Behavior Of Epitaxial Thin Films Research Articles

Excess Lithium in Transition Metal Layers of Epitaxially Grown Thin Film Cathodes of Li2MnO3 Leads to Rapid Loss of Covalency during First Battery Cycle

We have investigated the initial-cycle battery behavior of epitaxial thin films of Li2MnO3-cathodes by employing resonant inelastic X-ray scattering (RIXS) at the O K- and Mn L3-edges. Thin films ( ...

Uniaxial strain-induced ferroelectric phase with a giant axial ratio in a (110) BiFeO3thin film

Strain engineering, which employs biaxial misfit strain to deform the crystal structure, is a powerful tool to tune the physical behavior of epitaxial thin films. Here we show that a 10-nm-thick BiFeO${}_{3}$ film is uniaxially strained by (110)-oriented LaAlO${}_{3}$ substrate, which exhibits a monoclinic lattice with a giant $c$/$a$ \ensuremath{\sim} 1.24 and a unique stripe ferroelectric domain configuration, as revealed by high resolution synchrotron x-ray diffraction and piezoelectric force microscopy. A strain-phase diagram for BiFeO${}_{3}$ under uniaxial strain condition is predicted by first-principles calculations, suggesting that monoclinic $Pm$ phase with a large polarization of \ensuremath{\sim}130 \ensuremath{\mu}C/cm${}^{2}$ is the lowest-in-energy phase when strained by (110)-oriented LaAlO${}_{3}$ substrate. Our results provide a potential route to tune physical behavior of epitaxial ferroelectric thin films by uniaxial strain in (110) orientation, instead of widely investigated biaxial strain in (001) orientation.

Effect of magnetic fields on the abnormal electroresistance behavior in epitaxial thin films of La 0.8Ca 0.2MnO 3

We report an unusual electroresistance (ER) behavior induced by a current and its response to magnetic fields in La 0.8Ca 0.2MnO 3 epitaxial thin films. These thin films were fabricated on SrTiO 3 (1 0 0) substrate using pulsed laser deposition (PLD) technique. It is found that the electric resistivity in these films is significantly enhanced by applying a dc current over a threshold value. Simultaneously, an abnormal electroresistance behavior appears in the temperature range from 10 to 300 K. The enhanced resistance turns out to be very sensitive to a weak current. Even a very small dc current can remarkably depress the high resistance, showing an unusual colossal ER effect. The ER reaches ∼1175% at temperatures lower than ∼50 K, and ∼705% at 300 K for a current changing from 0.72 to 10.5 μA. The influence of magnetic fields on the transport was also studied. The I– V curves can be strongly influenced by a low magnetic field even at room temperature. The deduced magnetoresistance (MR) reaches 120% at 300 K upon applying a magnetic field of 0.25 T. An interesting phenomenon is that the observed MR is current dependent.

Magnetoresistance of epitaxial thin films of ferromagnetic metallic oxide SrRuO3 with different domain structures

We have studied the magnetoresistive behavior of epitaxial thin films of the conductive ferromagnetic oxide SrRuO3 with different domain structures grown on both miscut (001) SrTiO3 and exact (001) LaAlO3 substrates. A strong anisotropic magnetoresistance (MR) has been observed in the single domain SrRuO3 thin film on miscut (001) SrTiO3 substrate. In contrast, the SrRuO3 thin film on (001) LaAlO3 substrate shows identical MR behavior in two orthogonal directions on the film due to the presence of 90° domains in the plane. For both the films, large negative magnetoresistance effects (∼10%) were observed when the current and the applied magnetic field are parallel. This is attributed to a reduction in spin fluctuations near Tc and to magnetization rotation leading to a change in the angle between the current and magnetization at low temperatures.

Strain stabilized metal–insulator transition in epitaxial thin films of metallic oxide CaRuO3

We report the observation of both metallic and semiconducting behavior in epitaxial thin films of the metallic oxide CaRuO3 deposited under identical conditions. X-ray diffraction studies showed that while semiconducting films with enlarged unit cells were obtained on single-crystal (100) SrTiO3 substrates, metallic films with lattice parameters close to the bulk material grew on (100) LaAlO3 substrates and poor crystalline quality SrTiO3 substrates. It is believed that a strain induced substitution of the small Ru4+ cations by the larger Ca2+ cations occurs, breaking the conduction pathway within the three-dimensional network of the RuO6 octahedra and leading to a metal–insulator transition. This unique phenomenon, which is not observed in bulk material, can be significant in technologically important epitaxial perovskite oxide heterostructures.

Effect of Lattice Strain on Growth Mechanisms and Electrical Transport Behavior of Epitaxial CaRuO3 Thin Films

ABSTRACTWe have observed both metallic and semiconducting behavior in epitaxial thin films of the metallic oxide CaRuO3 deposited under identical conditions on (100) SrTiO3 substrates of varying crystalline quality. X-ray diffraction studies showed that while semiconducting films with enlarged unit cells were obtained on single crystal SrTiO3 substrates, metallic films with lattice parameters close to the bulk material grew on poor crystalline quality SrTiO3 substrates. The films deposited on (100) LaAlO3 substrates consistently showed metallic behavior. Atomic force microscope images suggest that the semiconducting films had a coherent two dimensional nucleation. In contrast, three dimensional island-like incoherent growth was seen in the metallic films. It is believed that in the coherent films a strain induced substitution of the small Ru4+ cations by the larger Ca2+ cations occurs, breaking the conduction pathway within the three dimensional network of the RUO6 octahedra and leading to a metal-insulator transition. This unique phenomenon - which is not observed in bulk material - can be significant in technologically important epitaxial perovskite oxide heterostructures.

Correlation of structural quality with superconducting behavior in epitaxial thin films of Ba2YCu3O7−δ on LaAlO3(100)

The correlations between structural quality and superconducting behavior in 1000-Å-thick Ba2YCu3O7−δ (BYCO) films grown on LaAlO3(100) from the coevaporation of BaF2, Y, and Cu, followed by an optimized ex situ annealing process are reported. Epitaxial films with smooth, laminar morphology and excellent crystallinity can be grown to have critical current density Jc values nearly identical to single crystals. This finding contrasts with the typical observation that Jc values in thin films of BYCO are very high compared to those of single crystals. This is attributed to a greater density of flux pinning sites due to structural defects within the films. The most crystalline films presented here have penetration length λ∼2000 Å with temperature dependencies described well by the Bardeen–Cooper–Schrieffer (BCS) theory. Material disorder of two types can be controlled by the high-temperature stage Ta of the annealing process. The first type is point defects and dislocations the same size or smaller than the coherence length ξab, which Rutherford backscattering/channeling suggests decrease in number with increasing Ta. The second is crevices, pinholes, and microcracks, which are at least one to two orders of magnitude larger than ξab. At Ta < 850 °C, crevices, which create areas of nonuniform thickness, occur due to incomplete epitaxial growth and correlate with the presence of weak links. Hence film resistivity is high, Tc is low, and λ is large. As Ta is increased, the film morphology becomes smoother and all electrical properties improve, except for Jc in nonzero applied magnetic fields, since the improved epitaxy correlates with reduced flux pinning. By Ta= 900 °C, the BYCO films are similar to single crystals in both cation alignment and Jc behavior. Above this annealing temperature, pinholes and microcracks develop and increase in both size and density with increasing Ta. Although these relatively large defects do not act as weak links, they do affect magnetic screening (and hence λ), to result in an anomalous temperature dependence that masks the intrinsic BCS behavior.