Ferromagnetic resonance and damping properties of CoFeB thin films as free layers in MgO-based magnetic tunnel junctions

We have investigated the magnetization dynamics of sputtered Co₄₀Fe₄₀B₂₀ thin films in a wide range of thicknesses used as free layers in MgO-based magnetic tunnel junctions, with the technique of broadband ferromagnetic resonance (FMR). We have observed a large interface-induced magnetic perpendicular anisotropy in the thin film limit. The out-of-plane angular dependence of the FMR measurement revealed the contributions of two different damping mechanisms in thick and thin film limits. In thinner films (< 2 nm), two-magnon scattering and inhomogeneous broadening are significant for the FMR linewidth, while the Gilbert damping dominates the linewidth in thicker films (≥ 4 nm). Lastly, we have observed an inverse scaling of Gilbert damping constant with film thickness, and an intrinsic damping constant of 0.004 in the CoFeB alloy film is determined.