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## Evaluation of Several Carbon-Supported Nanostructured Ni-Doped Manganese Oxide Materials for the Electrochemical Reduction of Oxygen

GARCIA, Amanda C.; HERRERA, Alberto Diaz; TICIANELLI, Edson A.; CHATENET, Marian; POINSIGNON, Christiane
Fonte: ELECTROCHEMICAL SOC INC Publicador: ELECTROCHEMICAL SOC INC
Tipo: Artigo de Revista Científica
ENG
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Physical and electrochemical properties of nanostructured Ni-doped manganese oxides (MnO(x)) catalysts supported on different carbon powder substrates were investigated so as to characterize any carbon substrate effect toward the oxygen reduction reaction (ORR) kinetics in alkaline medium. These NiMnO(x)/C materials were characterized using physicochemical analyses. Small insertion of Ni atoms in the MnO(x) lattice was observed, which consists of a true doping of the manganese oxide phase. The corresponding NiMnO(x) phase is present in the form of needles or agglomerates, with crystallite sizes in the order of 1.5-6.7 nm (from x-ray diffraction analyses). Layered manganite (MnOOH) phase has been detected for the Monarch 1000-supported NiMnO(x) material, while different species of MnO(x) phases are present at the E350G and MM225 carbons. Electrochemical studies in thin porous coating active layers in the rotating ring-disk electrode setup revealed that the MnO(x) catalysts present better ORR kinetics and electrochemical stability upon Ni doping. The ORR follows the so-called peroxide mechanism on MnO(x)/C catalysts, with the occurrence of minority HO(2)(-) disproportionation reaction. The HO(2)(-) disproportionation reaction progressively increases with the Ni content in NiMnO(x) materials. The catalysts supported on the MM225 and E350G carbons promote faster disproportionation reaction...

## Fabricação e testes de células a combustível de óxido sólido a etanol direto usando camada catalítica; Solid oxide fuel cells fabrication and operation running direct ethanol using a catalytic layer

Nobrega, Shayenne Diniz da
Fonte: Biblioteca Digitais de Teses e Dissertações da USP Publicador: Biblioteca Digitais de Teses e Dissertações da USP
Tipo: Tese de Doutorado Formato: application/pdf
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## Properties of Ca1−xHoxMnO3 perovskite-type electrodes

Ferreira, B M; Melo Jorge, M E; Lopes, M E; Nunes, M R; Silva Pereira, M I
Tipo: Artigo de Revista Científica
ENG
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In this work, Ca1−xHoxMnO3 (x = 0, 0.1 and 0.2) perovskite oxide pelleted electrodes were prepared from the respective powders obtained by the citrate route method at 1173 K. The electrodes exhibit particle size that decreases with the holmium content in the oxide. All the samples reveal semiconductor behaviour and the presence of holmium induces a marked decrease in the electrical resistivity. The results can be well attributed to the changes in the Mn4+/Mn3+ ratio. Electrodes were characterized by cyclic voltammetry and chronopotentiometry. Cyclic voltammetric studies indicate a similar behaviour of the electrodes, irrespective of their composition. Two pairs of peaks were identified and associated, one to the Mn4+/Mn3+ redox couple and the other to the Mn7+/Mn4+ and Mn6+/Mn4+ redox couples. The voltammetric data provide evidence that the electrodes roughness factor increases with the introduction of Ho-ions in the oxide structure, what is consistent with the crystallite size obtained by X-ray diffraction (XRD) and the morphology observed by scanning electron microscopy (SEM). The Ho substituted electrodes present higher current density when compared with CaMnO3 electrodes what can be attributed both to higher electrical conductivity and smaller particle size. The chronopotentiometric studies have shown that the discharge occurs by different mechanisms for the oxide electrodes with and without Ho.

## Characterization and electrochemical behaviour of nanostructured calcium samarium manganite electrodes fabricated by RF-Magnetron Sputtering

Barrocas, B.; Sério, S.; Rovisco, A.; Nunes, Y.; Sá, A. I. Correia de; Pereira, M. I. da Silva; Jorge, M. E. Melo
Tipo: Artigo de Revista Científica
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Ca1-xSmxMnO3(0 . x . 0.4) films were successfully fabricated on Indium Tin Oxide (ITO) coated quartzglass substrates by radio frequency magnetron sputtering technique (RF- magnetron sputtering) fromcompacted nanosized powder targets, and subsequent annealing at 800.C in air, for 6 h. X-ray diffrac-tion shows a pure typical perovskite phase for x . 0.1. Scanning electron microscopy and atomic forcemicroscopy revealed that the films surface is dense, with low roughness, depending on the Sm content, even though a few cracks were observed. Crystallite size was found to decrease with the Sm content. Theelectrodes were characterized by cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS). The oxide electrode fs capacitance was estimated using both techniques and the corresponding roughness factors evaluated. The values obtained from the two methods show a good agreement. A com-parison between the voltammetric data and those referred in the literature allowed finding out that theredox reaction occurring at the electrode surface involves the pair Mn4+/Mn3+. EIS measurements con-firm the voltammetric data and they also give additional information about the film porosity and the charge transfer resistance. This last parameter is associated with the oxidation and reduction of the pairMn3+/Mn4+and after normalized by the roughness factor shows an increase with samarium content.

## Thin film deposition of metal oxides in resistance switching devices: electrode material dependence of resistance switching in manganite films

Nakamura, Toshihiro; Homma, Kohei; Tachibana, Kunihide
Tipo: Artigo de Revista Científica
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The electric-pulse-induced resistance switching in layered structures composed of polycrystalline Pr1−xCaxMnO3 (PCMO) sandwiched between Pt bottom electrode and top electrodes of various metals (metal/PCMO/Pt) was studied by direct current current–voltage (I-V) measurements and alternating current impedance spectroscopy. The I-V characteristics showed nonlinear, asymmetric, and hysteretic behavior in PCMO-based devices with top electrode of Al, Ni, and Ag, while no hysteretic behavior was observed in Au/PCMO/Pt devices. The PCMO-based devices with hysteretic I-V curves exhibited an electric-pulse-induced resistance switching between high and low resistance states. Impedance spectroscopy was employed to study the origin of the resistance switching. From comparison of the impedance spectra between the high and low resistance states, the resistance switching in the PCMO-based devices was mainly due to the resistance change in the interface between the film and the electrode. The electronic properties of the devices showed stronger correlation with the oxidation Gibbs free energy than with the work function of the electrode metal, which suggests that the interface impedance is due to an interfacial oxide layer of the electrode metal. The interface component observed by impedance spectroscopy in the Al/PCMO/Pt device might be due to Al oxide layer formed by oxidation of Al top electrode. It is considered that the interfacial oxide layer plays a dominant role in the bipolar resistance switching in manganite film-based devices.

## Surface Nanostructures in Manganite Films

Gambardella, A.; Graziosi, P.; Bergenti, I.; Prezioso, M.; Pullini, D.; Milita, S.; Biscarini, F.; Dediu, V. A.
Fonte: Nature Publishing Group Publicador: Nature Publishing Group
Tipo: Artigo de Revista Científica
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Ultrathin manganite films are widely used as active electrodes in organic spintronic devices. In this study, a scanning tunnelling microscopy (STM) investigation with atomic resolution revealed previously unknown surface features consisting of small non-stoichiometric islands. Based upon this evidence, a new mechanism for the growth of these complex materials is proposed. It is suggested that the non-stoichiometric islands result from nucleation centres that are below the critical threshold size required for stoichiometric crystalline growth. These islands represent a kinetic intermediate of single-layer growth regardless of the film thickness, and should be considered and possibly controlled in manganite thin-film applications.

## Local Tunneling Magnetoresistance probed by Low-Temperature Scanning Laser Microscopy

Werner, Robert; Weiler, Mathias; Petrov, Aleksandr Yu.; Davidson, Bruce A.; Gross, Rudolf; Kleiner, Reinhold; Goennenwein, Sebastian T. B.; Koelle, Dieter
Tipo: Artigo de Revista Científica
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Tunneling magnetoresistance (TMR) in a vertical manganite junction was investigated by low-temperature scanning laser microscopy (LTSLM) allowing to determine the local relative magnetization M orientation of the two electrodes as a function of magnitude and orientation of the external magnetic field H. Sweeping the field amplitude at fixed orientation revealed magnetic domain nucleation and propagation in the junction electrodes. For the high-resistance state an almost single-domain antiparallel magnetization configuration was achieved, while in the low-resistance state the junction remained in a multidomain state. Calculated resistance $R_\mathrm{calc}(H)$ based on the local M configuration obtained by LTSLM is in quantitative agreement with R(H) measured by magnetotransport.

## Large domain wall resistance in self-organised manganite film

Granja, L.; Hueso, L. E.; Sanchez, D.; Prieto, J. L.; Levy, P.; Mathur, N. D.
Tipo: Artigo de Revista Científica
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The electrical resistance of magnetic domain walls in ferromagnetic metallic manganites can be enhanced to 10-12 Ohm.m2 by patterning nanoconstrictions [J. Appl. Phys. 89, 6955 (2001)]. We show equally large enhancements in a phase separated La0.60Ca0.40MnO3 manganite film without recourse to nanopatterning. The domain walls were measured in the current-perpendicular-to-the-plane (CPP) geometry between ferromagnetic metallic La0.70Ca0.30MnO3 electrodes patterned like magnetic tunnel junctions.; Comment: Devices are set in the context of metallic tunnel junctions, and magnetic data is included

## Novel Quantum Spin Assisted Tunneling in Half Metallic Manganite Tunnel Junctions

Gu, R. Y.; Sheng, L.; Ting, C. S.
Tipo: Artigo de Revista Científica
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The electron tunneling in half metallic manganite tunnel junctions is studied by using a quantum mechanically treated double exchange model. We show that the stimulation of spin excitations, caused by the strong Hund's coupling between the conduction $e_g$ electrons and the localized quantum spins of the manganite ions, would assist electrons to tunnel through the junction even with the antiparallel aligned magnetizations of the electrodes. This mechanism gives rise to an extra tunnel conductance, in addition to that predicted by Julliere's model. Our theory is in good agreement with the voltage dependence of the tunnel conductance in manganite tunnel junctions observed by experiments.; Comment: 5 pages, 1 figure

## Low frequency 1/f noise in doped manganite grain-boundary junctions

Philipp, J. B.; Alff, L.; Marx, A.; Gross, R.
Tipo: Artigo de Revista Científica
Relevância na Pesquisa
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We have performed a systematic analysis of the low frequency 1/f-noise in single grain boundary junctions in the colossal magnetoresistance material La_{2/3}Ca_{1/3}MnO_{3-delta}. The grain boundary junctions were formed in epitaxial La_{2/3}Ca_{1/3}MnO_{3-delta} films deposited on SrTiO_3 bicrystal substrates and show a large tunneling magnetoresistance of up to 300% at 4.2 K as well as ideal, rectangular shaped resistance versus applied magnetic field curves. Below the Curie temperature T_C the measured 1/f noise is dominated by the grain boundary. The dependence of the noise on bias current, temperature and applied magnetic field gives clear evidence that the large amount of low frequency noise is caused by localized sites with fluctuating magnetic moments in a heavily disordered grain boundary region. At 4.2 K additional temporally unstable Lorentzian components show up in the noise spectra that are most likely caused by fluctuating clusters of interacting magnetic moments. Noise due to fluctuating domains in the junction electrodes is found to play no significant role.; Comment: 9 pages, 7 figures

## Transport and spectroscopic properties of superconductor - ferromagnet - superconductor junctions of $La_{1.9}Sr_{0.1}CuO_4$ - $La_{0.67}Ca_{0.33}MnO_3$ - $La_{1.9}Sr_{0.1}CuO_4$

Tipo: Artigo de Revista Científica
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Transport and Conductance spectra measurements of ramp-type junctions made of cuprate superconducting $La_{1.9}Sr_{0.1}CuO_4$ electrodes and a manganite ferromagnetic $La_{0.67}Ca_{0.33}MnO_3$ barrier are reported. At low temperatures below $T_c$, the conductance spectra show Andreev-like broad peaks superposed on a tunneling-like background, and sometimes also sub-gap Andreev resonances. The energy gap values $\Delta$ found from fits of the data ranged mostly between 7-10 mV. As usual, the gap features were suppressed under magnetic fields but revealed the tunneling-like conductance background. After field cycling to 5 or 6 T and back to 0 T, the conductance spectra were always higher than under zero field cooling, reflecting the negative magnetoresistance of the manganite barrier. A signature of superparamagnetism was found in the conductance spectra of junctions with a 12 nm thick LCMO barrier. Observed critical currents with barrier thickness of 12 nm or more, were shown to be an artifact due to incomplete milling of one of the superconducting electrodes.; Comment: 10 figures

## Oxide spintronics

Bibes, Manuel; Barthelemy, Agnes
Tipo: Artigo de Revista Científica
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Concomitant with the development of metal-based spintronics in the late 1980's and 1990's, important advances were made on the growth of high-quality oxide thin films and heterostructures. While this was at first motivated by the discovery of high-temperature superconductivity in perovskite Cu oxides, this technological breakthrough was soon applied to other transition metal oxides, and notably mixed-valence manganites. The discovery of colossal magnetoresistance in manganite films triggered an intense research activity on these materials, but the first notable impact of magnetic oxides in the field of spintronics was the use of such manganites as electrodes in magnetic tunnel junctions, yielding tunnel magnetoresistance ratios one order of magnitude larger than what had been obtained with transition metal electrodes. Since then, the research on oxide spintronics has been intense with the latest developments focused on diluted magnetic oxides and more recently on multiferroics. In this paper, we will review the most important results on oxide spintronics, emphasizing materials physics as well as spin-dependent transport phenomena, and finally give some perspectives on how the flurry of new magnetic oxides could be useful for next-generation spintronics devices.; Comment: Invited review paper published in a Special Issue of IEEE Transactions on Electron Devices on Spintronics

## Decoupled magnetic and electrical switching in manganite trilayer

Hueso, L. E; Granja, L.; Levy, P.; Mathur, N. D.
Tipo: Artigo de Revista Científica
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We report magnetic and electrical transport studies of an epitaxially grown trilayer thin film structure comprising La0.59Ca0.41MnO3 sandwiched between La0.67Ca0.33MnO3 electrodes. Since La0.59Ca0.41MnO3 lies at the edge of the thin film ferromagnetic metallic phase field, phase separation effects are expected. These effects can explain the observed magnetic isotropy of the middle layer. By contrast, the electrode material is magnetically uniaxial. Easy axis magnetic field sweeps of the trilayer produce two sharp magnetic transitions, but only one sharp transition in current-in-plane resistance measurements.

## Transformation of spin information into large electrical signals via carbon nanotubes

Hueso, Luis E.; Pruneda, Jose M.; Ferrari, Valeria; Burnell, Gavin; Valdes-Herrera, Jose P.; Simons, Benjamin D.; Littlewood, Peter B.; Artacho, Emilio; Fert, Albert; Mathur, Neil D.
Tipo: Artigo de Revista Científica
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Spin electronics (spintronics) exploits the magnetic nature of the electron, and is commercially exploited in the spin valves of disc-drive read heads. There is currently widespread interest in using industrially relevant semiconductors in new types of spintronic devices based on the manipulation of spins injected into a semiconducting channel between a spin-polarized source and drain. However, the transformation of spin information into large electrical signals is limited by spin relaxation such that the magnetoresistive signals are below 1%. We overcome this long standing problem in spintronics by demonstrating large magnetoresistance effects of 61% at 5 K in devices where the non-magnetic channel is a multiwall carbon nanotube that spans a 1.5 micron gap between epitaxial electrodes of the highly spin polarized manganite La0.7Sr0.3MnO3. This improvement arises because the spin lifetime in nanotubes is long due the small spin-orbit coupling of carbon, because the high nanotube Fermi velocity permits the carrier dwell time to not significantly exceed this spin lifetime, because the manganite remains highly spin polarized up to the manganite-nanotube interface, and because the interfacial barrier is of an appropriate height. We support these latter statements regarding the interface using density functional theory calculations. The success of our experiments with such chemically and geometrically different materials should inspire adventure in materials selection for some future spintronics; Comment: Content highly modified. New title...

## Conditions for the growth of smooth La0.7Sr0.3MnO3 thin films by pulsed electron ablation

Graziosi, Patrizio; Prezioso, Mirko; Gambardella, Alessandro; Kitts, Catherine; Rakshit, Rajib Kumar; Riminucci, Alberto; Bergenti, Ilaria; Borgatti, Francesco; Pernechele, Chiara; Solzi, Massimo; Pullini, Daniele; Busquets-Mataix, David Jeronimo; Dediu,
Tipo: Artigo de Revista Científica
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We report on the optimisation of the growth conditions of manganite La0.7Sr0.3MnO3 (LSMO) thin films prepared by Channel Spark Ablation (CSA). CSA belongs to pulsed electron deposition methods and its energetic and deposition parameters are quite similar to those of pulsed laser deposition. The method has been already proven to provide manganite films with good magnetic properties, but the films were generally relatively rough (a few nm coarseness). Here we show that increasing the oxygen deposition pressure with respect to previously used regimes, reduces the surface roughness down to unit cell size while maintaining a robust magnetism. We analyse in detail the effect of other deposition parameters, like accelerating voltage, discharging energy, and temperature and provide on this basis a set of optimal conditions for the growth of atomically flat films. The thicknesses for which atomically flat surface was achieved is as high as about 10-20 nm, corresponding to films with room temperature magnetism. We believe such magnetic layers represent appealing and suitable electrodes for various spintronic devices.; Comment: original paper, thin film optimization, 25 pages, 9 figures

## Large Magnetoresistance in a Manganite Spin-Tunnel-Junction Using LaMnO3 as Insulating Barrier

Yunoki, S.; Dagotto, E.; Costamagna, S.; Riera, J. A.
Tipo: Artigo de Revista Científica
Relevância na Pesquisa
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A spin-tunnel-junction based on manganites, with La$_{1-x}$Sr$_x$MnO$_3$ (LSMO) as ferromagnetic metallic electrodes and the undoped parent compound LaMnO$_3$ (LMO) as insulating barrier, is here theoretically discussed using double exchange model Hamiltonians and numerical techniques. For an even number of LMO layers, the ground state is shown to have anti-parallel LSMO magnetic moments. This highly resistive, but fragile, state is easily destabilized by small magnetic fields, which orient the LSMO moments in the direction of the field. The magnetoresistance associated with this transition is very large, according to Monte Carlo and Density Matrix Renormalization Group studies. The influence of temperature, the case of an odd number of LMO layers, and the differences between LMO and SrTiO$_3$ as barriers are also addressed. General trends are discussed.; Comment: 14 pages, 18 figures

## Growth and physical property study of single nanowire (diameter ~ 45nm) of half doped Manganite

Datta, Subarna; Chandra, Sayan; Samanta, Sudeshna; Das, K.; Srikanth, H.; Ghosh, Barnali
Tipo: Artigo de Revista Científica
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We report here the growth and characterization of functional oxide nanowire of hole doped manganite of La0.5Sr0.5MnO3 (LSMO). We also report four probe electrical resistance measurement of single nanowire of LSMO (diameter ~ 45nm) using FIB fabricated electrodes. The wires were fabricated by hydrothermal method using autoclave at a temperature of 270 oC. The elemental analysis and physical property like electrical resistivity were studied at individual nanowire level. The quantitative determination of Mn valency and elemental mapping of constituent elements was done by using Electron Energy Loss Spectroscopy (EELS) in the Scanning Transmission Electron Microscopy (STEM) mode. We addressed the important issue of whether as a result of size reduction the nanowires can retain the desired composition, structure and physical properties. The nanowires used were found to have a ferromagnetic transition (TC) at around 325 K which is very close to the bulk value of around 330 K found in single crystal of the same composition confirming that the functional behavior is likely to be retained even after size reduction of the nanowires to a diameter of 45 nm. The electrical resistivity shows insulating behavior within the temperature range measured...

## Giant Magnetoresistance in an all-oxide spacerless junction

Singh, Mangala Prasad; Carvello, Baptiste; Ranno, Laurent
Tipo: Artigo de Revista Científica
Relevância na Pesquisa
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We report the fabrication of an oxide-specific type of magnetoresistive junction, which is a ferromagnetic bilayer. Both electrodes are high spin-polarization oxides: magnetite (Fe3O4) and manganite (La0.7Sr0.3MnO3). Negligible magnetic coupling between both ferromagnetic electrodes is realised, which allows to obtain parallel and antiparallel magnetic configurations of the electrodes when sweeping the applied magnetic field. The structure exhibits negative giant magnetoresistance (GMR) at low temperatures. This negative MR shows that both electrodes stay spin-polarized at the interface and have opposite spin polarizations, i.e. the Fe3O4 layer has a negative spin polarization at low temperature. Maximum GMR (-5%) is obtained at 55K.; Comment: The following article appeared in Applied Physics Letters and may be found at http://link.aip.org/link/APPLAB/v89/p022504 . Copyright 2006 American Institute of Physics. This article may be downloaded for personal use only. Any other use requires prior permission of the author and the American Institute of Physics

## Magnetoresistance in an all-manganite heterostructure

Salafranca, J.; Calderon, M. J.; Brey, L.
Tipo: Artigo de Revista Científica