AF-biased CoFe multilayer films with FMR frequency at 5 GHz and beyond

Antiferromagnetic/ferromagnetic/antiferromagnetic (AF/F/AF) multilayers are investigated. The AF layer is 500-Å-thick Ni₅₀Mn₅₀. Films are studied as a function of the thickness of the ferromagnetic layer (e_F). Films with Ni₈₀Fe₂₀, Co₅₀Fe₅₀, and Co₉₀Fe₁₀ exhibit interfacial exchange coupling energy densities (J_ex) of 0.22, 0.82, and 0.97 erg.cm^-2 respectively. High J_ex ensures strong exchange coupling (H_ex) and large uniaxial anisotropy fields (H_ua) at 90° from the pinning direction. This promotes soft magnetism in CoFe films while they are not soft naturally. With e_F=150 Å,H_ua is of 570 and 755 Oe for Co₅₀Fe₅₀ and Co₉₀Fe₁₀, respectively. Thicknesses of ∼0.3 μm are also shown possible for practical use by means of multiple alternations (n ≥ 20). Microwave characterizations confirm coherent reversible magnetization processes. Co₅₀Fe₅₀ multilayers are the best candidates with the highest μ´ and the lowest μ´´ at high frequency because an exceptional combination of a high 4πM_s of 21 kG and a small damping factor of 0.015. ferromagnetic resonance frequencies of ∼5 GHz are reported, and values of ∼10 GHz are shown realistic, for the first time.