Vanadium dioxide (VO2) with its unique sharp resistivity change at the metal-insulator transition (MIT) has been extensively considered for the near-future terahertz/infrared devices and energy harvesting systems. Controlling the epitaxial quality and microstructures of vanadium dioxide thin films and understanding the metal-insulator transition behaviors are therefore critical to novel device development. The metal-insulator transition behaviors of the epitaxial vanadium dioxide thin films deposited on Al2O3 (0001) substrates were systematically studied by characterizing the temperature dependency of both Raman spectrum and Fourier transform infrared spectroscopy. Our findings on the correlation between the nucleation dynamics of intermediate monoclinic (M2) phase with microstructures will open a new avenue for the design and integration of advanced heterostructures with controllable multifunctionalities for sensing and imaging system applications.

There has been significant progress on the fundamental science and technological applications of complex oxides and multiferroics. Among complex oxide thin films, barium strontium titanate (BST) has become the material of choice for room-temperature-based voltage-tunable dielectric thin films, due to its large dielectric tunability and low microwave loss at room temperature. BST thin film varactor technology based reconfigurable radio frequency (RF)/microwave components have been demonstrated with the potential to lower the size, weight, and power needs of a future generation of communication and radar systems. Low-power multiferroic devices have also been recently demonstrated. Strong magneto-electric coupling has also been demonstrated in different multiferroic heterostructures, which show giant voltage control of the ferromagnetic resonance frequency of more than two octaves. This manuscript reviews recent advances in the processing, and application development for the complex oxides and multiferroics, with the focus on voltage tunable RF/microwave components. The over-arching goal of this review is to provide a synopsis of the current state-of the-art of complex oxide and multiferroic thin film materials and devices, identify technical issues and technical challenges that need to be overcome for successful insertion of the technology for both military and commercial applications, and provide mitigation strategies to address these technical challenges. V C 2013 AIP Publishing LLC.

Fast ion conductors are at the foundation of a number of important technologies, ranging from fuel cells to batteries to gas separators. Recent results suggest that strained interfaces and thin films may offer new mechanisms for achieving enhanced ionic transport. In this work, we investigate strained 40-nm films of perovskite La0.5Sr0.5CoO3−δ, which is an important material for solid oxide fuel cell cathodes and oxygen separation membranes. We demonstrate that a strained thin film of La0.5Sr0.5CoO3−δ on SrTiO3 can have dramatically different anion and cation thermodynamics and kinetics than bulk La0.5Sr0.5CoO3−δ. We use synchrotron X-ray diffraction to show that La0.5Sr0.5CoO3−δ thin films form an ordered phase at 650 K. The ordered phase consists of La and Sr cations in planes parallel to the surface and is associated with coherent expansion in the c-direction of ∼5%. This chemical ordering is not observed in the bulk material and is ascribed to the interplay between the epitaxial strain imposed by the substrate, changes in oxygen vacancy content and cation mobility, and the ordering of oxygen vacancies.

On the IrlOOl} surface an Ir adatom is surprisingly found to jump along the (001) direction instead of along the smoother closely packed <110> atomic channel of the substrate, probably by an atomic exchange mechanism. The sites visited by one diffusing adatom therefore form a c(2x2) net of the substrate lattice. The displacement distributions derived are consistent with those expected from a discrete random walk with the atomic jump length of the nearest-neighbor distance of the c(2x2) net. PACS numbers: 68.35.Fx, 61.50.Cj, 68.35.MdThe transport of atoms on a surface is achieved by random-walk diffusion of single atoms and possibly also of small atomic clusters. One of the subjects of great current interest is how an adsorbed atom, or adatom, moves or jumps on the surface. In general, one can expect an adatom to jump along the surface channel direction of the least corrugation height based on the hardsphere model of the surface. In fact, most experimental data are consistent with this assumption.' As far as we are aware, there are few exceptions to this rule. An exception is that on the fee (110) surface of some metals, an adatom can jump either along the surface channel direction or over the surface channel by an exchange diffusion.^ A recent theoretical calculation by Feibelman^ concludes that it is energetically favorable to have an Al adatom on the AlfooU surface displace along the (001> direction by an exchange diffusion of the adatom with a substrate atom instead of displacing along the smoother (110) surface channel direction by atomic hopping. His argument is that Al atom has the valency of three. If an Al adatom moves in the (001) direction, one of the lattice Al atoms will be displaced upward and move together with the adatom. In such a concerted motion of two Al atoms, during the transition each of them will have three chemical bonds formed, two with substrate Al atoms and one with the other jumping atom. Thus such a mode of diffusion is energetically favored. He further argues that as a result of this form of exchange diffusion, the surface sites visited by the diffusing atom will form a c(2x2) net of the substrate lattice. Although decent field-ion-microscopic (FIM) images of aluminum can be obtained,^ it is very unlikely that detailed atomic steps in the surface diffusion of single atoms can be studied for this metal. This is because the evaporation field of Al is too low to allow a good image resolution of the FIM and, in addition, the surface is too unstable for a lengthy measurement. We present here an FIM study of the atomic jump direction in random-walk diffusion of an Ir adatom on the Ir{00l} surface by a measurement of both the two-dimensional displacement distribution and the structure of the surface net visited by one diffusion adatom.Techniques and procedures for studying the behavior of single atoms and small atomic clusters on metal surfaces using the FIM are now very well developed. ^ They can be found in the literature and will not be described in detail here. In this study, we pay gre...

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.

Thin films consisting of perovskite ferromagnetic nanorod (La,Sr)MnO 3 with highly epitaxial quality and well-defined nanorod separation have been synthesized with pulsed laser processing. Each nanorod has an in-plane dimension of about 20−30 nm and length equal to the thin film thickness that can vary from tens to hundreds of nm. Magnetic property measurements revealed that the magnetizations of the as-grown films are highly dependent on the evolved self-assembled structures with strong anisotropic behavior.

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.