H. Bouyanfif

Phase stability frustration on ultra-nanosized anatase TiO2.

This work sheds light on the exceptional robustness of anatase TiO2 when it is downsized to an extreme value of 4 nm. Since at this size the surface contribution to the volume becomes predominant, it turns out that the material becomes significantly resistant against particles coarsening with temperature, entailing a significant delay in the anatase to rutile phase transition, prolonging up to 1000 °C in air. A noticeable alteration of the phase stability diagram with lithium insertion is also experienced. Lithium insertion in such nanocrystalline anatase TiO2 converts into a complete solid solution until almost Li1TiO2, a composition at which the tetragonal to orthorhombic transition takes place without the formation of the emblematic and unwished rock salt Li1TiO2 phase. Consequently, excellent reversibility in the electrochemical process is experienced in the whole portion of lithium content.

Highly constrained ferroelectric [BaTiO3](1−x)Λ/[BaZrO3]xΛ superlattices: X-ray diffraction and Raman spectroscopy

We report an x-ray diffraction (XRD) and a Raman-scattering investigation of ferroelectric/paraelectric superlattices [BaTiO3] (1−x)Λ/[BaZrO3]xΛ for which the composition varied, 0.15 ≤ x ≤ 0.85, while the superlattice (SL) modulation period Λ was kept constant at about 100 A. The samples were epitaxially grown by pulsed laser deposition on MgO substrates buffered with La0.5Sr0.5CoO3. Based on the XRD analysis and on polarized Raman spectra, we have showed that the large strain in SLs induced ferroelectricity in BaZrO3 (BZ) for all SLs, a material that is paraelectric in the bulk form at any temperature and in the single film. The induced polar axis in BZ layers is perpendicular to the plane of substrate while BaTiO3 (BT) layers exhibit in-plane polar orientation. Raman spectroscopy revealed a lattice ordering in SLs due to the misfit strain generated by the large lattice mismatch between the alternating BZ and BT layers. This strain induced a huge upward frequency of the lowest E(1TO) soft mode from 60 c...

Phonon and magnon excitations in Raman spectra of an epitaxial bismuth ferrite film

An epitaxial film of bismuth ferrite BiFeO3 on a MgO(001) single-crystal substrate has been prepared by pulsed laser deposition using SrTiO3 and SrRuO3 buffer layers. At room temperature, the polarization characteristics of the Raman spectra of the BiFeO3 film under study suggest a monoclinic symmetry. The high-temperature (295–1100 K) investigations of the Raman spectra have been performed in the frequency range 20 cm−1 < ν < 1600 cm−1. Particular attention has been paid to the high-frequency region with a band observed at 610 cm−1, which corresponds to the maximum density of states of the magnon branch at the Brillouin zone boundary, and an intense band in the second-order Raman spectra with the maximum at ∼1250 cm−1, which corresponds to the density of states of two-magnon excitations. It has been found that the intensity of the band at ∼1250 cm−1 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−1 suggests that this film undergoes an antiferromagnetic phase transition at a temperature of ∼670 K.

Comparison of structural, microstructural, and electrical analyses of barium strontium titanate thin films

The results of structural and electrical characterizations of a barium strontium titanate (Ba0.5Sr0.5TiO3) film, including low temperature x-ray diffraction, low temperature, field dependent Raman spectroscopy, transmission electron microscopy, and low temperature, field dependent microwave measurements, are compared and contrasted. The structural characterization showed the film to be a good single crystal, epitaxial with the (001) MgO substrate. In the ferroelectric state, the tetragonal axis lies in plane leading to a 90° domain structure. A high density of misfit and threading dislocations was observed in the film. The transition to the paraelectric state on warming was shown to be diffuse and appeared to be strongly influenced by local variations in strain which were attributed to the defective microstructure. The dielectric response was also dominated by such effects.

Temperature Dependent Structural Properties of PbMg 1/3 Nb 2/3 O 3 Thin Films

We present structural results on Pb(Mg 1/3 Nb 2/3 )O 3 (PMN) thin films grown by pulsed laser deposition (PLD). Reflection high energy electron diffraction (RHEED), room temperature θ-2θ and phi scan x-ray diffraction measurements indicate that the films are epitaxially oriented. We have performed x-ray diffraction measurements as a function of temperature (10 K < T < 900 K) on two PMN films, 1500 Å and 7000 Å thick, grown on (100) MgO substrates buffered with La 1/2 Sr 1/2 CoO 3 (LSCO). We compare the temperature dependent evolution of the films out of plane lattice parameter with that of single crystal PMN. At first sight the dependencies are markedly different but can be explained by taking into account the thermal expansion of the MgO substrate. Asymmetric rocking curves taken about the {303} families of planes indicate that the substrate induces a tetragonal distortion in the films which exists over the entire temperature range investigated.

Structural Studies of Relaxor/Ferroelectric Pb(Mg1/3Nb2/3)O3/PbTiO3 Superlattices

Abstract We have synthesized [Pb(Mg1/3Nb2/3)O3](1− x)Λ/[PbTiO3] x Λ (PMN(1− x)ΛPT(x)Λ) superlattices by pulsed laser ablation onto MgO substrates buffered with La1/2Sr1/2CoO3 (LSCO). We varied the composition (0.1 ≤ x ≤ 0.8), and kept the modulation period Λ constant at about 140 ÅX-ray diffraction indicates that the polar c-axis of the PT layers lies in the plane of the films. This is confirmed by the Raman spectra. There is a shift to lower frequencies of the PT soft mode due to stress but we observe no indications of stress dependence in the Raman spectra of the PMN layers.

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

An 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 μC/cm2. The transport mechanisms were investigated from 90 to 400 K and compared to several models. It was found that the nearest neighbor hopping and variable range hopping mechanisms dominate the conduction above and below 270 K, 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.

Resistive Switching Hysteresis in Thin Films of Bismuth Ferrite

We 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.

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 A to 25 A 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 A), while Pnma LaFeO3-like structures are observed for small periodicity Λ ≤ 50 A. 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.Artificial (BiFeO3)0.5Λ/(LaFeO3)0.5Λ superlattices have been grown by pulsed laser deposition. The periodicity Λ was varied from 150 A to 25 A 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 A), while Pnma LaFeO3-like structures are observed for small periodicity Λ ≤ 50 A. 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.

Structural investigation of (111) oriented (BiFeO3)(1-x)Λ/(LaFeO3)xΛ superlattices by X-ray diffraction and Raman spectroscopy

(BiFeO3)(1-x)Λ/(LaFeO3)xΛ 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 a different family of planes was 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.