LaMnO3 Films Research Articles

Development of a metal-organic deposition process for growth of lanthanum manganate buffer layers for coated conductors

One embodiment of YBa2Cu3O7-x (YBCO)-coated conductors consists of YBCO film grown using a metal-organic deposition (MOD) process on a buffer layer stack deposited on a Ni-W substrate. A possible alternative is to replace the multilayer buffer stack with a single layer of LaMnO3 (LMO) grown by MOD. A suitable temperature (T) – oxygen partial pressure (PO2) process-window to grow LMO films via MOD on Ni substrates has been identified. Untextured LMO was grown on pure Ni. However, we have not been able to grow phase-pure LMO films on either bare or Y2O3-coated, biaxially textured Ni-5W (at.%) due to the incongruent, overlapping requirements of having the PO2 low enough to not oxidize the Ni-5W substrate but high enough to convert the metal-organic precursors to LMO and the relatively high temperatures needed to form an epitaxial film in an MOD process. The main problem is that tungsten from the substrate reacts with cations in the overlying film forming tungstates when the films are processed in the T-PO2 window.

Size effect of substitutional alkaline-earth elements on the electrical and structural properties of LaMnO3 films

Tolerance factor controlled Mn-based colossal magnetoresistance (CMR) thin films (La0.7Ca0.3MnO3, La0.7Sr0.3MnO3, La0.7Sr0.11Ba0.19MnO3, and La0.7Ba0.3MnO3) which have the same content of divalent cation and Mn3+/Mn4+ ratio were deposited on amorphous SiO2/Si substrate by rf magnetron sputtering at 350°C substrate temperature. Post annealing treatment for 1 h at 600°C was also carried out to investigate the effects of internal strain and chemical bonding nature from different divalent ions on the electrical properties of the films by maintaining a similar crystalline state. The films crystallized with pseudo cubic structure in spite of different tolerance factors. The sheet resistance of films changed according to crystallization and Mn-O bonding character. Mn L-edge X-ray absorption spectra revealed that Mn3+/Mn4+ ratio did not change in all the films and Mn 2p core level X-ray photoelectron spectra showed that Mn-O bonding property changed to more covalence as increasing tolerance factor by substitution with larger size divalent cation. O K-edge X-ray absorption spectra observed t2g and eg electron states and low resistivity after post anneal could be explained by the promotion of electrons to low binding energy state. Temperature coefficient of resistance (TCR) values were about -2.24 ∼ -2.57%/K of as deposited CMR films and these values were reasonable for uncooled microbolometer applications.

Signature of Jahn–Teller distortion and oxygen stoichiometry in Raman spectra of epitaxial LaMnO3+δ thin films

Epitaxial thin films of LaMnO3 were grown by pulsed laser deposition in identical deposition conditions but in situ annealed in different oxygen environments. All the films showed an insulator to metal transition. It also showed a magnetic order that correlates well with an insulator to metal behavior. The transition temperature critically depends on annealing oxygen pressure and increases with increasing oxygen content. It is observed that instead of strain, oxygen content plays a major role in deciding the transport properties. X-ray diffraction and Raman spectroscopy measurements revealed that the increasing oxygen content results in a decrease in unit cell volume along with a reduction in Jahn–Teller distortion. It is shown that the intensities of the two Jahn–Teller activated modes in Raman spectra are highly sensitive to the oxygen stoichiometry of the films and that the intensity ratio can be effectively used to characterize oxygen content in these films.

Chemical solution deposition of LaMnO3-based films for coated conductors

LaMnO3-based films were prepared using the chemical solution deposition method. It was found that the films on perovskite oxide single crystal substrates are highly (h00)-oriented when the annealing atmosphere is oxygen or air; however, when the substrate is yttrium-stabilized ZrO2, only the La1-xNaxMnO3 films are highly (h00)-oriented, and other LaMnO3-based films are (110)-oriented. Under a reducing annealing atmosphere, the atmosphere must be wet in order to create a suitable oxygen partial pressure to crystallize the LaMnO3-based films. After annealing under a wet reducing atmosphere the LaMnO3-based films are (110)-oriented when the films are directly deposited on Ni tapes; however, when SrTiO3-buffered Ni tapes are used, the LaMnO3 films are (h00)-oriented, which is suitable for subsequent growth of YBCO. The results suggest that it is possible to tune the orientation of buffer layers using suitable templates, which can widen the selection of buffer layers for coated conductors in the all metallorganic deposition approach.

Impedance investigation on porous Sr-doped LaMnO3 films onto Sr-Mg-doped LaGaO3 electrolyte

The use of Sr-Mg-doped LaGaO3 (LSGM), a highly conducting oxygen ion electrolyte, in intermediate temperature solid oxide fuel cells (IT-SOFC) technology requires suitable electrode materials. Because the Sr-doped LaMnO3 (LSM) cathode coupled with the YSZ electrolyte had shown relatively good performances, it has been also suggested for LSGM-based cells. As cathode overpotential is the main performance limitation, the optimization of the LSM/LSGM interface might be of fundamental relevance in the technology development of LSGM-based IT-SOFC. LSM films with different porosities were screen printed on both faces of the LSGM pellets; their morphology and electrical properties were investigated by scanning electron microscopy and impedance spectroscopy, respectively. Porosity was induced by the addition of icing sugar (3–5 wt%) to the LSM powder during ink preparation. Homogeneous electrode layers with a thickness of about 30 μm were obtained. The higher the sugar amount, the higher the film porosity and the lower the LSM grain size. The Nyquist plots show two low frequency arcs to which the charge transfer and mass transfer process or oxygen dissociation process were associated, respectively. One can expect that the best electrochemical performances are obtained using highly porous electrode.

The effect of magnetic order and thickness in the Raman spectra of oriented thin films ofLaMnO3

Low-temperature Raman studies on (100)-orientedLaMnO3 thin films of different thicknesses deposited onLaAlO3(100) substrate by using the pulsed laser deposition technique are presented. The phonon modefrequencies depend strongly on the thickness of the film and in comparison to theLaMnO3 single crystal shift to higher values with decreasing thickness. The broadening of Ramanline widths was also observed with decreasing film thickness. Anomalous softening of theB2g mode (Jahn–Teller distortion activated) was observed for all films below the magnetic orderingtemperature. A signature of lattice anomalies in the form of changes in line width of theB2g mode correlated with the magnetic ordering temperature was also seen. The results wereinterpreted by considering strong spin–phonon coupling and reduction in Jahn–Tellerdistortion driven polarons due to magnetic order.

Approaches towards ferroelectric control of thin film magnetism†

We discuss three different approaches towards ferroelectric control of thin film magnetism and recent experimental results. Firstly, ferromagnetic films of doped LaMnO3 were grown on piezoelectric Pb(Mg1/3Nb2/3)0.72Ti0.28O3(001) crystals and have been strained reversibly by up to 0.15%. The strain modulation is accompanied by large magnetization changes (for example, of 25% at ambient temperature in La0.7Sr0.3MnO3), strong resistance modulations and a shift of the magnetic Curie temperature. Secondly, the concept of a multiferroic field-effect transistor comprising a magnetic channel and a ferroelectric gate is introduced. We discuss the effect of piezoelectric gate strain. Finally, the first results on single-phase multiferroic films of HoMnO3 are described. Epitaxial films have been prepared by the pulsed laser deposition technique. †We would like to devote this article to Prof. Hartmut Fueß on the occasion of his official retirement. He has been active in the area of structural research on oxides throughout many years of his working life.

Microstructural properties of solution-deposited La0.7Sr0.3MnO3 and LaMnO3 thin films

The microstructural properties of solution-deposited La0.7Sr0.3MnO3 and nondoped LaMnO3 films have been studied. Several factors such as the choice of substrate, method of surface cleaning, annealing atmosphere, and precursor solution concentration were considered. Films with smoother surfaces were observed on SrTiO3 (100) and LaAlO3 (100) crystals, due to the near matching of the film-substrate lattice constants and thermal-expansion coefficients. In contrast, substrates such as Si (100) yield films with a rougher morphology. Variations in the crystalline quality and surface morphology of the films were demonstrated, simply by altering the precursor solution concentration and the surface condition of substrate. The local structure of the films was studied by Raman spectroscopy, which indicated enrichment in the oxygen content, differing from ideal oxygen stoichiometry (or from films annealed in inert gas atmosphere). This latter effect was shown to be more effective in the case of LaMnO3, where a structural transition from rhombohedral to orthorhombic was observed by changing the annealing atmosphere from ambient air to pure N2 gas, and is suggested to be the main origin of the anomalous conductivity increase reported in the literature in air or O2-annealed manganite films with respect to stoichiometric bulk La1−x(Sr,Ca)xMnO3.

MOCVD of LaMnO3 on Biaxially Textured Ni‐based Substrates in a Reducing Atmosphere

AbstractThe metal–organic (MO) CVD process was modified to allow the in‐situ deposition of perovskite manganites on biaxially textured Ni‐based metal tapes (RABiTS). The modification consists of the creation of the protective atmosphere by simultaneous introduction of ammonia and water. Such an atmosphere prevents the oxidation of Ni and the reduction of manganites into non‐perovskite phases. MOCVD experiments were accomplished in the band‐type reactor using the band evaporation system at 800 °C and 850 °C. Orthorhombic LaMnO3 film demonstrates (101) orientation (i.e. (001) orientation in cubic perovskite notation) minimizing lattice mismatch with the nickel substrate. The film stoichiometry, total pressure, water and ammonia flows greatly influence the quality of the film orientation. The oriented perovskite manganite films on Ni tapes can be used as buffers for high Tc superconductors for high current applications as well as magnetoresistance‐based sensors of the magnetic field.

Resonant X-ray scattering and cooperative Jahn–Teller effect in LaMnO3 thin films

Abstract Epitaxial thin films of LaMnO3 were grown on MgO and SrTiO3. The cooperative Jahn–Teller transition occurred at different temperatures for the two films. It is shown that the intensity of the resonant (1 0 0) peak at the Mn K-edge is proportional to the 3 2 power of the orthorhombic strain for both films at all temperatures below the transition. This provides an evidence in favor of the band effect in accounting for the resonant scattering in LaMnO3. A significant intensity difference in the resonant scattering in the two films is pointed out.

IR Absorption of Trapped and Free Jahn–Teller Polarons in 3D-Crystals and Films of LaMnO3 and La2/3Sr1/3MnO3

The experimental and theoretical study of IR-absorption in LaMnO3 (LMO) single crystals has led to the observation of new five well-defined peaks. Their positions at RT are the following: 0.161 eV, 0.147 eV, 0.130 eV, 0.117 eV, and a much more broadened peak at ∼0.24 eV. The IR-absorption and its temperature and heat treatment dependence was studied with the help of a Fourier IR spectrometer Bruker IFS 120 HR. Analogous peaks of IR-absorption in LMO films are shifted to the lower energy side. The La2/3Sr1/3MnO3 film IR-absorption is characterized by additional broadening. The results were interpreted as direct detection of Jahn–Teller hole free polarons, as well as trapped hole and electronic polarons in LMO and related systems. Two type of Jahn–Teller hole states were used in this model. There were free Jahn–Teller hole polarons, and Jahn–Teller hole polarons within localized charge transfer vibronic excitons (Mn4+—Mn2+ pairs). IR-absorption peak induced by electronic polaron within such type exciton mentioned above was also interpreted.

Composition and Growth-Temperature Effect on the Microstructure of Epitaxial La1−x Srx MnO3 (x = 0, 0.2) Thin Films on (001) LaAlO3

La0.8Sr0.2MnO3 (LSMO) and LaMnO3 (LMO) thin films epitaxially grown on (001) LaAlO3 substrate at 700 and 800°C were studied using cross-section and plan-view transmission electron microscopy. In both LSMO and LMO films deposited at 700°C, a two-layered structure was observed: a continuous cubic perovskite layer epitaxially grown on the substrate followed by epitaxially grown orthorhombic column nanostructures. The orthogonal nano columns in LSMO were very well defined with a narrow size distribution. The LMO films exhibited a somewhat distorted orthogonal shape for the nanostructured columns and a wider size distribution. LSMO and LMO epitaxial films deposited at 800°C consisted of only a continuous single layer of cubic perovskite. These results reveal that formation of epitaxial column nanostructures in the La1-xSrxMnO3 thin films strongly depends on the growth temperature. Sr substitution on La sites in La1-xSrxMnO3 can improve uniformity of column size distribution and perfection of the orthogonal shape of nano columns.

Origin of metal–insulator transition temperature enhancement in underdoped lanthanum manganite films

The possibility of controlling transport properties of colossal magnetoresistance manganite films using substrate-induced strain has attracted great interest. We have investigated transport properties of La0.9Ca0.1MnO3, La0.92Ba0.08MnO3, La0.8Ba0.2MnO3, and LaMnO3 films. When the films were post-annealed at proper conditions, all of them showed metal–insulator transitions. Their transition temperatures TMI were much higher than the corresponding bulk values, irrespective of the type of substrate-induced biaxial strain. This surprising fact demonstrated that strain could not be the main origin of the TMI enhancement observed in the underdoped (dopant concentration x<0.3) manganite films. We suggested that TMI enhancements should be attributed mostly to the cationic vacancies in the post-annealed films.

Single Buffer Layer Technology for YBCO Coated Conductors

ABSTRACTIn an effort to develop alternative single buffer layer technology for YBa2Cu3O7-δ (YBCO) coated conductors, we have investigated LaMnO3 (LMO) as a potential buffer layer. High-quality LMO films were grown directly on biaxially textured Ni and Ni-W (3%) substrates using rf magnetron sputtering. YBCO films were then grown on LMO buffers using pulsed laser deposition. Detailed X-ray studies have shown that both YBCO and LMO layers were grown with a single epitaxial orientation. Rutherford backscattering spectroscopy (RBS) analyses have indicated the ratio of La to Mn ratio is 1:1. SEM micrographs indicated that 3000-Å-thick LMO films on biaxially textured Ni (100) substrates were dense, continuous and crack-free. A high Jc of over 1 MA/cm2 at 77 K and self-field was obtained on YBCO films grown on LMO-buffered Ni or Ni-W substrates. We have identified LaMnO3 as a good diffusion barrier layer for Ni and it also provides a good template for growing high current density YBCO films.

Preparation of Sr-substituted LaMnO3 thick films as cathode for solid oxide fuel cell

Thick films of Sr-substituted LaMnO3 (LSM) were deposited on impervious YSZ electrolyte by the screen printing technique. The LSM powders used in this investigation were synthesized by autoignition of carboxylate-nitrate gel. Multiple coatings were applied to increase the thickness of LSM layer. Effects of coating thickness and sintering temperature on the microstructure and electrical properties of the films were studied. The properties of the films obtained from the present study, fulfill the requirements of the cathode of a solid oxide fuel cell.

Growth and Magnetic Structure of La0.67Sr0.33MnO3 Films

ABSTRACTGrowth of LaMnO3 films that exhibit colossal magnetoresistance (CMR) has concentrated heavily on Ca doped materials. However, since the 33% Sr doped films are ferromagnetic at room temperature, they are ideal candidates for dual growth-magnetic structure studies using scanned probe techniques. In this study, interest was focused on the relations between growth/processing parameters, film morphology, and electronic/magnetic properties. In addition, films were grown on both LaA1O3 (LAO) and SrTiO3 (STO) to examine the results of stress induced by different substrate mismatches. La0.67Sr0.33MnO3 (LSMO) was grown using pulsed laser deposition (PLD) at temperatures between 500 °C and 800 °C. The film microstructure, crystallinity, and magnetic and electrical properties were characterized by room temperature scanning tunneling microscopy (STM), atomic force microscopy (AFM), magnetic force microscopy (MFM), x-ray diffraction, and temperature dependent transport and magnetization measurements. The growth trends follow those previously reported for Ca doped films. Grains increase in size with increasing temperature and coalesce into extended layers after annealing. Although topographic contributions complicate interpretation of some MFM data, local magnetic structure observed here is generally associated with film defects.

Preparation and Characteristics of LaMnO3 Thin Film Electrode on YSZ by a Vapor-Phase Process

Abstract Dense and thin LaMnO3/YSZ/La0.85Sr0.15MnO3 cell was perpared by CVD-EVD process. The prepared LaMnO3 film was sufficiently uniform and dense, no pore or crack was observed in the LaMnO3 film. AC impedance measurements indicated that the electrode interface conductivity was linearly dependent on the electrode surface area. For dense and thin film electrode of LaMnO3, oxygen electrochemical reacion proceeded predominantly at the electrode surface region.

Magnetocaloric properties of doped lanthanum manganite films

LaMnO3 films doped with Ca, Ba, or Sr have been fabricated using the metalorganic decomposition technique. These films exhibit paramagnetic-to-ferromagnetic phase transitions at 250, 300, and 350 K, respectively. By measuring the film magnetization as a function of field and temperature we have determined the entropy change associated with these transitions. The large magnetization of these materials results in a total entropy change a factor of five less than that of gadolinium, the prototypical high-temperature magnetocaloric material. Improvements in film morphology and composition may provide a further increase in the magnetization and total entropy change in these materials.

Oxygen Reduction at the Interface YSZ / LaMnO3 prepared by a Vapor Phase Processes

Dense and thin LaMnO3 and YSZ was deposited on porous La0.85(Sr)0.15MnO3 substrate in order to study oxygen redox behavior at an interface of LaMnO3/YSZ. YSZ film was deposited by the CVD - EVD method on porous substrate at first, LaMnO3 film was deposited on the YSZ film by the EVD method. Results of the AC impedance measurements showed that electrode interface conductivity (σe) was directly proportional to the surface area, while σe v. s. TPB (Three Phase Boundary) length plot did not show linear relationship. It was concluded that LaMnO3 surface as well as TPB region, is an active site for oxygen redox reaction when the LaMnO3 film electrode is very thin (about few μm).