Tunnel junctions (TJ) consisting of two ferromagnetic (pinned and free) layers separated by an insulator are strong candidates for MRAMs. Due to spin dependent tunneling one is able to obtain two resistance (R) states corresponding to parallel or antiparallel FM-layer magnetizations. Recently, reversible R-changes induced by an electrical current were found in thin TJs [1] and attributed to electromigration (EM) in nanoconstrictions in the insulating barrier [2]; we thus obtain Current Induced Switching (CIS). Here we report a study on the transport properties (R, MR and CIS) of thin magnetic MnIr/CoFe/AlO_x/CoFe and non-magnetic MnIr/CoFe/Ta/AlO_x/Ta/CoFe tunnel junctions. CIS at room temperature amounts to a 60% resistance change, and this effect is discussed in terms of nanostructural rearrangements of metallic ions at the FM/insulator and NM/insulator interfaces. Interestingly, the current direction for which R-switching occurs in non-magnetic TJs is opposite to that of magnetic ones. This will be related to the dominance of different EM contributions (direct/wind forces) in CoFe and Ta layers. After switching (in magnetic TJs) some ions return to their initial sites when the electrical current is reduced. This relaxation is thermally assisted and two energy barriers emerge in the R-evolution over time. The physical origin of these effects is discussed, including ion EM between different types of sites near the metal/insulator interfaces. In contrast, no relaxation is observed in non-magnetic TJs, indicating that migrated Ta ions are buried inside stable local energy minima. In both types of tunnel junctions the CIS magnitude decreases with decreasing temperature, as expected for a temperature assisted process. A three resistance state magnetic TJ device is demonstrated, using a suitable combination of MR (magnetic) and CIS (structural) switching. [1] Y. Liu et al., Appl. Phys. Lett. 82, 2871 (2003). [2] A. Deac et al., J. Appl. Phys. 95, 6792 (2004).

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