Transport properties of ferromagnetic/non-magnetic/ferromagnetic single electron transistors are investigated as a function of external magnetic field, temperature, bias and gate voltage. By designing the magnetic electrodes to have different switching fields, a two-mode device is realized having two stable magnetization states, with the electrodes aligned in parallel and antiparallel. Magnetoresistance of approximately 100% is measured in Co/AlO$_{X}$/Al/AlO$_{X}$/Co double tunnel junction spin valves at low bias, with the Al spacer in the superconducting state. The effect is substantially reduced at high bias and temperatures above the $T_{C}$ of the Al. The experimental results are interpreted as due to spin imbalance of charge carriers resulting in suppression of the superconducting gap of the Al island.
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