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Navarro, H., M. Sirena, J. González Sutter, H. E. Troiani, P. G. del Corro, P. Granell, F. Golmar, and N. Haberkorn. "Electrical transport across nanometric SrTiO3 and BaTiO3 barriers in conducting/insulator/conducting junctions." Materials Research Express 5, no. 1 (2018): 016408.
Abstract: We report the electrical transport properties of conducting/insulator/conducting heterostructures by studying current–voltage IV curves at room temperature. The measurements were obtained on tunnel junctions with different areas (900, 400 and 100 μ m 2 ) using a conducting atomic force microscope. Trilayers with GdBa 2 Cu 3 O 7 (GBCO) as the bottom electrode, SrTiO 3 or BaTiO 3 (thicknesses between 1.6 and 4 nm) as the insulator barrier, and GBCO or Nb as the top electrode were grown by DC sputtering on (100) SrTiO 3 substrates For SrTiO 3 and BaTiO 3 barriers, asymmetric IV curves at positive and negative polarization can be obtained using electrodes with different work function. In addition, hysteretic IV curves are obtained for BaTiO 3 barriers, which can be ascribed to a combined effect of the FE reversal switching polarization and an oxygen vacancy migration. For GBCO/BaTiO 3 /GBCO heterostructures, the IV curves correspond to that expected for asymmetric interfaces, which indicates that the disorder affects differently the properties at the bottom and top interfaces. Our results show the role of the interface disorder on the electrical transport of conducting/insulator/conduction heterostructures, which is relevant for different applications, going from resistive switching memories (at room temperature) to Josephson junctions (at low temperatures).
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Curiale, J., R. D. Sánchez, H. E. Troiani, C. A. Ramos, H. Pastoriza, A. G. Leyva, and P. Levy. "Magnetism of manganite nanotubes constituted by assembled nanoparticles." Phys. Rev. B 75 (2007): 224410.
Abstract: We present the study of the magnetic properties of manganite nanotubes with 800 nm of external diameter and 3-5 μm length. The study includes two ferromagnetic compounds, La0.67Ca0.33MnO3 and La0.67Sr0.33MnO3, and one with ferromagnetic and charge ordered phase coexistence, La0.325Pr0.300Ca0.375MnO3. In this case, the walls are formed by grains and have approximately thickness of 150 nm and the most probable grain size is 70 nm. For the ferromagnetic nanotubes smaller values are observed, between 45 and 60 nm for the thickness and 25 nm for the grain size, respectively. In La0.325Pr0.300Ca0.375MnO3 we observe distinctive features comparing with the ferromagnetic nanotubes. After cooling below T$_C$ the samples at $H=0$, the first magnetization curve shows a gradual growth of the magnetization until reach a maximum value, which depends of H. After this, the material presents expected ferromagnetic hysteresis loops. Also, significant differences between the cooling and warming magnetization processes and large linewidth in the ferromagnetic resonances are observed. These experimental facts are explained in terms of the competition between the charge ordered and the ferromagnetic phases, including strong interaction among these last regions. Observations of dipolar interactions between the magnetic grains are evidenced by isothermal remanent magnetization and direct current demagnetization experiments.
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Curiale, J., R. D. Sánchez, H. E. Troiani, H. Pastoriza, P. Levy, and A. G. Leyva. "Morphological and magnetic characterization of manganites oxide based nanowires and nanotubes." Physica B 354, no. 1-4 (2004): 98–103.
Abstract: A morphological and magnetic study of sub-micrometric nanostructures of a manganese oxide-based compound La0.325Pr0.300Ca0.375MnO3 is presented. Nanowires of 50 and 100 nm and tubes with external diameters of 200 and 800 nm were synthesized. Scanning electron microscopy with EDS microanalysis (SEM) and transmission electron microscopy (TEM) allowed to characterize the morphology and microstructure of these nanostructures. The walls of the nanotubes are composed of on average 40 nm grain size particles, while in nanowires the grains are much smaller, in the range between 4 and 20 nm. In both situations, the small grains present a random crystalline orientation, as it is confirmed by TEM diffraction experiments. Magnetization measurements and hysteresis loops are presented and discussed based on magnetic interaction between the grains, anisotropy considerations and reversal magnetization mechanisms.
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Haberkorn, N., J. Guimpel, S. Suárez, H. E. Troiani, P. Granell, F. Golmar, J. - H. Lee, S. H. Moon, and H. Lee. "Strong influence of the oxygen stoichiometry on the vortex bundle size and critical current densities J c of GdBa 2 Cu 3 O x -coated conductors grown by co-evaporation." Superconductor Science and Technology 30, no. 9 (2017): 095009.
Abstract: We report on the influence of oxygen stoichiometry on the vortex creep mechanism of GdBa 2 Cu 3 O x -coated conductors produced by co-evaporation. The oxygen stoichiometry of the films, x , was modified in a controlled way between 6.85 and 7, which resulted in systematic and reversible control of the superconducting critical temperature between about 78 and 93 K. The change in the oxygen stoichiometry produces a strong reduction in the self-field critical current densities J c without significantly modifying the power-law dependence at intermediate magnetic fields, which indicates a negligible contribution of oxygen vacancies to the pinning. In addition, the characteristic glassy exponent μ shows a systematic diminution from about 1.63 at x = 7 to about 1.12 at x = 6.85. The results are compared with those obtained for proton- and oxygen-irradiated films, in which the vortex dynamics is determined by a balance between the improved pinning, originating from nanocluster inclusion, and the suppressed superconducting properties due to disorder in the nanoscale.
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