Thin Films Of La0 Research Articles

Textured nano-agglomerates embedded in the thin film of La0.8Sr0.2MnO3

We have observed nano-scale chemical composition separation in the thin film of La0.8Sr0.2MnO3 by means of high-angle-angular-dark-field imaging, elemental mapping, and composition line-scanning in a transmission electron microscope. It is found that highly dense and dimensionally uniform nano-agglomerates are embedded in the film. The ratio of Mn∕La in the nano-agglomerates is fluctuated and generally much higher than that in their surrounding medium. Such compositional inhomogeneities result in a textured microstructure, which is one of the most important parameters for tunning electronic properties in the colossal magnetoresistance oxides.

Giant planar Hall effect in colossal magnetoresistive La0.84Sr0.16MnO3 thin films

The transverse resistivity in thin films of La0.84Sr0.16MnO3 (LSMO) exhibits sharp field-symmetric jumps below TC. We show that a likely source of this behavior is the giant planar Hall effect combined with biaxial magnetic anisotropy. The effect is comparable in magnitude to that observed recently in the magnetic semiconductor Ga(Mn)As. It can be potentially used in applications such as magnetic sensors and nonvolatile memory devices.

Biaxial strain induced electrical inhomogenities and phase separation in the ferromagnetic metallic phase in thin films of La0.7Ca0.3MnO3: A scanning tunneling potentiometry/spectroscopy study.

ABSTRACTWe have investigated the effect of biaxial strain on local electrical/electronic properties in thin films of La0.7Ca0.3MnO3 with varying degrees of biaxial strain in them. The local electrical properties were investigated as a function of temperature by scanning tunneling spectroscopy (STS) and scanning tunneling potentiometry (STP), along with the bulk probe like conductance fluctuations.The results indicate a positive correlation between the lattice mismatch biaxial strain and the local electrical/electronic inhomogenities observed in the strained sample. This is plausible since the crystal structure of the manganites interfere rather strongly with the magnetic/electronic degrees of freedom. Thus even a small imbalance (biaxial strain) can induce significant changes in the electrical properties of the system.

Laser ablation of La0.9Sr0.1Ga0·8Mg0.2O2.85 — plume and film characterization

Thin films of La0.9Sr0.1Ga0·8Mg0.2O2.85 (LSGMO), an electrolyte material for the solid oxide fuel cells (SOFC), have been prepared by pulsed laser deposition method. Preparation of thin films under high vacuum gives amorphous films, which on annealing at 730 °C in air give stochiometric thin films. Highly oriented, polycrystalline and amorphous films of LSGMO are obtained on SrTiO3 (001), sapphire (0001) and quartz substrates, respectively. The ionic conductivity value for the film deposited on sapphire substrate is an order of magnitude larger than for the film deposited on SrTiO3. The plume composition is analyzed by a mass spectrometer.

Electronic structure of La0.7Sr0.3MnO3 thin films for hybrid organic/inorganic spintronics applications

Recently, hybrid organic/inorganic interfaces have been used in prototype spin valves, with thin films of La0.7Sr0.3MnO3 as the spin-polarized charge carrier injecting electrode. We have used x-ray photoelectron spectroscopy and ultraviolet photoelectron spectroscopy to study the surface properties of La0.7Sr0.3MnO3 thin films prepared by the channel-spark ablation technique. In particular, preparation of the surfaces by annealing in ultra-high-vacuum surface segregation, the valence electronic structure and work function were studied in order to provide important information for charge injection behavior. It is shown that annealing in vacuum at T<500 °C removes surface contamination and stabilizes oxygen content. The work-function values change from 4.2 to 4.8 eV depending on surface treatment. A surface layer consisting of SrO and SrCO3 of a few angstrom thick was found, which can significantly influence the spin injection properties at the interfacial region.

Ru-doped La0.7(Ba–Ca)0.3MnO3 thin films: indirect evidence of phase separation

Magnetic and transport properties of thin films ofLa0.7BaxCa0.3−xMn1−yRuyO3 (wherex = 0.1, 0.2, 0.3and y = 0,0.1) fabricated using the pulsed laser deposition technique have been investigated. WhileLa–Ba–Ca–Mn–O samples with no doping on B-sites show very strong ferromagnetism with a rather highTC, Ru-doped sampleshave a much lower TC.This reduction of ferromagnetism is consistent with the results obtained from transport measurements: underzero field, La0.7Ba0.1Ca0.2Mn0.9Ru0.1O3 thin films shows an insulator-to-metal transition at 75 K, the transition temperature shiftsto lower temperatures under applied magnetic fields, and a positive magnetoresistance(MR) was observed. It is likely that Ru doping creates some antiferromagneticinsulating matrix which separates a basically ferromagnetic background into isolatedferromagnetic domains so that, even though the phase is ferromagnetic, it is notconductive. A ferromagnetic-insulating state appears in the composition ofx = 0.3 hole-doped concentration, which is very unexpected, and the positive MR is also veryanomalous.

Microstructure of new colossal magnetoresistance La1–xTexMnO3 (x= 0.1, 0.2) thin films

We have used transmission electron microscopy and characterized the microstructure in thin films ofLa1–xTexMnO3 (x = 0.1, 0.2) compounds, which are new additions to the family of colossal magnetoresistance (CMR) oxides. Electron diffraction and high-resolution imaging reveal that Te-doped thin filmswith a thickness of 100 nm for x = 0.1 and 200 nm for x = 0.2 are epitaxially grown on the SrTiO3 (001) substrate. On the basis of electron diffraction patterns obtained from cross-section and plan-view specimens, the new compounds are determined to have an orthorhombic unit cell with lattice parameters ao ≈ 21/2ap = 0.54 nm, bo ≈ 2ap = 0.77 nm, co ≈ 21/2ap = 0.54 nm, where ap is the lattice parameter of an ideal cubic perovskite structure, 0.39 nm. The microstructures in orthorhombic La1–xTexMnO3 compounds are clarified in terms of two 90°-oriented domains, both of which are grown with [101] axis perpendicular to the substrate surface. (© 2003 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Microdomains in thin films of rhombohedral La0.7Sr0.3MnO3

Thin films of La0.7Sr0.3MnO3 with thickness of 300 nm have been grown by computer-controlled laser molecular beam epitaxy on SrTiO3 (001) substrate and were characterized by transmission electron microscopy. It is seen that the films are epitaxially grown on SrTiO3 substrate. Microstructures in the films are clarified by oriented domains of the rhombohedral variant derived from the perovskite structure. Relationships between each domain and the SrTiO3 substrate are discussed.

Microdomains in thin films of rhombohedral La0.7Sr0.3MnO3

Thin films of La0.7Sr0.3MnO3 with thickness of 3 00 nm have been grown by computer-controlled laser molecular beam epitaxy on SrTiO3 (001) substrate and were characterized by transmission electron microscopy. It is seen that the films are epitaxially grown on SrTiO3 substrate. Microstructures in the films are clarified by oriented domains of the rhombohedral variant derived from the perovskite structure. Relationships between each domain and the SrTiO3 substrate are discussed.

Stress-induced suppression of piezoelectric properties in PbTiO3:La thin films via scanning force microscopy

Sol-gel derived polycrystalline La-doped PbTiO3 films are investigated by scanning force microscopy (SFM) in a piezoelectric contact mode. The SFM signal proportional to the effective piezoelectric coefficient, deff, is measured inside individual domains as a function of the mechanical force exerted by the SFM tip on the film’s surface. It is found that the piezoelectric signal can be fully suppressed under sufficiently high force (∼20–22 μN). The suppression is qualitatively different for domains of opposite polarities where deff may change its sign depending on the orientation of polarization vector. The piezoelectric hysteresis loops acquired under increasing force gradually shrink with an asymmetric decrease of the piezoelectric coefficients. This leads to a pronounced vertical shift of the loops (deff offset). The obtained results are discussed in terms of the competing electrostatic and piezoelectric contributions to the measured signal and stress-induced polarization instabilities in the vicinity of the SFM tip.

Direct observation of electron doping inLa0.7Ce0.3MnO3using x-ray absorption spectroscopy

We report on a X-ray absorption spectroscopic (XAS) study on a thin film of La0.7Ce0.3MnO3, a manganite which was previously only speculated to be an electron doped system. The measurements clearly show that the cerium is in the Ce(IV) valence state and that the manganese is present in a mixture of Mn2+ and Mn3+ valence states. These data unambiguously demonstrate that La0.7Ce0.3MnO3 is an electron doped colossal magnetoresistive manganite, a finding that may open up new opportunities both for device applications as well as for further basic research towards a better modelling of the colossal magnetoresistance phenomenon in these materials.

Correlation of thickness and magnetization in LCMO film

High quality thin films of La0.67Ca0.33MnO3 (LCMO) of different thickness were grown on LAO substrates by pulsed laser deposition (KrF, λ = 278 nm). The AFM images suggest a twodimensional step-growth. DC magnetization measurements of the films in a field of 500 Oe show that the magnetic ordering temperature is the same for all the films in both FC and ZFC conditions and is the same as that for the bulk. However, a difference is seen between the FC and ZFC magnetization of the films. There seems to be a systematic in this difference with respect to the thickness of the film, with the difference decreasing with thickness. We suggest that the difference in the magnetization under FC and ZFC conditions may be due to strain-induced anisotropy arising from the lattice mismatch between the substrate and the film or due to the shape anisotropy due to epitaxial growth

Intrinsic inhomogeneities in manganite thin films investigated with scanning tunneling spectroscopy.

Thin films of La0.7Sr0.3MnO3 on MgO show a metal insulator transition and colossal magnetoresistance. The shape of this transition can be explained by intrinsic spatial inhomogeneities, which give rise to a domain structure of conducting and insulating domains at the submicrometer scale. These domains then undergo a percolation transition. The tunneling conductance and tunneling gap measured by scanning tunneling spectroscopy were used to distinguish and visualize these domains.

Direct observation of percolation in a manganite thin film.

Upon cooling, the isolated ferromagnetic domains in thin films of La0.33Pr0.34Ca0.33MnO3 start to grow and merge at the metal-insulator transition temperature TP1, leading to a steep drop in resistivity, and continue to grow far below TP1. In contrast, upon warming, the ferromagnetic domain size remains unchanged until near the transition temperature. The jump in the resistivity results from the decrease in the average magnetization. The ferromagnetic domains almost disappear at a temperature TP2 higher than TP1, showing a local magnetic hysteresis in agreement with the resistivity hysteresis. Even well above TP2, some ferromagnetic domains with higher transition temperatures are observed, indicating magnetic inhomogeneity. These results may shed more light on the origin of the magnetoresistance in these materials.

Determination of Oxygen Vacancy Concentration in a Thin Film of La0.6Sr0.4CoO3 − δ by an Electrochemical Method

Equivalent circuit analysis was undertaken on a mixed conductor electrode/solid oxide electrolyte system. In a limited case where surface reaction is the predominant rate-controlling process, the equivalent circuit was simplified to parallel connection of a resistor (surface reaction resistance) and a capacitor (chemical capacitance due to oxygen nonstoichiometry). Equilibrium oxygen vacancy concentration was correlated with the chemical capacitance. The model was applied to a dense film of deposited on a sintered plate of by a laser ablation method. Frequency response of the electrochemical impedance was measured under dc bias at 873-1073 K in gas mixtures. The observed capacitance was extremely large, e.g., around 0.1 to 1 for the 1.5 μm thick film. The oxygen vacancy concentration in the film was calculated from the capacitance and compared with the literature data measured by thermogravimetry. The film was found to show smaller oxygen nonstoichiometry. The enthalpy for oxygen vacancy formation in the film was about 40 kJ mol larger than the bulk. © 2002 The Electrochemical Society. All rights reserved.

Oriented domains in a thin film of La0.8Sr0.2MnO3prepared by laser molecular-beam epitaxy

A thin film of La0.8Sr0.2MnO3. prepared by computer-controlled laser molecular-beam epitaxy, on a single-crystal SrTiO3 substrate, has been characterized by transmission electron microscopy. Electron microdiffraction and high-resolution imaging reveal that the as-received thin film with a thickness of 200 nm is epitaxially grown on the SrTiO3(001) substrate. The microstructures in the whole film are clarified in terms of the oriented microdomains. The crystallographic relationships of these domains are discussed on the basis of an orthorhombic unit cell. Theoretical calculations based on a geometrical model that was recently proposed and applied to a number of epitaxial systems have been carried out to rationalize the present observations.

Oriented domains in a thin film of La 0.8 Sr 0.2 MnO 3 prepared by laser molecular-beam epitaxy

A thin film of La0.8Sr0.2MnO3. prepared by computer-controlled laser molecular-beam epitaxy, on a single-crystal SrTiO3 substrate, has been characterized by transmission electron microscopy. Electron microdiffraction and high-resolution imaging reveal that the as-received thin film with a thickness of 200 nm is epitaxially grown on the SrTiO3(001) substrate. The microstructures in the whole film are clarified in terms of the oriented microdomains. The crystallographic relationships of these domains are discussed on the basis of an orthorhombic unit cell. Theoretical calculations based on a geometrical model that was recently proposed and applied to a number of epitaxial systems have been carried out to rationalize the present observations.

Multilayered Structures of SrNb0.1Ti0.9O3/La0.8Sr0.2MnO3/SrTiO3 Prepared by Laser Molecular Beam Epitaxy

Multilayer oxides of SrNb0.1Ti0.9O3/La0.8Sr0.2MnO3/SrTiO3 have been grown by computer-controlled laser molecular beam epitaxy and characterized by transmission electron microscopy. Electron microdiffractions and high-resolution imaging reveal that the as-prepared thin film of La0.8Sr0.2MnO3 with thickness of 200 nm is epitaxially grown on the SrTiO3(001) substrate and the SrNb0.1Ti0.9O3 with thickness of 250 nm epitaxially on the as-received La0.8Sr0.2MnO3 film. The microstructures in the La0.8Sr0.2MnO3 film are clarified in terms of the oriented microdomains. In contrast, microstructures in SNTO are featured by the formation of superstructures due to charge ordering. Crystallographic relationships of these domains are discussed on the basis of an orthorhombic cell and rationalized by theoretical calculations based on a geometrical model.

Double-strain effect in doped La0.7Ca0.3MnO3 PLD grown thin films and fabrication of high-resistance tunnel junction using a novel nano-scale insulating tunnel barrier

Thin films of La0.65R0.05Ca0.3MnO3 (R = Ho and Y) grown using pulsed laser deposition (PLD) technique on (100) NdGaO3 (NGO) single-crystal substrates have shown a decrease in TP farther from the corresponding values obtained on bulk samples. This decrease could be attributed to an additional substrate strain effect. The uniqueness of the present result is that Ho-doped La0.7Ca0.3MnO3 (LCMO) films have shown drastic reduction in resistance (ρmax) compared to that of Y-doping similar to what has been observed in the bulk samples. We have also fabricated a novel FM/I/FM (FM = ferromagnet (La2/3Ba1/3MnO3 (LBMO) in this case); I = insulator) tunnel junction by replacing the widely used SrTiO3 (STO) insulating layer by an insulating phase of Ho-doped LCMO composition (La0.55Ho0.15Ca0.3MnO3) to study the MR and electronic correlation effects. High resistance values have been realized across junctions made up of the LCMO-based insulating layer. This result could open up new avenues in TMR research.

A scanning tunneling microscopy and potentiometry study of epitaxial thin films of La0.7Ca0.3MnO3

ABSTRACTTo investigate the role of grain boundaries and other growth related microstructure in manganite films, a scanning tunneling microscope is used to simultaneously probe surface topography and local potential distribution under current flow at nanometer level in films of epitaxial thin films of La0.7Ca0.3MnO3deposited on single crystal SrTiO3and NdGaO3substrate by laser ablation. We have studied two types of films strained and strain relaxed. Thin (50nm) films (strained due to lattice mismatch between substrate and the film) show step growth (unit cell steps) and have very smooth surfaces. Relatively thicker films (strain relaxed, thickness 200nm) do not have these step growths and show rather smooth well connected grains. Charge transport in these films is not uniform on the nanometer level and is accompanied by potential jumps at the internal surfaces. In particular scattering from grain boundaries results in large variations in the local potential resulting in fields as high as 104-105V/cm located near the grain boundaries. We discuss the role of local strain and strain inhomogeneties in determining the current transport in these films and their resistance and magnetoresistivity. In this paper we attempt to correlate between bulk electronic properties with microscopic electronic conduction using scanning tunneling microscopy and scanning tunneling potentiometry.