Magnetic-field-induced enhancement of the vortex pinning in the overdoped regime of La _2-xSr _xCuO ₄: Relation to the microscopic phase separation

In order to investigate the inhomogeneity of the superconducting (SC) state in the overdoped high-T _c-cuprates, we have measured the magnetic susceptibility, χ, of La _2-x,Sr _xCuO ₄ (LSCO) single crystals in the overdoped regime in magnetic fields parallel to the c-axis up to 7 T on warming after zero-field cooling. It has been found for x = 0.198 and 0.219 that the temperature dependence of χ in 1 T shows a plateau, that is, χ is almost independent of temperature in a moderate temperature-range in the SC state. Moreover, a so-called second peak in the magnetization curve has markedly appeared in these crystals. These results indicate an anomalous enhancement of the vortex pinning and strongly suggest the occurrence of a microscopic phase separation into SC and normal-state regions in the overdoped high-T _c cuprates.