Effect of magnetic impurities on the vortex lattice properties in NbSe₂ single crystals

We report a pronounced peak effect in the magnetization of Co_xNbSe₂ single crystals with critical temperatures T_c ranging between 7.1 and 5.0 K, and Mn_xNbSe₂ single crystals with critical temperatures down to 3.4 K. We correlate the peak effect in magnetization with the structure of the vortex lattice across the peak-effect region using scanning-tunneling microscopy. Magnetization measurements show that the amplitude of the peak effect in the case of Co_xNbSe₂ exhibits a nonmonotonic behavior as a function of the Co content, reaching a maximum for concentration of Co of about 0.4 at. % (corresponding to a T_c of 5.7 K) and after that gradually decreasing in amplitude with the increase in the Co content. The normalized value of the peak position H_p/H_c2 has weak dependence on Co concentration. In the case of Mn_xNbSe₂ the features of the peak effect as a function of the Mn content are different and they can be understood in terms of strong pinning.