Ultrahigh frequency permeability of sputtered Fe–Co–B thin films

Amorphous FeCoB alloys can have high resistivity and relatively high saturation magnetization which are desired for ultrahigh frequency devices such as future write heads and wireless inductors. In this study, FeCoB films are observed to have a low easy axis coercivity (1–2 Oe), a field deposited induced anisotropy of ∼35 Oe and ${\text{4πM}}_s\text{∼17\hspace{0.05em}500\hspace{0.05em}}\mathrm{G}$ when sputtered in a typical configuration. However, samples sputtered in an off axis arrangement are found to have an additional uniaxial anisotropy as a result of the oblique incidence of the atomic flux onto the substrate. This extra anisotropy increases with increasing oblique angle. For these same samples, there is little change in the easy axis coercivity, saturation magnetization, and the Hoffmann structure factor $\text{(S∼0.05\hspace{0.05em}}{\mathrm{ergs/cm}}^2\mathrm{).}$ The obtained large anisotropy fields (>35 Oe) are found to push the ferromagnetic resonance frequency to at least above 1 GHz at the expense of the initial permeability.