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. Current is injected into one of the contacts while the voltage is monitored at the other, thus defining a four-probe resistance. At energies below the superconducting gap, we observe a negative four-probe resistance that can be explained by crossed Andreev reflection. PACS 74.45.+c; 85.75.-d; 03.67.Mn