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Cerâmicos magnetoresistivos à base de manganês -La1-xSrxMnO3

Sousa, Patrick Manuel Fernandes de
Fonte: Universidade de Aveiro Publicador: Universidade de Aveiro
Tipo: Dissertação de Mestrado
POR
Relevância na Pesquisa
26.34%
Durante os últimos anos tem-se desenvolvido um interesse crescente em materiais capazes de apresentar várias ordens ferróicas, isto é, capazes de apresentar simultaneamente uma ordem ferro ou antiferroeléctrica e também ferro ou antiferromagnética. Estes materiais têm aplicações potencialmente muito interessantes na recente electrónica de spin, denominada de spintrónica, em dispositivos (filtros de spin e junções de efeito de túnel) para memórias magnéticas não-voláteis (MRAMs, do acrónimo em Inglês, Magnetoresistive Random Access Memory). As junções de efeito de túnel são constituídas por eléctrodos ferromagnéticos com comportamento metálico separados por uma barreira isoladora aplicada sob a forma de filmes finos em heteroestruturas epitaxiais. Por estes aspectos, uma investigação sistemática destes materiais quer como cerâmicos para posterior utilização como alvos para deposição de filmes, quer como filmes é necessária e oportuna. As perovesquites à base de manganês La1-xAxMnO3 (A = Ca, Ba, ou Sr) são importantes para uma variedade de aplicações industriais e atraem uma enorme atenção devido à muito elevada magnetoresistência, consequência da mudança de muitas ordens de magnitude da resistividade eléctrica com a aplicação de um pequeno campo magnético...

Properties of Ca1−xHoxMnO3 perovskite-type electrodes

Ferreira, B M; Melo Jorge, M E; Lopes, M E; Nunes, M R; Silva Pereira, M I
Fonte: Elsevier Publicador: Elsevier
Tipo: Artigo de Revista Científica
ENG
Relevância na Pesquisa
36.55%
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.

Preparation and characterization of Ca1-xCexMnO3 perovskite electrodes

Lucas, C.; Eiroa, I.; Nunes, M.R.; Russo, P.A.; Ribeiro Carrott, M.M.L.; Silva Pereira, M.I.; Melo Jorge, M.E.
Fonte: Springer Publicador: Springer
Tipo: Artigo de Revista Científica
ENG
Relevância na Pesquisa
36.43%
The electrochemical properties of Ca1−xCexMnO3 perovskite-type oxide electrode have been investigated by cyclic voltammetry in Na2SO4 aqueous solutions with pH 14. The structural and morphological characterizations have also been investigated and the information used to interpret the electrochemical behavior. An estimation of the electrode’s capacitance and roughness factor has been obtained by means of cyclic voltammetry. The specific capacitance and consequently the roughness factor values are affected by the presence of Ce ions in the oxide. These findings are in agreement with the increase of the oxide-specific surface area by the introduction of Ce ion. The open-circuit potential and the voltammetric patterns are dependent on the presence of Ce ion in the electrodes and support that the surface electrochemistry of the perovskite oxide electrodes is governed by the Mn4+–Mn3+ redox couple.

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
Fonte: Elsevier Publicador: Elsevier
Tipo: Artigo de Revista Científica
Publicado em //2014 ENG
Relevância na Pesquisa
36.28%
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.

Spin polarised injection in sexithienyl thin films

Dediu, V.; Murgia, M.; Matacotta, F. C.; Barbanera, S.; Taliani, C.
Fonte: Universidade Cornell Publicador: Universidade Cornell
Tipo: Artigo de Revista Científica
Publicado em 10/04/2001
Relevância na Pesquisa
16.1%
Electroluminescence in organic semiconductors strongly depends on the relative population of excited singlet and triplet excitonic states resulting from recombination of injected carriers. In conventional organic light-emitting diodes (OLED) optical emission is usually based on fluorescence from excited singlet states1, although phosphorescence from triplets is also investigated2. Governing the spin statistics of the injected carriers would open the way to amplify a chosen electronic transition increasing therefore the OLED efficiency. Direct injection of carriers with high degree of spin polarisation along a given axis should lead to preferential population of either singlet or triplet excited states, depending on the relative electrodes polarisation. Here we report a first experimental evidence of direct spin polarised (SP) injection in sexithienyl (T6), a prototypical organic semiconductor, from half-metallic manganites. The spin penetration depth in T6 is about 250-300 nm at room temperature. The results are discussed taking into account possible spin-flip mechanisms in organic material and interface effects.; Comment: 6pages, 2 figures

Large domain wall resistance in self-organised manganite film

Granja, L.; Hueso, L. E.; Sanchez, D.; Prieto, J. L.; Levy, P.; Mathur, N. D.
Fonte: Universidade Cornell Publicador: Universidade Cornell
Tipo: Artigo de Revista Científica
Relevância na Pesquisa
16.1%
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

Oxide spintronics

Bibes, Manuel; Barthelemy, Agnes
Fonte: Universidade Cornell Publicador: Universidade Cornell
Tipo: Artigo de Revista Científica
Publicado em 20/06/2007
Relevância na Pesquisa
16.51%
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

Polarons, free charge localisation and effective dielectric permittivity in oxides

Maglione, Mario
Fonte: Universidade Cornell Publicador: Universidade Cornell
Tipo: Artigo de Revista Científica
Publicado em 18/06/2010
Relevância na Pesquisa
16.1%
This review will deal with several types of free charge localisation in oxides and their consequences on the effective dielectric spectra of such materials. The first one is the polaronic localisation at the unit cell scale on residual impurities in ferroelectric networks. The second one is the collective localisation of free charge at macroscopic interfaces like surfaces, electrodes and grain boundaries in ceramics. Polarons have been observed in many oxide perovskites mostly when cations having several stable electronic configurations are present. In manganites, the density of such polarons is so high as to drive a net lattice of interacting polarons. On the other hand, in ferroelectric materials like BaTiO3 and LiNbO3, the density of polarons is usually very small but they can influence strongly the macroscopic conductivity. The contribution of such polarons to the dielectric spectra of ferroelectric materials is described. Even residual impurities as for example Iron can induce well defined anomalies at very low temperatures. This is mostly resulting from the interaction between localised polarons and the highly polarisable ferroelectric network in which they are embedded. The case of such residual polarons in SrTiO3 will be described in more details...

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

Yunoki, S.; Dagotto, E.; Costamagna, S.; Riera, J. A.
Fonte: Universidade Cornell Publicador: Universidade Cornell
Tipo: Artigo de Revista Científica
Publicado em 06/02/2008
Relevância na Pesquisa
16.1%
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

Spin-dependent transport in oxide-based tunnel junctions

Galceran Vercher, Regina; Sort Viñas, Jordi
Fonte: [Barcelona] : Universitat Autònoma de Barcelona, Publicador: [Barcelona] : Universitat Autònoma de Barcelona,
Tipo: Tesis i dissertacions electròniques; info:eu-repo/semantics/doctoralThesis; info:eu-repo/semantics/publishedVersion
Publicado em //2015 ENG
Relevância na Pesquisa
16.43%
Aquesta tesi estudia les propietats de magnetotransport en unions túnel on un dels elèctrodes és l'òxid ferromagnètic La0.7Sr0.3MnO3 (LSMO). En concret, ens interessem per dos fenòmens diferents: (i) magnetoresistència (MR) en unions túnel amb un sol elèctrode magnètic i (ii) filtratge d'espí en unions túnel magnètiques. L'efecte túnel és extremadament dependent de les interfícies i una bona qualitat de les heteroestructures resulta crucial per a obtenir un bon rendiment dels dispositius. És per aquest motiu que bona part d'aquesta tesi es dedica al creixement (per polvorització catòdica) i caracterització de capes primes, a l'estudi de les interfícies de les heterostructures i a la fabricació de les unions. Pel que fa a les unions amb un únic elèctrode magnètic, ens centrem en l'estudi del transport túnel en funció de la temperatura i del camp magnètic aplicat en unions de Pt/LaAlO3/LSMO. En el nostre treball hem identificat diferents mecanismes físics que juguen un paper important en la MR d'aquest sistema: la magnetoresistència túnel anisòtropa (TAMR), de l'ordre de 4 % a baixa temperatura, i una altra contribució a la magnetoresistència, de l'ordre de 17%. A més, TAMR a baix camp magnètic s'atribueix a la rotació de dominis magnètics. En el cas d'unions amb dos elèctrodes magnètics...