Comparison of parallel and perpendicular ferromagnetic resonance in an exchange-biased bilayer

We have investigated the ferromagnetic resonance spectra of an exchange-biased Ni80Fe20/CoO bilayer between room temperature and 4 K. Primary attention has been paid to the effect of the antiferromagnetic CoO film on the temperature-dependent resonance field shift of the ferromagnetic Ni80Fe20 film with respect to that of an unbiased film. At low temperatures, the field shift with the magnetic field applied perpendicular to the plane was determined to be more than twice the magnitude of the parallel field shift, and of the same sign, while an unoxidized single ferromagnetic film has much smaller parallel and perpendicular low-temperature shifts (here defined with respect to room temperature) of opposite sign. This observation implies that the anisotropy axis can rotate with the applied field, provided that the primary cause of the anisotropy is the interaction between the adjacent ferromagnetic and antiferromagnetic films. Since the perpendicular shift is more than a factor of two larger than the parallel field shift, the rotatable anisotropy is, in fact, anisotropic in this bilayer