  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.
