Influence of grain growth on the high-frequency behaviour of ferromagnetic Fe-Co-(M)-N (M=Ta, Hf) nanocomposite films

The CMOS compatible ferromagnetic Fe-Co-(M)-N (M=Ta, Hf) films were investigated with regard to their grain size-dependent frequency behaviour. Predominantly Fe₃₃Co₄₀Ta₁₀N₁₇ films were deposited by reactive r.f. magnetron sputtering. These films were compared to Fe₃₆Co₄₄Hf₉N₁₁ films. In order to induce an in-plane uniaxial anisotropy H_u as well as to investigate the grain growth behaviour, the films were annealed in a static magnetic field. The in-plane uniaxial anisotropy field of around 4 mT as well as a good soft magnetic behaviour with a saturation polarisation of approximately 1.2-1.4 T could be observed after heat treatment. Ferromagnetic resonance frequencies (FMR) of approximately up to 2.4 GHz could be achieved according to the Kittel theory. Depending on the heat treatment, high-frequency losses through energy dissipation was made conspicuous by means of the full-width at half-maximum (FWHM) Δf_eff of the imaginary part of the frequency-dependent permeability which was between 0.4 and 1 GHz. This FWHM was basically discussed in terms of two-magnon scattering theories, in combination with the Herzer random anisotropy model. In order to correlate the resonance line broadening with a phenomenological damping parameter α_eff, which ranged from about 0.0125 to 0.028, the modified Landau-Lifschitz-Gilbert equation was used to fit and describe the permeability spectra of the ferromagnetic films.