LOCK-IN TRANSITION OF FLUX-LINES IN LAYERED SUPERCONDUCTORS

In an attempt to prove the existence of a lock-in transition of flux-lines in layered superconductors, we have synthesised Nb/Cu multilayers by means of evaporation under ultra-high vacuum conditions. Magnetic moment measurements by means of a high-sensitivity torque magnetometer show a series of maxima at fields B(n) = 1.01 +/- 0.13 x 10(-15) (n2/D2), with B(n) in Tesla and the total thickness D in nm. Maxima up to n = 7 have been observed in several Nb/Cu multilayers with different single layer thickness (varying from 4 to 20 nm). The effect disappears when the angle between B and the plane of the multilayer is larger than approximately 5-degrees. This is in good agreement with the critical angle predicted for lock-in of flux-lines in the layer plane. To our knowledge, this is the first experimental evidence for a lock-in transition and enhanced pinning in a layered superconductor with a coherence length xi comparable to the single layer thickness.