Peak and history effects in two-dimensional collective flux pinning

Flux pinning in amorphous ${\mathrm{Nb}}_3$Ge and ${\mathrm{Mo}}_3$Si films with thicknesses ranging from 60 nm to 3 μm 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 $B_{\mathrm{ST}}$. Between $B_{\mathrm{ST}}$ and $B_{c2}$ 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 $B_{\mathrm{ST}}$. A theoretical criterion derived for $B_{\mathrm{ST}}$ 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 $B_{\mathrm{ST}}$ 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.