Abstract
Magnetic field penetration and magnetization hysteresis loops (MHLs) have been measured in KxFe2−ySe2 single crystals. The magnetic field penetration shows a two-step feature with a very small full-magnetic-penetration field (≈300 Oe at 2 K), and accordingly the MHL exhibits an abnormal vanishing of the central peak near zero field below 13 K. The width of the MHL in KxFe2−ySe2 at the same temperature is in general much smaller than that measured in Ba0.6K0.4Fe2As2 and Ba(Fe0.92Co0.08)2As2, and the MHLs in the latter two samples show a normal central peak near zero field. All these anomalies found in KxFe2−ySe2 can be understood in the sense that the sample is percolative with weakly coupled superconducting islands.