Anthony Ferri

Tuning the atomic and domain structure of epitaxial films of multiferroic BiFeO3

Recent works have shown that the domain walls of room-temperature multiferroic BiFeO3 (BFO) 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 (001)-SrTiO3, as a function of thickness. The clamping of the substrate has been observed to exist in two different regimes: ultrathin, d 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 degrees 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 BiFeO3 thin films.

Nanoscale investigations of switching properties and piezoelectric activity in ferroelectric thin films using piezoresponse force microscopy

With respect to nanoscale ferroelectric thin films research, piezoresponse force microscopy (PFM) for domain imaging and local piezoelectric spectroscopy for switching properties and piezoelectric activity measurements require the control of experimental conditions associated to the experimental set-up. To avoid any misinterpretation of the results, stiffness of the cantilever, or frequency and amplitude of the driving AC voltage have to be carefully investigated. In this paper, optimal working conditions determined with our experimental set-up are described in order to evidence the architecture of domains, to measure the coercive voltage and to evaluate the piezoelectric activity of Pb(Zr, Ti)O3 thin films. Illustrations are carried out on highly oriented, unetched and ion beam etched tetragonal films; explanations of the behaviour of the ferroelectric material at the nanoscale level are given.

Ferroelectric Control of Organic/Ferromagnetic Spinterface

Organic multiferroic tunnel junctions based on La0.6 Sr0.4 MnO3 /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.

Reversible control of magnetic domains in a Tb0.3Dy0.7Fe2/Pt/PbZr0.56Ti0.44O3 thin film heterostructure deposited on Pt/TiO2/SiO2/Si substrate

Tb0.3Dy0.7Fe2/Pt/PbZr0.56Ti0.44O3 (Terfenol-D/Pt/PZT) magnetoelectric (ME) thin films were deposited on Pt/TiO2/SiO2/Si substrate. Ferroelectric and magnetic properties were characterized at room temperature. At zero dc magnetic field and out of mechanical resonance, a variation of the voltage across the ferroelectric film was obtained when a small external ac magnetic field was applied to the device. The corresponding ME voltage coefficient was 1.27 V/cm Oe. On the same sample, local magnetic domain patterns were imaged by magnetic force microscopy. Reversible changes in magnetic domain patterns were observed when a dc electric field of 120 to 360 kV/cm was applied to the ferroelectric layer. These results confirm that both magnetic control of ferroelectric polarization and electric control of magnetization are achievable on ME thin films devices deposited on silicon substrates.

Lead-Free α-La2WO6 Ferroelectric Thin Films

(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.

Epitaxial growth and nanoscale electrical properties of Ce2Ti2O7 thin films

(00l) epitaxial Ce2Ti2O7 thin films with a layered perovskite/monoclinic structure were grown on (110)-oriented Nb-doped SrTiO3 substrates via pulsed laser deposition and a sol–gel method associated with spin-coating. Using the sol–gel method, the Ce2Ti2O7 films were obtained by annealing at 950 °C under a reductive Ar/H2 atmosphere. Employing the pulsed laser deposition technique, they were directly grown under vacuum (10−6 mbar) with a controlled re-oxidation during the cooling step. The pole figure measurements provide the in-plane crystallographic relationships between the film and substrate: [001]SrTiO3//[100]Ce2Ti2O7 and [1−10]SrTiO3//[010]Ce2Ti2O7. Piezoresponse force microscopy measurements highlight the local ferroelectric character of the films synthetized. The switching capability was more reliable for the film grown via pulsed laser deposition, which was explained by the lower mosaic spread. Higher local conductivity was also detected using conductive-atomic force microscopy of the physically deposited film and was attributed to its lower thickness. Such epitaxially deposited functional oxides may be considered as promising candidates for integration into advanced electronic devices.

Thickness effect on nanoscale electromechanical activity in Pb(Mg1/3Nb2/3)O3-PbTiO3 thin films studied by piezoresponse force microscopy

0.7Pb(Mg1/3Nb2/3)O3–0.3PbTiO3 (PMN–PT) ferroelectric thin films with thickness ranging from 28 to 110 nm were sputter deposited onto LaNiO3/SiO2/Si substrates. Optical properties were determined by spectroscopic ellipsometry. We found B = 4.6 and λ0 = 209 nm, which is consistent for all PMN–PT samples with previous results shown in the literature. Nanoscale electromechanical activity was probed by using piezoresponse force microscopy in imaging and spectroscopic modes. Both piezoresponse images and local piezoloops recorded on each film highlighted an enhancement of piezoelectric vibration amplitude when the film thickness increased from 28 to 62 nm (∼1.06 to ∼1.34 mV), then saturation was observed for thicker films. This specific evolution was explained taking into account the low-permittivity interfacial Pb2Nb2O7 layer existing between bottom electrode and PMN-PT layer. Higher leakage current when thickness is decreasing was shown, which could also explain the particular behavior of the local electromec...

Nanoscale Investigations of α- and γ-Crystal Phases in PVDF-Based Nanocomposites

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 | d33| ≈ 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.

Fabrication of La 2 Ti 2 O 7 nanostructures by focused Ga 3+ ion beam and characterization by piezoresponse force microscopy

(012)-oriented lead-free La₂Ti₂O₇ thin films with the monoclinic/perovskite layered structure have been grown by a solgel route on (100)-oriented doped Nb:SrTiO₃ substrates. On nanoscale, both poling experiments performed via the tip of atomic force microscope and the existence of local piezoloops within the domains confirm the piezo-/ferro-electric behaviour of the films. Islands in the lateral range 300-500 nm have been fabricated by focused Ga³⁺ ion beam etching on platinum top electrode. As measured on piezoloops, electromechanical activity within the islands is shown to be similar to the one obtained for the virgin film; no piezoelectric degradation for La₂Ti₂O₇ islands is highlighted. These results confirm that La₂Ti₂O₇ is a highly resistant oxide to ion-beam irradiation. La₂Ti₂O₇ could be considered as a material of choice for the realization of lead-free piezoelectric nanostructures.

Microstructure and local electrical behavior in [(Nd2Ti2O7)4/(SrTiO3)n]10 (n = 4–8) superlattices

Artificial [(Nd2Ti2O7)4/(SrTiO3)n]10 superlattices (n = 4 and 8) were successfully epitaxially grown on SrTiO3 substrates by pulsed laser deposition using the in situ high energy electron diffraction reflection diagnostic. The crystallographic relationships between Nd2Ti2O7 (NTO) and SrTiO3 (STO) (layers and substrate) were: [100]NTO//[001]STO, [010]NTO//[10]STO, and (00l)NTO//(110)STO. Nanoscale current variation was detected on both superlattices, with the (NTO4/STO4)10 heterostructure showing a higher density. The (NTO4/STO4)10 sample did not show a piezoelectric response when measured by piezo-force microscopy (PFM), while ambiguous piezoactivity was observed on the (NTO4/STO8)10 superlattice. Scanning transmission electron microscopy energy dispersive spectroscopy analysis showed the diffusion of Nd3+ cations on Sr2+ sites in SrTiO3 structure into the multilayers, which was more pronounced when the value of n was lower. These particular nanoscale electrical behaviors, evidenced by electrical conducting channels and misleading PFM signals, were mainly attributed to the presence of oxygen vacancies in the SrTiO3 layers at higher concentrations near the interface and to the mixed valence state of the titanium (Ti3+/Ti4+). This work showed the strong influence of interface structure on nanoscale electrical phenomena in complex oxide superlattices.