Vortex-glass transition, pinning mechanisms, and vortex dynamics in NbRe films and NbRe/Py bilayers

We present a comparative study of vortex-glass (VG) phase, vortex pinning, and vortex dynamics in su- perconducting niobium-rhenium (NbRe) films and NbRe/permalloy bilayers fabricated by the dc magnetron sputtering technique, where permalloy (Py) serves as the ferromagnetic layer. Current-voltage characteristics were measured as a function of temperature and magnetic field to investigate the VG transition, analyzed through scaling laws to determine the VG phase line, Hg(T ). NbRe films exhibit systematically higher Hg(T ) magnitude than the NbRe/Py bilayers, indicating stronger vortex pinning in the former. The reduced Hg(T ) trend observed in the NbRe/Py bilayers is attributed to the superconducting proximity effect and the homogeneous magnetic exchange interaction at the NbRe/Py interface. To further elucidate the pinning properties, critical current densities and pinning forces were evaluated, revealing that vortex pinning is strongly influenced by structural inhomogeneities and disorder in both systems, with consistently larger pinning forces in NbRe films. The weaker pinning in NbRe/Py bilayers is reflected in their vortex-instability behavior, characterized significantly by their higher vortex velocities compared to NbRe films.