Metastability across the peak position of the peak effect in weakly pinned vortex matter

The peak positions of the classical peak effect (CPE) or the second magnetization peak (SMP) have often been considered to mark the limiting fields (or, temperatures), below which the supercooling effects occur. This notion holds so long as the SMP and the CPE remain distinct. The circumstances in which the two anomalies overlap, the peak position of the composite anomaly need not represent the limiting value, above which the metastability effects would be absent. On the basis of explorations done to study metastability effects in weakly pinned single crystals of 2H-NbSe2 and CeRu2 we propose a possible recipe to mark the spinodal line using ac susceptibility measurements, where one looks at the third harmonic of the real part of ac susceptibility. (author)