Calcium-Controlled Tuning of Zn0.25Co0.5Ca0.25Fe1.97Ba0.03−yCayO4 Ferrites for MHz-Range Electromagnetic Interference Shielding in Biomedical Electronic Systems

Background: Spinel ferrites are widely used for electromagnetic interference (EMI) shielding in biomedical devices and high-frequency electronics because of their tunable structural, magnetic, and dielectric properties. However, the effect of progressive Ca2+ substitution on the properties of Zn0.25Co0.5Ca0.25Fe1.97Ba0.03−yCayO4 ferrites has not been systematically investigated. Methods: Zn0.25Co0.5Ca0.25Fe1.97Ba0.03−yCayO4 (y = 0.00, 0.01, 0.02, 0.03) nanoparticles were synthesized using a sol–gel autocombustion method, and their structural, vibrational, magnetic, electrical, and dielectric properties were characterized using X-ray diffraction (XRD) with Rietveld refinement, Raman spectroscopy, vibrating sample magnetometry (VSM), temperature-dependent current–voltage measurements, and dielectric spectroscopy (4Hz–8 MHz). Results: XRD with Rietveld refinement revealed a predominantly single-phase cubic spinel structure with crystallite sizes of 15.10–18.06 nm and trace α-Fe2O3. Raman spectra revealed five characteristic modes, while peak shifts and A1g subband splitting indicated Fe2+/Fe3+ redistribution and increased inversion. The y = 0.02 sample exhibited the highest saturation magnetization (72.73 emu/g), coercivity (746.92 Oe), anisotropy constant (56,587 erg/cm3), and resonance frequency (16.06 GHz), whereas the y = 0.01 sample showed the highest alternating-current conductance. All samples showed semiconducting behavior and non-Debye dielectric relaxation. Conclusions: Controlled Ba2+/Ca2+ substitution effectively tailors the structural, magnetic, electrical, and dielectric properties of Zn–Co spinel ferrites. The y = 0.02 composition demonstrates the best overall performance and is therefore a promising candidate for MHz-range EMI shielding and biomedical electronic applications.