Microstructural, Morphological, and Magnetic Effects of NiFe2O4 Shell Formation Around Nanospherical ZnFe2O4 Cores

First-row transition metal oxides have relatively modest magnetic properties compared to those of permanent magnets based on rare earth elements. However, there is a hope that this gap might be bridged via proper compositional and structural adjustments. Bi-magnetic nanostructures with homogeneous interfaces often exhibit a combination or synergy of properties of both phases, resulting in improved performance compared to their monophasic magnetic counterparts. To gain a deeper insight into these complex structures, a bi-magnetic nanostructured material composed of superparamagnetic nanoparticles comprising a zinc ferrite core and a nickel ferrite shell was synthesized using the seed-mediated growth approach. The resulting ZnFe2O4@NiFe2O4 core–shell nanoparticles were characterized using a series of experimental techniques and were compared to the ZnFe2O4 cores. Most importantly, the formation of the NiFe2O4 shell around the ZnFe2O4 core improved the net crystallinity of the material and altered the particle morphology by reducing the convexity of the surface. Simultaneously, the magnetic measurements demonstrated the coherence of the interface between the core and the shell. These effects combined led to improved spin coupling and stronger magnetism, as evidenced by higher saturation magnetization and the doubling of the blocking temperature for the ZnFe2O4@NiFe2O4 core–shell particles relative to the ZnFe2O4 cores.