Search

Nanostructures of ferromagnetic oxides having Curie temperatures above room temperature have potential for applications in memory devices and future spin-based electronic applications. In this article, we report on the dc and high frequency magnetic properties of arrays of elliptical CoFe2O4 nanopillars, covering a large area, fabricated by combined electron beam lithography, and a sol-gel based chemical route. The nanopillars were successfully fabricated on insulating oxidized silicon substrates and on epitaxial thin films of ferroelectric BiFeO3. We performed magnetic force microscopy and ferromagnetic resonance spectroscopy on the arrays to probe their magnetic properties. Due to the possible existence of dominant pinning sites, the CoFe2O4 nanopillars are not single-domain even at nanometer size scales. [ABSTRACT FROM AUTHOR]

The dc and high frequency magnetic properties are described for arrays of elliptical Co[Fe.sub.2][O.sub.4] nanopillars, which has covered a large area, prepared by combined electron beam lithography and a sol-gel based chemical route. The studies have shown that the Co[Fe.sub.2][O.sub.4] nanopillars are not single-domain even at nanometer size scales due to the existence of dominant pinning sites.

Functional oxide superlattices offer new and exciting possibilities for the exploration of emergent properties at the nanoscale. While the behavior of La1−xSrxFeO3 films has been extensively investigated at low temperatures, few studies have been carried out at high temperatures, particularly for LaFeO3/SrFeO3 superlattice systems. Here, we investigate the transport behavior and optical properties of (LaFeO3)n/(SrFeO3)1 superlattices at 373 K and above. Using optical spectroscopy, we observe a low energy excitation at ∼1 eV, attributable to charge transfer between the O 2p and Fe 3d states of the δ-doped single SrFeO3 layer. From in-plane conductivity measurements on the superlattices, we determine activation energies that are much lower than those of alloyed samples and vary with the total number of SrFeO3 layers. This suggests that polaronic transport is confined near the SrFeO3 regions, permitting mobilities significantly enhanced over those in alloyed thin films. [ABSTRACT FROM AUTHOR]

Fast, reversible redox reactions in solids at low temperatures without thermomechanical degradation are a promising strategy for enhancing the overall performance and lifetime of many energy materials and devices. However, the robust nature of the cation's oxidation state and the high thermodynamic barrier have hindered the realization of fast catalysis and bulk diffusion at low temperatures. Here, we report a significant lowering of the redox temperature by epitaxial stabilization of strontium cobaltites (SrCoOx) grown directly as one of two distinct crystalline phases, either the perovskite SrCoO3−δ or the brownmillerite SrCoO2.5. Importantly, these two phases can be reversibly switched at a remarkably reduced temperature (200-300 °C) in a considerably short time (< 1 min) without destroying the parent framework. The fast, low-temperature redox activity in SrCoO3−δ is attributed to a small Gibbs free-energy difference between two topotatic phases. Our findings thus provide useful information for developing highly sensitive electrochemical sensors and low-temperature cathode materials. [ABSTRACT FROM AUTHOR]

The restricted volume above and below the sample on a six-circle diffractometer can limit the size and complexity of sample environments used in surface diffraction studies. An alternative configuration of the diffractometer, where the sample normal is aligned parallel to the χ axis, allows for ample space above and below the χ circle for instrumentation. The merits of this approach are outlined and angles are derived for the diffraction condition for constant-incident-angle, constant-sample-azimuthal-angle and specular geometries. Using a version of this code written for SPEC (http://www.certif.com/content/spec/), sample specular and nonspecular crystal truncation rods measured from a 5 nm-thick thin film are presented. [ABSTRACT FROM AUTHOR]

In this work, epitaxial (001) BiFeO3 thin films were deposited on SrTiO3 and TbScO3 single-crystal substrates and analyzed with high-resolution x-ray diffraction--reciprocal space mapping. A basic method was developed to extract structural details of the as-grown BiFeO3 film, including (i) epitaxial strain, (ii) ferroelastic domains, and (iii) lattice rotations. After demonstrating the method, extrinsic distortions at vertical twin walls were determined for specific BiFeO3 hetero-structures. A relatively large distortion (0.20° ± 0.08°) was measured in a multidomain (12) and incoherent film, while a nearly intrinsic distortion (0.04° ± 6 0.03°) was measured in a two-domain coherent film. This work offers insights into the structure of multiferroic BiFeO3 thin films with a general approach that is appropriate for low symmetry epitaxial heterostructures. [ABSTRACT FROM AUTHOR]

Devices that confine and process single electrons represent an important scaling limit of electronics. Such devices have been realized in a variety of materials and exhibit remarkable electronic, optical and spintronic properties. Here, we use an atomic force microscope tip to reversibly 'sketch' single-electron transistors by controlling a metal-insulator transition at the interface of two oxides. In these devices, single electrons tunnel resonantly between source and drain electrodes through a conducting oxide island with a diameter of ?1.5 nm. We demonstrate control over the number of electrons on the island using bottom- and side-gate electrodes, and observe hysteresis in electron occupation that is attributed to ferroelectricity within the oxide heterostructure. These single-electron devices may find use as ultradense non-volatile memories, nanoscale hybrid piezoelectric and charge sensors, as well as building blocks in quantum information processing and simulation platforms. [ABSTRACT FROM AUTHOR]

We report intermodulation distortion (IMD) in high-quality epitaxial YBa2Cu3O7-δ (YBCO) thick films and multilayers prepared by pulsed-laser deposition (PLD) using multiple targets. Two sets of films were prepared: single-layer and multilayer YBCO films in which a 20-nm-thick CeO2 inter layer is inserted between every 100-nm-thick YBCO layer. With increasing YBCO thickness from 200 nm to 1200 nm, single-layer films exhibited improvement of IMD, whereas multilayer films showed degradation of IMD. Similarly, YBCO crystalline quality in single-layer films was improved with increasing the thickness, while that in multi-layers films degraded. These results suggest that there is a strong correlation between crystalline quality and IMD for YBCO films. The comparison of experimental data with the theory of IMD suggests that improving crystalline quality is essential for YBCO films with thickness greater than 1 μm to achieve an intrinsic nonlinear behavior. PLD using multiple targets is a very useful method to grow thick YBCO films with high crystalline quality and low IMD. [ABSTRACT FROM AUTHOR]

We report the structure and chemistry of the self-assembled oxide nanopillars that form in superconducting Co-doped BaFe2As2 thin film grown by pulsed laser deposition. The oxide nanopillars consist of a BaFeO2 phase, form epitaxially on the SrTiO3 template, and grow coherently with the BaFe2As2 film. The nanopillars are square with a uniform size of 4-5 nm, which is close to twice the superconducting coherence length. Despite a volume content of ∼5%, the nanopillars do not degrade the structural quality of the BaFe2As2 matrix. Indeed the nanopillars provide exceptionally strong vortex pinning and high critical current density due to the very close correlation of pillar and vortex core diameters. [ABSTRACT FROM AUTHOR]

Control over electron transport at scales that are comparable to the Fermi wavelength or mean-free path can lead to a variety of electronic devices. Here we report electrical rectification in nanowires formed by nanoscale control of the metal-insulator transition at the interface between LaAlO3 and SrTiO3. Controlled in-plane asymmetry in the confinement potential produces electrical rectification in the nanowire, analogous to what occurs naturally for Schottky diodes or by design in structures with engineered structural inversion asymmetry. Nanostructures produced in this manner may be useful in developing a variety of nanoelectronic, electro-optic, and spintronic devices. [ABSTRACT FROM AUTHOR]

Epitaxial BiFeO3 thin films on miscut (001) SrTiO3 substrates relax via mechanisms leading to an average rotation of the crystallographic axes of the BiFeO3 layer with respect to the substrate. The angle of the rotation reaches a maximum in the plane defined by the surface normal of the film and the direction of the surface miscut. X-ray microdiffraction images show that each BiFeO3 mosaic block is rotated by a slightly different angle and contains multiple polarization domains. These effects lead to a complicated overall symmetry in BiFeO3 thin films. This relaxation mechanism can be extended to other complex oxides. [ABSTRACT FROM AUTHOR]