Abstract
The exchange bias field dependence on the Mn-oxide and its microstructure in NiFe/Mn-oxide bilayers was investigated. Transmission electron microscopy results have shown that the bilayer bottom consisted of either α-Mn, rocksalt MnO, or a composite of tetragonal Mn 3 O 4 + MnO, depending on the ratio of O 2 /Ar used during dual ion-beam deposition. Magnetometry results at 5 K indicate that the exchange bias field (H ex ∼-300 Oe) is largest in a NiFe/Mn (0% O 2 /Ar) bilayer. The MnO formation by in situ Mn oxidation results in a decrease in H ex in a NiFe/Mn-oxide (21% O 2 /Ar) bilayer. In contrast, a further increase in the O 2 /Ar ratio during deposition results in larger H ex and H c . This is attributed to the oxidation of MnO into a harder ferrimagnet, Mn 3 O 4 . Our results indicate that the antiferromagnetic Mn enabled stronger coupling with NiFe than MnO. In addition, we find that the MnO- Mn 3 O 4 coupling dominates the exchange bias effects at high oxygen concentrations. © 2008 American Institute of Physics.