OBSERVATION OF THE PEAK EFFECT AT HIGH FIELDS IN SINGLE-CRYSTALLINE YBA(2)CU(4)0(8)

The peak effect in the magnetic hysteresis of an untwinned YBa2Cu4O8 s ingle crystal has been measured at temperatures down to 5 K, using a h igh sensitivity susceptometer in pulsed magnetic fields up to 50 T. Th e peak effect is due to an increase of the flux pinning as the magneti c field increases and results in a magnetisation hysteresis loop in th e shape of a fishtail. The observed peak in the M(H) curve shows a cle ar temperature dependence in that the field B-max at which the peak oc curs increases with decreasing temperature.