The second peak effect observed in (La1-xSrx)(2)CuO4-delta single crystals

The oxygen deficiency, delta, dependence of the second peak effect in the magnetization hysteresis curves was investigated on (La1-xSrx)(2)CuO4-delta single crystals with various delta at T/T-c,=0.6. When the magnetic field was applied parallel to the c-axis, the second peak field, B-2pk, shifted towards higher fields with decreasing delta. The magnetic hysteresis width was suppressed below B-2pk with decreasing delta, while little delta dependence was detected above B-2pk. The current dependence of the flux potential barrier evaluated from the magnetic relaxation at T/T-c=0.7 changed its slope, \partial derivative U/partial derivative J\, drastically around B-2pk. The results indicate a crossover in the creep mechanism around B-2pk.