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

We report a pronounced peak effect in the magnetization of CoxNbSe2 single crystals with critical temperatures Tc ranging between 7.1 and 5.0 K, and MnxNbSe2 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 Cox NbSe2 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 Tc 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 Hp/Hc2 has weak dependence on Co concentration. In the case of Mnx NbSe2 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.