Flux pinning and phase separation in oxygen-rich La2-xSrxCuO4+y

We have studied the magnetic characteristics of a series of superoxygenated La2-xSrxCuO4+y samples. As shown in previous work, these samples spontaneously phase separate into an oxygen-rich superconducting phase with a T-C near 40 K and an oxygen poor magnetic phase that also orders near 40 K. The oxygenated samples studied are highly magnetically reversible even to low temperatures. Although the internal magnetic regions of these samples might be expected to act as pinning sites, our present study shows that they do not favor flux pinning. Flux pinning requires a matching condition between the defect size and the superconducting coherence length. Thus, our results imply that the magnetic regions are too large to act as pinning centers. This also implies that the much greater flux pinning in typical La2-xSrxCuO4 materials is the result of nanoscale inhomogeneities that grow to become the large magnetic regions in the superoxygenated materials. The superconducting regions of the phase separated materials are in that sense cleaner and more homogenous than in the typical cuprate superconductor.