Crossing point of the magnetization versus temperature curves and the Meissner fraction in granular La1.9Sr0.1CuO4 superconductors: Random orientation and inhomogeneity effects

In this paper, we first calculate the extrinsic random orientation effects on the Meissner fraction in granular layered superconductors with highly an isotropic single crystal grains. Then we present detailed measurements of t he crossing point of the excess magnetization versus temperature curves, De lta M(T)(H), in the reversible mixed state and of the Meissner fraction in two granular La1.9Sr0.1CuO4 samples, before and after grain alignment. The analyses of these experiments on the grounds of the calculations indicated above allow us to disentangle the different extrinsic effects on the excess magnetization at the crossing point, Delta M*, and on the field-cooled sus ceptibility at the crossing point temperature, chi(FC)(T*): random orientat ion, demagnetization, and inhomogeneities. We show then that once these two measured observables are adequately and separately corrected from random o rientation and demagnetizing effects, the remaining extrinsic effects on De lta M* (associated with structural and, mainly, stoichiometric inhomogeneit ies) may be tal;en into account through chi(FC)(T*), This seems to apply at least in samples with relatively good magnetic response [let us say, with \chi(ab)(FC)(T*)\ greater than or similar to 0.2]. The resulting intrinsic crossing point may be explained in terms of the theoretical approaches prop osed by Bulaevskii and co-workers and by Tesanovic and co-workers and based on thermal fluctuations of vortices. These last results extend to granular high-temperature superconductors (HTSC) our recent conclusions for highly anisotropic single crystalline HTSC. [S0163-1829(99)09405-9].