Interplay of composition and anisotropy on evolution of microstructural, static and dynamic magnetic properties of CoFeB thin films on annealing

Highlights: • Change in microstructure and anisotropy is connected to the magnetization dynamics. • High Boron content and large UMA in CoFeB results in partially crystalline state. • CoFeB films with lower B content and smaller UMA results in complete crystallization. • Complete crystallization causes larger damping compared to partial crystallization. • Two-magnon scattering is confirmed by non-linear frequency dependence of linewidth. The crystallization of CoFeB electrode is essential for achieving high tunneling magnetoresistance (TMR) in magnetic tunnel junctions (MTJs) with MgO barrier. In this study, we systematically investigated the influence of annealing on the structural and magnetic properties of CoFeB (23 nm) thin films using X-ray diffraction (XRD), magnetization (M) vs. magnetic field (H) and ferromagnetic resonance (FMR) measurements. The variations in composition and uniaxial magnetic anisotropy (UMA) of as-deposited CoFeB thin films results in a different crystalline state on annealing. It was found that the partial crystallization takes place for CoFeB compositions with high Boron content and exhibiting large UMA. This initial stage of crystallization is associated with the quick release of stress and will result in low coercivity, smaller crystallite size and a decrease in uniaxial anisotropy which will eventually lead to lower magnetic damping. However, on complete crystallization for other compositions with the formation of bcc-CoFe (110) phase, both the crystallite size and coercivity increases and cubic anisotropy emerge which results in very enhanced damping. The FMR linewidth has non-linear frequency dependence which gives direct evidence that relaxation is not exclusively governed by Gilbert damping, but it is also important to consider the contribution from two-magnon scattering (TMS). The TMS dominates the damping process in crystallized films and is mainly caused by the anisotropy dispersion created due to the grain growth. Thus, the work connects the change in microstructure and anisotropy to the magnetization dynamics, in particular, the FMR linewidth.