Vortex Confinement in Planar S/F Hybrid Structures*

We have investigated the effect of periodic stray fields of the ferromagnet on the vortex dynamics in superconductor/ferromagnet (S/F) systems. Magnetization measurements were performed using SQUID magnetometry for Permalloy/Niobium (Py/Nb) samples of varying Py domain widths and Nb thicknesses. The hysteresis loops show an increase of the critical current for some values of magnetic domain width and superconductor thickness in some portion of the H-T phase diagram. However, below a threshold temperature sudden jumps in magnetization are observed during a slow sweep of the external magnetic field, which indicate the occurrence of vortex avalanches. These avalanches have been confirmed by scanning probe microscopy and they can cause a collapse of the critical state below a threshold temperature. Static and dynamics of these systems will be discussed.