Temporal evolution of spin-polarization in ferromagnetic tunnel junctions

The spin polarization of the simple transition-metal ferromagnets Fe, Co, and Ni can be measured using ferromagnet/insulator/superconductor tunneling structures. Measurements, carried out over the past 20 years using superconducting aluminum electrodes and alumina tunneling barriers, have given a wide range of values, especially for Ni. In this letter, we show that high-spin-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 decreases over time, which we believe to result from a reaction between Ni and alumina. We conclude that the spin-polarization values measured in such studies are strongly influenced by the detailed structure of the ferromagnet-superconductor interface. © 2000 American Institute of Physics.