Interface biquadratic coupling and magnon scattering in exchange-biased ferromagnetic thin films grown on epitaxial FeF ₂

The magnetic anisotropy of ferromagnetic (FM) Ni, Co, and Fe polycrystalline thin films grown on antiferromagnetic (AF) FeF ₂(110) epitaxial layers was studied, as a function of temperature, using ferromagnetic resonance. In addition to an in-plane anisotropy in the FM induced by fluctuations in the AF short-range order, a perpendicular (biquadratic) magnetic anisotropy, with an out-of-plane component, was found which increased with decreasing temperature above the AF Neél temperature (T _N=78.4K). This is a surprising result given that the AFs uniaxial anisotropy axis was in the plane of the sample, but is consistent with prior experimental and theoretical work. The resonance linewidth had a strong dependence on the direction of the external magnetic field with respect to in-plane FeF ₂ crystallographic directions, consistent with interface magnon scattering due to defect-induced demagnetizing fields. Below T _N, the exchange bias field H _E measured via FMR for the Ni sample was in good agreement with H _E determined from magnetization measurements if the perpendicular out-of-plane anisotropy was taken into account. A low field resonance line normally observed at H0, associated with domain formation during magnetization in ferromagnets, coincided with the exchange bias field for T<T _N, indicating domain formation with the in-plane FM magnetization perpendicular to the AF easy axis. Thus, biquadratic FMAF coupling is important at temperatures below and above T _N.