  We report on magneto-optical imaging, magnetization, Hall effect and
magneto-resistance experiments in Nb/Al_2O_3/Co thin film heterostructures. The
magnetic field is applied perpendicularly to the plane of the film and gives
rise to abrupt flux penetration of dendritic form. A magnetization texture is
imprinted in the Co layer in perfect coincidence with these ramifications. The
spin domains that mimic the vortex dendrites are stable upon the field removal.
Moreover, the imprinted spin structure remains visible up to room temperature.
Complementary magnetization, Hall effect and magneto-resistance experiments
were performed in a similar sample where electrical contacts were placed on the
Co layer. In the region of the field - temperature diagram where flux
instabilities are known to occur in Nb films, irregular jumps are observed in
the magnetic hysteresis and large amplitude noise is detected in the
magneto-resistance and Hall resistivity data when measured as a function of the
field.
