Synthesis, structural characterization, and magnetic property of nanostructured ferrite spinel oxides (AFe2O4, A = Co, Ni and Zn)

Nanostructured ferrite spinels AFe2O4 (A = Co, Ni, Zn) were successfully synthesized via a co-precipitation method using oxalate salt as a precursor in an anionic surfactant system in combination with a simple calcination process. High crystallinity samples of nanoparticle spinels in a grain size range of 15–100 nm were obtained by varying the calcination temperature (300–700 °C) and time (1–5 h). Their pore sizes were controlled in a range of 3 nm up to a hundred nm by tailoring the calcination conditions. Raising the calcination temperature was found to decrease the Brunauer–Emmett–Teller (BET) surface area, and broaden the pore structure due to enhanced crystal growth and agglomeration of interparticles of spinels. Transmission electron microscopy (TEM) images of ferrite spinels calcined at 300 °C showed mesoporous structures with narrow pore size distribution, and the maximum BET surface area of CoFe2O4, NiFe2O4 and ZnFe2O4 were found at 201 (Co), 315 (Ni), and 273 (Zn) m2 g−1, respectively. The magnetic hysteresis loops of the ferrite spinels at room temperature demonstrated ferromagnetism in CoFe2O4, superparamagnetism–ferromagnetism in NiFe2O4, and paramagnetism in ZnFe2O4. The highest saturation magnetization (Ms), remanent magnetization (Mr), and coercivity (Hc) were obtained from high crystallinity spinels calcined at 700 °C. Nanostructured AFe2O4 with high surface area and mesoporosity promises potentials as novel magnetic catalysts.