Strain-induced anisotropic transport properties of LaBaCo₂O₅.₅+δ thin films on NdGaO₃ substrates

A time-shared switching scheme designed for multi-probe scanning tunneling microscope

We report the design of a time-shared switching scheme, aiming to realize the manipulation and working modes (imaging mode and transport measurement mode) switching between multiple scanning tunneling microscope (STM) probes one by one with a shared STM control system (STM CS) and an electrical transport characterization system. This scheme comprises three types of switch units, switchable preamplifiers (SWPAs), high voltage amplifiers, and a main control unit. Together with the home-made software kit providing the graphical user interface, this scheme achieves a seamless switching process between different STM probes. Compared with the conventional scheme using multiple independent STM CSs, this scheme possesses more compatibility, flexibility, and expansibility for lower cost. The overall architecture and technique issues are discussed in detail. The performances of the system are demonstrated, including the millimeter scale moving range and atomic scale resolution of a single STM probe, safely approached multiple STM probes beyond the resolution of the optical microscope (1.1 µm), qualified STM imaging, and accurate electrical transport characterization. The combinational technique of imaging and transport characterization is also shown, which is supported by SWPA switches with ultra-high open circuit resistance (909 TΩ). These successful experiments prove the effectiveness and the usefulness of the scheme. In addition, the scheme can be easily upgraded with more different functions and numbers of probe arrays, thus opening a new way to build an extremely integrated and high throughput characterization platform.

Electric field control of resistive switching and magnetization in epitaxial LaBaCo2O5+δ thin films

The low operating temperature and volatile characteristics of the magnetization change are the main obstacles for the practical applications of spintronic and magnetic memories. In this work, both the resistive switching and magnetization switching are realized in Pt/LaBaCo2O5+δ (LBCO)/Nb-doped SrTiO3 (Nb-STO) devices at room temperature through an electric field. Unlike the traditional approach of an external stress inducing a volatile magnetization change, the magnetization in the Pt/LBCO/Nb-STO device is modulated by an electrical field, along with the resistive switching. The resistive and magnetization switching can be attributed to the variation of the depletion layer width at the LBCO/Nb-STO interface via oxygen vacancy migration and the increase/decrease of the Co-O-Co bond length, respectively. The present device with the synchronous manipulation of both resistance and magnetization at room temperature can be applied in nonvolatile resistive memories and novel magnetic multifunctional devices.

Structure and magnetic properties of highly oriented LaBaCo2O5+δ films deposited on Si wafers with Pt/Ti buffer layer

High-quality crystalline LaBaCo₂O_5+δ films are successfully deposited on Si wafers with Pt/Ti buffer layer, and tunable electrical and magnetic properties are achieved.

Upgrade of a commercial four-probe scanning tunneling microscopy system

Upgrade of a commercial ultra-high vacuum four-probe scanning tunneling microscopy system for atomic resolution capability and thermal stability is reported. To improve the mechanical and thermal performance of the system, we introduced extra vibration isolation, magnetic damping, and double thermal shielding, and we redesigned the scanning structure and thermal links. The success of the upgrade is characterized by its atomically resolved imaging, steady cooling down cycles with high efficiency, and standard transport measurement capability. Our design may provide a feasible way for the upgrade of similar commercial systems.

Manipulation of Optical Transmittance by Ordered-Oxygen-Vacancy in Epitaxial LaBaCo2O5.5+δ Thin Films

Giant optical transmittance changes of over 300% in wide wavelength range from 500 nm to 2500 nm were observed in LaBaCo₂O_5.5+δ thin films annealed in air and ethanol ambient, respectively. The reduction process induces high density of ordered oxygen vacancies and the formation of LaBaCo₂O_5.5 (δ = 0) structure evidenced by aberration-corrected transmission electron microscopy. Moreover, the first-principles calculations reveal the origin and mechanism of optical transmittance enhancement in LaBaCo₂O_5.5 (δ = 0), which exhibits quite different energy band structure compared to that of LaBaCo₂O₆ (δ = 0.5). The discrepancy of energy band structure was thought to be the direct reason for the enhancement of optical transmission in reducing ambient. Hence, LaBaCo₂O_5.5+δ thin films show great prospect for applications on optical gas sensors in reducing/oxidizing atmosphere.

Anisotropic Strain Induced Directional Metallicity in Highly Epitaxial LaBaCo2O5.5+δ Thin Films on (110) NdGaO3

Highly directional-dependent metal-insulator transition is observed in epitaxial double perovskite LaBaCo2O5.5+δ films. The film exhibit metallic along [100], but remain semiconducting along [010] under application of a magnetic field parallel to the surface of the film. The physical origin for the properties is identified as in-plane tensile strain arising from oxygen vacancies. First-principle calculations suggested the tensile strain drastically alters the band gap, and the vanishing gap opens up [100] conduction channels for Fermi-surface electrons. Our observation of strain-induced highly directional-dependent metal-insulator transition may open up new dimension for multifunctional devices.

High-performance CuFe2O4 epitaxial thin films with enhanced ferromagnetic resonance properties

CuFe₂O₄ epitaxial films with superior FMR properties compared with bulk material have been successfully fabricated for the first time.

Gas Sensing Properties of Epitaxial LaBaCo2O5.5+δ Thin Films

Chemical reactivity and stability of highly epitaxial mixed-conductive LaBaCo₂O_5.5+δ (LBCO) thin films on (001) LaAlO₃ (LAO) single-crystalline substrates, fabricated by using pulsed laser deposition system, were systematically investigated. Microstructure studies from x-ray diffraction indicate that the films are c-axis oriented with the interface relationship of [100]_LBCO//[100]_LAO and (001)_LBCO//(001)_LAO. LBCO thin films can detect the ethanol vapor concentration as low as 10ppm and the response of LBCO thin film to various ethanol vapor concentrations is very reliable and reproducible with the switch between air and ethanol vapor. Moreover, the fast response of the LBCO thin film, as the p-type gas sensor, is better than some n-type oxide semiconductor thin films and comparable with some nanorods and nanowires. These findings indicate that the LBCO thin films have great potential for the development of gas sensors in reducing/oxidizing environments.