H-T PHASE-DIAGRAM OF NB-O SOLID-SOLUTION

Measurements of the magnetization as a function of temperature T, magn etic field H, and time are performed in Nb-O solid solution having T-c = 6.8 K. Two ''irreversibility lines'' are found in the H-T phase dia gram: one of those lies well below the H-c2(T) boundary, while the oth er is situated very close to H-c2(T). In both pinned vortex state regi ons Kohlrausch magnetic relaxation is observed, implying the occurrenc e of a strongly disordered vortex matter. The reentrant irreversible b ehavior of the magnetization close to H-c2(T) could be understood in t erms of a ''peak effect''. Similarity between vortex behavior in Nb-O and heavy-fermion systems is pointed out.