Crystal structures and magnetic properties of spinel ferrites synthesized from natural Fe–Mg–Ca carbonates

Spinel ferrites with tunable magnetic properties were synthesized by the thermal treatment of Mg-substituted siderite and ankerite at different temperatures. The changes in magnetization and Curie temperature were monitored as a function of calcination temperature. A rapid increase in magnetization up to 25 Am2/kg was observed at 600 °С owing to spinel ferrite formation from a magnetite–magnesioferrite solid solution, followed by a moderate increase in magnetization (up to 30 Am2/kg) at 900 °С. Simultaneously, the Curie temperature decreased from 500 °C to 280 °C owing to Fe oxidation and cation redistribution. The resulting magnetic product (σS ≥ 30 Am2/kg, TC: 280 °C) exhibited a mixed spinel structure with a maghemite-like stoichiometry, (Fe3+, Mg)(Fe3+, Mg)2O3+ẟ, a crystallite size of 22 nm at 700 °C and 34 nm at 900 °C, and an increased unit cell length (a = 8.41 Å).