Impedance of nanometer thickness ferromagnetic Co ₄₀Fe ₄₀B ₂₀ films

Nanocrystalline Co ₄₀Fe ₄₀B ₂₀ films, with film thickness t _f = 100 nm, were deposited on glass substrates by the magnetron sputtering method at room temperature. During the film deposition period, a dc magnetic field, h = 40 Oe, was applied to introduce an easy axis for each film sample: one with h-L and the other with h-w, where L and w are the length and width of the film. Ferromagnetic resonance (FMR), ultrahigh frequency impedance (IM), dc electrical resistivity (ρ), and magnetic hysteresis loops (MHL) of these films were studied. From the MHL and r measurements, we obtain saturation magnetization 4πM _s = 15.5 kG, anisotropy field H _k = 0.031 kG, and r = 168 mW.cm. From FMR, we can determine the Kittel mode ferromagnetic resonance (FMR-K) frequency fFMRK = 1,963 MHz. In the h-L case, IM spectra show the quasi-Kittel-mode ferromagnetic resonance (QFMR-K) at f ₀ and the Walker-mode ferromagnetic resonance (FMR-W) at f _n, where n = 1, 2, 3, and 4. In the h-w case, IM spectra show QFMR-K at F ₀ and FMR-W at F _n. We find that f ₀ and F ₀ are shifted from f _FMRK, respectively, and f _n = F _n. The in-plane spin-wave resonances are responsible for those relative shifts.