Magnetization dynamics of amorphous and nanocomposite CoFeMnNbSiB films with the addition of excess cobalt and boron

We report on the magnetic and structural characterization of Co _75.4Fe _2.3Mn _2.3Nb ₄Si ₂B ₁₄ (nominal) nanocomposite thin films and compositional derivatives formed by the co-sputtering of additional cobalt and boron followed by ex-situ vacuum annealing at 520 °C for different annealing times. The structural and compositional analyses of these samples were performed using transmission electron microscopy and atom probe tomography (APT). For the quasi-static magnetic measurements, vibrating sample magnetometry (VSM) was used and the magnetization dynamics was studied using a custom-built broadband ferromagnetic resonance (FMR) instrument with a frequency range of 1–65 GHz and fields up to 1.6 T. Two strong resonances were observed for the annealed nominal and B-added samples whereas a dominant single resonance was observed for all as-deposited and all annealed Co-added samples. This observation correlates with APT results where Mn-rich columnar structures were observed in the as-deposited nominal and Co-added annealed films. Whereas, a nearly equiaxed morphology of the crystalline phase was observed for annealed nominal and B-added films. Hence, FMR spectroscopy was found to be sensitive to the different phases present in the samples and differences in morphology. The quasi-static magnetic properties measured using VSM were not able to distinguish between the individual contributions. However, they provide important information regarding the overall magnetization. The VSM results suggested that short time annealing of the nominal composition results in better soft magnetic properties compared to the derivative compositions that were investigated in this study.