[en] 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 degreeC. The annealing temperature T_TMR^*, 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 degreeC in the normal junctions) increases with annealing temperature up to 380 degreeC and remains at a maximum until 450 degreeC. 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 photoelectron spectra for the Fe2p core level show that the FeOx contribution in the upper part of the inserted layer decreases upon annealing, while it increases in the inner part near the barrier, suggesting the FeOx decomposition and the oxygen diffusion toward the inner metallic Fe and Al barrier. The study of R x A values and barrier parameters versus annealing temperature for samples with 7 and 25 Aa Fe - FeOx also reflects the above structural changes in the inserted layer. [copyright] 2001 American Institute of Physics