Influence of Microwave-Assisted and Conventional Sintering on Magnetic and Magnetostrictive Properties of CoFe2O4 Ceramics

In this work, the cobalt ferrite ceramic was synthesized by using Pechini and densified using two different sintering routes: conventional sintering (CS) at 1200 °C for 3 h and microwave sintering (MS) at 1200 °C for 15 min. The influence of both sintering methods on the structural, microstructural, magnetic, and magnetostrictive properties was investigated. X-ray diffraction and Rietveld refinement confirmed the formation of a single-phase cubic spinel structure (Fd-3m) without secondary phases for both samples, and the crystal structure parameters exhibited only minor variations, indicating that the crystal structure remained essentially unaffected by the sintering route. In contrast, SEM micrographs revealed a remarkable reduction in average grain size from 7.72 µm (CS) to 0.74 µm (MS), demonstrating the effectiveness of microwave sintering in controlling grain growth (≈90% reduction in grain size). The magnetic measurements showed slightly higher saturation magnetization and coercive field values for sample MS, which were associated with the refined microstructure and enhanced magnetic anisotropy. Magnetostriction curves at room temperature yielded saturation values of −120 × 10−6 (CS) and −110 × 10−6 (MS) for samples, respectively. Despite the lower saturation magnetostriction, the microwave-sintered sample exhibited a significantly higher piezomagnetic coefficient (66 × 10−5 T−1) compared with the conventional sintered sample (35 × 10−5 T−1), it showed ≈8% decrease in saturation magnetostriction and ≈89% increase in piezomagnetic coefficient. Furthermore, a sign reversal of the magnetostrictive response was observed near 0.40 T for the MS sample, suggesting competition between magnetoelastic contributions associated with different crystallographic directions. The unstressed magnetostriction model successfully reproduced the experimental magnetostriction curves, confirming that grain-size refinement and grain-boundary effects play a dominant role in governing the magnetomechanical response of magnetic ceramics.