NdGaO3 Substrates Research Articles

Giant Polarization Sustainability in Ultrathin Ferroelectric Films Stabilized by Charge Transfer.

Ferroelectricity is generally deteriorated or even vanishes when the ferroelectric films are downsized to unit cell scale. To maintain and enhance the polarization in nanoscale ferroelectrics are of scientific and technological importance. Here, giant polarization sustainability is reported in a series of ultrathin PbTiO3 films scaled down to three unit cells grown on NdGaO3 (110) substrates with La0.7 Sr0.3 MnO3 as bottom electrodes. Atomic mappings via aberration-corrected scanning transmission electron microscopy demonstrate the robust ferroelectricity for the sub-10 nm thick film. For the 1.2 nm thick film, the polarization reaches ≈50 µC cm-2 . The 2 nm thick film possesses a polarization as high as the bulk value. The films ranging from 10 to 35 nm display a giant elongation of out-of-plane lattice parameter, which corresponds to a polarization of 100 µC cm-2 , 20% larger than that of the bulk PbTiO3 . The giant enhancement of polarization in the present films is proposed to result from the charge transfer at the La0.7 Sr0.3 MnO3 /PbTiO3 interface, as supported by the anomalous decrease of Mn valence measured from X-ray photoelectron spectroscopy. These results reveal the significant role of charge transfer at interfaces in improving large polarizations in ultrathin ferroelectrics and are meaningful for the development of future electronic devices.

Structural and compositional characterization of single crystal uranium dioxide thin films deposited on different substrates

Uranium dioxide thin films were deposited on single crystal TiO2, Al2O3, YSZ, ZnO and NdGaO3 substrates to optimize conditions for the growth of high quality single crystal films. X-ray diffraction results show that all the films have one growth direction and well defined peaks in the specular scans with the expected symmetry for each growth orientation. The UO2/Al2O3, TiO2, and ZnO films have high concentration of misfit dislocations that increase with the lattice mismatch. The UO2 film on YSZ is found to be in registry with the substrate. The film has narrow mosaic component that is imposed upon a broader component arises from the diffuse scattering due to defects in the film. Meanwhile, UO2/NdGaO3 film shows a splitting of the X-ray diffraction peaks which is attributed to the in-plane asymmetry of the orthorhombic substrate.

Long-range Stripe Nanodomains in Epitaxial (110) BiFeO3 Thin Films on (100) NdGaO3 Substrate

Here, we report the observation of ferroelectric and ferroelastic nanodomains in (110)-oriented BiFeO3 (BFO) thin films epitaxially grown on low symmetric (100) NdGaO3 (NGO) substrate. We observed long range ordering of ferroelectric 109° stripe nanodomains separated by periodic vertical domain walls in as-grown 130 nm thick BFO films. The effect of La0.67Sr0.33CoO3 (LSCO) conducting interlayer on domain configurations in BFO/NGO film was also observed with relatively short range-ordering of stripe domains due to the modified electrostatic boundary conditions in BFO/LSCO/NGO film. Additional studies on B-site doping of Nb ions in BFO films showed change in the domain structures due to doping induced change in lattice anisotropy while maintaining the stripe domain morphology with 109° domain wall. This long-range array of ferroelectric and ferroelastic domains can be useful for optoelectronic devices and ferroelastic templates for strain coupled artificial magnetoelectric heterostructures.

Experimental evidence for anisotropic double exchange interaction driven anisotropic transport in manganite heterostructures

An anisotropic double exchange interaction driven giant transport anisotropy is demonstrated in a canonic double exchange system of La2/3Sr1/3MnO3 ultrathin films epitaxially grown on NdGaO3 (110) substrates. The oxygen octahedral coupling at the La2/3Sr1/3MnO3/NdGaO3 interface induces a planar anisotropic Mn-O-Mn bond bending, which causes a significant anisotropic overlap of neighboring Mn orbitals. Due to the anisotropic double exchange interaction, it is found that the conductivity of the La2/3Sr1/3MnO3 film is enhanced when current is applied along the in-plane short crystalline axis. However, the anisotropic behavior is absent in the high temperature paramagnetic phase. Our results demonstrate anisotropic transport in the clean limit where phase separation is absent and the role of anisotropic phase percolation can be excluded.

Ultra-low magnetic damping of perovskite La0.7Sr0.3MnO3 thin films

The perovskite La0.7Sr0.3MnO3 (LSMO) films grown on different substrates were investigated by an angle resolved broadband ferromagnetic resonance technique. All films exhibited a four-fold magnetocrystalline anisotropy, which is in accord with the crystal structure. Moreover, a perpendicular uniaxial anisotropy changed from the (001)pc easy plane to the [001]pc easy direction when the strain of LSMO films varies from tensile to compressive. The ultra-low magnetic damping constant of 5.2 × 10−4 was obtained for a 44.6 nm LSMO film on an NdGaO3 (110) substrate. The breathing Fermi surface model in which the damping constant is proportional to the density of states at Fermi energy is the dominant mechanism for the intrinsic magnetic relaxation.

Surface Segregation in Lanthanum Strontium Manganite Thin Films and Its Potential Effect on the Oxygen Reduction Reaction

A limiting factor for solid oxide fuel cell (SOFC) performance is the oxygen reduction reaction (ORR) that occurs at the cathode surface. This study examines changes in the surface composition of (La0.8Sr0.2)0.95MnO3 (LSM-20), a well-known SOFC cathode material. Heteroepitaxial thin films of LSM-20 were grown on lattice matched single crystal LaAlO3 and NdGaO3 substrates. Total Reflection X-ray Fluorescence (TXRF) measurements showed surface manganese enrichment on annealing at 800°C in air. X-ray absorption near edge spectroscopy (XANES) showed that this leads to a redistribution of manganese 3+, 4+, and 2+ valence states at the surface. This was confirmed by defect modeling, which showed that surface Mn segregation leads to an increase in the surface concentration of oxygen vacancies and the Mn+4 species, both of which are expected to be beneficial for ORR.

Growth of a/c grain boundary with well-defined facet in single-crystalline YBa2Cu3O7 − δ film by liquid phase epitaxy

A novel a/c grain boundary with well-defined facet of the YBa2Cu3O7−δ (Y123) film featuring a single-crystalline nature was successfully grown by liquid phase epitaxy, differing from conventional ill-defined facet film possessing a polycrystalline characteristic by the prior art deposition techniques. Here the extremely low supersaturation as essential prerequisite was exploited on (110) NdGaO3 (NGO) substrates with partly-etched c-axis Y123 films. Based on the selective growth, preferred a-axis Y123 hetero-epitaxially grew on the bare NGO directly while c-axis Y123 homo-epitaxially proceeded on pre-existing Y123, consequently fabricating a distinct a/c grain boundary, which has great importance for fundamental studies and device applications.

Origin and Quenching of Novel ultraviolet and blue emission in NdGaO3: Concept of Super-Hydrogenic Dopants.

In this study we report the existence of novel ultraviolet (UV) and blue emission in rare-earth based perovskite NdGaO3 (NGO) and the systematic quench of the NGO photoluminescence (PL) by Ce doping. Study of room temperature PL was performed in both single-crystal and polycrystalline NGO (substrates and pellets) respectively. Several NGO pellets were prepared with varying Ce concentration and their room temperature PL was studied using 325 nm laser. It was found that the PL intensity shows a systematic quench with increasing Ce concentration. XPS measurements indicated that nearly 50% of Ce atoms are in the 4+ state. The PL quench was attributed to the novel concept of super hydrogenic dopant (SHD)”, where each Ce4+ ion contributes an electron which forms a super hydrogenic atom with an enhanced Bohr radius, due to the large dielectric constant of the host. Based on the critical Ce concentration for complete quenching this SHD radius was estimated to be within a range of 0.85 nm and 1.15 nm whereas the predicted theoretical value of SHD radius for NdGaO3 is ~1.01 nm.

Orbital configuration in CaTiO3 films on NdGaO3

Despite its use as a constituent layer for realization of a polar metal and interfacial conductivity, the microscopic study of electronic structure of CaTiO3 is still very limited. Here, we epitaxially stabilized CaTiO3 films on NdGaO3 (110) substrates in a layer-by-layer way by pulsed laser deposition. The structural and electronic properties of the films were characterized by reflection-high-energy-electron-diffraction, X-ray diffraction, and element-specific resonant X-ray absorption spectroscopy. To reveal the orbital polarization and the crystal field splitting of the titanium 3d state, X-ray linear dichroism was carried out on CaTiO3 films, demonstrating the orbital configuration of dxz/dyz < dxy < d3z2−r2 < dx2−y2. To further explore the origin of this configuration, we performed the first-principles density function theory calculations, which linked the orbital occupation to the on-site energy of Ti 3d orbitals. These findings can be important for understanding and designing exotic quantum states in heterostructures based on CaTiO3.

Effect of Sr Content and Strain on Sr Surface Segregation of La1-xSrxCo0.2Fe0.8O3-δ as Cathode Material for Solid Oxide Fuel Cells.

Strontium-doped lanthanum cobalt ferrite (LSCF) is a widely used cathode material due to its high electronic and ionic conductivity, and reasonable oxygen surface exchange coefficient. However, LSCF can have long-term stability issues such as surface segregation of Sr during solid oxide fuel cell (SOFC) operation, which can adversely affect the electrochemical performance. Thus, understanding the nature of the Sr surface segregation phenomenon and how it is affected by the composition of LSCF and strain are critical. In this research, heteroepitaxial thin films of La1-x SrxCo0.2Fe0.8O3-δ with varying Sr content (x = 0.4, 0.3, 0.2) were deposited by pulsed laser deposition (PLD) on single-crystal NdGaO3, SrTiO3, and GdScO3 substrates, leading to different levels of strain in the films. The extent of Sr segregation at the film surface was quantified using synchrotron-based total-reflection X-ray fluorescence (TXRF) and atomic force microscopy (AFM). The electronic structure of the Sr-rich phases formed on the surface was investigated by hard X-ray photoelectron spectroscopy (HAXPES). The extent of Sr segregation was found to be a function of the Sr content in bulk. Lowering the Sr content from 40% to 30% reduced the surface segregation, but further lowering the Sr content to 20% increased the segregation. The strain of LSCF thin films on various substrates was measured using high-resolution X-ray diffraction (HRXRD), and the Sr surface segregation was found to be reduced with compressive strain and enhanced with tensile strain present within the thin films. A model was developed correlating the Sr surface segregation with Sr content and strain effects to explain the experimental results.

Surface Segregation in Strontium Doped Lanthanum Cobalt Ferrite: Effect of Composition and Strain

The oxygen reduction reaction (ORR) at the cathode is often the rate-controlling step in the electrochemical reactions occurring in the solid oxide fuel cells (SOFCs). Strontium doped lanthanum cobalt ferrite (LSCF) is a widely used cathode material due to its high electronic and ionic conductivity, and reasonable oxygen surface exchange coefficient. However, LSCF can have long-term stability issues such as surface segregation of Sr during SOFC operation, which can adversely affect the electrochemical performance. Thus, understanding the nature of the Sr surface segregation phenomenon, and how it is affected by the composition of LSCF and strain are critical. In this research, heteroepitaxial thin films of La1-x Sr x Co0.2Fe0.8O3-δ with various Sr contents (x = 0.4, 0.3, 0.2) were deposited by Pulsed Laser Deposition (PLD) on single crystal NdGaO3, SrTiO3 and GdScO3 substrates, leading to different strains in the films. The LSCF thin films were annealed at 800 °C for 10 hours in air. The extent of Sr segregation at the post-annealed film surface was quantified using synchrotron-based total reflection X-ray fluorescence (TXRF) technique. The formation of secondary phases were found and their surface area coverage was quantified by Atomic Force Microscope (AFM). Transmission electron microscope (TEM) and related spectroscopy techniques were used for microstructural and quantitative elemental analyses of the secondary phases on surface which were found to be Sr-rich phases. The bonding environment of the Sr-rich phases formed on the surface were investigated by synchrotron-based hard X-ray photoelectron spectroscopy (HAXPES). The extent of Sr segregation was found to be a function of the Sr content in bulk. Lowering the Sr content from 40% to 30% reduced the surface segregation, but further lowering the Sr content to 20% increased the segregation. The strains of LSCF thin films on various substrates were measured using high-resolution X-ray diffraction (HRXRD) and the Sr surface segregation was found to be reduced with compressive strain and enhanced with tensile strain present within the thin films. A model was developed correlating the Sr surface segregation with Sr content and strain effects to explain the experimental results.

Antiferroelectric films of deuterated betaine phosphate

Thin films of partially deuterated betaine phosphate have been grown by the evaporation on Al2O3(110) and NdGaO3(001) substrates with a preliminarily deposited structure of interdigitated electrodes. The grown films have a polycrystalline block structure with characteristic dimensions of blocks of the order of 0.1–1.5 mm. The degree of deuteration of the films D varies in the range of 20–50%. It has been found that, at the antiferroelectric phase transition temperature Tcafe = 100–114 K, the fabricated structures exhibit an anomaly of the electrical capacitance C, which is not accompanied by a change in the dielectric loss tangent tanδ. The strong-signal dielectric response is characterized by the appearance of a ferroelectric nonlinearity at temperatures T >Tcafe, which is transformed into an antiferroelectric nonlinearity at T <Tcafe. With a further decrease in the temperature, double dielectric hysteresis loops appear in the antiferroelectric phase. The dielectric properties of the films have been described within the framework of the Landau-type thermodynamic model taking into account the biquadratic coupling ξP2η2 between the polar order parameter P and the nonpolar order parameter η with a positive coefficient ξ. The E–T phase diagram has been constructed.

Anisotropic transport properties in the phase-separated La0.67Ca0.33MnO3/NdGaO3 (001) films**Project supported by the National Basic Research Program of China (Grant Nos. 2011CB921801, 2012CB921403, and 2013CB921701) and the National Natural Science Foundation of China (Grant Nos. 11074285, 51372064, and 11134007).

The anisotropic transport property was investigated in a phase separation La0.67Ca0.33MnO3 (LCMO) film grown on (001)-oriented NdGaO3 (NGO) substrate. It was found that the resistivity along the b-axis is much higher than that along the a-axis. Two resistivity peaks were observed in the temperature dependent measurement along the b-axis, one located at 91 K and the other centered at 165 K. Moreover, we also studied the response of the resistivities along the two axes to various electric currents, magnetic fields, and light illuminations. The resistivities along the two axes are sensitive to the magnetic field. However, the electric current and light illumination can influence the resistivity along the b-axis obviously, but have little effect on the resistivity along the a-axis. Based on these results, we believe that an anisotropic-strain-controlled MnO6 octahedra shear-mode deformation may provide a mechanism of conduction filaments paths along the a-axis, which leads to the anisotropic transport property.

Direct observation of infinite NiO2 planes in LaNiO2 films

Epitaxial thin films of LaNiO2, which is an oxygen-deficient perovskite with “infinite layers” of Ni1+O2, were prepared by a low-temperature reduction of LaNiO3 single-crystal films on NdGaO3 substrates. We report the high-angle annular dark-field and bright-field scanning transmission electron microscopy observations of infinite NiO2 planes of c-axis-oriented LaNiO2 epitaxial thin films with a layer stacking sequence of NiO2/La/NiO2. Resistivity measurements on the films show T2 dependence between 400 and 150 K and a negative Hall coefficient.

Controlling the preferential orientation in sol-gel prepared CaCu3Ti4O12 thin films by LaAlO3 and NdGaO3 substrates

Researchers have paid considerable attention to CaCu3Ti4O12 (CCTO) due to the colossal dielectric constant over a wide range of frequency and temperature. Despite of the growing number of works dealing with CCTO, there have been few studies of the role played by the substrate in inducing structural and dielectric effects of this material. In this work, highly-oriented CCTO thin films have been deposited on LaAlO3(100), NdGaO3(100) and NdGaO3(110) substrates using a sol-gel method. These single crystal substrates were chosen in terms of small lattice mismatch between CCTO and the substrate. The X-ray diffraction patterns showed that the CCTO film layers grow with different orientations depending upon the substrate used. We show that the preferred orientation of CCTO thin films can be manipulated to a high degree by growing it on specific crystal planes of the substrates without the use of buffer layers. Colossal dielectric constants are observed in our films which appear to correlate with the film crystallinity and preferred orientation.

Microstructure and Compositional Defects Affects Proton Conductivity and Reactions in Y-doped BaZrO3 Thin Films

In this paper, we report on BaZr0.8Y0.2O3-x (BZY) thin films grown on highly mismatched NdGaO3 (110) substrates by RHEED assisted pulsed laser deposition. The conduction and electrochemical performances are studied by Electrochemical Impedance Spectroscopy and Electrochemical Strain Microscopy respectively. Conductivity, as well as electrochemical response improves while decreasing the thickness, indicating that the proton movement is prompt at the defective interface region. Structural defects at the interface between film and substrate are clearly displayed by x-ray diffraction, RHEED patterns and transmission electron microscopy. The role of chemical defects on BZY film properties is elucidated by Hard X-ray Photoelectron Spectroscopy. Our results demonstrate that both structural dislocations and chemical defects influence the proton conduction and reaction process in BZY thin films.

Controlled lateral anisotropy in correlated manganite heterostructures by interface-engineered oxygen octahedral coupling.

Controlled in-plane rotation of the magnetic easy axis in manganite heterostructures by tailoring the interface oxygen network could allow the development of correlated oxide-based magnetic tunnelling junctions with non-collinear magnetization, with possible practical applications as miniaturized high-switching-speed magnetic random access memory (MRAM) devices. Here, we demonstrate how to manipulate magnetic and electronic anisotropic properties in manganite heterostructures by engineering the oxygen network on the unit-cell level. The strong oxygen octahedral coupling is found to transfer the octahedral rotation, present in the NdGaO3 (NGO) substrate, to the La2/3Sr1/3MnO3 (LSMO) film in the interface region. This causes an unexpected realignment of the magnetic easy axis along the short axis of the LSMO unit cell as well as the presence of a giant anisotropic transport in these ultrathin LSMO films. As a result we possess control of the lateral magnetic and electronic anisotropies by atomic-scale design of the oxygen octahedral rotation.

YBa2Cu3O7-x bicrystal Josephson junctions with high IcRn-products and wide-ranging resistances for THz applications

Josephson junctions with high characteristic voltages $I_{c}R_{n}$ and resistances $R_{n}$ ranging from subohm values to several hundred ohms are required for various terahertz (THz) applications. Due to optimization of technology, [100]-tilt YBa2Cu3O7-x bicrystal Josephson junctions with an average $I_{c}R_{n}$ (5 K) value of 7 mV at an $R_{n}$ -range from 0.4 to 400 $\Omega$ have been fabricated on (320) bicrystal NdGaO3 substrates. Classical and frequency-selective detection of THz radiation have been demonstrated with the fabricated Josephson junctions. The main characteristics of Josephson oscillators, spectrum analyzers, admittance spectrometers, and classical detectors have been evaluated within this range of junction parameters, and promising results have been obtained.

Low-temperature deposition of meta-stable β-MoO3(011) epitaxial thin films using step-and-terrace substrates

We demonstrated the epitaxial growth of meta-stable β-MoO3(011) thin films on SrTiO3(100), NdGaO3(110), and NdGaO3(001) substrates, using dc reactive magnetron sputtering. Low-temperature deposition at ~200°C resulted in the suppression of the formation of thermodynamically stable α-MoO3, and we observed the growth of a β-MoO3 phase with mixed orientation. In contrast, depositions on substrate surfaces with a step-and-terrace structure resulted in uniaxially oriented β-MoO3(011) epitaxial thin films. The step-and-terrace structures were found to play an important role in controlling the epitaxial growth behavior of β-MoO3 thin films.

Ferroelectric Thin Films of Deuterated Betaine Arsenate

Ferroelectric thin films of partially deuterated betaine arsenate have been grown by evaporation method on NdGaO3 and α-Al2O3 substrates. The films consist of small single-crystal blocks, in which ferroelastic domains have been visualized by polarizing microscope. X-ray diffraction shows that the blocks form a polycrystalline structure of film. Ferroelectric phase transition in DBA films is followed by dielectric anomaly and appearance of dielectric hysteresis loops. The magnitude of dielectric anomaly is small as compared with DBA single crystals. Small value of anomaly is associated with polycrystalline structure of DBA films. The deuteration degree of betaine arsenate films is smaller than that of crystals used for film preparation due to substitution of deuterium by hydrogen ions during the crystallization process.