Epitaxial (LaBa)Co2O5+δ thin films were grown on (001) LaAlO3 single-crystal substrates using pulsed laser deposition. Microstructure characterizations from X-ray diffraction and electron microscopy indicate that the films are highly c-axis oriented with cube-on-cube epitaxy. Transport property measurements indicate that the films have typical semiconductor behavior with a novel phase transition and hysteresis phenomena at 540 K. The chemical dynamic studies reveals that the resistance of the film changes drastically with the change of redox environment, i.e., the magnitude of resistance changes, ΔR = 1 × 102 ⇔ 1 × 106 Ω, is found within a short response time (∼700 ms). These phenomena suggest that the as-grown (LaBa)Co2O5+δ film have extraordinary sensitivity to reducing-oxidizing environment and the exceedingly fast surface exchange rate.
Giant resistance switching behavior in mixed conductive ͑LaBa͒Co 2 O 5+␦ epitaxial thin film were discovered in high temperature and reducing environments during the reduction and reoxidation process. A reproducible resistance response of over 99% was achieved in the films during a change of 4% H 2 / 96% N 2 to oxygen at temperature range of 400-780°C. The results indicate that at, low oxygen partial pressure, the extension of oxygen deficiency is an essential factor to the high temperature physical properties of ͑LaBa͒Co 2 O 5+␦ and demonstrates its potential application as a chemical sensor device for reducing environments at high temperature. © 2010 American Institute of Physics. ͓doi:10.1063/1.3484964͔Cobalt based perovskite oxides have received increasing attention in the past decade due to their high mixed ionicelectronic conductivity in chemical sensor and green energy device development.1,2 Recent studies indicate that oxygen deficient doped double perovskite cobaltates ͑LnBa͒Co 2 O 5+␦ ͑Ln= La, Pr, Gd͒ have excellent mixed ionic-electronic conductivity with a fast surface exchange coefficient. [3][4][5] In this family of compounds, the A-site cationic ordered arrangement is favored due to the large difference in Ba 2+ and rare earth ionic radii, except ͑LaBa͒Co 2 O 5+␦ ͑LBCO͒. In LBCO, the doped bivalent Ba 2+ is not only inducing plenty of oxygen deficiency but also structurally providing the capability to achieve the smallest oxygen deficiency in this family of compounds due to the very similar ionic radii between Ba 2+ and La 3+ . Therefore, LBCO provides a unique platform with the geometrical stabilized perovskite phase and a wide range of oxygen deficiency, which enables one to study the electrical conductivity, defect structures and stability at high temperature over a wide range of oxygen partial pressure.Up to now, studies on LBCO are only limited to the bulk polycrystalline samples for low temperature transport and magnetic properties.6-8 The high temperature physical properties of LBCO are rarely studied due to the structure failure of the bulk material in a reducing environment. Recently, we have fabricated epitaxial single crystalline LBCO thin films on ͑001͒ LaAlO 3 ͑LAO͒, enabling one to systematically study the electrical transport properties of LBCO under various environments. A reproducibly dramatic resistance change is observed as the LBCO film is exposed to oxidizing and reducing environment over a wide range of temperature. Especially, it is interesting to note that the re-oxidation of the LBCO film has a very short response time, suggesting that an exceedingly fast oxygen exchange rate occurs at the film surface.A KrF excimer pulsed laser deposition system with a wavelength of 248 nm was employed to deposit the ͑LaBa͒Co 2 O 5+␦ thin films on ͑001͒ LaAlO 3 substrates. An energy density of 2.0 J / cm 2 and a laser repetition rate of 5 Hz were adopted during film deposition. A high density, single phase, stoichiometric ͑LaBa͒Co 2 O 5+␦ target was purchased from Praxair Inc. The deposit...
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.
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.
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