-to-charge current conversion in SrTiO3-based two-dimensional electron gases”, and was approved with honours. He is now a PhD of Sorbonne-Université. Congratulations Diogo !
In the presence of a perpendicular magnetic field, the trajectory of electrons is curved, which leads to a transverse voltage: this is the Hall effect. When electrons travels through certain types of non-collinear spin textures – such as skyrmions – they also experience the equivalent of a magnetic field, which produces a “topological” Hall effect. In magnetic thin films with perpendicular magnetic anisotropy, magnetization may reverse by the formation of bubbles that in some cases possess a spin structure similar to that of skyrmions. In our recent paper just out in Nature Physics, we have measured a large topological Hall effect in thin films of a weakly doped metallic manganite (Ce,Ca)MnO3. Its parent compound – CaMnO3 – is a Mott insulator, i.e. a material showing an insulating character due to strong repulsion effects between electrons (coined strong correlations). Magnetic force microscopy images reveal the presence of magnetic bubbles whose density shows a magnetic field dependence akin to that of the topological Hall effect. Interestingly, the amplitude of the effect strongly increases as Ce doping is reduced and the materials approaches the Mott insulating state, which suggests that the topological Hall effect is enhanced by strong correlations.
This work was performed in collaboration with Rutgers University, Nagoya University, ICMAB Barcelona and the Universidad Complutense de Madrid
Giant topological Hall effect in correlated oxide thin films
L. Vistoli et al, Nature Phys. doi:10.1038/s41567-018-0307-5 (2018)
Rare-earth titanates are Mott insulators, and emerge as promising building blocks to realize exotic electronic states at oxide interfaces. For small rare-earths such as Dy, these titanates are canted ferrimagnets. However, critical to this magnetic order is the 3+ valence of the Ti cations, which is hard to stabilize. In our recent paper just published in Advanced Materials, we report for the first time the growth of high-quality DyTiO3 thin films, with excellent, bulk-like magnetic properties at high thickness, and a surprisingly enhanced saturation magnetization at low thickness. This thickness dependence of the magnetic properties is reminiscent of dead-layer effects in more conventional materials in which magnetization is reduced at low thickness. Through a combination of X-ray spectroscopies and magnetometry we have shown that this “living-dead” magnetic layer arise from uncoupled, paramagnetic Dy ions with neighbouring non-magnetic Ti4+ present at the film surface.
This work was performed in collaboration with the Helmholtz-Zentrum Berlin, the University of Würzburg and the Paul-Scherrer Institute.
A Living-Dead Magnetic Layer at the Surface of Ferrimagnetic DyTiO3 Thin Films
R. Aeschlimann et al ; Adv. Mater. 10.1002/adma.201707489 (2018)
Through a combination of conductive-tip atomic force microscopy (CAFM) and X-ray photemission electron microscopy (XPEEM), we have investigated the phase separation ocurring at the metal-insulator transition of NdNiO3 thin films. Our images reveal the nucleation of ∼100–300 nm metallic domains in the insulating state that grow and percolate as temperature increases. In our paper just out in Nano Letters, 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.
This work was performed in collaboration with the Helmholtz-Zentrum Berlin and the Laboratoire de Physiques des Solides in Orsay.
Direct Mapping of Phase Separation across the Metal–Insulator Transition of NdNiO3
D. Preziosi et al ; Nano Lett. 10.1021/acs.nanolett.7b04728 (2018)
If you wonder how we perform our experiments and want to delve into the process of growing our oxide samples, you can have a look at this video published as a an article in JOVE, the Journal Of Visualized Experiments. It shows our group member Diogo Vaz growing an heterostructure combining an oxide grown by pulsed laser deposition and a metal deposited by sputtering. The sample is characterized by in-situ X-ray photoelectron spectroscopy to probe the formation of a 2-dimensional electron gas in SrTiO3, and characterized by magnetotransport. The manuscript is available in pdf here.
Growth and electrostatic/chemical phroperties of metal/LaAlO3/SrTiO3 eterostructures
D.C Vaz et al ; J. Vis. Exp. 132, e56951, doi:10.3791/56951 (2018).
Along with Dutch chemist Daniël Vanmaekelbergh our group member Manuel Bibes will receive the Descartes-Huygens Prize 2017. That was announced today by the Royal Netherlands Academy of Arts and Sciences (KNAW), the Embassy of France in the Netherlands and the Académie des Sciences. The two nanoscientists have been awarded the prize for their outstanding research and their contribution to Franco-Dutch relations. Manuel will use his Descartes-Huygens Prize to spend three months conducting research at the Center for Cognitive Systems and Materials at the University Groningen. He will also visit the nanolaboratories at the University of Twente. The three organisations aim to combine their expertise to develop low-power electronics.
Pictures from the Award Ceremony that took place on February 20, 2018 at the Académie des Sciences in Paris.
As the old adage goes, Seeing is believing! This is particularly acute for antiferromagnets that despite their interest for low-power, ultrafast spintronics applications remain practically impossible to image at the nanoscale since their the antiparallel order of neighbouring spins results in a vanishing magnetization. Usually, reciprocal-space, bulk averaging methods such as neutron diffraction are the techniques of choice to characterize antiferromagnets, but then microscopic details remain concealed. In our recent paper just published in Nature, we report the first real-space imaging of long-range periodic order in a complex antiferromagnet, bismuth ferrite (BiFeO3). We have used a new technique exploiting the extreme magnetic sensitivity of the photoluminescence response of a single nitrogen-vacancy (NV) center located near the apex of an atomic force microscope tip. In addition, we have also taken advantage of the magnetoelectric coupling present in BiFeO3 to evidence the electrical control of the cycloid propagation direction in real space. Besides highlighting the potential of NV magnetometry for imaging complex antiferromagnetic orders at the nanoscale, our results demonstrate how BiFeO3 can be used in the design of reconfigurable nanoscale spin textures on-demand.
This work was performed in collaboration with Lab. Charles Coulomb, CEA-Saclay, C2N, Synchrotron SOLEIL, Lab. Aimé Cotton and University of Basel. Left image on figure courtesy of P. Maletinsky.
Real-space imaging of non-collinear antiferromagnetic order with a single-spin magnetometer
I. Gross et al ; Nature 459, 252 (2017)
Our group member Agnès Barthélémy has just been awarded the Lazare-Carnot Prize of the French Academy of Science. This Prize created by the French Ministry of Defence acknowledges important advances in fundamental research with perspectives of civilian or military applications. Congratulations Agnès !
On July 13, 2017, Vincent Garcia defended his Habilitation Thesis in Palaiseau. He presented his ~10 year work on ferroelectric tunnel junctions, from their physical principle of operation through their combination with spintronics to their use as memristors. He also gave perspectives and proposed future directions of research. Lots of exciting new concepts to explore. Well done, Vincent !
The quasi two- dimensional electron system (q2DES) that forms at the interface between LaAlO3 (LAO) and SrTiO3 (STO) has attracted much attention from the oxide electronics community. One of its hallmark features is the existence of a critical LAO thickness of 4 unit-cells (uc) for interfacial conductivity to emerge; another is the extreme sensitivity of its transport properties to electrostatic boundary conditions. This surface-interface coupling was previously exploited to modulate both carrier densities and mobilities of the q2DES through the controlled adsorption of polar solvents and by capping with different materials. In our recent work just published in Advanced Materials, we investigate in detail the chemical, electronic and transport properties of several LAO(1-2 uc)/STO samples capped with different metals (Ti, Ta, Co, Ni80Fe20 – NiFe -, Nb, Pt, Pd and Au) grown in a ultra-high vacuum (UHV) system combining pulsed laser deposition (to grow the LAO), sputtering (to grow the metal) and in situ X-ray photoemission spectroscopy (XPS). The results confirm that for several metals a q2DES forms at 1-2 uc of LAO. Additionally, XPS shows that the appearance of interfacial conductivity is accompanied by a partial oxidation of the metal, a phenomenon that is strongly linked with the q2DES properties and with the formation of defects in this system. In contrast, for noble metals, the q2DES does not form at low LAO thicknesses and instead the critical thickness is increased above 4 unit cells. We discuss the results in terms of a hybrid mechanism that incorporates both electrostatic and chemical effects.
Tuning up or down the critical thickness in LaAlO3/SrTiO3 through in situ deposition of metal overlayers
D.C. Vaz et al ; Adv. Mater. 10.1002/adma.201700486 (2017)
More than 80% of known magnetic substances have dominant antiferromagnetic interactions. As it generates no stray field, the antiferromagnetic order is very discreet, which makes the processes of nucleation or growth of domains, as well as their responses to external stimuli at the microscopic scale, a virtually uncharted territory. The scarcity of real‑space imaging techniques devoted to this class of magnetic materials is a major bottleneck to understanding the fundamental basis of their manipulation.
In this paper just published in Nature Materials, we use second harmonic generation with unprecedented sub-micron resolution to image antiferromagnetic order in BiFeO3 thin films. We provide a direct visualization of the antiferromagnetic domains in a single ferroelectric one. these antiferromagnetic domains can be manipulated thanks to magnetoelectric coupling in this archetypal multiferroic. More unexpectedly, we are also able to manipulate the antiferromagnetic domains independently of the ferroelectric polarization, using electric fields significantly lower than the ferroelectric coercivity or using optical stimuli such as THz pulses generated by a femtosecond laser. This opens horizons to manipulate the antiferromagnetic order in multiferroics and brings new insights into the emerging field of antiferromagnetic spintronics.
This work was performed in collaboration with CEA-Saclay.
Multi-stimuli manipulation of antiferromagnetic domains assessed by second-harmonic imaging
J.-Y. Chauleau et al ; Nature Mater. 10.1038/nmat4899 (2017)
Rare-earth nickelates are intringuing perovskite oxides showing metal-insulator transition tuneable by the rare-earth size, and complex antiferromagnetic order at low temperature. Yet, a complete theoretical description of their rich phase diagram was missing. In this work just out in NPJ Quantum Materials, we have used first-principles simulations to describe their electronic and magnetic experimental ground state. We show that the insulating phase is characterized by a split of the electronic states of the two Ni sites (i.e. resembling low-spin 4+ and high-spin 2+) with a concomitant shift of the oxygen-2p orbitals toward the depleted Ni cations. Therefore, from the point of view of the charge, the two Ni sites appear nearly identical whereas they re in fact distinct. Performing such calculations for several nickelates, we have built a theoretical phase diagram that reproduces all their key features, namely a systematic dependence of the MIT with the rare-earth size and the crossover between a second to first order transition for R=Pr and Nd. Our results hint at strategies to control the electronic and magnetic phases of perovskite oxides by fine tuning of the level of covalence.
This work was performed thanks to collaboration with LIST.
Complete phase diagram of rare-earth nickelates from first-principles
J. Varignon et al ; NPJ Quant. Mater. 2, 21 (2017)
Artificial neural networks show enhanced performance for key applications such as data mining or pattern recognition, but need to be implemented in hardware to make these applications accessible to everyone. Memristors are the electronic equivalent of synapses, whose variable connecting strength is at the heart of the learning process.
Accurate modelling of memristor dynamics is essential for the development of autonomous learning in artificial neural networks. In this paper, we demonstrate that spike-timing-dependent plasticity can be harnessed from inhomogeneous polarization switching in ferroelectric memristors. Combining time-dependent transport measurements, ferroelectric domain imaging, and effective-Hamiltonian-based atomistic molecular dynamics simulations, we show that the ferroelectric switching underlying resistance changes in these devices can be described by a nucleation-limited model. Using this physical model, we can reliably predict the conductance evolution of ferroelectric synapses with varying neural inputs. These results pave the way toward low-power hardware implementations of billions of reliable and predictable artificial synapses in future brain-inspired computers.
Learning through ferroelectric domain dynamics in solid-state synapses
S. Boyn et al ; Nature Comm. 8, 14736 (2017)
Non collinear spin textures such as skyrmions are currently under focus owing to their fundamental interest (associated with the influence of the topology of the spin configuration on electronic properties) and to their potential for applications in data storage.
Multiferroic BiFeO3 naturally possesses a non collinear spin order which is also controlable by an electric field. In BiFeO3, spins order along a 62-nm-period cycloid. In our study just out in Advanced Materials, we have shown that when BiFeO3 is grown in thin film form, both epitaxial strain and the application of an external magnetic field lengthen this period and may eventually destroy the cycloidal state. By combining several stimuli (strain, magnetic field, electric field), it thus appears possible to stabilize various non-collinear spin states, which bears a strong potential for spintonics and magnonics.
This work was done in collaboration with the Laboratoire MPQ (Université Paris-Diderot), the GPM (Université de Rouen), the SPMS lab (Ecole Centrale Paris), the ESRF, the ISIS facility and the Moscow Institute of Science and Technology.
Strain and Magnetic Field Induced Spin-Structure Transitions in Multiferroic BiFeO3
A. Agbelele et al, Adv. Mater. 1602327 (2017) ; DOI: 10.1002/adma.201602327
See also the News piece by Labex NanoSaclay (in French).
On Monday, September 26, our group member Mathieu Grisolia successfully defended his PhD “Novel interfacial electronic states between correlated insulators“. Exploring novel concepts in correlated oxides was an exciting but arduous task, but Mathieu was certainly up to the challenge. Congratulations !
The spin–orbit interaction couples the electrons’ motion to their spin. As a result, a charge current running through a material with strong spin–orbit coupling generates a transverse spin current (spin Hall effect, SHE) and vice versa (inverse spin Hall effect, ISHE). The emergence of SHE and ISHE as charge-to-spin interconversion mechanisms offers a variety of novel spintronic functionalities and devices, some of which do not require any ferromagnetic material. However, the interconversion efficiency of SHE and ISHE (spin Hall angle) is a bulk property that rarely exceeds ten percent, and does not take advantage of interfacial and low-dimensional effects otherwise ubiquitous in spintronic hetero- and mesostructures. In our recent study just out in Nature Materials, we make use of an interface-driven spin–orbit coupling mechanism—the Rashba effect—in the oxide two-dimensional electron system (2DES) LaAlO3/SrTiO3 to achieve spin-to-charge conversion with unprecedented efficiency. Through spin pumping, we inject a spin current from a NiFe film into the oxide 2DES and detect the resulting charge current, which can be strongly modulated by a gate voltage. We discuss the amplitude of the effect and its gate dependence on the basis of the electronic structure of the 2DES and highlight the importance of a long scattering time to achieve ecient spin-to-charge interconversion.
This work was done in collaboration with the Spintec lab (CNRS/Univ. Grenoble Alpes).
Highly efficient and tunable spin to charge conversion through Rashba coupling at oxide interfaces
E. Lesne et al, Nature Mater. 15, 1261 (2016)
See also the News and Views piece by S. Caprara.
The 1st France-Japan joint Workshop of Oxide Electronics and Spintronics took place on May 19 and 20 in Paris and Palaiseau. Organized by Hiroshi Naganuma, an associate professor from Tohoku University and currently a visiting scientist at CNRS/Thales, this event gathered 40 participants from Japan and France to discuss recent results in the field. It was sponsored by the Japanese Society of Applied Physics and CNRS.
In collaboration with Shanghai Institute of Technical Physics and Centrale-Supélec, Paris, researchers from the group have shown that organic materials can be used for tunnel barriers in memory devices as a cheaper and eco-friendly replacement of their inorganic counterparts. In this paper, Tian et al. use poly(vinylidene fluoride) with 1-2 layer thickness to achieve giant tunnel electroresistance of 1,000% at room temperature. These results should stimulate further work combining organic spintronics with organic ferroelectrics and will open a new route for low cost, silicon-compatible or potentially rollable organic devices.
Tunnel electroresistance through organic ferroelectrics
B. Tian et al, Nature Commun. 7, 11502(2016)
Our group member Mathieu Grisolia (bottom right on picture) received the MRS Graduate Student Gold Award at the past MRS Spring meeting (Phoenix, AZ) for his work “Hybridization-Controlled Charge Transfer and Induced Magnetism at Correlated Oxide Interfaces” (see recent publication in Nature Physics). Congratulations to him!
Link to announcement on MRS website.
The control of optical fields is usually achieved through the electro-optic or acousto-optic effect in single-crystal ferroelectric or polar compounds such as LiNbO3 or quartz. In recent years, tremendous progress has been made in ferroelectric oxide thin film technology – a field which is now a strong driving force in areas such as electronics, spintronics, and photovoltaics. In our recent study published in Nature Communications, we have applied epitaxial strain engineering to tune the optical response of BiFeO3 thin films, and find a very large variation of the optical index with strain, corresponding to an effective elasto-optic coefficient larger than that of quartz. We observe a concomitant strain-driven variation in light absorption – reminiscent of piezochromism – which we show can be manipulated by an electric field. This constitutes a novel electrochromic effect that is reversible, remanent, and not driven by defects. These findings broaden the potential of multiferroics toward photonics and thin film acousto-optic devices, and suggest exciting device opportunities arising from the coupling of ferroic, piezoelectric, and optical responses.
Large elasto-optic effect and reversible electrochromism in multiferroic BiFeO3
D. Sando et al, Nature Commun. 7, 10718 (2016)
In oxide materials, atomic bonds are usually ionic, which means that electrons sit either on the metal ions or on the oxygens. However, in some compounds such as nickel or copper perovskites covalent bonding is favored: electrons are shared between the metal and the oxygens. Often the situation is in fact intermediate, and the fine tuning between ionic and covalent bonding plays a key role in the emergence of high-Tc superconductivity in cuprates. In the ongoing quest for novel high-Tc superconductors built from oxide multilayers, characterizing and tuning the level of covalence in the different oxide layers is therefore as important as controlling the overall number of electrons leaking from one layer to the other.
In our study (supported by ERC grant #615759 “MINT”) just out in Nature Physics , we show that at the interface between a nickel perovskite and an ionic material (here a titanium perovskite oxide, GdTiO3), electrons are transferred into the nickelate in an amount regulated by the local level of covalence. With weakly covalent NdNiO3, the total transferred charge is the largest while with LaNiO3, the strong covalent character thwarts electron transfer: added electrons tend to disturb the covalence level, which costs energy, and the material thus tries to keep covalence unchanged. Interestingly, while our interfaces do not show superconductivity they develop a novel ferromagnetic-like state, which does not exist in bulk nickelates, and whose properties are tuned by the covalence level. Further work is needed to clarify the magnetic coupling mechanism and achieve conductivity at the interface, but our finding identifies covalence as a new knob to guide research on correlated oxide heterostructures towards its holy grail.
Hybridization-controlled charge transfer and induced magnetism at correlated oxide interfaces
M. N. Grisolia et al, Nature Phys. 12, 484 (2016)
The 3rd edition of the International School of Oxide Electronics took place in Cargèse from October 12 to 24. It attracted over 70 participants and 26 speakers from 27 countries. Great science in a great place ! Next edition will take place in 2017.
Our group member Edouard Lesne succesfully defended his Phd thesis “Non-equilibrium spin accumulation phenomena at the LaAlO3/SrTiO3 quasi-two-dimensional electron system” on Friday, September 26. Bringing spins into this celebrated interface system was a formidable challenge, with very exiciting perspectives. Congrats to Edouard !
Ordered “ferroic” materials tend to divide into domains with different orientations of their order parameter. We have studied the interaction between two domain structures in a multiferroic heterostructure comprising a metamagnetic FeRh film grown epitaxially onto a ferroelectric and ferroelastic BaTiO3 crystal. We have used element-specific X-ray imaging to map the magnetic order and orientation of the FeRh while modulating temperature and electric field across BaTiO3, and have revealed local changes that are larger and more reversible than the global changes we previously reported. Our results emphasize the importance of understanding and harnessing nanoscale ferroic domain structure to achieved enhanced couplings in artificial multiferroics.
Local electrical control of magnetic order and orientation by ferroelastic domain arrangements just above room temperature
L. C. Phillips et al, Sci. Rep. 5, 10026 (2015)
We are very happy to welcome Hiroshi Naganuma in the group. “Hiro” is a visiting scientist from the University of Tohoku in Japan and he will stay with us for one year. More information about his profile is available here.
Ferroelectric tunnel junctions (FTJs) exhibit resistance switching associated with the reversal of the ferroelectric polarization in the barrier material. We have explored the influence of the top electrode materials (W, Co, Ni, Ir) on the electronic band profile in FTJs based on BiFeO3 barriers. Large variations of the transport properties are observed at room temperature. In particular, the analysis of current vs. voltage curves by a direct tunneling model indicates that the metal/ferroelectric interfacial barrier height increases with the top-electrode work function. While larger metal work functions result in larger OFF/ON ratios, they also produce a large internal electric field which results in large and potentially destructive switching voltages.
Engineering ferroelectric tunnel junctions through potential profile shaping
S. Boyn et al, APL Materials 3, 052503 (2015)
Welcome Daniele Preziosi ! He is joining us for a post-doc on interfaces between strongly correlated oxides (ERC Consolidator grant “MINT”). Daniele is an experimentalist and already has experience in magnetic and ferroelectric oxides. More information about his profile is available here.
This week, we received a new chamber (nicknamed “TURF”) to grow oxide thin films by pulsed laser deposition (PLD), equipped with high-pressure RHEED and laser heating. Through a cluster tool this chamber is now connected with another PLD system (“SURF”) and a sputtering chamber (to grow metallic layers). As gourmets know very well, Surf and Turf make for nice combinations ! For us, TURF greatly expands the range of materials that can be combined in our heterostructures.
Working with the group of Jacobo Santamaria in Madrid and the Helmholtz Zentrum Berlin, we have observed for the first time how magnetic domains mutually influence one another other at interfaces of all-oxide tunnel junctions. In epitaxial heterostructures combining layers of antiferromagnetic LaFeO3 (LFO) and ferromagnetic La0.7Sr0.3MnO3 (LSMO), we have found that a net magnetic moment is induced in the first few unit planes of LFO near the interface with LSMO. Using X-ray photoemission electron microscopy, we could show that the ferromagnetic domain structure of the manganite electrodes is imprinted into the antiferromagnetic tunnel barrier, endowing it with spin selectivity.
Insight into spin transport in oxide heterostructures from interface-resolved magnetic mapping
F.Y. Bruno et al, Nature Commun. 6, 6306 (2015)
In collaboration with teams from Université Paris-Sud, Ecole Centrale Paris, the University of Newcastle and the Helmholtz Zentrum Berlin, we have shown that a transition from an antiferromagnetic to a largely ferromagnetic state can be induced just above room temperature by applying a low electric field in heterostructures combining BaTiO3 and FeRh. The magnetization change is very large, corresponding to the largest magnetoelectric coupling ever reported. Structural data as well as first principles calculations indicate that the effect is mainly driven by voltage-dependent strain from BaTiO3. These results highlight the relevance of hybrid multiferroics combining oxides and transition metal alloys to achieve large, high temperature magnetoelectric effects. See also the news item at HZB and on the INP CNRS website.
Electric-field control of magnetic order above room temperature
R. O. Cherifi et al., Nature Mater. 13, 345 (2014)
Ferrite de bismuth, le solide surdoué : Great article on BiFeO3, the multitalented material in the French daily newspaper Le Monde.