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

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 Reχ is observed at 50-100 K, with an accompanying dissipation peak in Imχ. 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.