Negative four-terminal resistance as a probe of crossed Andreev   reflection
  We report on the experimental investigation of electronic transport in superconductor-ferromagnet spin-valve structures. Our samples consist of two ferromagnetic iron leads forming planar tunnel contacts to a superconducting aluminum wire. At energies below the superconducting gap, we observe a negative four-probe resistance that can be explained by crossed Andreev reflection. 
D. Beckmann H. v. L\"ohneysen