Novel High-Frequency Magneto-Dielectric Properties of CaO–SiO2 Co-Doped NiZnCo Spinel Ferrites for RF and Microwave Device Applications

In this paper, a series of NiZnCo spinel ferrites with the co-doping of CaO and SiO₂ (CaO-SiO₂) were studied for their potential application as high-frequency magneto-dielectric materials. The effects of CaO-SiO₂ on the crystalline phase composition, microstructure, and magnetic and dielectric properties over a wide frequency range of 0.1-5 GHz were systematically investigated. The results reveal that CaO-SiO₂ addition reduces the closed pores in NiZnCo ferrite and helps to enhance the permeability. With increasing CaO-SiO2 content (x = 0-0.6 wt%), the permeability μ' increases from ~7 to ~7.5 at 0.1 GHz. For permittivity ε', however, it declines evidently from ~9 to ~7.8 on account of the decreased grain size and density. Consequently, μ' and ε' are balanced to the matching and high values over a wide frequency range of 0.1-1 GHz in the samples with x = 0.3 and 0.6 wt%, accompanied with low loss characteristics that the magnetic loss tan δμ and dielectric loss tan δ_ε are on the order of 10^-2 and 10^-3, respectively.