Pair-breaking mechanisms in Nb/Gd/Nb films

Several series of Nb/Gd/Nb triple layers have been grown in UHV by e--beam evaporation onto sapphire (1120) substrates at 300 K. The superconducting transitions have been measured resistively and by dc-susceptibility, the ferromagnetic transition with the transverse magnetooptical Kerr effect. The Gd layers show long-range ferromagnetic order only above a layer thickness of 20 Å, which is attributed to the formation of a discontinuous film below this thickness. The superconducting properties of the films reflect this transition by a sudden decrease in the Tc(dGd)-curve with increasing dGd. This result indicates that pair breaking by the bulk-Gd exchange field is much more effective than pair breaking by spin-flip scattering. Depending on the thickness of the Nb layers, Tc(dGd) saturates for large values of dGd at a finite temperature or tends to zero.