Preparation and characterization of nanosized magnesium ferrite powders by a starch-gel process and corresponding ceramics

The synthesis and characterization of nanosized MgFe₂O₄ by a starch-gel method is described herein. A phase-pure nanosized MgFe₂O₄ powder (1a) was obtained after calcining a (MgFe)-starch gel at 550 °C. The powder has a specific surface area of 60.6 m²/g and a crystallite size of 9 nm. TEM investigations reveal particles in the range of 7–15 nm. The activation energy of the crystallite growth process was calculated as 89 ± 14 kJ/mol. The shrinkage and sintering behaviour of resulting compacts were studied. UV–Vis investigations of the nanosized powder 1a reveal an optical band gap of 2.38 eV, whereas calcination at 1100 °C (powder 1g) leads to a crystallite size of 129 nm and a band gap of 2.16 eV. Magnetization loops at 300 K and the temperature dependence of both the field-cooled and the zero-field-cooled magnetization indicate a superparamagnetic behaviour. The blocking temperature for powder 1a was determined as 140 K at a field of H = 500 Oe. We found different saturation magnetizations (M _s) depending on the calcination temperature. Calcination at 550 °C (1a) results in M _s = 20.0 emu/g which increases with calcination temperature to a maximum of 37.7 emu/g for powder 1e calcined at 900 °C. Ceramic bodies sintered between 1450 and 1600 °C exhibit M _s values of 25–28 emu/g. Magnetic investigations at 10 K on powders 1a–1g show hysteresis loops with coercivities up to 950 Oe, remanences to 10 emu/g and M _s values to 50.4 emu/g. Additionally, the nanoscaled powders show a shift of the hysteresis loops.