Growth of monodisperse nanospheres of MnFe₂O₄ with enhanced magnetic and optical properties

Highly dispersive nanospheres of MnFe₂O₄ are prepared by template free hydrothermal method. The nanospheres have 47.3-nm average diameter, narrow size distribution, and good crystallinity with average crystallite size about 22 nm. The reaction temperature strongly affects the morphology, and high temperature is found to be responsible for growth of uniform nanospheres. Raman spectroscopy reveals high purity of prepared nanospheres. High saturation magnetization (78.3 emu/g), low coercivity (45 Oe, 1 Oe = 79.5775 A·cm⁻¹⁾, low remanence (5.32 emu/g), and high anisotropy constant 2.84 × 10⁴ J/m³ (10 times larger than bulk) are observed at room temperatures. The nearly superparamagnetic behavior is due to comparable size of nanospheres with superparamagnetic critical diameter D_cr^spam. The high value of K_eff may be due to coupling between the pinned moment in the amorphous shell and the magnetic moment in the core of the nanospheres. The nanospheres show prominent optical absorption in the visible region, and the indirect band gap is estimated to be 0.98 eV from the transmission spectrum. The prepared Mn ferrite has potential applications in biomedicine and photocatalysis. (condensed matter: electronic structure, electrical, magnetic, and optical properties)