Single-electron tunneling in a double junction in which a nonmagnetic metal
lic island is separated from two ferromagnetic electrodes by tunnel barrier
s, is analysed theoretically in the sequential tunneling regime and for an
arbitrary intrinsic spin relaxation time on the island. It is shown that no
nequilibrium spin polarization of the island results in tunnel magnetoresis
tance due to rotation of the electrode magnetizations from antiparallel to
parallel alignment. It is also shown that discrete charging of the island g
ives rise to a fine structure in the voltage dependence of the island spin
polarization and tunnel magnetoresistance. (C) 1999 Elsevier Science B.V. A
ll rights reserved.