We investigate the ferroelectric phase transition and domain formation in a periodic superlattice consisting of alternate ferroelectric (FE) and paraelectric (PE) layers of nanometric thickness. We find that the polarization domains formed in the different FE layers can interact with each other via the PE layers. By coupling the electrostatic equations with those obtained by minimizing the Ginzburg-Landau functional we calculate the critical temperature of transition Tc as a function of the FE/PE superlattice wavelength Λ and quantitatively explain the recent experimental observation of a thickness dependence of the ferroelectric transition temperature in KTaO3/KNbO3 strained-layer superlattices.PACS numbers: 77.55.+f, 77.80.Dj, 77.80.Bh In the past decade refinements in deposition techniques have made it possible to fabricate nanoscale size oxide ferroelectric superlattices with the objective to merge and optimize the technological properties of the constitutive materials [1,2,3]. In designing such artificial structures an understanding of the physics of underlying processes is essential to determine whether the resulting characteristics are provided simply by the superposition of the bulk properties of the constituents or whether the interface and finite-size effects play a predominant role.Two competing types of phenomena that arise at the ferroelectric interface can affect the properties of the superlattices. The strain field, generated by the mechanical mismatch between the superlattice layers, influences the polarization orientation and generally increases the ferroelectric transition temperature T c [4]. In contrast, the electric depolarization field, produced by interfacial surface charges is unfavorable to the formation of the ferroelectric phase [5]. In fact, in cubic perovskite-like ferroelectrics the situation can be even more complex due the formation of both 180• ferroelectric [6] and 90• ferroelastic [4,7,8] domains. Although the properties of ferroelectric superlattices can be governed by domain structure, no systematic study of this effect has to our knowledge been performed.In the present paper, we address the question of ferroelectric domain formation in a periodic superlattice structure consisting of alternate ferroelectric (FE) and paraelectric (PE) layers of equal nanometric width 2a f = 2a p . So as to avoid the complications of the effect of 90• ferroelastic domains we assume that the ferroelectric layers have either natural or strain-induced c-oriented uniaxial symmetry. We will show that the domain patterns formed in the different FE layers interact with each other across the PE layers via the spatially inhomogeneous depolarization electric field emerging from the domains of the neighboring FE layers as shown in Fig. 1. This proximity type effect is dependent critically on the thickness of the PE layers. Our interest has also been motivated by a recent experimental study of FE/PE superlattices of KTaO 3 /KNbO 3 [9] in which, as the superlattice wavelength Λ = 2a f + 2a p decreases, t...

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 temperature is attributed to the stress effect induced by the substrate.

No abstract

We have used laser ablation to grow series of PbTiO 3 /BaTiO 3 ͑PTO/BTO͒ multilayers with a modulation wavelength ⌳ that varies between 50 ÅϽ⌳Ͻ360 Å. Modeling the x-ray-diffraction patterns of the multilayers indicates that the PTO layers are a-axis oriented. The Raman measurements reinforce this interpretation. The soft-mode Raman line shifts abruptly in frequency above ⌳ϭ240 Å due to possible strain relaxation of the multilayer, and the ⌳ dependent position of an asymmetric line in the vicinity of 200 cm Ϫ1 is modeled as a confined mode, a signature of a modulated structure. RAPID COMMUNICATIONS R6450PRB 61 F. Le MARREC et al.

No abstract

We have made magnetization and x-ray diffraction measurements on an epitaxial Pb(Fe1/2Nb1/2)O3 200 nm film. From the temperature dependence of the out-of-plane lattice parameter we can assign a Burns' temperature at Td ~ 640 K, a temperature at T* ~ 510 K, related to the appearance of static polar nanoregions, and an anomaly occurring at 200 K. The latter is precisely the N\'eel temperature TN determined from magnetization and points to spin-lattice coupling at TN ~ 200 K. We also observe "weak ferromagnetism" up to 300K and propose superantiferromagnetic clusters as a plausible scenario to explain this hysteresis above TN.Comment: 12 pages, 4 figure

We report a temperature-dependent high-resolution x-ray diffraction investigation of 200-nm epitaxial BiFeO 3 thin films grown on ͑001͒ SrTiO 3 . We find that BiFeO 3 undergoes two high-temperature transitions: a first-order ␣-␤ phase transition between 745 and 780°C and a more diffuse transition toward the ␥ phase at 860°C. Reciprocal space maps reveal that thin films remain monoclinic crossing the ␣-␤ phase transition. Linear extrapolation of the in-plane lattice parameters to higher temperatures appears to rule out cubic symmetry for the ␥ phase.

Superconducting upper critical fields H, 2(T), transition temperatures T"and normalstate electrical resistivities p"have been measured in the amorphous transition-metal alloy series Zri "Co, Zri "Ni", (Zri "Ti")Q7gNiQ22, and (Zri "Nb")Q78NiQ22. Structural integrity of these melt-spun alloys is indicated by x-ray, density, bend-ductility, normal-state electrical resistivity, superconducting transition width, and mixed-state flux-pinning measurements.