Gregory Collins

Interface engineered BaTiO3/SrTiO3 heterostructures with optimized high-frequency dielectric properties

Interface engineered BaTiO₃/SrTiO₃ heterostructures were epitaxially grown on (001) MgO substrates by pulsed laser deposition. Microstructural characterizations by X-ray diffraction and transmission electron microscopy indicate that the as-grown heterostructures are c-axis oriented with sharp interfaces. The interface relationships between the substrate and multilayered structures were determined to be [001](SrTiO₃)//[001](BaTiO₃)//[001](MgO) and (100)(SrTiO₃)//(100)(BaTiO₃)//(100)(MgO). The high-frequency microwave (∼18 GHz) dielectric measurements reveal that the dielectric constant and dielectric loss of the nanolayered heterostructures are highly dependent upon the stacking period numbers and layer thicknesses. With the increase in the periodic number, or the decrease in each layer thickness, the dielectric constant dramatically increases and the dielectric loss tangent rapidly decreases. The strong interface effect were found when the combination period is larger than 16, or each STO layer is less than 6.0 nm. The optimized dielectric performance was achieved with the best value for the loss tangent (0.02) and the dielectric constant (1320), which suggests that the BTO/STO heterostructures be promising for the development of the room-temperature tunable microwave elements.

Magnetic and Electrical Transport Properties of LaBaCo2O5.5+δ Thin Films on Vicinal (001) SrTiO3 Surfaces

Highly epitaxial LaBaCo(2)O(5.5+δ) thin films were grown on the vicinal (001) SrTiO(3) substrates with miscut angles of 0.5°, 3.0°, and 5.0° to systemically study strain effect on its physical properties. The electronic transport properties and magnetic behaviors of these films are strongly dependent on the miscut angles. With increasing the miscut angle, the transport property of the film changes from semiconducting to semimetallic, which results most probably from the locally strained domains induced by the surface step terraces. In addition, a very large magnetoresistance (34% at 60 K) was achieved for the 0.5°-miscut film, which is ~30% larger than that for the film grown on the regular (001) SrTiO(3) substrates.

Magnetic and transport properties of epitaxial (LaBa)Co2O5.5+δ thin films on (001) SrTiO3

The (LaBa)Co2O5+δ thin films were grown on (001) SrTiO3 single crystal substrates by using pulsed laser deposition. Microstructure studies from x-ray diffraction and electron microscopy show that the films have good epitaxial quality with a-axis orientation and sharp atomic interface. Transport property and isothermal magnetoresistance measurements have been used to understand the physical properties of the films with anomalous magnetic phenomena and the largest reported magnetoresistance value of 19% at 40 K.

Giant Magnetoresistance and Anomalous Magnetic Properties of Highly Epitaxial Ferromagnetic LaBaCo2O5.5+δ Thin Films on (001) MgO

Ferromagnetic thin films of the A-site nano-ordered double perovskite LaBaCo(2)O(5.5+δ) (LBCO) were grown on (001) MgO, and their structural and magnetic properties were characterized. The as-grown films have an excellent epitaxial behavior with atomically sharp interfaces, with the c-axis of the LBCO structure lying in the film plane and the interface relationship given by (100)(LBCO)//(001)(MgO) and [001](LBCO)//[100](MgO) or [010](MgO). The as-grown LBCO films exhibit a giant magnetoresistance (54% at 40 K under 7 T) and an anomalous magnetic hysteresis, depending strongly on the temperature and the applied magnetic field scan width.

PO2 dependant resistance switch effect in highly epitaxial (LaBa)Co2O5+δ thin films

This research was partially supported by Department of Energy under Grant No. DE-FG26-07NT43063, the National Science Foundation under Grant No. NSF-NIRT-0709293, the Texas ARP Program under Grant No. 003656-0103-2007, the State of Texas through the Texas Center for Superconductivity at the University of Houston, and the South Texas Technology Management Program.

Interface Effects on the Electronic Transport Properties in Highly Epitaxial LaBaCo2O5.5+δ Films

Single-crystalline perovskite LaBaCo2O5.5+δ thin films were grown on a (110) NdGaO3 single-crystal substrate in order to systematically investigate the effect of lattice mismatch on the electrical transport properties in comparison to the films on LaAlO3, SrTiO3, and MgO substrates. Microstructure studies reveal that all of the LaBaCo2O5.5+δ films are of excellent quality with atomically sharp interface structures. The electrical and magnetic transport property studies indicate that the resistivity, magnetoresistance, and magnetic moment of the film are very sensitive to the substrate materials because of the lattice mismatch/interface strain. The Curie temperature, however, is almost independent of the strain imposed by the substrate, probably because of the strong coupling between the nanodomain boundary and interface strain.

Thickness effects on the magnetic and electrical transport properties of highly epitaxial LaBaCo2O5.5+δ thin films on MgO substrates

The transport properties of double perovskite LaBaCo2O5.5+δ thin films with different thicknesses were systemically studied. A thin (7 nm in thickness), disordered LaBaCo2O5.5+δ layer was formed at the interface between the film and substrate. The films had a typical semiconductor behavior with antiferromagnetic and ferromagnetic behavior coexisting at low temperature. Although the Curie temperature was independent of the film thickness, the coercive fields and magnetizations increase with increasing the film thickness. An ultra large magnetoresistance effect value of about 44% was obtained at 60 K for the film of 82 nm.

Ultrafast oxygen exchange kinetics on highly epitaxial PrBaCo2O5+δ thin films

Symmetric half cell structures of highly epitaxial PrBaCo2O5+δ (PBCO)/Gd0.8Ce0.2.2O2:Y0.08Zr0.92O2/PrBaCo2O5+δ thin films were designed and fabricated on (001) LaAlO3. Microstructural studies indicate that the multilayer films have excellent single-crystal quality and epitaxial nature. The electrochemical impedance spectroscopy measurements reveal that the PBCO film electrodes have a polarization resistance as low as 0.109 Ω cm2 at 597 °C in air and an ultrafast surface exchange coefficient (0.006 cm/s at 598 °C) with low activation energy (0.77 eV). The low polarization resistance and rapid surface oxygen exchange kinetics suggest that the PBCO can be an excellent cathode material for intermediate-temperature solid oxide fuel cells.

Superfast oxygen exchange kinetics on highly epitaxial LaBaCo2O5+δ thin films for intermediate temperature solid oxide fuel cells

Superfast chemical dynamics on highly epitaxial LaBaCo2O5+δ (LBCO) heterostructures were systematically studied with symmetric half-cell LBCO/Gd0.2Ce0.8O2 (GCO):Y0.08Zr0.92O2 (YSZ)/LBCO heterostructures on (001) LaAlO3. The electrochemical impedance spectroscopy measurements reveal that the LBCO film electrodes have an ultralow polarization resistance as low as 0.11 Ω cm2 at 600 °C in air, a superfast surface exchange coefficient of 0.017 cm/s at 600°, and an extremely low activation energy value of 0.49 eV. These excellent physical chemistry properties and superfast chemical dynamics on the highly epitaxial LBCO thin films are considered to be somewhat related to the structure entropy of the nano ordered oxygen vacancy structure.

Ferroelectric BaTiO3/SrTiO3 multilayered thin films for room-temperature tunable microwave elements

Ferroelectric BaTiO3/SrTiO3 with optimized c-axis-oriented multilayered thin films were epitaxially fabricated on (001) MgO substrates. The microstructural studies indicate that the in-plane interface relationships between the films as well as the substrate are determined to be (001)SrTiO3//(001)BaTiO3//(001)MgO and [100]SrTiO3//[100]BaTiO3//[100]MgO. The microwave (5 to 18 GHz) dielectric measurements reveal that the multilayered thin films have excellent dielectric properties with large dielectric constant, low dielectric loss, and high dielectric tunability, which suggests that the as-grown ferroelectric multilayered thin films can be developed for room-temperature tunable microwave elements and related device applications.