Cobalt-doped Mg-Zn ferrites with equivalent permeability and permittivity for VHF antenna miniaturization applications

This study presents a systematic investigation of the crystalline phases, grain characteristics, dynamic hysteresis loops, and magnetodielectric attributes of Mg 0.9 Zn 0.1 Fe 2- x Co x O 4 ferrites ( x  = 0.00–0.10, with a increments of 0.02). The samples were fabricated by a solid-state synthesis route, and formed by sintering at 950 °C for 3 h. XRD analysis shows that all samples possess the standard ferrite phase. Due to the influence of sintering additive Bi 2 O 3 , the trace amounts of Bi 24 Fe 2 O 39 phase are present in the samples, which exert a positive influence on the material’s dielectric performance. Analysis of the grain characteristics indicates that the mean crystallite dimension of the samples gradually increases as Co 2 + ion substitution level rises, and the particle size distribution becomes more uniform. Upon applied magnetic field stimulation, all samples present outstanding soft magnetic properties. When the Co 2+ content is 0.06, the saturation magnetization of the material attains a peak value of 48.8 emu/g. Dynamic magnetic measurements reveal that the permeability decreases significantly (from 20 to 7.5) with the incorporation of Co 2+ ions, while the cutoff frequency correspondingly increases from 0.2 GHz to 1 GHz. At x  = 0.08, the sample exhibits excellent magneto-dielectric matching characteristics across the 10–300 MHz frequency band. The normalized impedance value is approximately 1, and the permeability and permittivity are nearly equal in magnitude ( μ' = 10, ε' = 10.1 at 100 MHz). Additionally, the magnetic and dielectric losses are remarkably low at 100 MHz, with tanδ μ = 0.054 and tanδ ε = 0.0048. Consequently, the magneto-dielectric material prepared in this work is a potential option for miniaturized VHF antenna substrate material.