Refining the exchange anisotropy paradigm: Magnetic and microstructural heterogeneity at the Permalloy-CoO interface

More than 50 years of extensive research into exchange anisotropy in ferromagnetic-antiferromagnetic bilayers has not produced a convincing explanation for any given system of its principal manifestations, namely, a shift of the hysteresis loop along the field axis (exchange bias) and enhanced coercivity. We have examined this issue in the prototypical polycrystalline Permalloy-CoO bilayer system with samples whose Permalloy thicknesses ranged from 1 to 25 nm. The heterogeneous magnetic and chemical microstructure of the ∼1-nm -thick interfacial region is responsible for the observed exchange bias and coercivity, and for their dependence on Permalloy thickness and on temperature. Approximately 75% of the interfacial moment is produced by magnetically hard particles which are exchange coupled to the CoO and are responsible for exchange bias and coercivity by virtue of their exchange coupling to the Permalloy. The remainder of the interfacial moment is produced by a magnetically soft phase that exhibits no exchange bias. The thickness dependence of the exchange bias agrees with the prediction of a random-field model in which the exchange coupling of the distributed hard particles provides a random field operating on the Permalloy. The coercivity is determined by the switching of the hard interfacial particles coupled to the Permalloy; it has a remarkably linear temperature dependence which can be explained by a simple thermal fluctuation model. The exchange bias exhibits the same temperature dependence as the CoO uncompensated spins and these uncompensated spins are on the interfacial {111} planes of the [111]-textured CoO. Finally, the kinetics of the chemical reactions responsible for the interfacial heterogeneity can contribute to the latent period during which the exchange bias can be substantially reversed by applying a field antiparallel to the cooling field.