N. Lemée

Epitaxial growth and magnetoelectric relaxor behavior in multiferroic 0.8Pb(Fe1/2Nb1/2)O3–0.2Pb(Mg1/2W1/2)O3 thin films

We present electric and magnetic properties of 0.8Pb(Fe1/2Nb1/2)O3–0.2Pb(Mg1/2W1/2)O3 films epitaxially grown on (001) SrTiO3 substrates using pulsed laser deposition. A narrow deposition window around 710 °C and 0.2 mbar has been identified to achieve epitaxial single-phase thin films. A typical Vogel–Fulcher relaxorlike dielectric and magnetic susceptibility dispersion is observed, suggesting magnetoelectric relaxor behavior in these films similar to the bulk. We determine a magnetic cluster freezing temperature of 36 K, while observing weak ferromagnetism via magnetic hysteresis loops up to 300 K.

Polarization Rotation in Ferroelectric Tricolor PbTiO3/SrTiO3/PbZr0.2Ti0.8O3 Superlattices.

In ferroelectric thin films, controlling the orientation of the polarization is a key element to controlling their physical properties. We use laboratory and synchrotron X-ray diffraction to investigate ferroelectric bicolor PbTiO3/PbZr0.2Ti0.8O3 and tricolor PbTiO3/SrTiO3/PbZr0.2Ti0.8O3 superlattices and to study the role of the SrTiO3 layers on the domain structure. In the tricolor superlattices, we demonstrate the existence of 180° ferroelectric stripe nanodomains, induced by the depolarization field produced by the SrTiO3 layers. Each ultrathin SrTiO3 layer modifies the electrostatic boundary conditions between the ferroelectric layers compared to the corresponding bicolor structures, leading to the suppression of the a/c polydomain states. Combined with the electrostatic effect, the tensile strain induced by PbZr0.2Ti0.8O3 in the PbTiO3 layers leads to polarization rotation in the system as evidenced by grazing incidence X-ray measurements. This polarization rotation is associated with the monoclinic Mc phase as revealed by the splitting of the (HHL) and (H0L) reciprocal lattice points. This work demonstrates that the tricolor paraelectric/ferroelectric superlattices constitute a tunable system to investigate the concomitant effects of strains and depolarizing fields. Our studies provide a pathway to stabilize a monoclinic symmetry in ferroelectric layers, which is of particular interest for the enhancement of the piezoelectric properties.

Structural and electrical properties of Bi0.5Na0.5 TiO3 based superlattices grown by pulsed laser deposition

Artificial superlattices of ferroelectric Bi0.5Na0.5TiO3 and BaTiO3 have been successfully grown on (001) insulator or conductive SrTiO3 substrates by pulsed laser deposition. In these epitaxial layered structures, the BaTiO3 layers were shown to contribute to an improvement of the two dimensional growth of the Bi0.5Na0.5TiO3 layers. The influence of the superlattice period Λ, between 5 and 20 nm, was investigated. We observe an increase in the in-plane tensile strain as Λ is reduced from 20 nm to 10 nm, accompanied by a decrease in the density of dislocations. A concomitant enhancement of the dielectric permittivity was measured, demonstrating the strain tunability of these superlattices. A significant reduction of the dielectric losses is also obtained with decreasing Λ. Furthermore, a minimum value of the coercive field of less than 70 kV/cm, close to that of Bi0.5Na0.5TiO3 bulk ceramics, was reached by decreasing the period. We demonstrate that the dielectric and ferroelectric properties can be explai...

Microwave dielectric dispersion in a multiferroic Pb(Fe1/2Nb1/2)O3 thin film

We present the dielectric dispersion in a Pb(Fe1/2Nb1/2)O3 (PFN) thin film grown on (001) SrTiO3 substrate from 20 MHz to 20 GHz in the temperature range of 270 to 340 K. In the radio frequency region, the hopping charge transport and associated polar nanoregion and/or domain-wall motion contribute to the spectrum. In the microwave region, relaxational dispersion was observed with a dielectric contribution of Δe ≈ 600 at room temperature having a characteristic frequency (fm)0.4 = 9.8 · (T−T0) Hz, where T0 is 660 K. It is associated with possible mode-softening behavior, related to the onset of polar nanoregions at T0. The dielectric permittivity shows similarities with 1-dimensional Ising model behaviour.

Strain effect in PbTiO3/PbZr0.2Ti0.8O3 superlattices: From polydomain to monodomain structures

Ferroelectric symmetric superlattices consisting of alternating layers of PbTiO3 and PbZr0.2Ti0.8O3, were grown by pulsed laser deposition on SrTiO3 and SrRuO3-coated SrTiO3 substrates. The superlattices, with wavelengths Λ ranging from 20 A to 200 A, were analyzed using x-ray diffraction (θ−2θ diffraction scans, rocking curves, and reciprocal space mapping), high resolution transmission electron microscopy, and piezoforce scanning microscopy. For large-period superlattices, the strain is relieved by the formation of an a/c polydomain structure which propagates through the whole film. We investigate the influence of the wavelength on the a-domain volume fraction, the lattice parameters, the in-plane strain exx, and the mosaicity of the samples. We show that with decreasing the wavelength, a reduction of the a-domain volume fraction is observed as well as a reduced tensile in-plane strain and a lower mosaicity. A concomitant improvement of the local ferroelectric response is detected. Below a critical wave...

Diffuse X-ray scattering from 180° ferroelectric stripe domains: polarization-induced strain, period disorder and wall roughness

A key element in ferroic materials is the presence of walls, separating domains with different orientations of the order parameter. We demonstrate that 180° stripe domains in ferroelectric films give rise to very distinct features in their diffuse X-ray scattering (DXS) intensity distributions. A model is developed that allows to determine not only the domain period but also the period disorder, the thickness and roughness of the domain walls, and the strain induced by the rotation of the polarization. As an example, the model is applied to ferroelectric/paraelectric superlattices. Temperature-dependent DXS measurements reveal that the polarization-induced strain dramatically decreases with increasing temperature and vanishes at the Curie temperature. The motion of ferroelectric domain walls appears as a collective process that does not create any disorder in the domain period, whereas the pinning by structural defects increases the wall roughness. This work will facilitate in-situ quantitative studies of ferroic domains and domain wall dynamics under the application of external stimuli, including electric fields and temperature.

Inhibition of polydomain formation in PbTiO3/PbZr0.2Ti0.8O3 superlattices by intercalation of ultra-thin SrTiO3 layers

We used pulsed laser deposition to grow a series of PbTiO3/PbZr0.2Ti0.8O3 superlattices on SrTiO3 and SrRuO3/SrTiO3 substrates. An a/c polydomain structure was evidenced by reciprocal space mapping and by transmission electron microscopy. Insertion of ultra-thin layers of SrTiO3 at the interfaces between PbTiO3 and PbZr0.2Ti0.8O3 layers has inhibited this polydomain formation. A strong decrease in the tetragonality indicates clearly that the polarization state in these superlattices has changed due to the insertion of the SrTiO3 layers. A purely elastic mechanism does not seem to explain the determined structural parameters.

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.

Ultraviolet vacuum ultraviolet optical functions for SrTiO3 and NdGaO3 crystals determined by spectroscopic ellipsometry

Complex dielectric functions e(E) = e1(E) + e2(E) were experimentally evaluated within the spectral range E = 2–25 eV and E = 2–20 eV for SrTiO3 and NdGaO3 single crystals, respectively, using synchrotron-based spectroscopic ellipsometry measurements. The ellipsometric spectra were evaluated within a framework of optical layer model taking into account sample surface roughness and anisotropy of NdGaO3. The parameters of Herzinger-Johs oscillator model were fitted to reproduce sufficiently all features of the optical spectra within the spectral range 2–10 eV. Only slight differences were revealed for spectra polarized along b and c crystallographic axes of the NdGaO3, which can confirm weak optical anisotropy.

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.