Synthesis of Mn–Zn ferrite nanoparticles by the oil-in-water microemulsion reaction method

The oil-in-water (O/W) microemulsion reaction method was used for the preparation of Mn–Zn ferrite magnetic nanoparticles (NPs). By variation of the precipitating agent and the oil phase concentration, Mn–Zn ferrite nanoparticles with different characteristics were obtained. HRTEM showed that at fixed microemulsion composition (surfactant:water weight ratio (S:W) 25:75, 12 wt% oil phase), the use of NaOH as precipitating agent resulted in smaller nanoparticle size than when tetramethylammonium hydroxide (TMAH) was used (2.4 nm vs. 5.2 nm, respectively). This is explained by the higher basic strength and concentration of NaOH compared to TMAH. When using two different oil phase concentrations (12 and 20 wt%) and TMAH as precipitating agent, particle size was increased upon increasing oil content (9.4 nm for 20 wt% oil); this is attributed to the higher concentration of precursor in the microemulsion as well as the higher processing temperature, which may favor growth and crystallization. X-ray diffraction patterns of as-obtained materials showed characteristic reflections of spinel structure, indicating that nanocrystalline materials were obtained at low temperature (T = 25–40 °C). Crystallite size varied depending on microemulsion composition and precipitating agent. As a consequence, the magnetic characteristics (blocking temperature and saturation magnetization) also changed as a function of the precipitating agent and microemulsion composition. All samples were superparamagnetic. The estimated saturation magnetization values were slightly smaller than that of the bulk material, suggesting the presence of a disordered Spin layer on the surface. The thickness of this magnetically inert layer was estimated from the size dependence of the saturation magnetization at 9.1 Å. These results demonstrate the potential of the O/W microemulsion reaction method as greener alternative for the synthesis of complex ceramics such as mixed oxide spinels with magnetic properties under mild conditions.