Search

Mechanical control of electrical properties in complex heterostructures, consisting of magnetic FeOx nanoparticles on top of manganite films, is achieved using atomic force microscope (AFM) based methods. Under applied pressure of the AFM tip, drop of the electrical conductivity is observed inducing an electrically insulating state upon a critical normal load. Current and surface potential maps suggest that the switching process is mainly governed by the flexoelectric field induced at the sample surface. The relaxation process of the electrical surface potential indicates that the diffusion of oxygen vacancies from the bulk of the manganite films towards the sample surface is the dominant relaxation mechanism. The magnetic FeOx nanoparticles, staying attached to the sample surface after the rubbing, protect the underlying manganite films and provide stability of the observed resistive switching effect. The employed mechanical control gives a new freedom in the design of resistive switching devices since it does not depend on the film thickness, and biasing is not needed.

In this work we report on the epitaxial growth of La2NiMnO6 double perovskite thin films on top of (001) oriented SrTiO3 substrates by RF magnetron sputtering. The influence of oxygen pressure (PO2 ) and growth temperature on the microstructure, stoichiometry of the films, and magnetic and transport properties is thoroughly investigated. It is found that high oxygen pressure promotes the growth of stoichiometric films, with a Ni/Mn ratio almost equal to 1. However, these films exhibit poor ferromagnetic properties with respect to the expected optimum values corresponding to ferromagnetic ordering mediated by superexchange interaction between Mn4+ and Ni2+ according to the Goodenough-Kanamori rules. Most interestingly, films grown at low PO2 exhibit Ni/Mn ratios below 1, but ferromagnetic properties close to the optimal ones. The valence balance between Ni and Mn ions in nonstoichiometric sample has been elucidated by X-ray absorption spectroscopy. The results indicate that Ni deficiency plays a crucial role in the puzzling insulating ferromagnetic behavior observed in nonstoichiometric samples.

Los dimorfos barbosalita y lipscombita, idealmente Fe2+Fe3+ 2(PO4)2(OH)2, se presentan en la pegmatita El Criollo (64º39¿9¿¿W, 31º21¿28¿¿S), Cerro Blanco, Córdoba, Argentina. En una de las zonas intermedias de esta pegmatita se encuentra un nódulo de triplita bordeado discontinuamente por la asociación portadora de barbosalita y lipscombita. La paragénesis primaria consiste en cuarzo con cristales euhedrales de flúor-apatita, zircón y sulfuros intersticiales (pirita>>calcopirita). La apatita está corroída y los sulfuros están reemplazados por una masa negra verdosa compuesta por granos anhedrales de barbosalita y lispcombita con numerosas inclusiones de fluorita, hematita y escasa fosfosiderita. Los parámetros de celda, obtenidos mediante refinamiento Rietveld, son (en Å): barbosalita (grupo espacial P21/n): a=7.326(2), b=7.446(2), c=7.401(2), ß=118.79(2)º; lipscombita (grupo espacial P43212): a=b=7.388(2), c=12.924(6). Análisis químicos de la barbosalita y lipscombita (microsonda de electrones, n=4) dieron (% en peso) P2O5: 35.72, SiO2: 0.07, FeO: 17.98, Fe2O3: 39.96, MnO: 0.40, Al2O3: 0.22, Na2O: 0.18, CaO: 0.12, MgO: 0.01, F: 0.27, H2O (calculado como 2-F): 4.45, Cl no detectado, total corregido por F=O (99.27). La cantidad de FeO y Fe2O3 se calculó para dar una relación Fe2+: Fe3+ de 1:2 en la fórmula estructural. En general, los fosfatos secundarios en estas pegmatitas indican ambiente oxidante y ácido y un fuerte control de la mineralogía primaria, subrayando la importancia del ambiente a escala muy local en las asociaciones de especies secundarias. Los minerales que cristalizan están muy influenciados por los aspectos cinéticos de las reacciones de disolución y precipitación, y no sólo por las configuraciones estructurales termodinámicamente estables. Las fases secundarias con H2O indican que la alteración continuó hasta temperaturas por debajo de los 250ºC, The dimorphs lipscombite and barbosalite, ideally Fe2+Fe3+ 2(PO4)2(OH)2, occur in El Criollo pegmatite (64º39¿9¿¿W, 31º21¿28¿¿S), Cerro Blanco, Córdoba, Argentina. In one of the intermediate zones of this pegmatite there is a nodule of triplite discontinuously rimmed by the lipscombite-barbosalite-bearing assemblage. The primary paragenesis consists of quartz with euhedral fluorapatite crystals, zircon, and interstitial sulfides (pyrite>>chalcopyrite). Apatite is corroded and sulfides have been replaced by a greenish black mass of anhedral grains of lipscombite and barbosalite, with myriads of inclusions of fluorite, hematite, and scarce phosphosiderite. Cell parameters, obtained by Rietveld refinement, are (in Å): barbosalite (space group P21/n): a=7.326(2), b=7.446(2), c=7.401(2), ß=118.79(2)º; lipscombite (space group P43212): a=b=7.388(2), c=12.924(6). Chemical analyses of barbosalite and lipscombite (electron microprobe, n=4) gave (wt. %) P2O5: 35.72, SiO2: 0.07, FeO: 17.98, Fe2O3: 39.96, MnO: 0.40, Al2O3: 0.22, Na2O: 0.18, CaO: 0.12, MgO: 0.01, F: 0.27, H2O (calculated as 2-F): 4.45, Cl not detected, total corrected for F=O (99.27). The amounts of FeO and Fe2O3 were calculated to give a Fe2+:Fe3+ ratio of 1:2 in the structural formula. In general, secondary phosphates in these pegmatites indicate an oxidizing and acidic environment and a strong control of the primary mineralogy, stressing the importance of the environment at a very local scale in determining the secondary species assemblage. Crystallizing minerals are very influenced by kynetic aspects of the dissolution and precipitation reactions, and not only by the thermodynamically stable structural configurations. Secondary phases containing H2O indicate that alteration continued to temperatures below 250ºC