Magnetoresistance oscillations due to changing effects in double ferromagnetic tunnel junctions

Electron tunneling in a double junction consisting of two ferromagnetic electrodes, with a small ferromagnetic metallic grain in between, is analyzed theoretically in the Coulomb blockade regime. A new phenomenon, that of oscillations in tunneling magnetoresistance due to discrete charging effects, is predicted. The corresponding oscillation period depends on the charging energy, and the oscillations disappear when both junctions have the same spin asymmetry. The interplay of the oscillations and a nonoscillatory voltage dependence of the magnetoresistance is also analyzed. [S0031-9007(97)05142-9].