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Through a direct observation, based on the spin-polarized tunneling technique, we explore the thermal stability of tunneling spin polarization in Al/Al[sub 2]O[sub 3]/Co junctions. Thermal robustness of this parameter, which is of key importance for magnetic tunnel junction performance, is established for in situ postdeposition anneal temperatures up to 500 °C. This stability is consistent with detailed in situ x-ray photoelectron spectroscopy measurements on the Al[sub 2]O[sub 3]/Co system which show no structural changes during the anneal. Our results imply that, for comparable magnetic tunnel junction devices, thermal stability is not limited by intrinsic processes in the Al[sub 2]O[sub 3] barrier and its interfaces. With ex situ postdeposition annealing in an Ar-atmosphere, which leads to severe degradation of the spin polarization above 250 °C, we demonstrate that the spin polarization is extremely vulnerable to diffusion of impurities. © 2004 American Institute of Physics. [ABSTRACT FROM AUTHOR]