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

Abstract

The CMOS compatible ferromagnetic Fe–Co–(M)–N (M=Ta, Hf) films were investigated with regard to their grain size-dependent frequency behaviour. Predominantly Fe 33Co 40Ta 10N 17 films were deposited by reactive r.f. magnetron sputtering. These films were compared to Fe 36Co 44Hf 9N 11 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) Δ f 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.