Sign of tunneling magnetoresistance in Cr O2-based magnetic tunnel junctions

Half-metallic ferromagnets are potentially useful as electrodes in magnetic tunnel junctions (MTJs) because of the possibility for 100% spin polarization near the Fermi energy. We report tunneling magnetoresistance (TMR) behavior of MTJs having epitaxial Cr O2 (100) as one electrode, CoFe as second electrode, and MgO as tunnel barrier, i.e., Ti O2 (100) Cr O2 (100) MgOCoFe. When Cr O2 is exposed to air, several monolayers of oxide surface decompose and negative MR is observed. Conversely, sample surfaces protected by thin Mg layers show small but positive MR consistent with intrinsic spin polarization. Thus, the TMR sign can be reversed simply by subtle modifications to the Cr O2 MgO interface.