We investigated the annealing effects of a magnetic tunnel junction on the Fe bottom pinned layer, of which the top surface was exposed to nitrogen plasma for a few tens of seconds prior to the deposition of the Al layer. The nitrogen-treated junction shows a lower magnetoresistance (MR) ratio and specific junction resistance (RA) than the untreated normal junction. However, after annealing at 230 °C, the MR ratio of the nitrogen-treated junction recovers to optimal values of the normal junction, while the RA remains lower than that of the normal junction. Furthermore, the nitrogen-treated junction shows less reduction of MR ratio and RA value with annealing at 270 °C for 3 h than the normal junction. From x-ray photoelectron spectroscopy and auger electron spectroscopy depth profiles, it is found that the nitrogen, which was initially at an interface between Al and Fe, diffuses into both the Al and FeMn layers after annealing at 230 °C. It seems that the nitrogen plays an important role in reducing Mn diffusion, as well as in improving the junction properties. © 2003 American Institute of Physics.