[Show abstract][Hide abstract] ABSTRACT: This paper describes a detailed analysis of the dependence of Raman scattering intensity on the polarization of the incident and inelastically scattered light in PbSc0.5Nb0.5O3 (PSN) single crystals and epitaxially compressed thin films grown on (100)-oriented MgO substrates. It is found that there are significant differences between the properties of the crystals and films, and that these differences can be attributed to the anticipated structural differences between these two forms of the same material. In particular, the scattering characteristics of the oxygen octahedra breathing mode near 810 cm-1 indicate a ferroelectric state for the crystals and a relaxor state for the films, which is consistent with the dielectric behaviors of these materials.
Journal of Advanced Dielectrics 06/2015; 05(02):1550013. DOI:10.1142/S2010135X15500137
[Show abstract][Hide abstract] ABSTRACT: A lattice strain of 0.3%–1.3% is achieved in epitaxial tetragonal BaTiO3
grown on (001)-oriented SrTiO3 single-crystal substrates. Our experimental studies of absorption spectra in the range of 0.74–9.0 eV demonstrate that epitaxy produces significant changes in the optical properties of the films compared with those of a reference polydomain BaTiO3 crystal: the absorption edge and the peak at 5 eV strongly blue-shift by 0.2–0.4 eV, the magnitude of the peak at 5 eV drops, and certain spectral features disappear, whereas the absorption peak at 8.5 eV remains unchanged. The observed behavior is attributed to ferroelectric
polarization, which is enhanced by epitaxial strain in the films. Our results indicate that epitaxially induced variations of ferroelectric
polarization may be used to tailor the optical properties of thin films for photonic and optoelectronic applications.
[Show abstract][Hide abstract] ABSTRACT: Perovskite-type ferroelectric (FE) crystals are wide bandgap materials with technologically valuable optical and photoelectric properties. Here, versatile engineering of electronic transitions is demonstrated in FE nanofilms of KTaO3, KNbO3 (KNO), and NaNbO3 (NNO) with a thickness of 10–30 unit cells. Control of the bandgap is achieved using heteroepitaxial growth of new structural phases on SrTiO3 (001) substrates. Compared to bulk crystals, anomalous bandgap narrowing is obtained in the FE state of KNO and NNO films. This effect opposes polarization-induced bandgap widening, which is typically found for FE materials. Transmission electron microscopy and spectroscopic ellipsometry measurements indicate that the formation of higher-symmetry structural phases of KNO and NNO produces the desirable red shift of the absorption spectrum towards visible light, while simultaneously stabilizing robust FE order. Tuning of optical properties in FE films is of interest for nanoscale photonic and optoelectronic devices.
Science and Technology of Advanced Materials 04/2015; 16(2). DOI:10.1088/1468-6996/16/2/026002 · 3.51 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: Very large lattice strain and strain-induced polarization are achieved in KNbO3 using epitaxial growth of a thin KNbO3 film onto a (001)-oriented SrTiO3 single-crystal substrate. We demonstrate experimentally that epitaxy produces dramatic changes of interband transitions in the film compared to those of a reference KNbO3 crystal: the energies of transitions change, some transitions are substantially suppressed and new ones appear in the film. A comparison of the experimental observations with theoretical calculations points to yet unexplored phenomena. Our results indicate that optical refraction and electro-optical coefficients of ferroelectric films can be controlled by epitaxial growth, which is of importance for emerging photonic and optoelectronic applications.
New Journal of Physics 04/2015; 17(4):043048. DOI:10.1088/1367-2630/17/4/043048 · 3.67 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: Quasi-static polarization induced by dc electric field was studied in a broad range of temperatures in epitaxial films of relaxor ferroelectric PbMg1/3Nb2/3O3 and PbSc1/2Nb1/2O3. The electric field was applied and the response was measured along the out-of-plane crystal direction of the epitaxial perovskite-structure (001) oriented films. The films remained in the relaxor state in zero-field cooling. A new polar state can be induced by electric field at a critical temperature below 100 K. The critical field and the induced polarization increased on cooling and reached the bulk-like values at 20 K. The orientational anisotropy of thin-film dipolar systems is discussed as a possible reason for the observed stable relaxor state.
[Show abstract][Hide abstract] ABSTRACT: Relaxor-to-ferroelectric transformations induced by varying electric fields and temperatures are studied experimentally in acube-on-cubetype epitaxial PbSc1/2Nb1/2O3 film grown on La1/2Sr1/2CoO3/MgO(001). Dielectric response, quasi-static and dynamic polarization, and dynamic current-voltage characteristics evidence the absence of spontaneous relaxor-to-ferroelectric transition. The electricfield-induced transformation from a glass-like relaxor state to a new dynamic polar state is detected at low temperatures below 100 K only. The frustration of ferroelectricity is discussed in relation to orientational anisotropy of the dipolar system in the epitaxial (001) film.
[Show abstract][Hide abstract] ABSTRACT: Optical index of refraction n is studied by spectroscopic ellipsometry in epitaxial nanofilms of NaNbO3 with thickness ~10 nm grown on different single-crystal substrates. The index n in the transparency spectral range (n ≈ 2.1 – 2.2) exhibits a strong sensitivity to atmospheric-pressure gas ambience. The index n in air exceeds that in an oxygen ambience by δn ≈ 0.05 – 0.2. The thermo-optical behaviour n(T) indicates ferroelectric state in the nanofilms. The ambience-sensitive optical refraction is discussed in terms of fundamental connection between refraction and ferroelectric polarization in perovskites, screening of depolarizing field on surfaces of the nanofilms, and thermodynamically stable surface reconstructions of NaNbO3.
Science and Technology of Advanced Materials 07/2014; 15(4):045001. DOI:10.1088/1468-6996/15/4/045001 · 3.51 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: The temperature evolution of local polarization properties in epitaxial PbZr0.65Ti0.35O3 films is studied by the low-temperature piezoresponse force microscopy (PFM). Pronounced changes in the film polarization state, including apparent polarization rotations and possible transitions between single-domain and polydomain states of individual ferroelectric nanocolumns, are revealed on cooling from the room temperature to 8 K using PFM imaging. More than two-fold increase in the coercive voltage extracted from the piezoresponse hysteresis loops is found on cooling from 240 to 8 K. The results are explained by the thermodynamic theory of strained epitaxial perovskite ferroelectric films.
[Show abstract][Hide abstract] ABSTRACT: Sodium niobate (NaNbO3, or NNO) is known to be antiferroelectric at temperatures between 45 and 753 K. Here we show experimentally the presence of the ferroelectric phase at temperatures between 100 and 830 K in the NNO crystals obtained by top-seeded solution growth. The ferroelectric phase and new phase transitions are evidenced using a combination of thermo-optical studies by variable angle spectroscopic ellipsometry, Raman spectroscopy analysis, and photoelectron emission microscopy. The possibility for strain-induced ferroelectricity in NNO is suggested.
[Show abstract][Hide abstract] ABSTRACT: Electric-field-induced transformations are studied experimentally in cube-on-cube-type epitaxial film of relaxor ferroelectric (FE) PbMg1/3Nb2/3O3 grown on (001) MgO substrate. The dielectric response, polarization, and current are measured along the out-of-plane <100> direction of the film and analyzed as a function of temperature, frequency, and applied field. Compared to the <100> crystal, transformation of the low-temperature relaxor state to a new state in the (001) film takes place at considerably lower temperatures and larger fields. Based on the found bisigmoidal shape and the two scaling regimes of the current-voltage curves, the two corresponding electric-field-induced processes are suggested to be dipolar flips and flow of a phase boundary in the film. The field-induced state in the film is dynamic and unstable, and it differs from the field-induced FE state in the crystal. The robustness of the relaxor state to electric field in the film is discussed in relation to spatial anisotropy of the dipolar system in the (001) film.
Physical Review B 02/2014; 89(9). DOI:10.1103/PhysRevB.89.094106 · 3.74 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: The optical properties of epitaxial perovskite-structure relaxor ferroelectric PbSc0.5Nb0.5O3 thin films are studied in broad spectral and temperature ranges by variable-angle spectroscopic ellipsometry. The films possess a metrically tetragonal crystal structure with a biaxial in-plane compressive strain of 0.1%–0.8%. The optical constants of the films with thickness of 10–50 nm are determined accurately using the advanced ellipsometry technique. The dramatic changes in the spectra of the dielectric functions and the absorption coefficient are found under various strain conditions. The characteristic energies of the spectra, including the bandgaps, vary by 0.1–0.5 eV. A frustration of the ferroelectric phase transition is evidenced by thermo-optical studies. A complex relationship between strain, polarization, and optical properties is discussed in terms of possible ionic displacements in metrically tetragonal PbSc0.5Nb0.5O3 films.
[Show abstract][Hide abstract] ABSTRACT: The use of high-performance perovskite-structure epitaxial relaxor ferroelectric films [ S. H. Baek et al. Science 334 958 (2011)] is hindered by the lack of knowledge of epitaxial effects therein. It is experimentally shown here that a biaxial epitaxial compression can favor the relaxor state over ferroelectricity. The absence of the ferroelectric transition and the existence of the low-temperature relaxor state are evidenced by a combination of x-ray diffraction, dielectric, polarization, and optical studies of PbSc0.5Nb0.5O3 films epitaxially grown on La0.5Sr0.5CoO3/MgO(001). This finding is beyond existing models of polarization in perovskite-structure epitaxial films and beyond the established ability to induce ferroelectricity by an epitaxial strain.
[Show abstract][Hide abstract] ABSTRACT: The possibility to tailor ferroelectricity by controlling epitaxial strain in thin films and heterostructures of complex metal oxides is well established. Here it is demonstrated that apart from this mechanism, 3D film growth during heteroepitaxy can be used to favor specific domain configurations that lead to step-like polarization switching and a giant nonlinear dielectric response in sub-switching ac electric fields. A combination of cube-on-cube epitaxial growth and the formation of columnar structures during pulsed laser deposition of Pb0.5Sr0.5TiO3 films on La0.5Sr0.5CoO3 bottom electrode layers and MgO (001) substrates stabilizes ferroelectric nanodomains with enhanced dynamic properties. In the Pb0.5Sr0.5TiO3 films, a- and c-oriented epitaxial columns grow from the bottom to the top of the film leading to random polydomain architectures with strong associations between the ferroelectric domains and the nanocolumns. Polarization switching in the two domain populations is initiated at distinctive fields due to domain wall pinning on column boundaries. Moreover, piezoelectric coupling between ferroelectric domains leads to strong interdomain elastic interactions, which result in an enhanced Rayleigh-type dielectric nonlinearity. The growth of epitaxial films with 3D columnar structures opens up new routes towards the engineering of enhanced ferroelectric and electromechanical functions in a broad class of complex oxide materials.
[Show abstract][Hide abstract] ABSTRACT: Phase transitions occurring in 130-nm-thick films of perovskite-structure ferroelectric Pb0.5Sr0.5TiO3 are experimentally studied by combining spectroscopic ellipsometry and low-frequency dielectric analysis. Polycrystalline and polydomain epitaxial films with relaxed misfit strain and columnar microstructure are investigated. The paraelectric and the ferroelectric states, and the temperatures and widths of the paraelectric-to-ferroelectric phase transitions, are identified from the temperature evolution of refractive index measured in transparency range. The temperatures at which transitions start on cooling are found to be considerably higher than the temperatures of the dielectric peaks. In contrast to the broad dielectric peaks, the transition width of 60 K in the polycrystalline film and that of 20 K in the polydomain epitaxial film are revealed. The discrepancies between optical and dielectric data are explained by the influence of extrinsic factors on the low-frequency response of the thin-film capacitors. It is suggested that fundamental mechanisms of ferroelectric phase transitions in thin films can be revealed by studies of thermo-optical properties.
[Show abstract][Hide abstract] ABSTRACT: The temperature dependences of the refractive index, n(T), calculated from ellipsometric data during the premartensitic and martensite transitions in a Ni2MnGa ferromagnetic shape memory alloy allow to detect all critical temperatures of the sample. The performed measurements reveal the behavior of a surface layer of a few tens nanometers depth due to the light absorption. Optical measurements reveal the earlier onset of premartensitic transition at the sample surface and shows unknown new features like time and temperature dependent effects related to the crystal surface. The underlying mechanisms of the observed temperature changes of the equilibrium n0 may be related to temperature dependent interactions between free electrons and phonons as well as between electrons and defects. A giant isothermal creep (up to 20%) of the refractive index was found for temperatures below T0 = 315 K. Below T0 the creep amplitude grows with the temperature difference T0-T. The creep disappears and the relaxation time becomes zero at temperatures above T0.
[Show abstract][Hide abstract] ABSTRACT: Epitaxial perovskite potassium tantalate (KTaO(3)) films with thicknesses of 7.4-36 nm are grown on SrTiO(3)(001) substrates by pulsed laser deposition. X-ray diffraction (XRD) analysis reveals evolution of lattice strain with increasing film thickness. A biaxial compressive in-plane strain as large as - 2.1% is obtained in the 7.4 nm-thick film. A bi-layer microstructure is detected in the 18 nm-thick film, suggesting the possibility for an abrupt strain relaxation.
[Show abstract][Hide abstract] ABSTRACT: We use computational and experimental methods to study d0 ferromagnetism at a
charge- imbalanced interface between two perovskites. In SrTiO3/KTaO3
superlattice calculations, the charge imbalance introduces holes in the SrTiO3
layer, inducing a d0 ferromagnetic half-metallic 2D electron gas at the
interface oxygen 2p orbitals. The charge imbalance overrides doping by
vacancies at realistic concentrations. Varying the constituent materials shows
ferromagnetism to be a gen- eral property of hole-type d0 perovskite
interfaces. Atomically sharp epitaxial d0 SrTiO3/KTaO3, SrTiO3 /KNbO3 and
SrTiO3 /NaNbO3 interfaces are found to exhibit ferromagnetic hysteresis at room
temperature. We suggest the behavior is due to high density of states and
exchange coupling at the oxygen t1g band in comparison with the more studied d
band t2g symmetry electron gas.
[Show abstract][Hide abstract] ABSTRACT: In ceramic BaTiO3 thin film prepared by chemical solution deposition, the influence of small grain size (10–150 nm) and grain boundaries on ferroelectric behavior is investigated by the studies of low‐frequency dielectric response. The apparent permittivity is suggested to result from an ensemble of grains possessing different properties and volume fractions, and having a nonferroelectric boundary layer. The effective permittivity of the boundary layer is found to be close to that of an interfacial layer in epitaxial thin‐film ferroelectrics, indicating possible fundamental resemblance of these layers. The maximum effective permittivity of the grain interiors is estimated to be about 1500. The observed small intrinsic grain permittivity, small Curie constant, broad dielectric peaks, and high‐temperature dielectric hysteresis are discussed in terms of size‐induced changes of the phase diagram.
Journal of the American Ceramic Society 04/2012; 95(4). DOI:10.1111/j.1551-2916.2011.04950.x · 2.61 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: The intrinsic ferroelectric phase transitions occurring in perovskite-structure epitaxial films of Pb0.5Sr0.5TiO3 are experimentally assessed and studied. The low-frequency dielectric response of thin-film heterostructure capacitors, polycrystalline films, and prototype ceramics is analyzed using phenomenological and equivalent circuit approaches. To explore the paraelectric to ferroelectric phase transitions, the derivative of inverse permittivity is used. In ceramics, the transition is of first order. In the epitaxial films, the order of the intrinsic transitions is difficult to determine in terms of classical phenomenology. Based on analysis of such factors as epitaxial strain, internal electric field, and domain configurations, we suggest that the configuration and dynamics of ferroelectric domains are responsible for this. The origin of apparently relaxor-looking dielectric peaks is discussed.
[Show abstract][Hide abstract] ABSTRACT: Epitaxial multilayers of Ba0.8Sr0.2TiO3 and Ba0.4Sr0.6TiO3 were grown by pulsed laser deposition on MgO substrates using an La0.5Sr0.5CoO3 bottom electrode. Some of the samples had a thin Ba0.6Sr0.4TiO3 buffer layer grown onto the bottom electrode to control the strains in the multilayers. The epitaxial relations, crystal perfection, and strains in the multilayers were studied by x-ray diffraction at room temperature. The dielectric properties were studied as a function of temperature. Correlation between lattice parameters, strain distributions, and width of the dielectric peaks is analyzed.