Temporal evolution of spin-polarization in ferromagnetic tunnel junctions

The spin polarization of the simple transition-metal ferromagnets Fe, Co, a nd Ni can be measured using ferromagnet/insulator/superconductor tunneling structures. Measurements, carried out over the past 20 years using supercon ducting aluminum electrodes and alumina tunneling barriers, have given a wi de range of values, especially for Ni. In this letter, we show that high-sp in-polarization values are found for Ni using superconducting films formed from Cu-doped aluminum deposited in a high-vacuum sputter-deposition system . However, we find that the magnitude of the spin polarization slowly decre ases over time, which we believe to result from a reaction between Ni and a lumina. We conclude that the spin-polarization values measured in such stud ies are strongly influenced by the detailed structure of the ferromagnet-su perconductor interface. (C) 2000 American Institute of Physics. [S0003-6951 (00)00132-7].