Softening of the flux line lattice in Ge/Pb multilayers

The authors measured the critical current density Jc of artificially grown Ge/Pb multilayers as a function of the magnetic field Hperpendicular to applied perpendicular to the layers. In these layered structures Jc(Hperpendicular to ) rapidly drops to a pronounced minimum at a field H*perpendicular to , then increases through a broad maximum and finally decays to zero at Hc2 perpendicular to . The systematic evolution of this minimum as a function of temperature, layer thickness and pinning strength allows us to conclude that this peculiar behavior is due to a thermally driven softening of the flux line lattice which can be related either to a magnetic decoupling or a melting of the vortex structure.