Giant magnetoresistance and anomalous magnetic properties of highly epitaxial ferromagnetic LaBaCo2O(5.5+δ) thin films on (001) MgO

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.

Formation of Ruddlesden-Popper Faults and Their Effect on the Magnetic Properties in Pr0.5Sr0.5CoO3 Thin Films

Epitaxial Pr_0.5Sr_0.5CoO₃ thin films have been grown on single-crystalline (La_0.289Sr_0.712)(Al_0.633Ta_0.356)O₃(001) substrates by the pulsed laser deposition technique. The magnetic properties and microstructure of these films are investigated. It is found that Ruddlesden-Popper faults (RP faults) can be introduced in the films by changing the laser repetition rate. The segregation of Pr at the RP faults is characterized by atomic-resolution chemical mapping. The formation of the RP faults not only contributes to the epitaxial strain relaxation but also significantly decreases the ferromagnetic long-range order of the films, resulting in lower magnetizations than those of the fault-free films. Our results provide a strategy for tuning the magnetic properties of cobalt-based perovskite films by modifying the microstructure through the film growth process.

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.

Catalytic Dynamics and Oxygen Diffusion in Doped PrBaCo2O(5.5+δ) Thin Films

The Sr and Fe codoped double perovskites PrBaCo2O5.5+δ (PrBCO) thin films of Pr(Ba0.5Sr0.5)(Co1.5Fe0.5)O5.5+δ (PBSCFO) were epitaxially grown for chemical catalytic studies. The resistance behavior of PBSCFO epitaxial films was monitored under the switching flow of reducing and oxidizing gases as a function of the gas flow time, t, using an electrical conductivity relaxation (ECR) experimental setup. The R(t) vs t relationships determined at various temperatures show the occurrence of two oxidation processes, Co(2+)/Co(3+) ↔ Co(3+) and Co(3+) ↔ Co(3+)/Co(4+). Mathematical fitting of the observed R(t) vs t relationships was carried out using Fick's second law for one-dimensional diffusion of charge carriers to derive the diffusivity D(T) and τ(T) for the two processes at various temperatures, T. The D(T) vs T relationships were analyzed in terms of the Arrhenius relationship to find the activation energies Ea for each process. Oscillations in the dR(t)/dt plots, observed under oxidation reactions, were discussed in terms of a layer-by-layer oxygen vacancy exchange diffusion mechanism. Our work suggests that thin films of LnBCO (Ln = lanthanide) with their A and B sites doped as in PBSCFO are excellent candidates for the development of low or intermediate temperature energy conversion devices and gas sensor applications.

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.

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

Thin films of double-perovskite structural LaBaCo2O5.5+δ were epitaxially grown on (110) NdGaO3 substrates by pulsed laser deposition. Microstructural studies by high-resolution X-ray diffraction and transmission electron microscopy revealed that the films have an excellent quality epitaxial structure. In addition, strong in-plane anisotropic strains were measured. Electrical transport properties of the films were characterized by an ultra-high-vacuum four-probe scanning tunneling microscopy system at different temperatures. It was found that the anisotropic in-plane strain can significantly tune the values of film resistance up to 590%.

Step terrace tuned anisotropic transport properties of highly epitaxial LaBaCo2O5.5+δ thin films on vicinal SrTiO3 substrates

Highly epitaxial LaBaCo2O5.5+δ (LBCO) thin films were grown on different miscut (001) SrTiO3 substrates (miscut angle of 0.5°, 3.0°, and 5.0°) to study the substrate surface step terrace effect on the in-plane electrical transport properties. The microstructure studies by X-ray diffraction and transmission electron microscopy indicate that the as-grown films are A-site disordered cubic perovskite structures with the c-axis highly oriented along the film growth direction. The four-probe scanning tunneling microscopy (STM) studies show that the LBCO thin films grown on the vicinal SrTiO3 substrates have a typical semiconductor behavior with the substrate surface terrace step inducing anisotropic electronic transport properties. These results indicate that in highly epitaxial thin films the surface terrace step induced local strains can play an important role in controlling the electronic transport properties and the anisotropic nature.

Ultrafast atomic layer-by-layer oxygen vacancy-exchange diffusion in double-perovskite LnBaCo2O5.5+δ thin films

Surface exchange and oxygen vacancy diffusion dynamics were studied in double-perovskites LnBaCo₂O_5.5+δ (LnBCO) single-crystalline thin films (Ln = Er, Pr; −0.5 < δ < 0.5) by carefully monitoring the resistance changes under a switching flow of oxidizing gas (O₂) and reducing gas (H₂) in the temperature range of 250 ~ 800°C. A giant resistance change ΔR by three to four orders of magnitude in less than 0.1 s was found with a fast oscillation behavior in the resistance change rates in the ΔR vs. t plots, suggesting that the oxygen vacancy exchange diffusion with oxygen/hydrogen atoms in the LnBCO thin films is taking the layer by layer oxygen-vacancy-exchange mechanism. The first principles density functional theory calculations indicate that hydrogen atoms are present in LnBCO as bound to oxygen forming O-H bonds. This unprecedented oscillation phenomenon provides the first direct experimental evidence of the layer by layer oxygen vacancy exchange diffusion mechanism.

Interface effects on the electronic transport properties in highly epitaxial LaBaCo2O(5.5+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.

Nanodomain induced anomalous magnetic and electronic transport properties of LaBaCo2O5.5+δ highly epitaxial thin films

A giant magnetoresistance effect (∼46% at 20 K under 7 T) and anomalous magnetic properties were found in a highly epitaxial double perovskite LaBaCo₂O_5.5+δ (LBCO) thin film on (001) MgO. Aberration-corrected Electron Microscopy and related analytical techniques were employed to understand the nature of these unusual physical properties. The as-grown film is epitaxial with the c-axis of the LBCO structure lying in the film plane and with an interface relationship given by (100)_LBCO || (001)_MgO and [001]_LBCO || [100]_MgO or [010]_MgO. Orderly oxygen vacancies were observed by line profile electron energy loss spectroscopy and by atomic resolution imaging. Especially, oxygen vacancy and nanodomain structures were found to have a crucial effect on the electronic transport and magnetic properties.

Magnetic and electrical transport properties of LaBaCo2O(5.5+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.