Damping and ferromagnetic resonance linewidth broadening in nanocrystalline soft ferromagnetic Fe–Co–Hf–N films

Abstract

In order to describe high-frequency damping mechanisms of ferromagnetic films by means of the imaginary part of the frequency-dependant permeability, CMOS compatible ferromagnetic Fe 36Co 44Hf 9N 11 films were deposited by reactive r.f. magnetron sputtering on oxidised 5×5 mm 2×380 μm (1 0 0)-silicon substrates with a 6-in. Fe 38Co 47Hf 15 target, as well as magnetic field annealing between 300 and 600 °C. An in-plane uniaxial anisotropy of around 4.5 mT as well as an excellent soft magnetic behaviour with a saturation polarisation of approximately 1.4 T could be observed after heat treatment at the above-mentioned temperatures, which drives these films to a high-frequency suitability. Ferromagnetic resonance frequencies of approximately up to 2.4 GHz could be obtained. The frequency-dependant permeability was measured with a broadband permeameter. Depending on the heat treatment, an increase of the full-width at half-maximum (FWHM) of the imaginary part of the frequency-dependant permeability is discussed in terms of two-magnon scattering, anisotropy-type competition and local resonance generation through predominant grain growth causing magnetisation and anisotropy inhomogeneities in the magnetic films. The grain size of the films was determined by (HRTEM) imaging and amounts from a few nanometres for films heat treated at 300 °C to more than 10 nm at 600 °C where the FWHM Δ f eff and the Landau–Lifschitz–Gilbert equation damping parameter α eff increases with d nm 2 and d nm (e.g. d nm is the grain diameter of the nonmagnetic Hf–N phase), respectively.