Ferromagnetic resonance studies of NiO-coupled thin films ofNi80Fe20

This paper describes ferromagnetic resonance (FMR) and magnetoresistive measurements of thin magnetic films coupled to antiferromagnetic films. First, FMR results for films of ${\mathrm{Ni}}_{80}{\mathrm{Fe}}_{20}$ show that coupling to NiO produces the angular variation in the resonance field of the type expected for unidirectional exchange anisotropy. However, unidirectional anisotropy values measured by in-plane ferromagnetic resonance are roughly 20% less than the loop shift measured via magnetoresistance. The difference is attributed in part to asymmetry in the coercivity. Second, in addition to the unidirectional anisotropy, coupling to NiO produces an isotropic negative resonance field shift that is larger than the exchange anisotropy field. This isotropic field shift is not consistent with models of exchange anisotropy in which the ferromagnet spins couple to a static antiferromagnet spin structure. It is consistent with the existence of a rotatable anisotropy, explained in terms of the energetics of domain configurations in the NiO. Third, using unpinned films as references, unidirectional anisotropy is measured for the first time with the magnetization rotated out of the film plane, and is found to be in reasonable agreement with in-plane measurements.