Large-distance rf- and dc-sputtering of epitaxial La1 − xSrxMnO3 thin films

Studies on large-distance sputtering as an effective alternative to molecular beam epitaxy, pulsed laser deposition or off-axis sputtering for the deposition of epitaxial La_1 − xSr_xMnO₃ (LSMO) thin films, are reported. The focus of this study is on the quality of the samples and their structural and magnetic properties. The dependence of the characteristics of the LSMO films on the sputtering mode (rf, dc) and the sputtering parameters, in particular on the oxygen partial pressure is established and discussed. It is shown that large-distance sputtering can provide high quality LSMO thin films without the need for post-annealing.     

Photoconductivity in BiFeO3/La0.7Sr0.3MnO3 heterostructure

In this letter, an oxide heterostructure has been fabricated by successively growing La_0.7Sr_0.3MnO₃ (LSMO) and BiFeO₃ (BFO) layers on LaAlO₃ (100) by pulsed laser deposition. Analysis of the leakage current at different temperature demonstrated that the Poole-Frenkel dominated the leakage current mechanism. Additionally, the BiFeO₃/La_0.7Sr_0.3MnO₃ heterostructure exhibits a positive colossal magnetoresistance (MR) effect over a temperature range of 50-320 K. The maximum MR values are determined to be about 45.32% at H = 0.5 T and 28.34% at H = 0.3 T. At last, we report photoconductivity in BiFeO₃/La_0.7Sr_0.3MnO₃ film under illumination from 160 mW/cm² and 200 mW/cm² green-light source, and photoconductivities increase with the intensity of light enhanced.     

Preparation and colossal magnetoresistance in a trilayer La0.67Sr0.33MnO3/La0.75MnO3/La0.67Sr0.33MnO3 device by dc magnetron sputtering

Epitaxial trilayer films of La_0.67Sr_0.33MnO₃ (LSMO)/La_0.75MnO₃ (L0.75MO)/La_0.67Sr_0.33MnO₃ (LSMO) have been prepared on (0 0 1) oriented LaAlO₃ substrates by dc magnetron sputtering. The structure and MR are studied. All as-deposited trilayer films exhibit a semiconductor to metal transition at temperature ranging from 116 to 185 K. The MR is also shown to be dependent on the thickness of the middle oxide layer. A maximum MR value of 32% (ΔR/R₀) has been obtained at 132 K under 0.4 T magnetic field for a LSMO (300 nm)/L0.75MO (70 nm)/LSMO (300 nm) trilayer film. The MR of trilayer film prefers to that of both LSMO and L0.75MO single layer films.     

Precursor chemistry for the solution deposition of epitaxial La0.66Sr0.33MnO3 (LSMO) thin films

A new chemical solution deposition method for the epitaxial growth of La_0.66Sr_0.33MnO₃ (LSMO) thin films from metal acetates, acetylacetonates and propionic acid is presented. Using this method, epitaxial LSMO thin films were grown on (001) SrTiO3 (STO) single crystalline substrates in the temperature range from 800 °C to 1100 °C, both in air and in oxygen atmosphere. The LSMO thin films exhibit good structural and electrical properties. The FWHM of the ω-scan for the (002) peak has a mean value of 0.06°. The Curie temperature of the LSMO thin films is about 320 K and 350 K for the annealed in oxygen and air, respectively.     

Magneto-electrical transport in V-patterned La0.7Sr0.3MnO3 nanostructures

We investigate the electrical transport and magnetic field dependence of nano-patterned La_0.7Sr_0.3MnO₃ devices. We find that the resistivity versus temperature dependence is the same as that observed in thin films, indicating that our nano-patterning preserves the fundamental properties of the material. At temperatures below 20 K there is resistivity upturn of ~ 5 % in the smallest and thinnest device. Structures in a “V” pattern were fabricated in order to investigate domain wall resistance. We find a much smaller resistance area product as compared to previous reports observed in nanoconstrictions and also that the switching field matches that in micromagnetic simulations.     

Effect of bottom electrodes on dielectric properties of high frequency Ba0.5Sr0.5TiO3 parallel plate varactor

Three types of Ba_0.5Sr_0.5TiO₃ (BST) thin film parallel plate varactor with different bottom electrodes were fabricated. The bottom electrodes of three types of varactor were perovskite conducting oxide La_0.7Sr_0.3MnO₃ (LSMO), Pt and Au, respectively. Dielectric properties of the BST thin films were characterized in the frequency range from 10 MHz to 15 GHz. The microstructure of the BST thin films was investigated by X-ray diffraction and scanning electron microscope. The microstructural analysis shows that the BST thin films grown on LSMO and Pt bottom electrodes are polycrystalline textured with columnar grains. Dielectric measurement indicates that the BST thin film grown on LSMO bottom electrode has a maximum dielectric constant and a little higher loss tangent.     

Microstructure and NO decomposition behavior of sol-gel derived (La0.8Sr0.2)0.95MnO3/yttria-stabilized zirconia nanocomposite thin film

(La_0.8Sr_0.2)_0.95MnO₃ and (La_0.8Sr_0.2)_0.95MnO₃/YSZ gel films were deposited by a spin-coating technique on scandium-doped zirconia (ScSZ) substrate using the precursor solution prepared from La(Oi-C₃H₇)₃, Sr(Oi-C₃H₇)₂, Mn(Oi-C₃H₇)₂ and 2-methoxyethanol. By heat-treating the gel films, the membrane reactors, (La_0.8Sr_0.2)_0.95MnO₃|ScSZ|Pt and (La_0.8Sr_0.2)_0.95MnO₃/YSZ|ScSZ|Pt were fabricated. It was found that the pre-firing temperature affected the microstructure evolution of (La_0.8Sr_0.2)_0.95MnO₃ and (La_0.8Sr_0.2)_0.95MnO₃/YSZ thin films. Pre-firing at low temperature resulted in high porosity and large grain size of the thin films. NO decomposition characteristics of the obtained membrane reactors were investigated at 600 °C in reactant gas, 1000 ppm of NO and 2% of oxygen. By applying a direct current to the membrane reactors, NO can be decomposed at the (La_0.8Sr_0.2)_0.95MnO₃ and (La_0.8Sr_0.2)_0.95MnO₃/YSZ composite cathode. By incorporating YSZ into (La_0.8Sr_0.2)_0.95MnO₃, the required consuming power to decompose NO could be reduced.     

Microstructure and electrical properties of Ba0.7Sr0.3(Ti1 − xZrx)O3 thin films prepared on copper foils with sol-gel method

Ba_0.7Sr_0.3(Ti_1 − xZr_x)O₃ (x = 0, 0.1, 0.2) (BSZT) thin films have been prepared on copper foils using sol-gel method. The films were annealed in an atmosphere with low oxygen pressure so that the substrate oxidation was avoided and the formation of the perovskite phase was allowed. The X-ray diffraction results show a stable polycrystalline perovskite phase, with the diffraction peaks of the BSZT films shifting toward the smaller 2θ with increasing Zr content. Scanning electron microscopy images show that the grain size of the BSZT thin films decreases with increasing Zr content. High resolution transmission electron microscopy shows the clear lattice and domain structure in the film. The dielectric peaks of the BSZT thin films broaden with increasing Zr content. Leakage current density of Ba_0.7Sr_0.3(Ti_1 − xZr_x)O₃ (x = 0.1) thin film is the lowest over the whole applied voltage.     

Preparation and characterization of La0.9Sr0.1Ga0.8Mg0.2O3−δ thin film electrolyte deposited by RF magnetron sputtering on the porous anode support for IT-SOFC

Thin films of solid electrolyte La_0.9Sr_0.1Ga_0.8Mg_0.2O_3−δ (LSGM) were deposited by RF magnetron sputtering onto porous La_0.7Sr_0.3Cr_0.5Mn_0.5O_3−δ (LSCM) anode substrates. The effects of substrate temperature, sputtering power density and sputtering Ar gas pressure on the LSGM thin film density, flatness and morphology were systematically investigated. RF sputtering power density of 7.8 W cm⁻², substrate temperature of 300 °C and sputtering Ar gas pressure of 5 Pa are identified as the best technical parameters. In addition, a three-electrode half cell configuration was selected to investigate the electrochemical performance of the thin film. The LSGM film deposited at optimum conditions exhibited a lower area specific ohmic resistance of 0.68 Ω cm⁻² at 800 °C, showing that the practicability of RF magnetron sputtering method to fabricate LSGM electrolyte thin film on porous LSCM anode substrates.     

Simple chemical solution deposition and thermoelectric properties of epitaxial La0.95Sr0.05CoO3 thin films

In this paper, simple chemical solution deposition method is used to prepare La_0.95Sr_0.05CoO₃ thin films on SrTiO₃ (001) substrates by acetate-based precursors. The derived film is characterized by x-ray diffraction, field-emission scanning electron microscopy and transmission electronic microscopy. The derived film is epitaxial growth with < 001>[100] La_0.95Sr_0.05CoO₃||<001>[100] SrTiO₃, indicating that the chemical solution deposition is an effective route to obtain the cobalt-based films. The resistivity, Seebeck coefficient and thermal power factor are 0.05Ω cm, 250 μV/K and 0.21 mWK^− 2m^− 1 at 300 K, respectively, which is higher than these of the ceramics, indicating epitaxial thin film is an effective route to enhance the thermoelectric properties of La_0.95Sr_0.05CoO₃.     

Synthesis,characterization and formaldehyde gas sensitivity of La0.7Sr0.3FeO3 nanoparticles assembled nanowires

La_0.7Sr_0.3FeO₃ nanoparticles assembled nanowires were synthesized by a hydrothermal method assisted with cetyltrimethylammonium bromide (CTAB). The hydrothermal temperature was 180 °C and the annealed temperature was 700 °C. Scanning electron microscopy (SEM), transmission electron microscopy (TEM), X-ray diffraction (XRD) and X-ray photoelectron spectroscopy (XPS) were used to characterize the morphology, composition and structural properties of the materials. The results showed that the La_0.7Sr_0.3FeO₃ nanoparticles assembled nanowires had a high aspect ratio (the largest aspect ratio >100); the size of the nanoparticles was about 20 nm and the diameter of the nanowires was about 100–150 nm. The growth mechanism of La_0.7Sr_0.3FeO₃ nanowires was discussed. Gas sensors were fabricated by using La_0.7Sr_0.3FeO₃ nanowires. Formaldehyde gas sensing properties were carried out in the concentration range of 0.1–100 ppm at the optimum operating temperature of 280 °C. The response and recovery times to 20 ppm formaldehyde of the sensor were 110 s and 50 s, respectively. The gas sensing mechanism of La_0.7Sr_0.3FeO₃ nanowires was investigated.     

Microwave assisted low temperature rapid synthesis of manganite system using La0.67Ce0.03Sr0.3MnO3 mini-cavity furnace

Amorphous La_0.7Sr_0.3MnO₃ (LSMO) and La_0.7Ca_0.3MnO₃ (LCMO) precursor powders synthesized by the citrate gel method at 673 K, have been found to crystallize by microwave irradiation in just 60 s using La_0.67Ce_0.03Sr_0.3MnO₃ (Ce-LSMO) as couplant. This is the lowest temperature treatment and synthesis time so far reported in literature for the formation of manganite systems. Using ceramic route, the same amorphous samples crystallize on heat treatment only at temperatures greater than 1000 K. The microwave heating through this method is novel and has tremendous potential for accelerating the evolution of the product phase in very shorter durations, with just low temperature processing of the precursors, which cannot be realized in normal process.     

The demonstration of epitaxial and electronic-magnetic properties for La0.67Sr0.33MnO3−δ films with (1 1 1) orientation

La_0.67Sr_0.33MnO_3−δ films, fabricated on (1 1 1) LaAlO₃ single-crystal substrates using a direct current magnetron sputtering technique, are demonstrated by X-ray diffraction patterns and pole figures to be high quality epitaxial films and there is a perfect matching relationship between the films and the substrates. We observed an obvious difference of the electronic-magnetic transportation properties among films sputtered on (1 1 1), (1 0 0) and (1 1 0) LaAlO₃ substrates, respectively. A mechanism for the difference is discussed briefly.     

Ferroic metal-oxide films grown by polymer assisted deposition

Polymer assisted deposition is a versatile technique to grow simple and complex metal-oxide thin films. In this paper we report the structural and electrical properties of ferroic materials, namely La_0.67M_0.33MnO₃ (M = Sr and Ca) and Ba_1 − xSr_xTiO₃ (x = 0.3, 0.5, and 0.7) prepared using this process. The films were prepared on single crystalline LaAlO₃ substrates. The films were highly c-axis oriented and epitaxial in nature. The ferromagnetic La_0.67Sr_0.33MnO₃ and La_0.67Ca_0.33MnO₃ films show intrinsic transport properties with maximum magnetoresistance values (at applied field of 5 T) of − 50% and − 88%, respectively. The highest dielectric constant (∼ 1010) and tunability (∼ 69%) of Ba_1 − xSr_xTiO₃ film occurs at x = 0.3 for films, which is at the phase boundary of tetragonal and cubic.     

New cathode compositions based on La0.84Sr0.16Mn1−xMxO3, where M = Al, Ga for solid oxide fuel cell

In order to identify new cathode compositions for the high temperature solid oxide fuel cell, we have investigated the effect of the trivalent cations Al and Ga at the Mn site of the well-studied cathode composition La_0.84Sr_0.16MnO₃. All the compositions have been prepared by the low temperature citrate-nitrate auto-ignition process and sintered within the temperature range of 1150-1350 °C for 4 h. In order to understand the compatibility of the prepared samples as alternative cathode materials, we compared their electrical conductivity and thermal expansion coefficient with those of La_0.84Sr_0.16MnO₃ and yttria-stabilized zirconia. A 10 mol% Al doped La_0.84Sr_0.16MnO₃ composition exhibited a conductivity of around 122 S cm⁻¹ at 950 °C and a thermal expansion coefficient of 11.04 × 10⁻⁶ K with a minimum reactivity towards yttria-stabilized zirconia. Though the conductivity of the new composition is lower than that of La_0.84Sr_0.16MnO₃ (169 S cm⁻¹ at 950 °C), it is still high enough for use as a cathode material.     

Broadening of the magnetic entropy change in La0.75Ca0.15Sr0.10MnO3

A broad table-like entropy change (ΔS) at room temperature has been observed in the ferromagnetic compound La_0.75Ca_0.15Sr_0.10MnO₃, which is analyzed in the concept of Landau theory and with critical exponent analysis obtained from the magnetization measurements. The change in entropy in La_0.75Ca_0.15Sr_0.10MnO₃ is discussed in the light of magnetoelastic coupling between the magnetization and the lattice distortion. Application aspects of this unusual broad magnetocaloric effect with relative cooling power of 107 J kg⁻¹ in an applied magnetic field of 1.6 T with an operating temperature range of 93 K around the room temperature are also discussed.     

Thermal treatment effects on interfacial layer formation between ZrO2 thin films and Si substrates

This paper describes the growth condition of stoichiometric ZrO₂ thin films on Si substrates and the interfacial structure of ZrO₂ and Si substrates. The ZrO₂ thin films were prepared by rf-magnetron sputtering from Zr target with mixed gas of O₂ and Ar at room temperature followed by post-annealing in O₂ ambient. The stoichiometric ZrO₂ thin films with smooth surface were grown at high oxygen partial pressure. The thick Zr-free SiO₂ layer was formed with both Zr silicide and Zr silicate at the interface between ZrO₂ and Si substrate during the post-annealing process due to rapid diffusion of oxygen atoms through the ZrO₂ thin films. After post annealing at 650-750 °C, the multi-interfacial layer shows small leakage current of less than 10⁻⁸ A/cm² that is corresponding to the high-temperature processed thermal oxidized SiO₂.     

Synthesis of La0.5A0.5MnO3 (A = Sr, Ba) by a hydrothermal method at low temperature

A mild hydrothermal method has been adopted to prepare La_0.5Sr_0.5MnO₃ and La_0.5Ba_0.5MnO₃, which is of interest for a number of possible applications. The results from X-ray diffraction (XRD) indicate that in the present work the temperature of 200 and 240 °C are sufficient to prepare phase pure La_0.5Sr_0.5MnO₃ and La_0.5Ba_0.5MnO₃ crystals. At 200 °C, La_0.5Sr_0.5MnO₃ nanowires are obtained. The average width and length of the nanowires are 40 nm and 4 μm, respectively. At 240 °C, La_0.5Ba_0.5MnO₃ powders obtained have a cubic structure with the average size of 3-5 μm.     

Anisotropic and excellent magnetocaloric properties of La0.7Ca0.3MnO3 single crystal with anomalous magnetization

Magnetic properties and the magnetocaloric effect (MCE) have been investigated in La_0.7Ca_0.3MnO₃ single crystal with applied field along both the ab-plane and the c-direction. Due to the magnetocrystalline anisotropy, the crystal exhibits anisotropy in the MCE. Upon application of a 5 T field, the magnetic entropy changes (ΔS_M), reaching values of 7.668 J/(kg K) and 6.412 J/(kg K) for both the ab-plane and the c-direction, respectively. A magnetic entropy change of 3.3 J/(kg K) was achieved for a magnetic field change of 1.5 T at the Curie temperature, T_C = 245 K. Due to the absence of grains in the single crystal, the ΔS_M distribution here is much more uniform than for gadolinium (Gd) and other polycrystalline manganites, which is desirable for an Ericsson-cycle magnetic refrigerator. For a field change of 5 T, the relative cooling power, RCP, reached 358.17 J/kg, while the maximum adiabatic temperature change of 5.33 K and a magnetoresistance (MR) ratio of 507.88% at T_C were observed. We analysed the magnetization of La_0.7Ca_0.3MnO₃ single crystal at T_C and estimated several parameters of spin fluctuation on the basis of a self-consistent renormalization theory of spin fluctuations, with reciprocal susceptibility above T_C. We found that the magnetic property of La_0.7Ca_0.3MnO₃ is weakly itinerant ferromagnetic. A large reversible MCE and no hysteresis loss with a considerable value of refrigerant capacity indicate that La_0.7Ca_0.3MnO₃ single crystal is a potential candidate as a magnetic refrigerant.     

La0.67Ca0.33MnO3 thin films on Si (1 0 0) by DC magnetron sputtering technique using nanosized powder compacted target

La_0.67Ca_0.33MnO₃ (LCMO) thin films were successfully fabricated by a DC magnetron sputtering technique on Si (1 0 0) substrates from chemically synthesized compacted powders. Powders of proper stochiometry composites were synthesized by a novel chemical technique [D.R. Sahu, B.K. Roul, P. Pramanik, J.L. Huang, Physica B 369 (2005) 209] and were found to be nanosized (≈40-50 nm). The sinterability of the powders were improved significantly due to their large surface area with a reduction of sintering temperature (up to 500 °C) as compared to the powders prepared by other solid-state reaction route. Bulk LCMO targets were prepared and preliminary structural and magnetic properties of target were investigated for colossal magnetoresistance (CMR) properties. Films deposition parameters like DC power, gas flow rate, deposition time, etc., were critically optimized to achieve desired thickness of film using above LCMO target by DC magnetron sputtering. LCMO films fabricated on Si (1 0 0) substrates showed enhanced magnetoresistance (MR) at low temperature. Maximum MR of about 1000% was observed at 100 K. Paramagnetic to ferromagnetic transitions were observed in films below room temperature and were found at approximately 240 K. However, as compared to bulk target prepared by a chemical route, it was found that Curie temperature (T_c) and MR response of bulk target were higher than the thin films. Preliminary point chemical analysis revealed the deficiency of Ca²⁺ ions in CMR films.