Peak effects in two- and three-dimensional collective pinning

Abstract

Flux pinning in amorphous Nb 3Ge and Mo 3Si films with thicknesses ranging from 60 nm to 20 μm has been studied as a function of temperature, magnetic induction and sample thickness. The theory of two-dimensional collective pinning agrees well with the critical current data. Close to the upper critical field, B c2, a peak in the volume pinning force, F p, is observed. It is demonstrated that this enhancement in F p is caused by the structural transition of the flux line lattice from an elastically deformed lattice below to a plastically distorted lattice in the peak. For thin samples (in our case typically d < 3 μm) the deformation of the flux line lattice remains two-dimensional, i.e., L c ⪢ d, whereas for thicker samples ( d > 5 μm) the transition is accompanied by a dimensional cross-over to three-dimensional deformation of the flux line lattice in the peak. The cross-over occurs when the longitudinal correlation length, L c, is approximately equal to d 2 . The cross-over is a first order phase transition and is characterized by a jump in the pinning force by up to a factor 10 and non-linear V I characteristics in the peak regime.