Interfacial Uncompensated Antiferromagnetic Spins: Role in Unidirectional Anisotropy in PolycrystallineNi81Fe19/CoOBilayers

The uncompensated spins on the surfaces of antiferromagnetic CoO films exhibit a thermoremanent magnetization after field cooling from $T_N$ that has the same temperature dependence as the exchange field of $\mathrm{Ni}{}_{81}\mathrm{Fe}{}_{19}/\mathrm{Co}\mathrm{O}$ bilayers after field cooling. This suggests that these interfacial uncompensated spins are responsible for unidirectional anisotropy. A model based on a calculation of the density of these interfacial uncompensated spins predicts the correct magnitude of the exchange field, as well as the observed inverse dependence on interfacial grain size.