Anti-polar state in BiFeO3_3/NdFeO3_3 superlattices

Antiferroelectrics are promising materials for high energy density capacitors and the search for environmentally-friendly and efficient systems is actively pursued. An elegant strategy to create and design new (anti)ferroic system relies on the use of nanoscale superlattices. We report here the use of such strategy and the fabrication of nanoscale BiFeO$_3$/NdFeO$_3$ superlattices and in depth characterization using high resolution X-ray diffraction and Transmission Electron Microscopy. The structural analysis at atomic scale demonstrates that such superlattices host anti-polar ordering most likely described by an antiferroelectric-like Pbnm symmetry. Temperature dependence of anti-polar state and structural transition further hint that the stability of the anti-polar state is controlled by the BiFeO$_3$ layer thickness within the stacking and, in a more moderate way, by interlayer strain. Discovery of such polar arrangement in superlattices and the possible generalization to the whole rare-earth family pave the way to new platforms for energy storage application as well as nano-electronic devices

Under pressure:Mechanochemical effects on structure and ion conduction in the sodium-ion solid electrolyte Na₃PS₄

10.1021/jacs.0c06668Journal of the American Chemical Society1424318422-1843

High-Temperature Lattice-Dynamics Evolution of YMnO3 and YbMnO3

International audienceThe exact mechanism responsible for the ferroelectricity in hexagonal manganite has been the subject of intense debate. Whether ferroelectricity and ferroelastic order appear at the same temperature and the role of covalency in the ferroelectric order are still discussed. High-temperature phase transitions are here investigated through the prism of lattice-dynamics evolution with temperature. Comparison is made between the YMnO3 and YbMnO3 behavior using polarized Raman spectroscopy. While YMnO3 shows two phase transitions (isosymetric at about 900 K and ferroelastic at 1200 K) YbMnO3 presents no lattice instability up to 1350 K. Phonons involved in the lattice instability and the zone tripling transition are identified. Moreover, peculiar hardening on heating of some YbMnO3 phonons is revealed and discussed, which further highlights the dynamical difference between these two hexagonal manganites

Resistive Switching Hysteresis in Thin Films of Bismuth Ferrite

International audienceWe have studied the resistive switching (RS) phenomenon in series of BiFeO3 thin films of thickness of 40 ? 154~nm deposited by PLD technique on conducting Nb-doped substrate of SrTiO3 and with Pt top electrodes. It was found that 154~nm film demonstrates the interface-provided I?V characteristic of Schottky diode when the applied voltage does not exceed the threshold value Vd = 1.3 V. The RS phenomenon appears as the current hysteresis loop during the 0 ? Vm ? 0 ? ?Vm ? 0 voltage sweep cycle, provided that the maximal stop-voltage Vm is larger than Vd. For thinner films neither diode-like I?V behavior nor substantial RS effect were observed. The results are interpreted in terms of the filamentary model of the mobile oxygen vacancies

Structural and magnetic properties of Co-doped ZnO thin films grown by ultrasonic spray pyrolysis method

International audienceCobalt-doped ZnO thin films with several different percentage of Co from 0 up to 15 at% were synthesized via a cheap, simple and versatile method i.e. ultrasonic spray pyrolysis at atmospheric pressure and a substrate temperature of 350 degrees C. The structure of the as prepared samples was characterized by X-ray diffraction (XRD), Raman spectroscopy and FTIR. The Co-doping effect is revealed by the presence of three additional peaks around 235, 470 and 538 cm(-1) respect to the Raman spectra of the unsubstituted film. Fourier transform infrared spectroscopy (FTIR) put in evidence the decrease of the bond force constant f with increasing Co-doping. By ultra-violet visible near infrared (UV-Vis-NIR) spectroscopy on Co-doped samples it was possible to show the presence of additional absorption bands at approximately 570, 620 and 660 nm suggesting that Co2+ ions do not change their oxidation when substituted to zinc and the ZnO lattice does not change its wurtzite structure as well. Finally, all our samples exhibit a paramagnetic behavior without any trace of intrinsic room temperature ferromagnetism. (C) 2016 Elsevier Ltd. All rights reserved

Under pressure:Mechanochemical effects on structure and ion conduction in the sodium-ion solid electrolyte Na₃PS₄

Fast-ion conductors are critical to the development of solid-state batteries. The effects of mechanochemical synthesis that lead to increased ionic conductivity in an archetypical sodiumion conductor Na3PS4 are not fully understood. We present here a comprehensive analysis based on diffraction (Bragg and pair distribution function), spectroscopy (impedance, Raman, NMR and INS), and ab initio simulations aimed at elucidating the synthesis-property relationships in Na3PS4. We consolidate previously reported interpretations regarding the local structure of ball-milled samples, underlining the sodium disorder and showing that a local tetragonal framework more accurately describes the structure than the originally proposed cubic one. Through variable-pressure impedance spectroscopy measurements, we report for the first time the activation volume for Na+ migration in Na3PS4, which is ∼30% higher for the ball-milled samples. Moreover, we show that the effect of ball-milling on increasing the ionic conductivity of Na3PS4 to ∼10-4 S/cm can be reproduced by applying external pressure on a sample from conventional high-temperature ceramic synthesis. We conclude that the key effects of mechanochemical synthesis on the properties of solid electrolytes can be analyzed and understood in terms of pressure, strain, and activation volume. </p

Phonon and Magnon Excitations in Raman Spectra of an Epitaxial Bismuth Ferrite Film

International audienceAn epitaxial film of bismuth ferrite BiFeO3 on a MgO(001) single crystal substrate has been pre pared by pulsed laser deposition using SrTiO3 and SrRuO3 buffer layers. At room temperature, the polariza tion characteristics of the Raman spectra of the BiFeO3 film under study suggest a monoclinic symmetry. The high temperature (295\textendash1100 K) investigations of the Raman spectra have been performed in the frequency range 20 cm\textendash1 < ν < 1600 cm\textendash1. Particular attention has been paid to the high frequency region with a band observed at 610 cm\textendash1, which corresponds to the maximum density of states of the magnon branch at the Bril louin zone boundary, and an intense band in the second order Raman spectra with the maximum at ~1250 cm\textendash1, which corresponds to the density of states of two magnon excitations. It has been found that the inten sity of the band at ~1250 cm\textendash1 decreases linearly with an increase in the temperature and, above 650 K, this band is absent. The extrapolation of the temperature dependence of the integrated intensity of the band at 1250 cm\textendash1 suggests that this film undergoes an antiferromagnetic phase transition at a temperature of ~670 K

Strain engineering of the magnetic anisotropy and magnetic moment in NdFeO 3 epitaxial thin films

International audienc

Strain engineering of the magnetic anisotropy and moment in NdFeO3 epitaxial thinfilms

Strain engineering is a powerful mean for tuning the various functionalities of ABO3 perovskite oxide thin films. Rare-earth orthoferrite RFeO3 materials such as NdFeO3 (NFO) are of prime interest because of their intriguing magnetic properties as well as their technological potential applications especially as thin films. Here, using a large set of complementary and advanced techniques, we show that NFO epitaxial thin films, successfully grown by pulsed laser deposition on (001)-SrTiO3, show a strong magnetic anisotropy below a critical thickness tc of ~54 nm, associated to the occurrence of structural modifications related to symmetry and domain pattern changes. By varying the tensile misfit strain through the decrease of film thickness below tc, the amplitudes of in- and out-of-plane magnetization can be continuously tuned while their ratio keeps constant. Furthermore, different low temperature magnetic behaviours are evidenced for strained and relaxed films suggesting that the strain-induced structural state impacts the magnetic phase stability