Low temperature enhancement of the remanent magnetization in CeCoIn$_{5}$

We investigated the vortex dynamics together with RF penetration depth measurements in the heavy fermions compound CeCoIn$_{5}$ down to~50~mK. No strong pining is observed and the relaxation curves are logarithmic as expected from Kim-Anderson theory. The temperature dependence of the relaxation rate, $S$, with a small but finite residual value indicate that quantum tunneling plays a role in the vortex creep only at very low temperatures. Remarkably, a new phase transition marked by a strong increase in the remnant magnetization, M$_{rem}$ is observed around $T$~=~0.3~K in very low magnetic fields. $M_{rem}$ increases roughly by a factor of two at 50 mK and we discuss if this can be caused solely by the change in the vortex lattice symmetry or underling magnetism has to play a role. Moreover, this anomaly is corroborated by the RF measurements at very low fields. We extended the vortex dynamics investigation to Pb irradiated CeCoIn$_{5}$. While the defects created by irradiation have a clear effect on the relaxation rates the enhancement of $M_{rem}$ still takes place at the same temperature.