Magnetic tunnel junction performance versus barrier thickness: NiFe/AlOx/NiFe junctions fabricated from a wedged Al layer

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The resistance–area product (R*A) and the magnetoresistance (MR) of NiFe/AlOx/NiFe spin-dependent tunnel junctions exhibit a strong dependence on the thickness of Al before oxidation. We obtain these data from wafers where we uniformly oxidize an Al layer with a wedged thickness profile, enabling us to reliably characterize the effect of Al thickness variations with subangstrom precision. The R*A drops from 104 to 102 Ω μm2 as the Al thickness decreases from 9 to 4 Å, respectively. The MR is highest (21%) for an Al thickness of 7 Å, where the Al layer is fully oxidized and the oxidation of the bottom NiFe electrode is minimal.