Contribution of low-temperature degrees of freedom to the anisotropy in Co/CoO exchange coupled bilayers

Abstract Measurements of the transverse magnetic AC-susceptibility χ ( T ) of exchange coupled polycrystalline Co/CoO bilayers have been made via the anisotropic magnetoresistance using a small AC field of 2–5 Oe and frequencies from 40 to 400 Hz. A step increase in Re χ is observed at 50–100 K, with an accompanying dissipation peak in Im χ . This feature is independent of the CoO thickness, but its dispersion with frequency suggests a thermally activated degree of freedom with an activation energy Ea∼150 K. This is consistent with a contribution to the anisotropy field due to the dynamics of the ferromagnetic rather than the antiferromagnetic grains.