Suchergebnisse

The surface of BiFeO3 single crystals has been characterized at the local level using several AFM-based techniques. We have observed the presence of two different epilayers showing electrical and mechanical properties different from those of the bulk: a ferroelectrically dead outer skin of 5 nm sitting upon a subsurface layer that displays an extremely fine pattern of hierarchical self-ordered nanodomains. Based on the size of the nanodomains and applying a Kittel-like analysis, we argue that the nanotwinned region should be confined in a layer less than a micron deep. The superficial phase transition at T 275 °C is restricted to the outer skin layer (the dead layer), while the nanotwinned layer is insensitive to this transition. In view of the photovoltaic properties and spin-dependent transport of domain walls in BiFeO3, the existence of nanodomains (and thus a high density of domain walls) in bulk single crystals is likely to be relevant for understanding their functional properties. © 2013 AIP Publishing LLC. ; This work has been supported by the Leverhulme Trust, the project MAT2010-17771 from the Spanish Government and DFG through SFB 762. N.D. wants to acknowledge the financial support from a RyC grant of the Spanish Ministerio de Economia y Competitividad. G.C. wants to acknowledge financial support from ICREA. ; Peer Reviewed

Flexoelectricity is a property of dielectric materials whereby they exhibit electric polarization induced by strain gradients; while this effect can be negligible at the macroscale, it can become dominant at the nanoscale, where strain gradients can turn out to be tremendous. Previous works have demonstrated that flexoelectricity coupled with piezoelectricity enables the mechanical writing of ferroelectric polarization. When considering ferroelectric materials with out-of-plane polarization, the coupling of piezoelectricity with flexoelectricity can insert a mechanical asymmetry to the system and enable the distinction of oppositely polarized domains, based on their nanomechanical response. Using atomic force microscopy and, more specifically, contact resonance techniques, the coupling of flexoelectricity to piezoelectricity can be exploited to mechanically read the sign of ferroelectric polarization in a non-destructive way. We have measured a variety of ferroelectric materials, from a single crystal to thin films, and domains that are polarized down always appear to be stiffer than oppositely polarized domains. In this article, we demonstrate experimentally that the phenomenon is size-dependent and strongly enhanced when the dimension of the material is reduced to nanoscale in thin films. Ultimately, we demonstrate how the sensitivity in mechanical reading of ferroelectric polarization can be improved by appropriately tuning the mechanical stiffness of the cantilevers. ; Financial support was obtained under projects from the Spanish Ministerio de Ciencia e Innovación (MICINN) under Project Nos. FIS2015-73932-JIN, MAT2016-77100-C2-1-P, PID2019-108573GB-C21, and PID2019-109931GB-I00. In addition, this work was partially funded by No. 2017-SGR-579 from the Generalitat de Catalunya. The ICN2 was supported by the Severo Ochoa Centres of Excellence Programme, funded by the Spanish Research Agency (AEI, Grant No. SEV-2017-0706). C.S. thanks BIST for the PREBIST Grant. This project has received funding from the European Union's Horizon 2020 research and innovation program under the Marie Skłodowska-Curie (MSCA) Grant Agreement No. 754558. Finally, E.L. acknowledges the individual fellowship No. MSCA-IF-GF-708129. E.L. is a Serra Húnter Fellow. K.C-E. acknowledges the Division II of the Swiss National Science Foundation under Project No. 200021_178782.

Electrochemical phenomena in ferroelectrics are of particular interest for catalysis and sensing applications, with recent studies highlighting the combined role of the ferroelectric polarisation, applied surface voltage and overall switching history. Here, we present a systematic Kelvin probe microscopy study of the effect of relative humidity and polarisation switching history on the surface charge dissipation in ferroelectric Pb(Zr0.2Ti0.8)O3 thin films. We analyse the interaction of surface charges with ferroelectric domains through the framework of physically constrained unsupervised machine learning matrix factorisation, Dictionary Learning, and reveal a complex interplay of voltage-mediated physical processes underlying the observed signal decays. Additional insight into the observed behaviours is given by a Fitzhugh–Nagumo reaction–diffusion model, highlighting the lateral spread and charge passivation process contributors within the Dictionary Learning analysis. ; The authors acknowledge Dr Sergei V. Kalinin of Oak Ridge National Laboratory, for helpful discussions about machine learning and the initial suggestion to explore reaction–diffusion modelling. This work was supported by Division II of the Swiss National Science Foundation under project 200021_178782. A.V. acknowledges support by the Spanish Government under the project PID2019-110907GB-I00 and the "Severo Ochoa" Program for Centres of Excellence in R&D (CEX2019-000917-S). N.D. acknowledges support by the Spanish Government under the project PID2019- 109931GB-I00. N.B.G. acknowledges support by the National Science Foundation under the project DMR-20269676. The authors would like to thank S. Muller for technical support. ; With funding from the Spanish government through the 'Severo Ochoa Centre of Excellence' accreditation (CEX2019-000917-S). ; Peer reviewed

Magneto-ionics is an emerging field in materials science where voltage is used as an energy-efficient means to tune magnetic properties, such as magnetization, coercive field, or exchange bias, by voltage-driven ion transport. We first discuss the emergence of magneto-ionics in the last decade, its core aspects, and key avenues of research. We also highlight recent progress in materials and approaches made during the past few years. We then focus on the "structural-ion"approach as developed in our research group in which the mobile ions are already present in the target material and discuss its potential advantages and challenges. Particular emphasis is given to the energetic and structural benefits of using nitrogen as the mobile ion, as well as on the unique manner in which ionic motion occurs in CoN and FeN systems. Extensions into patterned systems and textures to generate imprinted magnetic structures are also presented. Finally, we comment on the prospects and future directions of magneto-ionics and its potential for practical realizations in emerging fields, such as neuromorphic computing, magnetic random-access memory, or micro- and nano-electromechanical systems. ; Financial support by the European Research Council (SPIN-PORICS 2014-Consolidator Grant Agreement No. 648454, and the MAGIC-SWITCH 2019-Proof of Concept Grant, Agreement No. 875018), the Spanish Government (Nos. MAT2017-86357-C3-1-R, PDC2021-121276-C31, and PID2020-116844RB-C21), the Generalitat de Catalunya (2017-SGR-292 and 2018-LLAV-00032), and the European Regional Development Fund (Nos. MAT2017-86357-C3-1-R and 2018-LLAV-00032) were acknowledged. E.M. acknowledges support as a Serra Húnter Fellow. G.R. acknowledges support from Ayudas Ramon y Cajal No. RYC-2016-21412. ; With funding from the Spanish government through the 'Severo Ochoa Centre of Excellence' accreditation (CEX2019-000917-S). ; Peer reviewed

Using scanning probe microscopy, we measure the out-of-plane mechanical response of ferroelectric 180° domain walls and observe that, despite separating domains that are mechanically identical, the walls appear mechanically distinct—softer—compared to the domains. This effect is observed in different ferroelectric materials (LiNbO3, BaTiO3, and PbTiO3) and with different morphologies (from single crystals to thin films), suggesting that the effect is universal. We propose a theoretical framework that explains the domain wall softening and justifies that the effect should be common to all ferroelectrics. The lesson is, therefore, that domain walls are not only functionally different from the domains they separate, but also mechanically distinct. ; C. S. thanks BIST for the PREBIST Grant. This projecthas received funding from the European Union's Horizon 2020 research and innovation program under the Marie Skłodowska-Curie Grant Agreement No. 754558. E. L. acknowledges the funding received from the European Union's Horizon 2020 research and innovation program through the Marie Skłodowska-Curie Actions: Individual Fellowship-Global Fellowship (Ref. No. MSCA-IF-GF708129). M. S. and K. S. acknowledge the support of the European Research Council under the European Union's Horizon 2020 research and innovation program (Grant Agreement No. 724529), Ministerio de Economia, Industria y Competitividad through Grants No. MAT2016- 77100-C2-2-P and No. SEV-2015-0496, and the Generalitat de Catalunya (Grant No. 2017SGR 1506). G. C. acknowledge the support of the Ministerio de Economía, Industria y Competitividad, Agencia Estatal de Investigación/Fondo Europeo de Desarrollo Regional and European Union through Grant No. MAT2016-77100-C2-1-P (MINECO/AEI/FEDER, UE). ; Peer reviewed

The multifunctional (ferromagnetic and ferroelectric) response at room temperature that is elusive in single phase multiferroic materials can be achieved in a proper combination of ferroelectric perovskites and ferrimagnetic spinel oxides in horizontal heterostructures. In this work, lead-free CoFe2O4/BaTiO3 bilayers are integrated with Si(001) using LaNiO3/CeO2/YSZ as a tri-layer buffer. They present structural and functional properties close to those achieved on perovskite substrates: the bilayers are fully epitaxial with extremely flat surface, and exhibit robust ferromagnetism and ferroelectricity at room temperature. ; Financial support by the Spanish Government (projects MAT2014-56063-C2-1-R and FIS2013-48668-C2-1-P) and Generalitat de Catalunya (2014 SGR 734) is acknowledged. ICMAB-CSIC authors acknowledge financial support from the Spanish Ministry of Economy and Competitiveness, through the "Severo Ochoa" Programme for Centres of Excellence in R&D (SEV-2015-0496). IF acknowledges Beatriu de Pinós postdoctoral scholarship (2011 BP-A 00220 and 2011 BP-A_2 00014) from AGAUR-Generalitat de Catalunya and Juan de la Cierva - Incorporación postdoctoral fellowship (IJCI-2014-19102) from the Spanish Ministry of Economy and Competitiveness. MS acknowledges the FPU grant and JG acknowledges RyC contract (2012–11709) from the Spanish Government. Electron microscopy observations carried out at the ICTS-CNME at UCM. Authors acknowledge the ICTS-CNME for offering access to their instruments and expertise. Research at UCM was sponsored by Spanish MINECO MAT2015-66888-C3-3-R and Fundación BBVA. ; Peer reviewed

Under the terms of the Creative Commons Attribution License 3.0 (CC-BY). ; There is increasing evidence supporting the strong potential of twin walls in ferroic materials as distinct, spatially tunable, functional elements in future electronic devices. Here, we report an increase of about one order of magnitude in conductivity and more robust magnetic interactions at (100)-type twin walls in La0.7Sr0.3MnO3 thin films. The nature and microscopic origin of such distinctive behavior is investigated by combining conductive, magnetic, and force modulation scanning force microscopies with transmission electron microscopy techniques. Our analyses indicate that the observed behavior is due to a severe compressive strained state within an ∼1nm slab of material centered at the twin walls, promoting stronger Mn 3d-O2p orbital overlapping leading to a broader bandwidth and enhanced magnetic interactions. ; This research was sponsored by the Spanish MINECO (Grants No. MAT2011- 29081-C02, No. MAT2012-33207, and No. MAT2013-47869-C4-1-P) projects. We also acknowledge financial aid from the Generalitat de Catalunya (2014 SGR 501). N.B. and F.S. also acknowledge funding from the European Union Seventh Framework Programme under Grant Agreement No. 312483-ESTEEM2 (Integrated Infrastructure Initiative I3) for providing access to aberration corrected electron microscopes at CEMES (Toulouse) and LPS (Orsay). R.G. and N.B. thank the Spanish MINECO for financial support through the FPI program. F.S. acknowledges support from the Labex (Excellence Laboratory) NEXT through a visiting scientist fellowship at CEMES (Toulouse, France). ; Peer Reviewed

Achieving efficient spatial modulation of phonon transmission is an essential step on the path to phononic circuits using "phonon currents". With their intrinsic and reconfigurable interfaces, domain walls (DWs), ferroelectrics are alluring candidates to be harnessed as dynamic heat modulators. This paper reports the thermal conductivity of single-crystal PbTiO3 thin films over a wide variety of epitaxial-strain-engineered ferroelectric domain configurations. The phonon transport is proved to be strongly affected by the density and type of DWs, achieving a 61% reduction of the room-temperature thermal conductivity compared to the single-domain scenario. The thermal resistance across the ferroelectric DWs is obtained, revealing a very high value (≈5.0 × 10–9 K m2 W–1), comparable to grain boundaries in oxides, explaining the strong modulation of the thermal conductivity in PbTiO3. This low thermal conductance of the DWs is ascribed to the structural mismatch and polarization gradient found between the different types of domains in the PbTiO3 films, resulting in a structural inhomogeneity that extends several unit cells around the DWs. These findings demonstrate the potential of ferroelectric DWs as efficient regulators of heat flow in one single material, overcoming the complexity of multilayers systems and the uncontrolled distribution of grain boundaries, paving the way for applications in phononics. ; This work has received financial support from Ministerio de Economia y Competitividad (Spain) under project no. MAT2016-80762-R, Xunta de Galicia (Centro singular de investigacion de Galicia accreditation 2016-2019, ED431 G/09), the European Union (European Regional Development Fund-ERDF), and the European Commission through the Horizon H2020 funding by H2020-MSCA-RISE-2016 project no. 734187-SPICOLOST. E.L. acknowledges the funding received from the European Union's Horizon 2020 research and innovation program through the Marie Skłodowska-Curie Actions: Individual Fellowship-Global Fellowship (ref. MSCAIF-GF-708129). D.B. acknowledges financial support from MINECO (Spain) through an FPI fellowship (BES-2017- 079688). The work at Cornell was supported by the Army Research Office under grant W911NF-16-1-0315. H.P. acknowledges support from the National Science Foundation [Platform for the Accelerated Realization, Analysis, and Discovery of Interface Materials (PARADIM)] under cooperative agreement no. DMR-1539918. ; Peer reviewed

Achieving efficient spatial modulation of phonon transmission is an essential step on the path to phononic circuits using "phonon currents". With their intrinsic and reconfigurable interfaces, domain walls (DWs), ferroelectrics are alluring candidates to be harnessed as dynamic heat modulators. This paper reports the thermal conductivity of single-crystal PbTiO3 thin films over a wide variety of epitaxial-strain-engineered ferroelectric domain configurations. The phonon transport is proved to be strongly affected by the density and type of DWs, achieving a 61% reduction of the room-temperature thermal conductivity compared to the single-domain scenario. The thermal resistance across the ferroelectric DWs is obtained, revealing a very high value (≈5.0 × 10–9 K m2 W–1), comparable to grain boundaries in oxides, explaining the strong modulation of the thermal conductivity in PbTiO3. This low thermal conductance of the DWs is ascribed to the structural mismatch and polarization gradient found between the different types of domains in the PbTiO3 films, resulting in a structural inhomogeneity that extends several unit cells around the DWs. These findings demonstrate the potential of ferroelectric DWs as efficient regulators of heat flow in one single material, overcoming the complexity of multilayers systems and the uncontrolled distribution of grain boundaries, paving the way for applications in phononics ; This work has received financial support from Ministerio de Economía y Competitividad (Spain) under project no. MAT2016-80762-R, Xunta de Galicia (Centro singular de investigación de Galicia accreditation 2016-2019, ED431G/09), the European Union (European Regional Development Fund-ERDF), and the European Commission through the Horizon H2020 funding by H2020-MSCA-RISE-2016 project no. 734187-SPICOLOST. E.L. acknowledges the funding received from the European Union's Horizon 2020 research and innovation program through the Marie Skłodowska-Curie Actions: Individual Fellowship-Global Fellowship (ref. ...