CoFeFormula OmittedOFormula Omitted/ZnFeFormula OmittedOFormula Omitted bilayers were deposited by the pulsed laser deposition on amorphous fused quartz substrate at substrate temperature of 350Formula OmittedC and in oxygen pressure of 0.16 mbar. The films were studied after ex-situ annealing for 2 h in air at various temperatures up to 650Formula OmittedC. The magnetic properties of the bilayers were studied at 300 K and at 10 K. Ferromagnetic resonance was carried out at x-band frequencies at room temperature. It was found that as a result of annealing, the diffusion between Co ferrite and Zn ferrite starts around 350Formula OmittedC and leads to a large line width system having magnetization, which remains undetected by Ferromagnetic resonance.