Influence of Ca²⁺ doping on structural, dielectric and magnetic properties of Sr₂MgCo_2-xCa_xFe₂₄O₄₁ for potential antenna applications

Z-type hexaferrites Sr₂MgCo_2-xCa_xFe₂₄O₄₁ of stoichiometric composition where x=0.0, 0.2, 0.4, 0.6, 0.8 and 1.0 were prepared by sol-gel method. The obtained nanopowders were sintered at 950 °C/4 h using conventional sintering method. The samples were characterized by X-ray diffraction (XRD) and field emission scanning electron microscopy (FE-SEM). Phase analysis of Ca-doped Sr₂MgCo_2-xCa_xFe₂₄O₄₁ powders revealed pure Z-type hexaferrites phase. The room temperature dielectric and magnetic properties (ε′, ε″, μ′ and μ″) were studied using RF impedance analyzer in the frequency range of 1 MHz to 1.8 GHz. The dielectric responses of these compounds are found to be frequency dependent and show the frequency stability upto 1.4 GHz. The relaxation-type dielectric behavior is dominating in the low frequency region and resonance-type dielectric behavior is found to be dominating above 10⁹ Hz. It was inferred that the permeability was sensitively controlled by the substitution of Ca²⁺. The coercivity (H_c) increased gradually with an increase of calcium content. Frequency variation of AC conductivity is explained in terms of hopping of electrons between Fe²⁺ and Fe³⁺. Since loss tangent was kept low with the frequency ranging from 1 MHz to 1.4 GHz, such magneto-dielectric material could be used in this frequency range for antenna miniaturization.