Inverse transition of labyrinthine domain patterns in ferroelectric thin films.

Phase separation is a cooperative process, the kinetics of which underpin the orderly morphogenesis of domain patterns on mesoscopic scales1,2. Systems of highly degenerate frozen states may exhibit the rare and counterintuitive inverse-symmetry-breaking phenomenon3. Proposed a century ago4, inverse transitions have been found experimentally in disparate materials, ranging from polymeric and colloidal compounds to high-transition-temperature superconductors, proteins, ultrathin magnetic films, liquid crystals and metallic alloys5,6, with the notable exception of ferroelectric oxides, despite extensive theoretical and experimental work on the latter. Here we show that following a subcritical quench, the non-equilibrium self-assembly of ferroelectric domains in ultrathin films of Pb(Zr0.4Ti0.6)O3 results in a maze, or labyrinthine pattern, featuring meandering stripe domains. Furthermore, upon increasing the temperature, this highly degenerate labyrinthine phase undergoes an inverse transition whereby it transforms into the less-symmetric parallel-stripe domain structure, before the onset of paraelectricity at higher temperatures. We find that this phase sequence can be 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. Computational modelling and experimental observation of the inverse dipolar transition in BiFeO3 suggest the universality of the phenomenon in ferroelectric oxides. The multitude of self-patterned states and the various topological defects that they embody may be used beyond current domain and domain-wall-based7 technologies by enabling fundamentally new design principles and topologically enhanced functionalities within ferroelectric films.

Author Correction: Electric and antiferromagnetic chiral textures at multiferroic domain walls.

An amendment to this paper has been published and can be accessed via a link at the top of the paper.

Electric and antiferromagnetic chiral textures at multiferroic domain walls.

Chirality, a foundational concept throughout science, may arise at ferromagnetic domain walls1 and in related objects such as skyrmions2. However, chiral textures should also exist in other types of ferroic materials, such as antiferromagnets, for which theory predicts that they should move faster for lower power3, and ferroelectrics, where they should be extremely small and possess unusual topologies4,5. Here, we report the concomitant observation of antiferromagnetic and electric chiral textures at domain walls in the room-temperature ferroelectric antiferromagnet BiFeO3. Combining reciprocal and real-space characterization techniques, we reveal the presence of periodic chiral antiferromagnetic objects along the domain walls as well as a priori energetically unfavourable chiral ferroelectric domain walls. We discuss the mechanisms underlying their formation and their relevance for electrically controlled topological oxide electronics and spintronics.

Surface Proximity Effect, Imprint Memory of Ferroelectric Twins, and Tweed in the Paraelectric Phase of BaTiO3.

We have used energy-filtered photoemission electron microscopy (PEEM) at the photoemission threshold to carry out a microscopic scale characterization of the surface charge and domain structure of the (001) surface in BaTiO3. Signatures of ferroelectric and ferroelastic domains, and tweed, dominate the surface structure of BaTiO3 at room temperature. The surface ferroic signatures are maintained on heating to temperature (~550 K), well above the transition temperature (393 K). This surface proximity effect provides the mechanism for memory of the bulk ferroelectric domain arrangement up to 150 K above TC and thus can be considered as a robust fingerprint of the ferroelectric state near the surface. Self-reversal of polarization is observed for the tweed below TC and for the surface domains above TC. Annealing at higher temperature triggers the dynamic tweed which in turn allows a full reorganization of the ferroic domain configuration.

Strain and Magnetic Field Induced Spin-Structure Transitions in Multiferroic BiFeO3.

The magnetic-field-dependent spin ordering of strained BiFeO3 films is determined using nuclear resonant scattering and Raman spectroscopy. The critical field required to destroy the cycloidal modulation of the Fe spins is found to be significantly lower than in the bulk, with appealing implications for field-controlled spintronic and magnonic devices.

Tunnel electroresistance through organic ferroelectrics.

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.

Local electrical control of magnetic order and orientation by ferroelastic domain arrangements just above room temperature.

Ferroic materials (ferromagnetic, ferroelectric, ferroelastic) usually divide into domains with different orientations of their order parameter. Coupling between different ferroic systems creates new functionalities, for instance the electrical control of macroscopic magnetic properties including magnetization and coercive field. Here we show that ferroelastic domains can be used to control both magnetic order and magnetization direction at the nanoscale with a voltage. We use element-specific X-ray imaging to map the magnetic domains as a function of temperature and voltage in epitaxial FeRh on ferroelastic BaTiO3. Exploiting the nanoscale phase-separation of FeRh, we locally interconvert between ferromagnetic and antiferromagnetic states with a small electric field just above room temperature. Imaging and ab initio calculations show the antiferromagnetic phase of FeRh is favoured by compressive strain on c-oriented BaTiO3 domains, and the resultant magnetoelectric coupling is larger and more reversible than previously reported from macroscopic measurements. Our results emphasize the importance of nanoscale ferroic domain structure and the promise of first-order transition materials to achieve enhanced coupling in artificial multiferroics.

Strain engineering of perovskite thin films using a single substrate.

Combining temperature-dependent x-ray diffraction, Raman spectroscopy and first-principles-based effective Hamiltonian calculations, we show that varying the thickness of (Ba0.8Sr0.2)TiO3 (BST) thin films deposited on the same single substrate (namely, MgO) enables us to change not only the magnitude but also the sign of the misfit strain. Such previously overlooked control of the strain allows several properties of these films (e.g. Curie temperature, symmetry of ferroelectric phases, dielectric response) to be tuned and even optimized. Surprisingly, such desired control of the strain (and of the resulting properties) originates from an effect that is commonly believed to be detrimental to functionalities of films, namely the existence of misfit dislocations. The present study therefore provides a novel route to strain engineering, as well as leading us to revisit common beliefs.

Control of ferroelectricity and magnetism in multi-ferroic BiFeO3 by epitaxial strain.

Recently, strain engineering has been shown to be a powerful and flexible means of tailoring the properties of ABO3 perovskite thin films. The effect of epitaxial strain on the structure of the perovskite unit cell can induce a host of interesting effects, these arising from either polar cation shifts or rotation of the oxygen octahedra, or both. In the multi-ferroic perovskite bismuth ferrite (BiFeO3-BFO), both degrees of freedom exist, and thus a complex behaviour may be expected as one plays with epitaxial strain. In this paper, we review our results on the role of strain on the ferroic transition temperatures and ferroic order parameters. We find that, while the Néel temperature is almost unchanged by strain, the ferroelectric Curie temperature strongly decreases as strain increases in both the tensile and compressive ranges. Also unexpected is the very weak influence of strain on the ferroelectric polarization value. Using effective Hamiltonian calculations, we show that these peculiar behaviours arise from the competition between antiferrodistortive and polar instabilities. Finally, we present results on the magnetic order: while the cycloidal spin modulation present in the bulk survives in weakly strained films, it is destroyed at large strain and replaced by pseudo-collinear antiferromagnetic ordering. We discuss the origin of this effect and give perspectives for devices based on strain-engineered BiFeO3.

Electric-field control of magnetic order above room temperature.

Controlling magnetism by means of electric fields is a key issue for the future development of low-power spintronics. Progress has been made in the electrical control of magnetic anisotropy, domain structure, spin polarization or critical temperatures. However, the ability to turn on and off robust ferromagnetism at room temperature and above has remained elusive. Here we use ferroelectricity in BaTiO3 crystals to tune the sharp metamagnetic transition temperature of epitaxially grown FeRh films and electrically drive a transition between antiferromagnetic and ferromagnetic order with only a few volts, just above room temperature. The detailed analysis of the data in the light of first-principles calculations indicate that the phenomenon is mediated by both strain and field effects from the BaTiO3. Our results correspond to a magnetoelectric coupling larger than previous reports by at least one order of magnitude and open new perspectives for the use of ferroelectrics in magnetic storage and spintronics.

The direct magnetoelectric effect in ferroelectric-ferromagnetic epitaxial heterostructures.

Ferroelectric (FE) and ferromagnetic (FM) materials engineered in horizontal heterostructures allow interface-mediated magnetoelectric coupling. The so-called converse magnetoelectric effect (CME) has been already demonstrated by electric-field poling of the ferroelectric layers and subsequent modification of the magnetic state of adjacent ferromagnetic layers by strain effects and/or free-carrier density tuning. Here we focus on the direct magnetoelectric effect (DME) where the dielectric state of a ferroelectric thin film is modified by a magnetic field. Ferroelectric BaTiO3 (BTO) and ferromagnetic CoFe2O4 (CFO) oxide thin films have been used to create epitaxial FE/FM and FM/FE heterostructures on SrTiO3(001) substrates buffered with metallic SrRuO3. It will be shown that large ferroelectric polarization and DME can be obtained by appropriate selection of the stacking order of the FE and FM films and their relative thicknesses. The dielectric permittivity, at the structural transitions of BTO, is strongly modified (up to 36%) when measurements are performed under a magnetic field. Due to the insulating nature of the ferromagnetic layer and the concomitant absence of the electric-field effect, the observed DME effect solely results from the magnetostrictive response of CFO elastically coupled to the BTO layer. These findings show that appropriate architecture and materials selection allow overcoming substrate-induced clamping in multiferroic multi-layered films.

Crafting the magnonic and spintronic response of BiFeO3 films by epitaxial strain.

Multiferroics are compounds that show ferroelectricity and magnetism. BiFeO3, by far the most studied, has outstanding ferroelectric properties, a cycloidal magnetic order in the bulk, and many unexpected virtues such as conductive domain walls or a low bandgap of interest for photovoltaics. Although this flurry of properties makes BiFeO3 a paradigmatic multifunctional material, most are related to its ferroelectric character, and its other ferroic property--antiferromagnetism--has not been investigated extensively, especially in thin films. Here we bring insight into the rich spin physics of BiFeO3 in a detailed study of the static and dynamic magnetic response of strain-engineered films. Using Mössbauer and Raman spectroscopies combined with Landau-Ginzburg theory and effective Hamiltonian calculations, we show that the bulk-like cycloidal spin modulation that exists at low compressive strain is driven towards pseudo-collinear antiferromagnetism at high strain, both tensile and compressive. For moderate tensile strain we also predict and observe indications of a new cycloid. Accordingly, we find that the magnonic response is entirely modified, with low-energy magnon modes being suppressed as strain increases. Finally, we reveal that strain progressively drives the average spin angle from in-plane to out-of-plane, a property we use to tune the exchange bias and giant-magnetoresistive response of spin valves.

Field-induced percolation of polar nanoregions in relaxor ferroelectrics.

A first-principles-based effective Hamiltonian is used to investigate low-temperature properties of Ba(Zr,Ti)O(3) relaxor ferroelectrics under an increasing dc electric field. This system progressively develops an electric polarization that is highly nonlinear with the dc field. This development leads to a maximum of the static dielectric response at a critical field, E(th), and involves four different field regimes. Each of these regimes is associated with its own behavior of polar nanoregions, such as shrinking, flipping, and elongation of dipoles or change in morphology. The clusters propagating inside the whole sample, with dipoles being parallel to the field direction, begin to form at precisely the E(th) critical field. Such a result, and further analysis we perform, therefore, reveal that field-induced percolation of polar nanoregions is the driving mechanism for the transition from the relaxor to ferroelectric state.

Strain dependence of polarization and piezoelectric response in epitaxial BiFeO3 thin films.

Epitaxial strain has recently emerged as a powerful means to engineer the properties of ferroelectric thin films, for instance to enhance the ferroelectric Curie temperature (T(C)) in BaTiO(3). However, in multiferroic BiFeO(3) thin films an unanticipated strain-driven decrease of T(C) was reported and ascribed to the peculiar competition between polar and antiferrodistortive instabilities. Here, we report a systematic characterization of the room-temperature ferroelectric and piezoelectric properties for strain levels ranging between -2.5% and +1%. We find that polarization and the piezoelectric coefficient increase by about 20% and 250%, respectively, in this strain range. These trends are well reproduced by first-principles-based techniques.

Multiferroic phase transition near room temperature in BiFeO3 films.

In multiferroic BiFeO(3) thin films grown on highly mismatched LaAlO(3) substrates, we reveal the coexistence of two differently distorted polymorphs that leads to striking features in the temperature dependence of the structural and multiferroic properties. Notably, the highly distorted phase quasiconcomitantly presents an abrupt structural change, transforms from a standard to a nonconventional ferroelectric, and transitions from antiferromagnetic to paramagnetic at 360±20 K. These coupled ferroic transitions just above room temperature hold promises of giant piezoelectric, magnetoelectric, and piezomagnetic responses, with potential in many applications fields.

BiFeO3 films under tensile epitaxial strain from first principles.

Density-functional calculations are performed to predict structural and magnetic properties of (001) BiFeO(3) films under tensile epitaxial strain. These films remain monoclinic (Cc space group) for misfit strains between 0% and ≈8%, with the polarization, tilt axis and magnetization all rotating when varying the strain. At a tensile strain ≈8%, these films undergo a first-order phase transition towards an orthorhombic phase (Ima2 space group). In this novel phase, the polarization and tilt axis lie in the epitaxial plane, while the magnetization is along the out-of-plane direction and the direction of the antiferromagnetic vector is unchanged by the phase transition. An unexpected additional degree of freedom, namely, an antiphase arrangement of Bi atoms, is also found for all tensile strains.

Orthorhombic polar Nd-doped BiFeO3 thin film on MgO substrate.

A Nd-doped BiFeO(3) thin film deposited on MgO substrate was studied by synchrotron diffraction. The ferroelectric nature of the film is proven by in-plane remanent polarization measurement. The highest possible symmetry of the film is determined to be orthorhombic, within the Fm2m space group. Such a structure is rotated by 45° with respect to the substrate and is consistent with tilts of oxygen octahedra doubling the unit cell. This polar structure presents a rather unusual strain-accommodation mechanism.

Bridging multiferroic phase transitions by epitaxial strain in BiFeO3.

We report the influence of epitaxial strain on the multiferroic phase transitions of BiFeO3 films. Using advanced characterization techniques and calculations we show that while the magnetic Néel temperature hardly varies, the ferroelectric Curie temperature TC decreases dramatically with strain. This is in contrast with the behavior of standard ferroelectrics where strain enhances the polar cation shifts and thus TC. We argue that this is caused by an interplay of polar and oxygen tilting instabilities and that strain can drive both transitions close together to yield increased magnetoelectric responses.

Low-symmetry phases in ferroelectric nanowires.

Ferroelectric nanostructures have recently attracted much attention due to the quest of miniaturizing devices and discovering novel phenomena. In particular, studies conducted on two-dimensional and zero-dimensional ferroelectrics have revealed original properties and their dependences on mechanical and electrical boundary conditions. Meanwhile, researches aimed at discovering and understanding properties of one-dimensional ferroelectric nanostructures are scarce. The determination of the structural phase and of the direction of the polarization in one-dimensional ferroelectrics is of technological importance, since, e.g., a low-symmetry phase in which the polarization lies away from a highly symmetric direction typically generates phenomenal dielectric and electromechanical responses. Here, we investigate the phase transition sequence of nanowires made of KNbO(3) and BaTiO(3) perovskites, by combining X-ray diffraction, Raman spectroscopy, and first-principles-based calculations. We provide evidence of a previously unreported ferroelectric ground state of monoclinic symmetry and the tuning of the polarization's direction by varying factors inherent to the nanoscale.