The weakly pinned single crystals of the hexagonal 2H-NbSe₂ compound have emerged as prototypes for determining and characterizing the phase boundaries of the possible order-disorder transformations in the vortex matter. We present here a status report based on the ac and dc magnetization measurements of the peak effect phenomenon in three crystals of 2H-NbSe₂, in which the critical current densities vary over two orders of magnitude. We sketch the generic vortex phase diagram of a weakly pinned superconductor, which also utilizes theoretical proposals. We also establish the connection between the metastability effects and pinning.

We have studied the peak effect (PE) phenomenon in single crystals of weakly pinned superconductors CeRu₂ and 2H-NbS₂. 2H-NbS₂ is iso-structural and iso-electronic to 2H-NbSe₂, whose similarity with CeRu₂ as regards the PE representing the order-to-disorder transformation of the flux line lattice was claimed some time ago. We report on the step change in equilibrium magnetization across the peak effect in CeRu₂. We also present the vortex phase diagram of 2H-NbS₂ obtained from the magnetization data, and compare the PE phenomenon in 2H-NbS₂ and 2H-NbSe₂.

A comparative study of the thermomagnetic memory effects of J_c in two weakly pinned low T_c superconductors, Ca₃Rh₄Sn₁₃ (CaRhSn) and Yb₃Rh₄Sn₁₃ (YbRhSn), is presented. In both the systems, the peak effect (PE) phenomenon appears as an order-disorder transformation through stepwise amorphization of the flux line lattice (FLL). However, in CaRhSn. we can witness another disorder-driven transition (Bragg glass (BG) to a vortex glass (VG) in a distinct manner as in a single crystal of high T_c YBa₂Cu₃O_7-δ for H‖c.