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

International audience; The archetypical antiferroelectric, PbZrO3, is currently attracting a lot of interest, but no consensus can be clearly established on the nature of its ground state as well as on the influence of external stimuli over its physical properties. Here, the antiferroelectric state of 45-nm-thick epitaxial thin films of PbZrO3 is established by observing the characteristic structural periodicity of antiparallel dipoles at the atomic scale, combined with clear double hysteresis of the polarization-electric field response related to antiferroelectric–to–ferroelectric phase transitions. Surprisingly, while the antiferroelectric state is identified as the ground state, temperature-dependent measurements show that a transition to a ferroelectric-like state appears in a large temperature window (100 K). Atomistic simulations further confirm the existence, and provides the origin, of such ferroelectric state in the films. Electric-field-induced ferroelectric transitions are also detected by the divergence of the piezoresponse force microscopy response. Using this technique, we further reveal the signature of a ferroelectric ground state for 4-nm-thick PbZrO3 films. Compared with bulk crystals, these results suggest a more complex competition between ferroelectric and antiferroelectric phases in epitaxial thin films of PbZrO3.

We report on the formation of topological defects emerging from the cycloidal antiferromagnetic order at the surface of bulk BiFeO$_3$ crystals. Combining reciprocal and real-space magnetic imaging techniques, we first observe, in a single ferroelectric domain, the coexistence of antiferromagnetic domains in which the antiferromagnetic cycloid propagates along different wavevectors. We then show that the direction of these wavevectors is not strictly locked to the preferred crystallographic axes as continuous rotations bridge different wavevectors. At the junctions between the magnetic domains, we observe topological line defects identical to those found in a broad variety of lamellar physical systems with rotational symmetries. Our work establishes the presence of these magnetic objects at room temperature in the multiferroic antiferromagnet BiFeO$_3$, offering new possibilities for their use in spintronics., Comment: 7 pages, 4 figures, supplemental material as ancillary file

Antiferromagnetic thin films attract significant interest for future low-power spintronic devices [1]. Multiferroics, such as bismuth ferrite BiFeO3, in which antiferromagnetism and ferroelectricity coexist at room temperature, appears as a unique platform for spintronic [2] and magnonic devices [3]. The nanoscale structure of its ferroelectric domains has been widely investigated with piezoresponse force microscopy (PFM), revealing unique domain structures and domain wall functionalities [4]. However, the nanoscale magnetic textures present in BiFeO3 and their potential for spin-based technology remain concealed. In this report, we present two different antiferromagnetic spin textures in multiferroic BiFeO3 thin films with different epitaxial strains, using a commercial non-invasive scanning Nitrogen-Vacancy (NV) magnetometer based on a single NV defect in diamond, with a calibrated NV flying height of 60 nm and a proven DC field sensitivity of 1 T/Hz. Two BiFeO3 samples were grown on DyScO3 (110) and SmScO3 (110) substrates (later mentioned as BFO/DSO and BFO/SSO, respectively) using pulsed laser deposition. The striped ferroelectric domains in both samples are first observed by the in-plane PFM. The scanning NV magnetometry (SNVM) confirms the existence of the spin cycloid texture, with zig-zag wiggling angles of 90 and 127, and propagation wavelength of DSO=64 nm andSSO=103 nm, respectively. At the local scale, the combination of PFM and SNVM allows to identify the relative orientation of the ferroelectric polarization and cycloid propagation directions on both sides of a domain wall. For the BFO/DSO sample, the 90-degree in-plane rotation of the ferroelectric polarization imprints the 90-degree in-plane rotation of the cycloidal propagation direction along k1=[-1 1 0], corresponding to the type-I cycloid. On the contrary, in the BFO/SSO sample, the propagation vectors are found to be along k1'=[-2 1 1] and k2'= [1 -2 1] directions in the neighboring domains separated by the 71 domain wall. It is worth to mentioned that in the previous report [5], BFO/SSO, prepared in another growth chamber, showed G-type antiferromagnetic textures, compared to the observed type-II cycloid here. Our results here shed the light on future potential for reconfigurable nanoscale spin textures on multiferroic systems by strain engineering.

Helium implantation in epitaxial thin films is a way to control the out-of-plane deformation independently from the in-plane strain controlled by epitaxy. In particular, implantation by means of a helium microscope allows for local implantation and patterning down to the nanometer resolution, which is of interest for device applications. We present here a study of bismuth ferrite (BiFeO3) films where strain was patterned locally by helium implantation. Our combined Raman, XRD and TEM study shows that the implantation causes an elongation of the BiFeO3 unit cell and ultimately a transition towards the so-called super-tetragonal polymorph via states with mixed phases. In addition, TEM reveals the onset of amorphization at a threshold dose that does not seem to impede the overall increase in tetragonality. The phase transition from the R-like to T-like BiFeO3 appears as first-order in character, with regions of phase coexistence and abrupt changes in lattice parameters.

The temperature-dependent ferroelectric properties of super-tetragonal ${\mathrm{BiFeO}}_{3}$ are investigated using surface-sensitive low-energy electron microscopy (LEEM). We use epitaxial oxide ${\mathrm{BiFeO}}_{3}/{\mathrm{Ca}}_{0.96}{\mathrm{Ce}}_{0.04}{\mathrm{MnO}}_{3}$ bilayers grown by pulsed laser deposition on ${\mathrm{YAlO}}_{3}$ substrates. Ferroelectric, micrometer-scale domains are written by piezoresponse force microscopy and subsequently observed by LEEM from room temperature up to about 950 K. Kelvin probe force microscopy and LEEM spectroscopy reveal that the surface potential is efficiently (g50%) screened by adsorbates that are only released after annealing above 873 $\ifmmode\pm\else\textpm\fi{}$ 50 K in ultrahigh vacuum. The surface structure and chemistry of the ferroelectric thin films are analyzed using scanning transmission electron microscopy, electron energy loss spectroscopy, and x-ray photoelectron spectroscopy, discarding the occurrence of a putative ``skin layer'' effect. While its magnetic and structural transitions were reported in the literature, the true, ferroelectric Curie temperature of super-tetragonal ${\mathrm{BiFeO}}_{3}$ has not been determined so far. Here, we measure a Curie temperature of 930 $\ifmmode\pm\else\textpm\fi{}$ 30 K for the super-tetragonal ${\mathrm{BiFeO}}_{3}$ surface and corroborate it with volume-sensitive, temperature-dependent x-ray diffraction measurements. These results suggest that LEEM can be used as a powerful tool to probe surface charge and ferroelectric transitions in ultrathin films.

Perovskite rare-earth titanates are prototypical Mott insulators in which ${\mathrm{Ti}}^{3+}$ ions with $3{d}^{1}$ electronic configuration exhibit ferromagnetic or antiferromagnetic spin order, depending on the rare-earth size. This peculiar magnetic behavior has, however, been barely studied with element-specific probes, either in bulk or in thin films. The recent finding of fingerprints of ferromagnetism in two-dimensional electron gases at oxide interfaces involving rare-earth titanates has produced a surge of the interest in these complex materials. Harnessing the interfacial magnetic states in these heterostructures calls for a better understanding of their insufficiently explored magnetic states in bulk and especially in thin film form. In this paper, we combine high-resolution transmission electron microscopy with x-ray absorption spectroscopy and x-ray magnetic circular dichroism (XMCD) to determine the structural, electronic, and magnetic structure of $\mathrm{GdTi}{\mathrm{O}}_{3}$ in bulk and thin film form. In both cases, we find that the sample surface is strongly overoxidized but a few nm below, Ti is mostly 3+ and shows a large XMCD. We provide evidence for the ferrimagnetic nature of $\mathrm{GdTi}{\mathrm{O}}_{3}$ with antialigned Gd and Ti sublattices and show that, just as in antiferromagnetic $\mathrm{LaTi}{\mathrm{O}}_{3}$ or ferromagnetic $\mathrm{YTi}{\mathrm{O}}_{3}$, Ti carries no orbital moment.

Multiferroics 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 {\color{black} 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 voltage-controlled and reconfigurable magnonic systems. Imprinting a remnant electrical polarization in thin multiferroic $\mathrm{BiFeO_3}$ with a periodicity of $500\,\mathrm{nm}$ yields a modulation of the effective magnetic field in the micron-scale, ferromagnetic $\mathrm{La_{2/3}Sr_{1/3}MnO_3}$ 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 $>20 \,\mathrm{dB}$ rejection.

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.

International audience; Chirality, a foundational concept throughout science, may arise at ferromagnetic domain walls 1 22 and in related objects such as skyrmions 2. However, chiral textures should also exist in other types 23 of ferroics such as antiferromagnets for which theory predicts that they should move faster for 24 lower power 3 , and ferroelectrics where they should be extremely small and possess unusual 25 topologies 4,5. Here we report the concomitant observation of antiferromagnetic and electric chiral 26 textures at domain walls in the room-temperature ferroelectric antiferromagnet BiFeO 3. 27 Combining reciprocal and real-space characterization techniques, we reveal the presence of 28 periodic chiral antiferromagnetic objects along the domain walls as well as a priori energetically 29 unfavorable chiral ferroelectric domain walls. We discuss the mechanisms underlying their 30 formation and their relevance for electrically controlled topological oxide electronics and 31 spintronics. 32 33 Metallic ferromagnets have been the elemental bricks of spintronics for the last three decades and 34 continue to hold promises on the basis of non-collinear chiral spin textures such as skyrmions. These 35 topologically protected objects are envisioned to be the future of magnetic data storage thanks to 36 their specific stability, dynamics, and scalability 2. In parallel, antiferromagnets (AFs) are emerging as a 37 new paradigm for spintronics 6. They are intrinsically stable (being insensitive to spurious magnetic 38 fields), scalable (no cross talk between neighbouring memory cells), and fast (switching frequencies 39 in the THz regime). The opportunity of gathering the best of these two worlds and realize 40 "antiferromagnetic skyrmions" is then tremendously appealing but faces at least two major 41 challenges. The first one is to achieve antiferromagnetic chirality and the second one is to identify 42 appropriate control stimuli to create, annihilate and move these chiral objects. 43 On one hand, chirality may naturally emerge at domain walls. The antiferromagnetic domain wall 44 structure is a virtually uncharted territory but this is where translational symmetry is broken and spin 45 rotation favoured. On the other hand, AF manipulation is hampered by the intrinsic lack of net 46 magnetization, which prevents a straightforward magnetic actuation. This fundamental issue may be 47

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. 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.

Antiferromagnetic thin films are currently generating considerable excitement for low dissipation magnonics and spintronics. However, while tuneable antiferromagnetic textures form the backbone of functional devices, they are virtually unknown at the submicron scale. Here we image a wide variety of antiferromagnetic spin textures in multiferroic BiFeO3 thin films that can be tuned by strain and manipulated by electric fields through room-temperature magnetoelectric coupling. Using piezoresponse force microscopy and scanning NV magnetometry in self-organized ferroelectric patterns of BiFeO3, we reveal how strain stabilizes different types of non-collinear antiferromagnetic states (bulk-like and exotic spin cycloids) as well as collinear antiferromagnetic textures. Beyond these local-scale observations, resonant elastic X-ray scattering confirms the existence of both types of spin cycloids. Finally, we show that electric-field control of the ferroelectric landscape induces transitions either between collinear and non-collinear states or between different cycloids, offering perspectives for the design of reconfigurable antiferromagnetic spin textures on demand., Tailoring antiferromagnetic domains is critical for the development of low-dissipative spintronic and magnonic devices. Here the authors demonstrate the control of antiferromagnetic spin textures in multiferroic bismuth ferrite thin films using strain and electric fields.

International audience; Two-dimensional electron gases (2DEGs) can form at the surface of oxides and semiconductors or in carefully designed quantum wells and interfaces. Depending on the shape of the confining potential, 2DEGs may experience a finite electric field, which gives rise to relativistic effects such as the Rashba spin-orbit coupling. Although the amplitude of this electric field can be modulated by an external gate voltage, which in turn tunes the 2DEG carrier density, sheet resistance and other related properties, this modulation is volatile. Here, we report the design of a "ferroelectric" 2DEG whose transport properties can be electrostatically switched in a nonvolatile way. We generate a 2DEG by depositing a thin Al layer onto a SrTiO 3 single crystal in which 1% of Sr is substituted by Ca to make it ferroelectric. Signatures of the ferroelectric phase transition at 25 K are visible in the Raman response and in the temperature dependences of the carrier density and sheet resistance that shows a hysteretic dependence on electric field as a consequence of ferroelectricity. We suggest that this behavior may be extended to other oxide 2DEGs, leading to novel types of ferromagnet-free spintronic architectures.

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 NdNiO

International audience; Keywords: metal-insulator transition, phase separation, domain percolation, nanodevice J.H.L. and F.T. contributed equally to this work. Competition between co-existing 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, e.g., 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 260 down to 40 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 coalescence of individual metallic domains at the nanogap. Our observation provides a direct insight into percolation at the MIT of rare-earth nickelates.

The influence of film orientation, strain relaxation, and flexoelectric fields on the stability of the spin cycloid in (110)-oriented $\mathrm{BiFe}{\mathrm{O}}_{3}$ epitaxial films grown on $\mathrm{LaAl}{\mathrm{O}}_{3}$ substrates is investigated. By means of advanced x-ray-diffraction techniques, we show that thinner films have very large strain gradients which give rise to high flexoelectric fields. Using low-energy Raman spectroscopy and conversion electron M\"ossbauer spectroscopy (CEMS) we show that films up to 53 nm thick possess collinear antiferromagnetic order, with no cycloidal modulation. This suppression of the cycloid is proposed to be from strain and strain-gradient-induced flexoelectric fields. On the other hand, films thicker than 90 nm show a complex spin texture consistent with two separate cycloids, likely with different propagation directions. Interestingly, CEMS analysis suggests that the two cycloids have the same spin rotation plane. The multiple cycloids are suggested to arise from different ferroelastic domains (in turn influenced by twinning in the substrate) with different strain relaxation behaviors. These results offer insight into the factors that influence cycloid stability in the less common (110) film orientation and have implications for future magnonic devices.

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

Strong electronic correlations can produce remarkable phenomena such as metal–insulator transitions and greatly enhance superconductivity, thermoelectricity or optical nonlinearity. In correlated systems, spatially varying charge textures also amplify magnetoelectric effects or electroresistance in mesostructures. However, how spatially varying spin textures may influence electron transport in the presence of correlations remains unclear. Here we demonstrate a very large topological Hall effect (THE) in thin films of a lightly electron-doped charge-transfer insulator, (Ca,Ce)MnO3. Magnetic force microscopy reveals the presence of magnetic bubbles, whose density as a function of magnetic field peaks near the THE maximum. The THE critically depends on carrier concentration and diverges at low doping, near the metal–insulator transition. We discuss the strong amplification of the THE by correlation effects and give perspectives for its non-volatile control by electric fields., The authors thank V. Cros, V. Dobrosavljevic, J. Iñiguez, J.-V. Kim, D. Maccariello, J. Matsuno, I. Mertig, N. Nagaosa and N. Reyren for useful discussions, J.-Y. Chauleau and M. Viret for second harmonic generation experiments, N. Jaouen for resonant magnetic X-ray diffraction, J. Varignon for preparing Fig. 1a and J.-M. George for his help with some magnetotransport measurements. This research received financial support from the ERC Consolidator grant ‘MINT’ (contract no. 615759) and ANR project ‘FERROMON’. This work was also supported by a public grant overseen by the ANR as part of the ‘Investissement d’Avenir’ programme (LABEX NanoSaclay, ref. ANR-10-LABX-0035) through projects ‘FERROMOTT’ and ‘AXION’ and by the Spanish Government through project no. MAT2014-56063-C2-1-R and MAT2017-85232-R (AEI/FEDER, UE), and Severo Ochoa SEV-2015-0496 and the Generalitat de Catalunya (2014SGR 734 project). B.C. acknowledges grant no. FPI BES-2012-059023, R.C. acknowledges support from CNPq-Brazil, and J.S. thanks the University Paris-Saclay (D’Alembert programme) and CNRS for financing his stay at CNRS/Thales. Work at Rutgers was supported by the Office of Basic Energy Sciences, Division of Materials Sciences and Engineering, US Department of Energy under award no. DE-SC0018153. H.K. is supported by JSPS KAKENHI grants nos. 25400339, 15H05702 and 17H02929. K.N. is supported by a Grant-in-Aid for JSPS Research Fellow grant no. 16J05516, and by a Program for Leading Graduate Schools ‘Integrative Graduate Education and Research in Green Natural Sciences’.

Organic electronics is emerging for large-area applications such as photovoltaic cells, rollable displays or electronic paper. Its future development and integration will require a simple, low-power organic memory, that can be written, erased and readout electrically. Here we demonstrate a non-volatile memory in which the ferroelectric polarisation state of an organic tunnel barrier encodes the stored information and sets the readout tunnel current. We use high-sensitivity piezoresponse force microscopy to show that films as thin as one or two layers of ferroelectric poly(vinylidene fluoride) remain switchable with low voltages. Submicron junctions based on these films display tunnel electroresistance reaching 1,000% at room temperature that is driven by ferroelectric switching and explained by electrostatic effects in a direct tunnelling regime. Our findings provide a path to develop low-cost, large-scale arrays of organic ferroelectric tunnel junctions on silicon or flexible substrates., Ferroelectric organic materials can be used for tunnel barriers in memory devices as a cheaper and eco-friendly replacement of their inorganic counterparts. Here, Tian et al. use poly(vinylidene fluoride) with 1–2 layer thickness to achieve giant tunnel electroresistance of 1,000% at room temperature.

International audience; In 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.

The electric field control of functional properties is a crucial goal in oxide-based electronics. Non-volatile 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 Ca1-xCexMnO3, thus providing insight on how interface-engineering can enhance these switching effects., Work supported by ERC Consolidator grant MINT (Contract No. 615759)

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. Here, we apply 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 an electrochromic effect that is reversible, remanent and not driven by defects. These findings broaden the potential of multiferroics towards photonics and thin film acousto-optic devices, and suggest exciting device opportunities arising from the coupling of ferroic, piezoelectric and optical responses., Work supported by ERC Consolidator grant MINT (Contract No. 615759)

A first-principles-based effective Hamiltonian technique is used to investigate the polarization switching mechanisms in two polymorphic phases of ${\mathrm{BiFeO}}_{3}$ 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 ${\mathrm{BiFeO}}_{3}$ 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.

While ferromagnets are at the heart of daily life applications, their large magnetization and resulting energy cost for switching bring into question their suitability for reliable low-power spintronic devices. Non-collinear antiferromagnetic systems do not suffer from this problem and often possess remarkable extra functionalities: non-collinear spin order may break space-inversion symmetry and thus allow electric-field control of magnetism, or produce emergent spin-orbit effects, which enable efficient spin-charge interconversion. To harness these unique traits for next-generation spintronics, the nanoscale control and imaging capabilities that are now routine for ferromagnets must be developed for antiferromagnetic systems. Here, using a non-invasive scanning nanomagnetometer based on a single nitrogen-vacancy (NV) defect in diamond, we demonstrate the first real-space visualization of non-collinear antiferromagnetic order in a magnetic thin film, at room temperature. We image the spin cycloid of a multiferroic BiFeO$_3$ thin film and extract a period of $\sim70$ nm, consistent with values determined by macroscopic diffraction. In addition, we take advantage of the magnetoelectric coupling present in BiFeO$_3$ to manipulate the cycloid propagation direction by an electric field. Besides highlighting the unique potential of NV magnetometry for imaging complex antiferromagnetic orders at the nanoscale, these results demonstrate how BiFeO$_3$ can be used as a versatile platform for the design of reconfigurable nanoscale spin textures.

International audience; Among the variety of magnetic textures available in nature, antiferromagnetism is one of the most 'discrete' because of the exact cancellation of its staggered internal magnetization. It is therefore very challenging to probe. However, its insensitivity to external magnetic perturbations, together with the intrinsic sub-picosecond dynamics, make it very appealing for tomorrow's information technologies . Thus, it is essential to understand the microscopic mechanisms governing antiferromagnetic domains to achieve accurate manipulation and control. Using optical second-harmonic generation , a unique and laboratory-available tool , we succeeded in imaging with sub-micrometre resolution both electric and antiferromagnetic orders in the model multiferroic BiFeO$_3$. We show here that antiferromagnetic domains can be manipulated with low power consumption, using sub-coercive electric fields and sub-picosecond light pulses. Interestingly, we also show that antiferromagnetic and ferroelectric domains can behave independently, thus revealing that magneto-electric coupling can lead to various arrangements of the two orders.

In all-oxide ferroelectric (FE) - superconductor (S) bilayers, due to the low carrier concentration of oxides compared to transition metals, the FE interfacial polarization charges induce an accumulation (or depletion) of charge carriers in the S. This leads either to an enhancement or a depression of its critical temperature depending on FE polarization direction.Here we exploit this effect at a local scale to define planar weak-links in high-temperature superconducting wires. This is realized in BiFeO3(FE)/YBa2Cu3O7(S)bilayers in which the remnant FE domain structure is written at will by locally applying voltage pulses with a conductive-tip atomic force microscope. In this fashion, the FE domain pattern defines a spatial modulation of superconductivity. This allows us to write a device whose electrical transport shows different temperature regimes and magnetic field matching effects that are characteristic of Josephson coupled weak-links. This illustrates the potential of the ferroelectric approach for the realization of high-temperature superconducting devices.

The control of magnetism by electric fields is an important goal for the future development of low-power spintronics. Various approaches have been proposed on the basis of either single-phase multiferroic materials or hybrid structures in which a ferromagnet is influenced by the electric field applied to an adjacent insulator (usually having a ferroelectric, piezoelectric, or multiferroic character). The electric field effect on magnetism can be driven by purely electronic or electrostatic effects or can occur through strain coupling. Here we review progress in the electrical control of magnetic properties (anisotropy, spin order, ordering temperature, domain structure) and its application to prototype spintronic devices (spin valves, magnetic tunnel junctions). We tentatively identify the main outstanding difficulties and give perspectives for spintronics and other fields.

In ferroelectric tunnel junctions, the tunnel resistance depends on the polarization orientation of the ferroelectric tunnel barrier, giving rise to tunnel electroresistance. These devices are promising to be used as memristors in neuromorphic architectures and as non‐volatile memory elements. For both applications device scalability is essential, which requires a clear understanding of the relationship between polarization reversal and resistance change as junction size shrinks. Here we show robust tunnel electroresistance in BiFeO3‐based junctions with diameters ranging from 1200 to 180nm. We demonstrate that the tunnel electroresistance and the corresponding fraction of reversed ferroelectric domains change drastically with the junction diameter: while micron‐size junctions display reversal in less than 10% of the area, the smallest junctions show an almost complete polarization reversal. Modeling the electric‐field distribution, we highlight the critical role of the bottom electrode resistance which significantly diminishes the actual electric field applied to the ferroelectric barrier in the mixed polarization state. A polarization‐dependent critical electric field below which further reversal is prohibited is found to explain the large differences between the ferroelectric switchability of nano‐ and micron‐size junctions. Our results indicate that ferroelectric junctions are downscalable and suggest that specific junction shapes facilitate complete polarization reversal.

Spin-polarized charge transfer between a ferromagnet and a molecule can promote molecular ferromagnetism 1, 2 and hybridized interfacial states3, 4. Observations of high spin-polarization of Fermi level states at room temperature5 designate such interfaces as a very promising candidate toward achieving a highly spin-polarized, nanoscale current source at room temperature, when compared to other solutions such as half-metallic systems and solid-state tunnelling over the past decades. We will discuss three aspects of this research. 1) Does the ferromagnet/molecule interface, also called an organic spinterface, exhibit this high spin-polarization as a generic feature? Spin-polarized photoemission experiments reveal that a high spin-polarization of electronics states at the Fermi level also exist at the simple interface between ferromagnetic cobalt and amorphous carbon6. Furthermore, this effect is general to an array of ferromagnetic and molecular candidates7. 2) Integrating molecules with intrinsic properties (e.g. spin crossover molecules) into a spinterface toward enhanced functionality requires lowering the charge transfer onto the molecule8 while magnetizing it1,2. We propose to achieve this by utilizing interlayer exchange coupling within a more advanced organic spinterface architecture. We present results at room temperature across the fcc Co(001)/Cu/manganese phthalocyanine (MnPc) system9. 3) Finally, we discuss how the Co/MnPc spinterface’s ferromagnetism stabilizes antiferromagnetic ordering at room temperature onto subsequent molecules away from the spinterface, which in turn can exchange bias the Co layer at low temperature10. Consequences include tunnelling anisotropic magnetoresistance across a CoPc tunnel barrier11. This augurs new possibilities to transmit spin information across organic semiconductors using spin flip excitations12.

Using an experimental setup designed to scan a submicron sized light spot and collect the photogenerated current through larger electrodes, we map the photovoltaic response in ferroelectric BiFeO$_3$ single crystals. We study the effect produced by a unique 180° ferroelectric domain wall (DW) and show that the photocurrent maps are significantly affected by its presence and shape. The effect is large in its vicinity and in the Schottky barriers at the interface with the Au electrodes, but no extra photocurrent is observed when the illuminating spot touches the DW, indicating that this particular entity is not the heart of specific photoelectric properties. Using 3D modeling, we argue that the measured effect is due to the spatial distribution of internal fields which are significantly affected by the charge of the DW due to its distortion.

When electrons tunnel through an ultrathin ferroelectric, large variations of the tunnel transmission can result from the switching of the ferroelectric polarization. High-quality ultrathin films of BiFeO3 with atomically flat terraces and single unit-cell steps on epitaxial electrodes of LaNiO3 have been fabricated. The films crystallize in the pure polymorph of BiFeO3 with giant tetragonality; they show tunnel transport characteristics up to ten unit cells and a clear ferroelectric signal from scanning probe techniques. Sub-micrometer solid-state tunnel junctions defined from these heterostructures are switched by nanosecond voltage pulses. A complete reversal of the ferroelectric polarization results in large variations of the junction resistance, but partial switching of the polarization favors the reversibility of the resistance switching. Finally, the electrical transport characteristics of fully patterned tunnel junctions are investigated in the 300 K–80 K temperature range. Owing to the low resistivity of LaNiO3 electrodes, the junctions display record tunnel electroresistance values of more than 106 at 80 K combined with weak temperature dependences as expected for electron tunneling transport. These results will motivate further work to trigger metal–insulator transitions by electric-field effects in ferroelectric tunnel junctions combined with strongly correlated nickelates.

Resistive switching through electroresistance (ER) effect in metal-ferroelectric-metal (MFM) capacitors has attracted increasing interest due to its potential applications as memories and logic devices. However, the detailed electronic mechanisms resulting in large ER when polarisation switching occurs in the ferroelectric barrier are still not well understood. Here, ER effect up to 1000% at room temperature is demonstrated in C-MOS compatible MFM nanocapacitors with a 8.8 nm-thick poly(vinylidene fluoride) (PVDF) homopolymer ferroelectric, which is very promising for silicon industry integration. Most remarkably, using theory developed for metal-semiconductor rectifying contacts, we derive an analytical expression for the variation of interfacial barrier heights due to space-charge effect that can interpret the observed ER response. We extend this space-charge model, related to the release of trapped charges by defects, to MFM structures made of ferroelectric oxides. This space-charge model provides a simple and straightforward tool to understand recent unusual reports. Finally, this work suggests that defect-engineering could be an original and efficient route for tuning the space-charge effect and thus the ER performances in future electronic devices.

With only a few known useful room-temperature multiferroics, other ways of achieving materials showing magnetism as well as electrical polarization are sought. The discovery that the ferroelectric BaTiO3 also shows magnetism at room temperature at the interface with iron or cobalt marks a new approach to achieving multiferroic properties.

In the version of this Letter originally published, Hiroshi Kohno’s affiliation was incorrectly listed as Department of Earth and Space Science, Graduate School of Science, Osaka University, Osaka, Japan; it should have been Department of Physics, Nagoya University, Nagoya, Japan. This has been corrected in all versions of the Letter.

Anisotropies in the response of ferromagnetic electrodes attached to a gold nanoparticle lead to Coulomb blockade and spin-valve-like magnetoresistance phenomena. Such behaviour could allow the development of magnetically gated single-electron transistors composed of just two terminals.

We have studied the fabrication of atomic force microscope (AFM) based nanotemplates using electrically controlled indentation (ECI) and a composite barrier (photoresist/alumina) that is resistant to the lithography process and presents good mechanical properties for indentation. The indentation process is affected by several factors such as the indentation speed, the trigger voltage and the barrier type. We have used the nanotemplate technique to fabricate small gold–gold nanocontacts (1–10 nm). In this limit, the size of the contacts that is obtained through the indentation process seems to be stochastic. However, low dimension, clean metallic contacts were achieved with high temporal stability and compatible with low temperature measurements. The fabricated nanotemplates are versatile and can be used in a wide range of applications, from nanojunctions to connecting a single nano-object. Small area metallic contacts can be used to study spin injection or ballistic transport.

Synchronized spin-valve oscillators may lead to nanosized microwave generators that do not require discrete elements such as capacitors or inductors. Uniformly magnetized oscillators have been synchronized, but offer low power. Gyrating magnetic vortices offer greater power, but vortex synchronization has yet to be demonstrated. Here we find that vortices can interact with each other through the mediation of antivortices, leading to synchronization when they are closely spaced. The synchronization does not require a magnetic field, making the system attractive for electronic device integration. Also, because each vortex is a topological soliton, this work presents a model experimental system for the study of interacting solitons.

The epitaxial BiFeO3 films on different substrates were prepared by metal organic chemical vapor deposition (MOCVD). The ferroelectric properties of the films were controlled by Piezoresponse Force...

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 300 K. 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.

Some interfaces in semiconductors or insulators structurally cause a valence mismatch, which leads to a two-dimensional space charge that must be balanced by localised or mobile charge carriers. Screening by mobile electrons presents a lot of theoretical as well as practical interests. However it is extremely rare, so that we are aware of only one case, on which we focus here: the (0 0 1) interface between LaAlO 3 and TiO 2 -terminated SrTiO 3 . Theoretically, this interface between two insulators is positively charged. Electron conductivity is observed in this system, but whether it is associated with the interface screening or an extrinsic unintended doping is not yet settled. Here, we use the literature and our own numerical and practical experiments to discuss the physics of this system.

We have developed a new reliable method combining template synthesis and nanolithography-based contacting technique to elaborate current perpendicular-to-plane giant magnetoresistance spin valve nanowires, which are very promising for the exploration of electrical spin transfer phenomena. The method allows the electrical connection of one single nanowire in a large assembly of wires embedded in anodic porous alumina supported on Si substrate with diameters and periodicities to be controllable to a large extent. Both magnetic excitations and switching phenomena driven by a spin-polarized current were clearly demonstrated in our electrodeposited NiFe/Cu/ NiFe trilayer nanowires. This novel approach promises to be of strong interest for subsequent fabrication of phase-locked arrays of spin transfer nano-oscillators with increased output power for microwave applications.

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 La(0.1)Bi(0.9)MnO(3) (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.

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.

A reliable method enabling electrical measurements on single nanowires prepared by electrodeposition in an alumina template is described. This technique is based on electrically controlled nanoindentation of a thin insulating resist deposited on the top face of the template filled by the nanowires. We show that this method is very flexible, allowing us to electrically address single nanowires of controlled length down to 100 nm and of desired composition. Using this approach, current densities as large as 10(9) A cm(-2) were successfully injected through a point contact on a single magnetic multilayered nanowire. This demonstrates that the technique is very promising for the exploration of electrical spin injection in magnetic nanostructures.