Critical fields and the critical current density of La1.85Sr

A detailed investigation of the magnetic and transport properties of La/sub 1.85/Sr/sub 0.15/CuO/sub 4/ in the superconducting state is presented. It is concluded that the magnetic and transport behavior is that of a granular type-II superconductor in which strongly superconducting grains are coupled via weak superconducting links. Five characteristic fields are identified. H/sub c//sub 1//sup w/ (approx. =1 Oe at 4 K) is the field at which the first vortex line penetrates the weak links. At H = H/sup w/ (approx. =11 Oe at 4.2 K) the London penetration depth in the weak links is very large compared to their dimensions. H/sub c//sub 1//sup g/ (approx. =300 Oe at 4.2 K) defines the first flux thread entering the grains themselves. H/sup g/ is the value above which the magnetic behavior of the grain is thought to be governed by Abrikosov's negative surface energy model. The critical current density measured by the resistivity technique is about 8 A cm/sup 2/ and is imposed by the sample size and compositional inhomogeneities and by H/sub c//sub 1//sup w/, whereas the bulk (or intragrain) critical density could attain 10/sup 5/--10/sup 6/ A/cm/sup 2/.