Anomalous positive exchange bias in Ni80Fe20∕NixFe1−xO thin-film bilayers induced by ion-beam deposition effects

We present results on a Ni80Fe20∕NixFe1−xO thin-film bilayer that shows a positive exchange bias loop shift of ∼90Oe at 10 K under zero-field-cooled conditions. Zero-field-cooled and field-cooled hysteresis loops were double shifted at temperatures below 200 K. This behavior is due to the presence of a range of antiferromagnetic crystallite sizes in addition to multiple magnetic phases (e.g., FeO, Fe2O3, and NiO). Furthermore, the positive exchange bias loop shift decreases linearly with increasing temperature, with a compensation temperature Tcomp∼220K, after which negative exchange bias is measured. This temperature dependence of the exchange bias reflects the competition between the Ni80Fe20 ferromagnet and antiferromagnetic Fe oxide and NiO phases as well as a range of local blocking temperatures. We attribute the appearance of a positive exchange bias loop shift at low temperatures to temperature-dependent changes in the interfacial pinning and exchange coupling due to a complex NixFe1−xO structure from energetic ion-beam bombardment effects during the film deposition.