Valentin Garcia-Vazquez

Selected error sources in resistance measurements on superconductors

In order to investigate the causes that produce some of the unwanted effects observed in the resistance versus temperature profiles, a variety of sources of error for resistance measurements in superconductors, using a standard four-probe configuration, have been studied. A piece of superconducting Y1Ba2Cu3O7−x ceramic material has been used as the test sample, and the resulting effects in both accuracy and precision in its temperature dependent resistance are reported here. Studied measurement error sources include thermal emf’s, temperature sweep rates, Faraday currents, electrical-contact failures at the sample’s surface, thermal contractions at mechanically attached instrumental wires, external electromagnetic fields, and slow sampling rates during data acquisition. Details of the experimental setup and its measurement error function are also given.

Mechanosynthesis of MFe2O4 (M = Co, Ni, and Zn) Magnetic Nanoparticles for Pb Removal from Aqueous Solution

Adsorption of Pb(II) from aqueous solution using MFe2O4 nanoferrites (M = Co, Ni, and Zn) was studied. Nanoferrite samples were prepared via the mechanochemical method and were characterized by X-ray powder diffraction (XRD), Fourier transform infrared spectroscopy (FTIR), micro-Raman, and vibrating sample magnetometry (VSM). XRD analysis confirms the formation of pure single phases of cubic ferrites with average crystallite sizes of 23.8, 19.4, and 19.2 nm for CoFe2O4, NiFe2O4, and ZnFe2O4, respectively. Only NiFe2O4 and ZnFe2O4 samples show superparamagnetic behavior at room temperature, whereas CoFe2O4 is ferromagnetic. Kinetics and isotherm adsorption studies for adsorption of Pb(II) were carried out. A pseudo-second-order kinetic describes the sorption behavior. The experimental data of the isotherms were well fitted to the Langmuir isotherm model. The maximum adsorption capacity of Pb(II) on the nanoferrites was found to be 20.58, 17.76, and 9.34 mg·g−1 for M = Co, Ni, and Zn, respectively.

Perpendicular magnetic anisotropy in composite MgO/CoFeB/Ta/[Co/Pd]n structures

The impact of a non-magnetic Ta spacer layer on the perpendicular magnetic anisotropy (PMA) of composite magnetic structures constituted by ultra-thin Co/Pd multilayers (MLs) and MgO/CoFeB was studied. Composite structures lacking a Ta layer present in-plane magnetic anisotropy. The strong perpendicular anisotropy observed in sole Co/Pd MLs is not sufficient to pull the magnetic moment out of the film plane, not even after annealing at 300 or 350 °C. PMA with squareness values close to unity and annealing stability up to 350 °C is observed after the insertion of an ultra-thin Ta layer. Our study demonstrates that Ta layer is essential for obtaining perpendicular magnetic axis in MgO/CoFeB/Ta/[Co/Pd]6. The exchange coupling between the MgO/CoFeB bilayer and the Co/Pd MLs is ferromagnetic with sharp switching characteristics. Perpendicular composite structures with sharp magnetization reversal and annealing stability are relevant in perpendicular CoFeB-based magnetic tunnel junctions for the development of gigabit-scale nonvolatile memory.

Biased four-point probe resistance

The implications of switching the current polarity in a four-point probe resistance measurement are presented. We demonstrate that, during the inversion of the applied current, any change in the voltage V produced by a continuous drop of the sample temperature T will induce a bias in the temperature-dependent DC resistance. The analytical expression for the bias is deduced and written in terms of the variations of the measured voltages with respect to T and by the variations of T with respect to time t. Experimental data measured on a superconducting Nb thin film confirm that the bias of the normal-state resistance monotonically increases with the cooling rate dT/dt while keeping fixed dV/dT; on the other hand, the bias increases with dV/dT, reaching values up to 13% with respect to the unbiased resistance obtained at room temperature.

Study on the structural and magnetic properties of Ru 1−x Mo x Sr 2 GdCu 2 O 8±z system

The structural and magnetic properties of Ru_1-xMo_xSr₂GdCu₂O₈ with 0.0 ≤ x ≤ 0.50 were investigated through Mo substitution in the Ru sites. Samples were synthesized by solid-state reaction at ambient pressure at temperature between 960 °C and 1060 °C. X-ray diffraction results indicate that the partial substitution of Ru by Mo within the studied level of doping takes place isostructurally in a tetragonal structure (space group P4/mmm) with a solubility up to x = 0.50. Rietlveld- refinement analysis indicates that the Cu-O(2) and Ru-O(3) bond lengths increase with Mo content, while the Cu-O(1) and Ru-O(1) bond lengths, as well as Cu-O(1)-Ru bond angles decrease with increasing x with a minimum at approximately x = 0.30, and then increase for x > 0.30 resulting in inverse bell-type curves. Results of the DC magnetic susceptibility measurements on samples indicate that all samples exhibit ferromagnetic ordering, with T^Curie values depend on Mo content with a dependence that seems to be similar to Cu-O(1)-Ru bond angle. These results suggest a relationship between the structural and magnetic properties in the Ru_1-xMo_xSr₂GdCu₂O₈ system.

Study on the Structural, Electrical and Magnetic Properties of the

Samples of the Ru(Sr2-xBax)GdCu2O8±z system, with x = 0.00, 0.05, 0.10, 0.15, 0.20, 0.25 and 0.50 were prepared through solid-state reaction at ambient pressure at 980-1025°C. From the X-ray diffraction (XRD) patterns, it was observed that the substitution of Sr by Ba takes place iso-structurally into a tetragonal structure (space group P4/mmm). Rietveld refinement analysis indicates that both Cu-O(1) and Ru-O(1) bond lengths strongly depend on Ba content. The temperature dependence of the electrical resistance for the samples with 0.0 ≤ × ≤ 0.25 annealed in flowing oxygen at 960°C for 2 hrs. show a semiconducting behavior, whereas samples annealed during 30 hrs. present a semiconducting-to-metal transition. Results of the DC magnetic susceptibility measurements on samples annealed for 2 hrs. indicate that all samples exhibit ferromagnetic ordering, with magnetic transition temperature TCurrie that depend on Ba content with a dependence that seems to be similar to one found for the bond lengths.