Persistence of ferroelectricity above the Curie temperature at the surface of Pb(Zn1/3Nb2/3)O3-12%PbTiO3

Under the terms of the Creative Commons Attribution License 3.0 (CC-BY).Relaxor-based ferroelectrics have been known for decades to possess a relatively thick surface layer ("skin") that is distinct from its interior. Yet while there is consensus about its existence, there are controversies about its symmetry, phase stability, and origin. In an attempt to clarify these issues, we have examined the surface layer of PZN-12%PT. While the bulk transitions from a ferroelastically twinned tetragonal ferroelectric state with in-plane polarization to a cubic paraphase at Tc=200C, the skin layer shows a robust labyrinthine nanodomain structure with out-of-plane polarization that persists hundreds of degrees above the bulk Curie temperature. Cross-sectional transmission electron microscopy analysis shows that the resilience of the skin's polarization is correlated with a compositional imbalance: lead vacancies at the surface are charge-compensated by niobium enrichment; the excess of Nb5+ - a small ion with d0 orbital occupancy - stabilizes the ferroelectricity of the skin layer.We acknowledge financial support from the Spanish Ministerio de Ciencia e Innovación (Contracts No. MAT2010-17771, No. MAT2011-29081-C02, and No. FIS2013-48668- C2-1-P), and the Generalitat de Catalunya (Project 2014 SGR 1216). N.D. thanks the Spanish Ministerio de Ciencia e Innovación for the Ramon y Cajal Research Grant (No. RYC-2010-06365), and G.C. acknowledges an ERC Starting Grant (Project reference: ERC-SG-308023). ICN2 acknowledges support from the Severo Ochoa Program (MINECO, Grant SEV-2013-0295).Peer Reviewe

Chemical strain and oxidation-reduction kinetics of epitaxial thin films of mixed ionic-electronic conducting oxides determined by x-ray diffraction

This is an open access article distributed under the terms of the Creative Commons Attribution Non-Commercial No Derivatives 4.0 License.X-ray diffraction, at high T's and switching between N2/air atmospheres, was used to compare the chemical expansion due oxygen non-stoichiometry variations between epitaxial films of different mixed ionic-electronic conductors: La0.6Sr0.4CoO3−δ(LSC), Ba0.5Sr0.5Co0.8Fe0.2O3−δ(BSCF), LaNiO3−δ(LNO), La2NiO4+δ(L2NO) and GaBaCo2O5.5+δ(GBCO) and La0.7Sr0.3MnO3−δ(LSM). LSC and BSCF show the largest relative change in the cell parameter Δc/c = +0.5%, while L2NO and GBCO show negative Δc/c = −0.2% and −0.1%, respectively. LNO and LSM show either reduced or negligible chemical expansions. In all cases the values correspond to their particular defect equilibrium and degree of charge localization. The oxygen surface exchange kinetics was also evaluated from in-situ time-resolved analyses of the cell parameter variations. LSC, LNO and GBCO films show fast oxygen reduction kinetics, kchem = 5·10−6, 3·10−6, and 2·10−7 cm/s at 700°C, respectively, in relative agreement with reported values, while BSCF films show much slower kinetics than expected, below kchem = 10−7 cm/s at 650°C, related to the degradation process observed in the films.The authors acknowledge the Spanish Ministry of Education and Culture for financial support (MAT2011-29081-C02-01 and CONSOLIDER-INGENIO CSD2008-0023 projects). R. M. and J. R. thank the Spanish Ministry of Education for a FPI grant and PTA contracts.Peer Reviewe

Self-arranged misfit dislocation network formation upon strain release in La0.7Sr0.3MnO3/LaAlO3(100) epitaxial films under compressive strain

Santiso, José et al.Lattice-mismatched epitaxial films of LaSrMnO (LSMO) on LaAlO (001) substrates develop a crossed pattern of misfit dislocations above a critical thickness of 2.5 nm. Upon film thickness increases, the dislocation density progressively increases, and the dislocation spacing distribution becomes narrower. At a film thickness of 7.0 nm, the misfit dislocation density is close to the saturation for full relaxation. The misfit dislocation arrangement produces a 2D lateral periodic structure modulation (Λ≈ 16 nm) alternating two differentiated phases: one phase fully coherent with the substrate and a fully relaxed phase. This modulation is confined to the interface region between film and substrate. This phase separation is clearly identified by X-ray diffraction and further proven in the macroscopic resistivity measurements as a combination of two transition temperatures (with low and high T). Films thicker than 7.0 nm show progressive relaxation, and their macroscopic resistivity becomes similar than that of the bulk material. Therefore, this study identifies the growth conditions and thickness ranges that facilitate the formation of laterally modulated nanocomposites with functional properties notably different from those of fully coherent or fully relaxed material.This research was funded by the Spanish MINECO (projects: MAT2011-29081-C02, MAT2012-33207 and MAT2013-47869-C4-1-P, Consolider-Ingenio CSD2008-00023) and the European Union Horizon 2020 research and innovation programme under the Marie Sklodowska-Curie grant agreement No. 645658. We also acknowledge financial aid from the Generalitat de Catalunya (2014 SGR 501 and 2014 SGR 1216). J. S. thanks the support of Ministry of Education and Science through program “Salvador de Madariaga” for a grant (ref: PRX14/00297) to perform a stay at MIT. Z.K. is grateful for the support from the Ministry of Education, Science, and Technological Development of the Republic of Serbia through Project III45018. Q.L. and B.Y. thank the National Science Foundation for support through the MIT Center of Materials Science and Engineering MRSEC under grant no. DMR-1419807. This work made use of the Shared Experimental Facilities supported in part by the MRSEC Program of the National Science Foundation under award number DMR – 1419807. We thank Dr. Belén Ballesteros (ICN2, Barcelona) and Prof. Marie-Jo Casanova (CEMES, Toulouse) for their assistance with transmission electron microscopy experiments. We are grateful to J. Rubio-Zuazo and the ESRF for providing assistance in using beamline BM25. We also thank HZB for the allocation of synchrotron radiation beamtime at Bessy.Peer Reviewe

Chemical strain and oxidation-reduction kinetics of epitaxial thin films of mixed ionic-electronic conducting oxides determined by x-ray diffraction

X-ray diffraction, at high T's and switching between N/air atmospheres, was used to compare the chemical expansion due oxygen non-stoichiometry variations between epitaxial films of different mixed ionic-electronic conductors: LaSrCoO(LSC), BaSrCoFeO(BSCF), LaNiO(LNO), LaNiO(L2NO) and GaBaCoO(GBCO) and LaSrMnO(LSM). LSC and BSCF show the largest relative change in the cell parameter Δc/c = +0.5%, while L2NO and GBCO show negative Δc/c = -0.2% and -0.1%, respectively. LNO and LSM show either reduced or negligible chemical expansions. In all cases the values correspond to their particular defect equilibrium and degree of charge localization. The oxygen surface exchange kinetics was also evaluated from in-situ time-resolved analyses of the cell parameter variations. LSC, LNO and GBCO films show fast oxygen reduction kinetics, k = 5·10, 3·10, and 2·10 cm/s at 700°C, respectively, in relative agreement with reported values, while BSCF films show much slower kinetics than expected, below kchem = 10 cm/s at 650°C, related to the degradation process observed in the films

Antisite Defects and Chemical Expansion in Low-damping, High-magnetization Yttrium Iron Garnet Films

Yttrium iron garnet is widely investigated for its suitability in applications ranging from magneto-optical and microwave devices to magnonics. However, in the few-nanometer thickness range, epitaxial films exhibit a strong variability in magnetic behavior that hinders their implementation in technological devices. Here, direct visualization and spectroscopy of the atomic structure of a nominally stoichiometric thin film, exhibiting a small damping factor of 3.0 ⋅ 10, reveals the occurrence of Y-excess octahedral antisite defects. The two-magnon strength is very small, Γ≈10 Oe, indicating a very low occurrence of scattering centers. Notably, the saturation magnetization, 4πM=2.10 (±0.01) kOe, is higher than the bulk value, in consistency with the suppression of magnetic moment in the minority octahedral sublattice by the observed antisite defects. Analysis of elemental concentration profiles across the substrate-film interface suggests that the Y-excess is originated from unbalanced cationic interdiffusion during the early growth stages

Formation of self-organized Mn3O4 nanoinclusions in LaMnO3 films

et al.We present a single-step route to generate ordered nanocomposite thin films of secondary phase inclusions (Mn3O4) in a pristine perovskite matrix (LaMnO3) by taking advantage of the complex phase diagram of manganese oxides. We observed that in samples grown under vacuum growth conditions from a single LaMnO3 stoichiometric target by Pulsed Laser Deposition, the most favorable mechanism to accommodate Mn2+ cations is the spontaneous segregation of self-assembled wedge-like Mn3O4 ferrimagnetic inclusions inside a LaMnO3 matrix that still preserves its orthorhombic structure and its antiferromagnetic bulk-like behavior. A detailed analysis on the formation of the self-assembled nanocomposite films evidences that Mn3O4 inclusions exhibit an epitaxial relationship with the surrounding matrix that it may be explained in terms of a distorted cubic spinel with slight (~9°) c-axis tilting. Furthermore, a Ruddlesden-Popper La2MnO4 phase, helping to the stoichiometry balance, has been identified close to the interface with the substrate. We show that ferrimagnetic Mn3O4 columns influence the magnetic and transport properties of the nanocomposite by increasing its coercive field and by creating local areas with enhanced conductivity in the vicinity of the inclusions.Financial support from the Spanish Ministry of Economy and Competitiveness, through the “Severo Ochoa” Program for Centres of Excellence in R&D (SEV-2015-0496 and SEV 2013-0295), Projects MAT2011-29081 and MAT2015-71664-R and Ministry of Education and Science of Serbia (Grant—III45018) is acknowledged. This work has received funding from the European Union's Horizon 2020 research and innovation programme under the Marie Sklodowska-Curie grant agreement No. 645658 (DAFNEOX Project). NB thanks the Spanish MINECO for financial support through the FPI program.Peer reviewedPeer Reviewe

Engineering the microstructure and magnetism of La2CoMnO6-δ thin films by tailoring oxygen stoichiometry

Under the terms of the Creative Commons Attribution (CC BY) license to their work.We report on the magnetic and structural properties of ferromagnetic-insulating La2CoMnO6-δ thin films grown on top of (001) SrTiO3 substrates by means of RF sputtering technique. Careful structural analysis, by using synchrotron X-ray diffraction, allows identifying two different crystallographic orientations that are closely related to oxygen stoichiometry and to the features (coercive fields and remanence) of the hysteresis loops. Both Curie temperature and magnetic hysteresis turn out to be dependent on the oxygen stoichiometry. In situ annealing conditions allow tailoring the oxygen content of the films, therefore controlling their microstructure and magnetic properties.We acknowledge financial support from the Spanish MEC (MAT2011-29081 and MAT2012-33207), CONSOLIDER (CSD2007-00041), and FEDER program. R.G., L.L.-M. and N.B. thank the Spanish MINECO for the financial support through the FPI program. The research leading to these results has received funding from the European Community's Seventh Framework Programme (FP7/2007-2013) under Grant Agreement No. 312284.Peer Reviewe

Tunneling anisotropic magnetoresistance in La2/3Sr1/3MnO3/LaAlO3/Pt tunnel junctions

The magnetotransport properties of La2/3 Sr1/3 MnO3 (LSMO)/ LaAlO3 (LAO)/Pt tunneling junctions have been analyzed as a function of temperature and magnetic field. The junctions exhibit magnetoresistance (MR) values of about 37%, at H=90 kOe at low temperature. However, the temperature dependence of MR indicates a clear distinct origin than that of conventional colossal MR. In addition, tunneling anisotropic MR (TAMR) values around 4% are found at low temperature and its angular dependence reflects the expected uniaxial anisotropy. The use of TAMR response could be an alternative of much easier technological implementation than conventional MTJs since only one magnetic electrode is required, thus opening the door to the implementation of more versatile devices. However, further studies are required in order to improve the strong temperature dependence at the present stage

Self-Arranged Misfit Dislocation Network Formation upon Strain Release in La0.7Sr0.3MnO3/LaAlO3(100) Epitaxial Films under Compressive Strain

Lattice-mismatched epitaxial films of LaSrMnO (LSMO) on LaAlO (001) substrates develop a crossed pattern of misfit dislocations above a critical thickness of 2.5 nm. Upon film thickness increases, the dislocation density progressively increases, and the dislocation spacing distribution becomes narrower. At a film thickness of 7.0 nm, the misfit dislocation density is close to the saturation for full relaxation. The misfit dislocation arrangement produces a 2D lateral periodic structure modulation (Λ≈ 16 nm) alternating two differentiated phases: one phase fully coherent with the substrate and a fully relaxed phase. This modulation is confined to the interface region between film and substrate. This phase separation is clearly identified by X-ray diffraction and further proven in the macroscopic resistivity measurements as a combination of two transition temperatures (with low and high T). Films thicker than 7.0 nm show progressive relaxation, and their macroscopic resistivity becomes similar than that of the bulk material. Therefore, this study identifies the growth conditions and thickness ranges that facilitate the formation of laterally modulated nanocomposites with functional properties notably different from those of fully coherent or fully relaxed material

Atomic and electronic structure of self-organized defects in epitaxial films of functional perovskite-type oxides

The epitaxial thin films of functional perovskite-type oxides (ABO3) present interfacial coupling and misfit relaxation mechanisms governed by a complex interplay of chemical, electronic and structural degrees of freedom. The relaxation mechanisms of strained films may accommodate defects, such as misfit dislocations or twin walls, which exhibit a strong tendency towards self-organization with characteristic length scales of tens of nanometres. The core lattice structure of these defects is different from the bulk of the material and thus may be considered as a nano-phase with likely different physical properties, leading to the formation of functional nanostructures. The correlation between defect structure and functionality, together with the capacity of these defects to self-organize, offers a unique opportunity for the bottom-up elaboration of functional complex oxides nanodevices. This thesis focuses on the characterization of the microstructure, interface and self-organized defects of epitaxial films and functional nanostructures of oxide materials by using advanced transmission electron microscopy. Special emphasis is put on the atomic and chemical structure of the interfaces and generated defects, such as dislocations, twin walls and phase segregations, as well as on the strain fields and their correlation with chemical heterogeneities. In this regard, two different systems composed of lanthanum manganites are considered: LaMnO3:MnOx nanocomposite grown on (001)SrTiO3 and on (001)LaAlO3 substrates; and La0.7Sr0.3MnO3 films with self-organized defects grown on (001)SrTiO3 and on (001)LaAlO3. The materials studied in this work may be regarded as nanostructured films resulting from the self-organization of misfit relieving defects as follows: nanoinclusions of a MnOx phase (volume defects) in LaMnO3; twin walls between twin domains (planar defects) in La0.7Sr0.3MnO3 on SrTiO3; and misfit dislocations (line defects) in La0.7Sr0.3MnO3 on LaAlO3. In the LaMnO3:MnOx nanocomposite, the formation of regular vertically aligned nanoinclusions of a manganese oxide (MnOx) embedded in an LaMnO3 film is analysed via microstructural characterization. This analysis includes the determination of the LaMnO3 matrix microstructure with respect to the substrate together with the identification of the manganese oxide phase and a secondary phase: a La-rich layer close to LaMnO3-substrate interface. In the case of La0.7Sr0.3MnO3 on (001)SrTiO3 substrates, a detailed analysis of twin walls and their implications on the functional properties is performed. Local changes in the physical and structural properties of the TWs lead to the view of a twinned film as a self-organized nanostructure consisting of vertical nano-sheets of strongly compressed La0.7Sr0.3MnO3 embedded in a matrix of tensile strained La0.7Sr0.3MnO3. In the case of La0.7Sr0.3MnO3 ultrathin films grown on (001)LaAlO3, the relaxation mechanism of this films is analysed. These films relieve the misfitstrain by the formation of misfit dislocations above a critical film thickness of 2.5 nm. A detailed study of structural, chemical and electronic changes associated with the dislocation is also performed paying particular attention to the influence of strain fields on chemical composition at the nanoscale. A chemical reorganization occurs to accommodate the strain at the dislocations core region. The dependence of the degree of order of the dislocation pattern on film thickness is also explored. Finally, the implications of the dislocation strain field on surface topography and electrical transport are analysed, demonstrating that the multiscale nature of dislocations holds great promise for the creation of spontaneous surface ordered functional nanostructures in complex oxide thin films.The results and main conclusions obtained in this work open new perspectives for the development of functional self-organized nanostructures based on strain relieving defects.Peer Reviewe