Spin tunnel junctions fabricated with one interposed Fe–FeOx layer between the Al2O3 barrier and the top CoFe pinned electrode show large tunneling magnetoresistance (TMR) (40%) for anneals up to 380 °C. The annealing temperature TTMR*, where maximum TMR occurs, increases with the inserted Fe–FeOx layer thickness. For samples with thicker inserted layer, the pinned layer moment (which usually starts to decay below 300 °C in the normal junctions) increases with annealing temperature up to 380 °C and remains at a maximum until 450 °C. The large TMR at high temperature is related with the diffusion of extra Fe (from the Fe–FeOx layer) into the electrode interfacial region and the as-deposited paramagnetic FeOx decomposition into metallic Fe, and possibly the formation of some Fe3O4, which compensate the interface polarization loss associated with Mn interdiffusion. Rutherford backscattering spectrometry analysis confirms partial Fe diffusion into the top CoFe electrode after anneal. Meanwhile, x-ray photoele...