Efeitos de desordem química da densidade de corrente e na força de

Abstract

This work relates to the realization of an experimental study about the effects coming from the Ba partial substitution for Sr in the YBa2Cu3O7-δ superconductor upon the critical current, JC(H,T), and flux pinning mechanisms. For this purpose were grown single crystals samples of YBa2-xSrxCu3O7- with (x = 0; 0.02; 0.1; 0.25; 0.37). The crystals structure was analyzed by x-ray diffraction, (XRD), polarized light microscopy (PLM) and transmission electron microscopy (TEM) techniques. The XRD showed, for the indicated concentrations, that there are not different phases in addition to the orthorhombic structure of the YBa2Cu3O7-. The PLM and TEM images for the single crystal YBa1,75Sr0,25Cu3O7- showed the existence of a structure made by the arrangement of twins and small inclusions, possible associated by the presence of Sr atoms clusters. The DC magnetic measures as a function of the magnetic field, M(H and temperature, M(T), were performed done in an SQUID MPMS-XL magnetometer with the purpose to get the magnetic irreversibility field, Hirr(T) and critical current density, JC(H,T) that was calculated by the extended Bean model application. The behavior of the higher density pinning force, FP,Max(H,T), was obtained by the expression 𝐹𝑃 =𝐽𝐶 𝑥𝐵 . The pinning mechanisms were analyzed by the determination of the behavior of the reduced pinning force, f(h), where f(h) = FP/FP,Max and h=H/Hirr. The arrangement of the f(h) data was compared with the Dew-Hughes model scale functions. In a usual way, the results of JC(H,T) and f(h) showed that the dominant pinning mechanisms in the YBa2Cu3O7-, when the Ba was replaced by at least 18.5% for Sr, are the normal point and normal surface pinning mechanisms. Where the normal point pinning relevance was observed to x < 12.5% that coincided with the observation of the highest JC(T) values transported by 1% and 5% Sr doped samples when compared with the pure sample.