  We study Superconductor/Ferromagnet/Superconductor junctions with CuNi, PtNi,
or Ni interlayers. Remarkably, we observe that supercurrents through Ni can be
significantly larger than through diluted alloys. The phenomenon is attributed
to the dirtiness of disordered alloys leading to a short coherence length
despite a small exchange energy. To the contrary, pure Ni is clean resulting in
a coherence length as long as in a normal metal. Analysis of temperature
dependencies of critical currents reveals a crossover from short (dirty) to
long (clean) range proximity effects in Pt1-xNix with increasing Ni
concentration. Our results point out that structural properties of a
ferromagnet play a crucial role for the proximity effect and indicate that
conventional strong-but-clean ferromagnets can be advantageously used in
superconducting spintronic devices.
