Structural investigation of (111) oriented (BiFeO3)(1-x){\Lambda}/(LaFeO3)x{\Lambda} superlattices by X-ray diffraction and Raman spectroscopy

(BiFeO3)(1-x){\Lambda}/(LaFeO3)x{\Lambda} superlattices (SLs) with varying x have been grown by pulsed laser deposition on (111) oriented SrTiO3 substrates. In order to obtain good epitaxy and flat samples a conducting SrRuO3 buffer has been deposited prior to the superlattices to screen the polar mismatch for such (111) SrTiO3 orientation. X-ray diffraction reciprocal space mapping on different family of planes were collected and evidenced a room temperature structural change at x=0.5 from a rhombohedral/monoclinic structure for rich BiFeO3 to an orthorhombic symmetry for rich LaFeO3. This symmetry change has been confirmed by Raman spectroscopy and demonstrates the different phase stability compared to similar SLs grown on (100) SrTiO3. The strongly anisotropic strain and oxygen octahedral rotation/tilt system compatibility at the interfaces probably explain the orientation dependence of the phase stability in such superlattices.Comment: 17 pages, 3 figure

X-ray study of structural domains in the near-surface region of SrTiO₃ substrates with Y_0.6Pr_0.4Ba₂Cu₃O₇/La_2/3Ca_1/3MnO₃ superlattices grown on top

We investigated with synchrotron x-ray diffraction and reflectometry the formation of structural domains in the near-surface region of single crystalline SrTiO₃ (001) substrates with Y0.6Pr0.4Ba₂Cu₃O₇/La2/3Ca1/3MnO₃ superlattices grown on top. We find that the antiferrodistortive cubic to tetragonal transition, which occurs at TSTO=104  K in the bulk and at a considerably higher temperature of at least 120 K in the surface region of SrTiO₃, has only a weak influence on the domain formation. The strongest changes occur instead in the vicinity of the tetragonal to orthorhombic transition in SrTiO₃ around 65 K where pronounced surface facets develop that reach deep (at least several micrometers) into the SrTiO₃ substrate. These micrometer-sized facets are anisotropic and tilted with respect to one another by up to 0.5° along the shorter direction. Finally, we find that a third structural transition below 30 K gives rise to significant changes in the spread of the c-axis parameters. Overall, our data provide evidence for a strong mutual interaction between the structural properties of the SrTiO₃ surface and the multilayer grown on top

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

Interlayer strain effects on the structural behavior of BiFeO3/LaFeO3 superlattices

Artificial (BiFeO3)0.5Λ/(LaFeO3)0.5Λ superlattices have been grown by pulsed laser deposition. The periodicity Λ was varied from 150 Å to 25 Å and the relative ratio between BiFeO3 (BFO) and LaFeO3 (LFO) is kept constant in each period. X-ray diffraction, transmission electron microscopy, and Raman spectroscopy investigations indicate antiferroelectric-like structures for large periodicity (Λ ≥ 76 Å), while Pnma LaFeO3-like structures are observed for small periodicity Λ ≤ 50 Å. Room temperature magnetic measurements were obtained by vibrating sample magnetometry and suggest antiferromagnetic ordering with weak ferromagnetism. Temperature dependent x-ray diffraction studies show an important shift of paraelectric-antiferroelectric phase transition scaling with BFO thickness. Strain and size effects explain this behavior and discussion is also made on the possible role of the oxygen octahedral rotation/tilt degree of freedom

Influence of temperature and wavelength on the switchable photovoltaic response of a BiFe0.95Mn0.05O3 thin film

International audienceThe photovoltaic (PV) response of an epitaxial BiFe0.95Mn0.05O3 thin film grown using pulsed laser deposition has been investigated on a broad range of temperature. The wavelength dependent photovoltaic effect shows the contribution of the manganese doping and the presence of vacancies (Bi and O vacancies). The temperature dependent response of the PV response rules out electromigration and/or Schottky barriers as dominant mechanisms. This is corroborated with the observed switchable photovoltaic effect that can be explained either by the depolarizing field or by the bulk photovoltaic effect. In addition, the PV response shows a strong correlation with the low temperature polaronic-like conduction mechanism, and a high open circuit voltage (2.5V) is detected in the investigated vertical capacitive geometry. Published by AIP Publishing

Conduction mechanism in epitaxial BiFe0.95Mn0.05O3 thin film

International audienceAn epitaxial BiFe0.95Mn0.05O3 thin film of about 280 nm thickness was deposited on conductive SrRuO3 on a (001) LaAlO3 substrate by pulsed laser deposition. At room temperature, x-ray diffraction and Raman spectroscopy evidenced a pure rhombohedral structure and the hysteresis loop measurements showed a remanent polarization of Pr = 73 mu C/cm(2). The transport mechanisms were investigated from 90 to 400K and compared to several models. It was found that the nearest neighbor hopping and variable range hopping mechanisms dominate the conduction above and below 270K, respectively. Bulk like limited transport is attributed to hopping from and within trap levels depending on the temperature. Defects (vacancies) and manganese multiple valences play a major role in the electronic transport of such a strongly correlated system and should be considered to explain the recently observed photovoltaic effect in similar heterostructures. Published by AIP Publishing