Peak and history effects in two-dimensional collective flux pinning

Abstract

Flux pinning in amorphous Nb3Ge and Mo3Si films with thicknesses ranging from 60 nm to 3m has been studied as a function of perpendicular field, temperature, and history of the flux-line lattice (FLL). The theory of two-dimensional collective pinning agrees well with the critical-current data up to the field BST. Between BST and Bc2 a peak effect is observed for which possible origins are discussed in detail. It is concluded that elastic instabilities generated by local fluctuations of the pinning force on individual vortices induce a structural transition (ST) of the FLL at BST. A theoretical criterion derived for BST is in good agreement with the data. Isofield experiments clearly show for the first time the presence of flux-line dislocations in the peak regime. They cause an enhancement of the pinning force by the local reduction of the shear modulus. In the field region around BST distinct effects of the formation history of the FLL are observed. It is shown that the FLL can exist in metastable states which structurally relax upon a sufficiently fast movement of the lattice past the pinning centers. © 1986 The American Physical Society.