The superconducting transition of the ?-pyrochlore oxide KOs2O6 with Tc =9.60 K is studied by resistivity measurements under various magnetic fieldsusing a high-quality single crystal. The reentrant behavior ofsuperconductivity is observed near Tc in low magnetic fields below 2 T. Therecovered resistance probably due to flux flow exhibits an anomalous angledependence, indicating that flux pinning is enhanced in magnetic fields alongcertain crystallographic directions such as [110], [001] and [112]. It issuggested that there is an intrinsic pinning mechanism coming from the specificcrystal structure of the ?-pyrochlore oxide.
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