Oriented LaAlO3 Substrates Research Articles

Probing the domain structure of BiFeO3 epitaxial films with three-dimensional reciprocal space mapping

High-resolution 3-Dimensional Reciprocal Space Mapping (3D-RSM) has been performed on mixed-phase BiFeO3 (BFO) epitaxial films on (001)-oriented LaAlO3 substrates. Our results demonstrate that 3D-RSM is an effective way to present a structural overview of the different BFO polymorphs, domain variants, and even the interfacial regions between coexisting triclinic phases. The dislocation-free boundaries between the triclinic phases revealed by these 3D-RSMs are believed to be responsible for the large electromechanical response found in mixed-phase BFO films. This study demonstrates the unique merits of the 3D-RSM technique for the structural characterization of ferroic films with complicated domain structures.

The effect of interface oxygen content on magnetoelectric effect of epitaxial La0.7Sr0.3MnO3/BaTiO3 bilayer

The epitaxial La0.7Sr0.3MnO3/BaTiO3 (LSMO/BTO) bilayer films are grown on (001) oriented LaAlO3 substrate by pulsed laser deposition technique. The oxygen-rich interface is obtained through in-situ annealing process in oxygen, and the oxygen-deficient interface is obtained without the annealing process. The results show that the ferromagnetic properties of the LSMO film and the magnetoelectric effect (ME) of the bilayer films strongly correlate to the oxygen content at the interface of LSMO/BTO. The saturated magnetization and the ME voltage coefficient of the oxygen-rich bilayer film are higher than that of oxygen-deficient one. It suggests a more effective ME coupling at the LSMO/BTO interface, which are generated through not only the interface strain but also the spin polarized carriers.

Exchange bias induced by the fully strained La2/3Ca1/3MnO3 dead layers

A pure compressively strained La2/3Ca1/3MnO3 (LCMO) dead layer grown on (001)-oriented LaAlO3 substrate can show all the rich phenomenon of large bias field shift, coercive field enhancement, and high blocking temperature. The obtained exchange bias field (∼350 Oe) and the enhanced coercivity of about 1160 Oe at 5 K under 500 Oe cooling field are superior to that have been reported in LCMO-based ferromagnetic/antiferromagnetic superlattices or nanoscale systems. Our results clearly demonstrate that the inhomogeneous magnetic dead layer of LCMO can induce a strong exchange bias effect, which may be exploited as a very simple structure for spin-valve device application.

Thickness Dependence of Structural and Superconducting Properties of Single-Crystal-Like GdBCO Films Prepared by Photo-Assisted MOCVD

High-quality GdBa2Cu3O7-δ (GdBCO) films with thicknesses of 0.14-1.62 μm were deposited on (100)-oriented LaAlO3 substrates by photo-assisted metal-organic chemical vapor deposition technique. Evaluation of film microstructure and strain-relaxation phenomenon in the growing films and their effects on the crystallographic and superconducting properties of the GdBCO films have been investigated. A single-crystal-like cross-sectional morphology with dense and crack-free surface and no grain boundary visible was observed. The c-axis lattice parameters of the GdBCO films grown with different thicknesses were used to determine the out-of-plane strain. The result shows that the strain could be gradually relieved as film thickness increases. Highly biaxially textured GdBCO films have been obtained. For a 0.31-μm GdBCO film sample, the full-widths at half-maximum were 0.08° and 0.47° for out-of-plane and in-plane orientations, respectively. Such low values were similar to that of single-crystal GdBCO. The highest critical current density (Jc) value is ~ 2.5MA/cm2 (77 K and 0 T). As films were getting thicker, their Jc values decrease gradually. This is mainly attributed to a lack of pinning centers in the thicker films.

Influence of Oxygen to Argon Ratio on the Structural and Morphological Properties of Nb-Doped SrTiO<sub>3</sub> Epitaxial Films Grown by Reactive Ion Beam Sputter Deposition

Nb-doped SrTiO3 (STNO) films were grown on (001)-oriented LaAlO3 substrates by a reactive ion beam sputter deposition at various mixing ratios (OMRs) with a substrate temperature of 800oC. The STNO films exhibited good crystallinity with an epitaxial orientation as characterized by high-resolution X-ray diffraction, grazing-incidence X-ray diffraction, and in-plane pole figure analysis. A decrease of out-of-plane and in-plane lattice constants was observed with an increase of OMR. The surface morphology of the STNO films showed a very dense fine-grain structure. The root-mean-square roughness was found to be increased as the OMR increased. Moreover, the elemental compositions of the STNO films were examined by X-ray photoelectron spectroscopy.

Structural and magnetic properties of tetragonal and orthorhombic Pr0.5Sr0.5MnO3 thin films

Pr0.5Sr0.5MO3 (PSMO) films with tetragonal and orthorhombic structures were epitaxially grown on a [0 0 1]-oriented LaAlO3 substrate (LAO) and a BaTiO3 substrate (BTO), respectively. It was found that the M(T) curves of the tetragonal PSMO films exhibit one/two distinguished magnetic transition peaks depending on the thickness of the film, which is related to the competition between ferromagnetic (FM) and antiferromagnetic (AFM) interactions in the films. The tetragonal film shows an insulator-to-metal transition (MIT) as well as a metal-to-semiconductor transition with increasing temperature from 10 to 300 K, and the AFM insulating state of the film can be transformed into the metallic state under a magnetic field of 5 T due to the collapse of the AFM insulator state. For the orthorhombic film on BTO substrate, only a FM metallic state is observed without MIT from 10 to 300 K. In addition, a significant colossal magnetoresistance effect is observed with a wide temperature range from TC (182 K) to 10 K for the tetragonal films, while only around TC (240 K) for the orthorhombic films.

Incipient ferroelectricity in 2.3%tensile-strained CaMnO3films

Epitaxial CaMnO3 films grown with 2.3% tensile strain on (001)-oriented LaAlO3 substrates are found to be incipiently ferroelectric below 25 K. Optical second harmonic generation (SHG) was used for the detection of the incipient polarization. The SHG analysis reveals that CaMnO3 crystallites with in-plane orientation of the orthorhombic b axis contribute to an electric polarization oriented along the orthorhombic a (resp.\ c) axis in agreement with the predictions from density functional calculations.

Room temperature Mott metal-insulator transition and its systematic control in Sm1−xCaxNiO3 thin films

We report an epitaxial growth and electronic transport properties of Ca-doped SmNiO3 (Sm1−xCaxNiO3, 0≤x≤0.1) thin films deposited on (001)-oriented LaAlO3 substrates by the pulsed laser deposition method. Due to strong electron correlations of the Sm1−xCaxNiO3 films, the Mott metal-insulator (MI) transition appears around 370 K, which decreases to room temperature only by the 1%–2% Ca doping, and dramatically shifts to lower temperatures by increasing the Ca content. For x≥0.1, the film reveals metallic conductivity down to the lowest temperature measured. In the insulating phase of x<0.04, we observe another resistivity anomaly around 200 K corresponding to an antiferromagnetic ordering of the Ni sublattice of SmNiO3 matrix. A complete electronic phase diagram of the Sm1−xCaxNiO3 thin film is unveiled by this work.

10.1007/s11455-008-1012-z

We have studied the structure and the electro-and magnetotransport properties of 25-nm-thick epitaxial La0.67Ca0.33MnO3 (LCMO) films mechanically strained during nucleation and growth on (001)-oriented LaAlO3 substrates. The unit cell parameters of such LCMO films measured parallel and perpendicular to the substrate plane are significantly different (a ‖ = 3.790 Å, a ⊥ = 3.948 Å). The magnetocrystalline anisotropy and phase separation induced by the unit cell distortion in the film lead to the appearance of clearly pronounced hysteresis loops on the plots of electric resistance versus magnetic field.

Low-temperature magnetoresistance of biaxially strained 24-nm-thick La0.67Ca0.33MnO3 films

The lateral unit cell parameter in nanodimensional La0.67Ca0.33MnO3 (LCMO) films grown on (001)-oriented LaAlO3 substrates is significantly (approximately 4%) smaller than the value measured along the normal to the substrate plane. At T < 140 K, the temperature dependence of the resistivity ρ of LCMO films follows the relation ρ − ρ (T = 4.2 K) ≈ρ2(H)T 4.5, where ρ2 is independent of the temperature but decreases with increasing magnetic field H. It is shown that this decrease is related both to a decay of the spin waves in ferromagnetic domains and to the transformation of antiferromagnetic phase inclusions into ferromagnetic ones.

Electrical transport and photoinduced properties of an Sr0.7Ce0.3MnO3 film

Direct current (dc) electrical transport and photoinduced properties of a high-quality epitaxial film of Sr0.7Ce0.3MnO3 on a (100)-oriented LaAlO3 substrate have been investigated. The sample exhibits semiconductive behavior and the electrical transport result agrees with Mott's VRH model. Laser irradiation results in a decrease of the resistance of the film in the entire temperature range measured, which is attributed to the photogenerated electron–hole pairs and the electron hopping from the valence band to the conduction band.

Structural, dielectric, and insulating properties of barium strontium titanate thin films grown on various oriented LaAlO3 substrates

( Ba 0.6 Sr 0.4 ) TiO 3 (BST) thin films were deposited on (100) and (110) oriented LaAlO3 (LAO) single-crystal substrates by radio-frequency magnetron sputtering system, respectively. X-ray diffraction θ-2θ and Φ-scans results showed that the BST thin films grown on LaAlO3 (100) displayed (h00) preferred orientation, while the films grown on LaAlO3 (110) indicated (110) preferred orientation. Atomic force microscope revealed that BST films grown on LAO (110) had smoother surface and smaller grain size than that BST films grown on LAO (100). Compared with BST films grown on LAO (100), the BST films grown on LAO (110) had higher tunability and better figure of merit. Additionally, the current-voltage (I-V) measurement indicated that the leakage current density of BST films grown on LAO (110) was obviously reduced compared to that of BST films grown on LAO (100).

Electric resistance and magnetoresistance of 30-nm-Thick La0.67Ca0.33MnO3 films elastically compressed in the (110) plane

We have studied the structure, electric resistance, and magnetoresistance of 30-nm-thick (110)La0.67Ca0.33MnO3 (LCMO) films grown by laser deposition on (001)-oriented LaAlO3 substrates. The unit cell parameters a and b (along the [100] and [010]LCMO axes, respectively) of these manganite films are significantly (by ∼1.2%) increased as compared to the corresponding values in the pseudocubic unit cell of bulk stoichiometric LCMO crystals. At T < 150 K, the temperature dependence of the resistivity of LCMO films is well described by the relation ρ = ρ1 + ρ2 (H) T4.5. The value of ρ 2 decreases with increasing magnetic field and is close to the analogous coefficient for manganite films grown on substrates with small lattice misfit.

Magnetotransport characteristics of biaxially strained 25-nm-thick La0.67Ca0.33MnO3 films

We have studied the structure and the electro-and magnetotransport properties of 25-nm-thick epitaxial La0.67Ca0.33MnO3 (LCMO) films mechanically strained during nucleation and growth on (001)-oriented LaAlO3 substrates. The unit cell parameters of such LCMO films measured parallel and perpendicular to the substrate plane are significantly different (a ‖ = 3.790 A, a ⊥ = 3.948 A). The magnetocrystalline anisotropy and phase separation induced by the unit cell distortion in the film lead to the appearance of clearly pronounced hysteresis loops on the plots of electric resistance versus magnetic field.

Realization of biferroic properties in La0.6Sr0.4MnO3∕0.7Pb(Mg1∕3Nb2∕3)O3–0.3(PbTiO3) epitaxial superlattices

A set of symmetric and asymmetric superlattices with ferromagnetic La0.6Sr0.4MnO3 (LSMO) and ferroelectric 0.7Pb(Mg1∕3Nb2∕3)O3–0.3(PbTiO3) as the constituting layers was fabricated on LaNiO3 coated (100) oriented LaAlO3 substrates using pulsed laser ablation. The crystallinity, and magnetic and ferroelectric properties were studied for all the superlattices. All the superlattice structures exhibited a ferromagnetic behavior over a wide range of temperatures between 10 and 300K, whereas only the asymmetric superlattices exhibited a reasonably good ferroelectric behavior. Strong influence of an applied magnetic field was observed on the ferroelectric properties of the asymmetric superlattices. Studies were conducted toward understanding the influence of conducting LSMO layers on the electrical responses of the heterostructures. The absence of ferroelectricity in the symmetric superlattice structures has been attributed to their high leakage characteristics. The effect of an applied magnetic field on the ferroelectric properties of the asymmetric superlattices indicated strong influence of the interfaces on the properties. The dominance of the interface on the dielectric response was confirmed by the observed Maxwell-Wagner-type dielectric relaxation in these heterostructures.

Advances in ${\hbox{YBa}}_{2}{\hbox{Cu}}_{3}{\hbox{O}}_{7-\delta}$ Grain Boundary Biepitaxial Josephson Junctions: Transport Properties and Mesoscopic Effects

We report on the latest developments in the study of transport properties of YBa2Cu3O7-delta (YBCO)-based grain boundary (GB) biepitaxial Josephson junctions. We have successfully extended the fabrication process, originally developed for (110) oriented SrTiO3 substrates, to (110) oriented LaAlO3 substrates in view of microwave applications. We will report, also, on the observation of conductance fluctuations, we believe related to the coherent diffusion of quasi-particles across the GB. These results, together with the recent observation of macroscopic quantum tunneling and energy level quantization in these junctions, could help cast some light on the mechanism involved in transport across grain boundaries and in revising the role and the effect of quasi particles in d-wave transport.

Electrical behavior of Y-doped Ba0.6Sr0.4TiO3 thin films

Ba0.6Sr0.4TiO3 (BST) and 1.5 at% Y-doped Ba0.6Sr0.4TiO3 (Y-BST) thin films have been deposited on single-crystal (100) oriented LaAlO3 substrates using pulsed-laser deposition technique (PLD), respectively. X-ray diffraction (XRD) scanning revealed that the two kinds of films could be epitaxially grown in pure single-oriented perovskite phases, but Y-BST thin films showed an enhanced crystallization effect. The dielectric properties of the pure and Y-BST thin films were measured at 10 kHz and 300 K with a parallel-plate capacitor configuration. The results revealed that the addition of Y as an acceptor doping is very effective to increase dielectric tunability, and to reduce leakage current of BST thin films. The figure-of-merit (FOM) factor value increases from 17.32 for BST to 25.84 for Y-BST under an applied electric field of 300 kV/cm. The leakage current density of the BST thin films at a negative bias field of 300 kV/cm decreases from 2.45 × 10−4 A/cm2 to 1.55 × 10−6 A/cm2 by Y doping. The obtained results indicated that the Y-doped BST thin film is a promising candidate material for tunable microwave devices.

Raman spectroscopy evidence of strong spin-phonon coupling in epitaxial thin films of the double perovskite La2NiMnO6

The Raman spectra of epitaxial thin films of La2NiMnO6 on (001)-oriented LaAlO3 substrates have been measured in several exact scattering configurations between 12 and 773K. The evolution of the spectra with increasing temperature is consistent with a transition from the monoclinic P121∕n1 to rhombohedral R3¯ structure. The spin-phonon coupling results in significant softening of the phonon mode involving stretching vibrations of the (Ni∕Mn)O6 octahedra. The anomalous softening extends above TC=275K, indicating the presence of short range ordering up to 400K.

Precursor Evolution and Nucleation Mechanism of YBa2Cu3Ox Films by TFA Metal−Organic Decomposition

We describe the conversion of yttrium, barium, and copper trifluoracetate-derived solid precursors to epitaxial YBa2Cu3Ox superconducting ceramics on (001)-oriented LaAlO3 substrates. Transmission electron microscopy, electron energy loss spectroscopy, energy-dispersive X-ray analysis, and X-ray diffraction are used to characterize the reaction path and nucleation mechanism yielding high critical current YBa2Cu3O7. Our results show that the pyrolysis of the trifluoracetate solutions yields a nanostructured, partially amorphous Ba1-xYxF2+x matrix having a Ba/Y ratio close to 2, with homogeneously dispersed CuO nanoparticles. Upon heating, the chemical trajectory of the fluoride matrix and the overall microstructural evolution of the ceramic precursor prior to YBa2Cu3O7 nucleation is driven by the decomposition and oxidation of this solid solution. The Y solid solubility decreases with temperature yielding Y2O3 which reacts with the CuO particles forming Y2Cu2O5 at about 700 °C. In addition, electron energy loss spectroscopy reveals a high oxygen concentration and almost no Y in the matrix quenched from 795 °C, at a stage where the YBa2Cu3Ox phase still forms disconnected 50−100 nm thick islands spaced by 1−2 μm. The observed evolution from Ba1-xYxF2+x to a barium oxyfluoride mostly occurs prior to the heteroepitaxial nucleation of YBa2Cu3Ox at about 700 °C. Hence, a microstructural scenario is defined which favors competitive nucleation growth between heteroepitaxial YBa2Cu3Ox and bulk Y2Cu2O5. X-ray diffraction pole-figure analysis reveals that the oxyfluoride phase is heavily textured, exhibiting two epitaxial relationships with the (001)-LaAlO3 substrate: (001)OF//(001)LaAlO3, [110]OF//[100]LaAlO3 and (111)OF//(001)LaAlO3, [110]OF//[100]LaAlO3 (OF stands for oxyfluoride). High-resolution observations of the growth front support that (111)-oriented oxyfluoride regions provide low-barrier nucleation sites for c-axis-oriented YBa2Cu3Ox on the buried (001)-LaAlO3 substrate. However, owing to its high mismatch, this orientation only represents roughly 15% percent of the total OF volume. Considering that the nucleation of YBa2Cu3Ox is confined to those regions, this would lead to an anomalously large internuclei spacing, as is indeed observed, favoring the formation of large YBa2Cu3Ox grains and films with a low mosaic spread.

Capacitance of (001)SrRuO3/(001)SrTiO3 and (110)SrRuO3/(110)SrTiO3 interfaces in epitaxial heterostructures grown on LaAlO3 substrates

The temperature dependence of the permittivity ɛ measured at T = 120−300 K for SrTiO3 layers in SrRuO3/SrTiO3/SrRuO3 heterostructures grown on (001)-and (110)-oriented LaAlO3 substrates is well approximated by the relation ɛ0/ɛ = C 0 −1 (T − T CW) + ɛ0/ɛI, where the C 0 and T CW values virtually coincide with the Curie constant and the Curie-Weiss temperature, respectively, for SrTiO3 single crystals. The specific capacitance of the (001)SrRuO3/(001)SrTiO3 and (110)SrRuO3/(110)SrTiO3 interfaces calculated using the obtained ɛI values is 2.7 and 6.4 μF/cm2, respectively.