Solvothermal synthesis of CoFe2O4 submicron compact spheres and tunable coercivity induced via low-temperature thermal treatment

•Compact CoFe2O4 spheres were prepared using solvothermal synthesis method.•We discussed the formation mechanism of compact spheres in detail.•The spheres exhibited superparamagnetic-like properties.•A SPM–FM transition was induced by low temperature thermal treatment.•The transition was due to the enhanced exchange coupling as the organics removed.

Compact CoFe2O4 submicron spheres were successfully prepared by a typical solvothermal synthesis method using potassium acetate as protective agent. The as-prepared spheres exhibited the onset of superparamagnetism. The saturation magnetization (Ms), remanent magnetization (Mr), coercivity (Hc) and remanence ratio R (Mr/Ms) were 46.79 emu/g, 0.84 emu/g, 18.4 Oe and 0.018, respectively. Followed by thermal treatment at 250–600 °C, the annealed spheres exhibited a sharp increment in coercivity without significant growth in crystal size. The coercivity of the sample annealed at 250 °C was 597.5 Oe and the sample annealed at 600 °C was increased to 1371.7 Oe. The probable mechanism of the increment in coercivity was suggested to be induced by the enhanced exchange interactions as the organics degraded in thermal treatment.

Graphical abstractSubmicron compact spheres that prepared by solvothermal synthesis method exhibited a SPM–FM transition induced by low-temperature thermal treatment.Figure optionsDownload full-size imageDownload as PowerPoint slide