Resonance frequency above 20 GHz in superparamagnetic NiZn-ferrite

Abstract

We investigate the frequency dispersion of complex permeability in the GHz range in superparamagnetic nickel–zinc ferrite thin films with different Ni/Zn ratios using a microstrip probe. The films, comprising crystallites as small as 3 nm and deposited by a microwave-irradiation-assisted solvothermal method, exhibit the coexistence of two resonance characteristics—a ferromagnetic resonance peak ([Formula: see text]) at ∼2 GHz and a superparamagnetic resonance peak ([Formula: see text]) above 20 GHz, breaching Snoek's limit. The high value of [Formula: see text] is attributed to the high surface anisotropy and far-from-equilibrium distribution of cations in the lattice, while [Formula: see text] is attributed to the thermally driven superparamagnetic relaxation of ferrite nanocrystallites in the thin films. This work demonstrates the feasibility of employing superparamagnetic ferrite thin films so deposited as excellent CMOS-integrable magnetic components for high-speed and high-frequency electromagnetic device applications.