F. Le Marrec

Ferroelectric transition in an epitaxial barium titanate thin film: Raman spectroscopy and x-ray diffraction study

We have performed x-ray diffraction and Raman spectroscopy measurements in the temperature range of 300–873 K on a single phase epitaxially oriented BaTiO3 thin film grown by pulsed laser deposition on a single crystal MgO substrate. The θ–2θ room temperature diffraction measurements and asymmetric rocking curves indicate that the film is very weakly tetragonal with the c-axis parallel to the plane of the film. X-ray diffraction measurements up to high temperature reveal only a change in slope in the perpendicular to the plane lattice parameter around 450 K (in bulk Tc=395 K) indicating that a diffuse-like of phase transition is taking place. Room temperature polarized Raman spectra show that the film is indeed tetragonal with C4v symmetry and with the a-axis perpendicular to the film plane. Monitoring of the overdamped soft mode and the 308 cm−1 mode confirms that the phase transition is taking place over a wide temperature range according to the x-ray results. The increase of the phase transition temper...

Absence of a PbTiO3 phase transition in PbTiO3/BaTiO3 superlattices

Abstract We have performed X-ray diffraction and Raman spectroscopy on three PbTiO 3 /BaTiO 3 superlattices (PTO/BTO) from room temperature (RT) to 600°C. The superlattices were 4000 A thick and the modulation wavelengths Λ, with equally thick layers of PTO and BTO, were 136, 187 and 357 A. The samples were laser deposited onto single crystal MgO substrates buffered with 50 A of SrTiO 3 (STO). We observe no X-ray evidence of a phase transition in the PTO layers of these three superlattice samples up to 600°C. This result was reinforced by X-ray modeling of the samples. We also monitored the E(1TO) or “soft “ mode by Raman spectroscopy up to 550°C and found only a linear decrease of mode frequency with temperature for all three superlattices: another sign that no phase transition is taking place in this temperature range.

Ferroelectric Phase in Barium Titanate Epitaxial Thin Film

X-ray diffraction and Raman spectroscopy were used to examine the structure and the ferroelectric state of highly oriented barium titanate thin film deposited by the pulsed laser deposition on (001)MgO substrate. It is found that the film is weakly tetragonal and domains are a-axis oriented. The temperature dependencies of the out plane lattice parameter and the tetragonal Raman peak at 308 cm m 1 show that the ferro-paraelectric phase transition occurs above the the bulk transition temperature and can be considered as diffuse. This behaviour is attributed to the stress effect.

Structure and vibrational properties of some PbTiO3-based ferroelectric superlattices

Abstract Ferroelectric PbTiO3(PTO)/BaTiO3(BTO) superlattices have been grown on SrTiO3 buffered MgO substrates by laser ablation. An extended X-ray study has shown that the orientation of individual layers is c-domain (i.e., c-axis of the tetragonal phase perpendicular to the substrate) for BTO and a-domain for PTO, contrary to what is observed for single PTO or BTO thin films, which are both c-oriented. Raman scattering reinforces this assertion, and also defines the critical modulation period A above which stress relaxation occurs via misfit dislocations. Confined modes have been also observed, in the same way as in semiconductor superlattices. The temperature variation of the soft mode frequency is shown to depend upon A.

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.

Mixed orientation PbTiO3/BaTiO3 superlattices: X-ray diffraction and raman spectroscopy

Abstract We have used pulsed laser ablation to grow a series of PbTiO3/BaTiO3 (PTO/BTO) multilayers with a modulation wavelength Λ that varies between 50Å<Λ<360 Å. By modeling the x-ray diffraction patterns we have determined that the PTO layers are a-axis oriented and the BTO layers are c-axis oriented. This is in contrast to individual thin films of PTO in which the c-axis is perpendicular to the plane of the film. Raman measurements of the multilayers reinforce this x-ray model determination. We also observe that the soft mode Raman line shifts abruptly in frequency above Λ=240Å. We ascribe this to the strain relaxation in the multilayer, in which misfit dislocations appear at a critical wavelength Λc. A calculation of Λc for a-axis PTO/c-axis BTO multilayers based on elasticity theory yields a value of 260Å, in good agreement with the Raman results.

Ferroic phase transition sequence in epitaxial BiFeO3 thin films

We report a temperature-dependent high-resolution X-ray diffraction investigation of 200 nm epitaxial BiFeO3 thin films grown on (001) SrTiO3. We were successful in making epitaxial thin films thermally stable, a key requirement for this study. Our results provide evidence that BiFeO3 undergoes three high-temperature transitions: an antiferromagnetic transition at 425°C, a first-order α–β phase transition between 745°C and 780°C and a smoother transition toward the γ-phase at 860°C. Both the α–β and β–γ transitions take place 60°C lower than the bulk values whereas the antiferromagnetic transition occurs 55°C higher. This underscores the part played by strain and also emphasizes that BiFeO3 is not a conventional ferroelectric perovskite. Reciprocal space maps reveal the unusual result that the thin films remain monoclinic on crossing the α–β phase transition. Linear extrapolation of the in-plane lattice parameters to higher temperatures rules out cubic symmetry for the γ-phase.

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.

Structural and raman spectrocsopic investigation of epitaxial Pb(Zr0.52Ti0.48)O3-based heterostructures

Abstract We have successfully grown in situ, by laser ablation, the following heterostructures: Pb(Zr0.52Ti0.48)O3 / YBa2Cu3O7 / CeO2 / (1102) Al2O3(PZT/YBCO/CeO2/Al2O3), Pb(Zr0.52Ti0.48)O3 / YBa2Cu3O7 / (100) SrTiO3(PZT/YBCO/STO), and Pb(Zr0.52Ti0.48)O3 / SrTiO3 / (100) MgO (PZT/STO/MgO). By combining RHEED. x-ray diffraction and atomic force microscopy, we have determined that (a) all the buffer and electrode layers are atomically flat, (b) PZT grows in the 2D to 3D Stransky-Krastanov mode, (c) there is an epitaxial relationship between the in-plane axes of the layers and the substrates, and (d) the c-axis of PZT is perpendicular to the plane of the film. Raman scattering shows a significant difference in spectra between a PZT thin film heterostructure and a ceramic PZT target. This is further evidence for the epitaxial nature of the PZT films and their fundamental difference with respect to ceramic samples.

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.