Recent works have shown that the domain walls of room-temperature multiferroic BiFeO 3 ͑BFO͒ thin films can display distinct and promising functionalities. It is thus important to understand the mechanisms underlying domain formation in these films. High-resolution x-ray diffraction and piezoforce microscopy, combined with first-principles simulations, have allowed us to characterize both the atomic and domain structure of BFO films grown under compressive strain on ͑001͒-SrTiO 3 , as a function of thickness. The clamping of the substrate has been observed to exist in two different regimes: ultrathin, d Ͻ 18 nm, and thin, d Ͼ 18 nm. When this is taken into account in the calculations, an excellent agreement between the predicted and observed lattice parameters is shown. We derive a twinning model that describes the experimental observations and could explain why the 71°domain walls are the only ones showing insulating character. This understanding of the exact mechanism for domain formation provides us with a new degree of freedom to control the structure and, thus, the properties of BiFeO 3 thin films.

The impact of carbon nanotube (CNT) incorporation into semicrystalline poly(vinylidene fluoride), PVDF, was investigated at both the macro and nanoscales. A special effort was devoted to probe the local morphology and the mechanical, ferroelectric, piezoelectric, and electrical conductivity response by means of atomic force microscopy. Incorporation of CNTs mainly induces the development of the polar γ-phase, and as a consequence, the coexistence of the γ-phase with the most stable nonpolar α-phase is observed. A maximum γ-phase content is reached at 0.7 wt % CNT loading. The spherulitic morphology of the PVDF α-phase is assessed, in conjunction with the lack of any ferroelectric response, while the presence of the polar γ-phase is confirmed, owing to clear piezoresponse signals. Local piezoelectric measurements on γ-phase domains yield a maximum effective coefficient | d| ≈ 13 pm/V, thus underlining the potential for applications of such functional PVDF-based nanocomposites in advanced piezoelectric devices. An increase in macroscopic conductivity with CNT content is observed, with a percolation threshold achieved for a composition close to 0.7 wt %. Nanoscale investigation of the electrical conductivity confirms the presence of some infinite CNT cluster homogeneously distributed over the surface. The macroscopic viscoelastic behavior of the composite reflects the reinforcing effect of CNTs, while the nanomechanical characterization yields a local contact modulus of the γ-phase domains larger than that of its α-phase counterpart, in agreement with the fact that the CNTs act as γ-phase promoters and subsequently reinforce the γ-domains.

Organic multiferroic tunnel junctions based on La Sr MnO /poly(vinylidene fluoride) (PVDF)/Co structures are fabricated. The tunneling magneto-resistance sign can be changed by electrically switching the ferroelectric polarization of PVDF barrier. It is demonstrated that the spin-polarization of the PVDF/Co spinterface can be actively controlled by tuning the ferroelectric polarization of PVDF. This study opens new functionality in controlling the injection of spin polarization into organic materials via the ferroelectric polarization of the barrier.

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(001)-Epitaxial La2WO6 (LWO) thin films are grown by pulsed laser deposition on (001)-oriented SrTiO3 (STO) substrates. The α-phase (high-temperature phase in bulk) is successfully stabilized with an orthorhombic structure (a = 16.585(1) Å, b = 5.717(2) Å, c = 8.865(5) Å). X-ray-diffraction pole-figure measurements suggest that crystallographic relationships between the film and substrate are [100]LWO ∥ [110]STO, [010]LWO ∥ [11̅0]STO and [001]LWO ∥ [001]STO. From optical properties, investigated by spectroscopic ellipsometry, we extract a refractive-index value around 2 (at 500 nm) along with the presence of two absorption bands situated, respectively at 3.07 and 6.32 eV. Ferroelectricity is evidenced as well on macroscale (standard polarization measurements) as on nanoscale, calling for experiments based on piezo-response force-microscopy, and confirmed with in situ scanning-and-tunneling measurements performed with a transmission electron microscope. This work highlights the ferroelectric behavior, at room temperature, in high-temperature LWO phase when stabilized in thin film and opens the way to new functional oxide thin films dedicated to advanced electronic devices.

At very low temperature (450 • C), (111)-oriented and polycrystalline 0.7Pb(Mg 1/3 Nb 2/3 ) O 3 -0.3PbTiO 3 (PMN-PT) thin films have been grown on platinum (Pt) and lanthium niobate (LaNiO 3 ) bottom electrodes respectively. Macroscopic measurements reveal lower coercive fields for PMN-PT grown on LaNiO 3 compared to on platinum, while the piezoelectric coefficient d 33 is greater. At the nanometer scale, local piezoelectric hysteresis loops show that the voltages required for domain switching and piezoelectric response are the highest for PMN-PT deposited on LaNiO 3 . The electrical results can be explained by taking into account the effects induced by both electrodes on the surface morphology and structural properties of the films.

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Ferroelectric domains were investigated using piezoresponse force microscopy in superlattices composed of multiferroic BiFeO 3 and SrTiO 3 layers. Compared to single BiFeO 3 thin films, a reduction in the domains size and a suppression of the in-plane orientation of domains are observed in a superlattice of (BiFeO 3 ) 4 (SrTiO 3 ) 8 , suggesting a constrained ferroelectric domain orientation along the out-of-plane <001> direction. Such modification of domain size and orientation in BiFeO 3 -based heterostructures could play a vital role on engineering the domains and domain wall mediated functional properties necessary for device applications. 1 prellier@ensicaen.frRecent study on the strain effect on epitaxial (001) BFO thin films shows that, though the magnitude of the polarization remains unchanged, the polarization variants in BFO could be altered by strain. 9 A strain-induced out-of-plane rotation of polarization from the (111)