AVALANCHELIKE MAGNETIC-RELAXATION IN THE PEAK-EFFECT REGIME OF A NB-OSOLID-SOLUTION

An increase of the pinning of vortices near the upper critical field b oundary, the peak effect (PE), is studied in a Nb-O solid solution by means of the magnetization measurements. The PE onset is understood as a disorder-induced transition between a relatively ordered vortex lat tice and an entangled vortex state. It is found that a characteristic feature of the vortex state in the PE region is a relaxation occurring via a large size magnetization jump or avalanche. It is also found th at the temperature and field-dependent time t(j)(H,T), at which the ju mp occurs, quasi diverges at the peak field H-p(T).