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Superconductivity, Spintronics and Surface Science Center

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Representative Recent Publications
 
   
  • T. Petrisor Jr., R. B. Mos, M. Nasui, M. S. Gabor, A. Augieri, G. Celentano, D. De Felicis,  E. Bemporad,  L. Ciontea, T. Petrisor, "The Vortex Path Model Analysis of the Field Angle Dependence of the Critical Current Density in Nanocomposite YBa2Cu3O7−x– BaZrO3 Films Obtained by Low Fluorine Chemical Solution Deposition", J. Supercond. Nov. Magn. (2014) 27:2493–2500; DOI 10.1007/s10948-014-2712-z

Abstract
In the present paper, we analyze the role of in situ grown BaZrO3 (BZO) inclusions in YBa2Cu3O7−x (YBCO) thin films prepared by chemical solution deposition using a low fluorine coating solution, on the field angle dependence of the critical current density, Jc(θ), data using the vortex path model. In order to form a coherent picture on the BZO doping influence on the pinning properties of the YBCO matrix, detailed structural analyses performed by Xray diffraction techniques and microstructural evaluation by transmission electron microscopy are also presented. The evaluation of different contributions to the overall, Jc, permitted us to prove the effectiveness of the BZO inclusions acting as isotropic pinning centers, reflected in a uniform component of high relative value with respect to other components. For the studied 10 mol% BZO doping concentration, a threefold increase in the critical current density, Jc, of the YBCO host is measured, in self-field at 77 K, corresponding to a value of Jc = 2.9MA/cm2, whereas a factor 10 is measured at 1 T (Jc = 0.35 MA/cm2).


     
  • M.S. Gabor, C. Tiusan, T. Petrisor Jr.,T. Petrisor., M. Hehn,Y. Lu, E. Snoeck, "Structural defects analysis versus spin polarized tunneling in Co2FeAl/MgO/CoFe magnetic tunnel junctions with thick MgO barriers", J. Magn. Magn. Matter. 347, 79–85, (2013), DOI information: 10.1016/j.jmmm.2013.07.028.
Abstract
We report on spin polarization reduction by incoherent tunneling in single crystal Co2FeAl/MgO/Co50Fe50 magnetic tunnel junctions (MTJs). A large density of misfit dislocations in the Heusler based MTJs has been provided by a thick MgO barrier and its 3.8% lattice mismatch with the Co2FeAl electrode. Our analysis implicates a correlated structural-transport approach. The crystallographic coherence in the real space has been investigated by High Resolution Transmission Electron Microscopy phase analysis. The electronic transport experiments in variable temperature, fitted with a theoretical extended-Glazman–Matveev model, address different levels of the tunneling mechanisms from direct to multi-center hopping. We demonstrate a double impact of dislocations, as extended defects, on the tunneling polarization. Firstly, the breaking of the crystal symmetry destroys the longitudinal and lateral coherence of the propagating Bloch functions. This affects the symmetry filtering efficiency of the Δ1Δ1 states across the (001) MgO barriers and reduces the associated effective tunneling polarization. Secondly, dislocations provide localized states within the MgO gap. This determines temperature activated spin-conserving inelastic tunneling through chains of defects which are responsible for the one order of magnitude drop of the tunnel magnetoresistance from low to room temperature.

Fig. 7 a) HRTEM image of the CFA/MgO/CoFe trilayer and (b) corresponding phase image; (c) HRTEM image of the Fe/MgO/Fe trilayer and (d) corresponding phase image; (e) and (f) schematic representation of the misfit dislocations inside the MgO barrier as resulted from (b) and (d), the coherent (CH1) and incoherent (CH2) conduction channels were also indicated; (g) and (h) bias voltage dependence of differential tunneling conductance dI/dV curves for the CFA/MgO/CoFe and Fe/MgO/Fe MTJs, respectively.