Structural, Magnetic, and Optical Characterization of ${\rm MnFe}_{2}{\rm O}_{4}$ Nanoparticles Synthesized Via Sol-Gel Method

Nanoparticles of MnFe₂O₄ were synthesized via sol-gel method, with calcination processes at 400 ^°C and 500 ^°C in air. The effects of the different calcinations in the formation of the crystal structure and magnetic and optical properties were studied with X-ray diffraction (XRD), electron microscopy, SQUID magnetometer, and optical absorption. The XRD studies reveal the formation of a phase corresponding to cubic spinel structure in both samples and the presence of a second phase identified as Fe₂O₃, in the case of the sample with higher temperature treatment. The TEM images of the first sample show small nonuniform nanoparticles with a mean size of 7.8 nm, with a strong tendency to form agglomerates. Magnetization studies as a function of temperature were carried following field-cooled (FC)-Zero-field-cooled (ZFC) routines, where the ZFC curves exhibit blocking temperatures close to 250 K in both cases, and the behavior of the samples below this temperature suggests strong interaction between the particles. In the magnetization as a function of magnetic field studies, the curves display a tendency to saturate at low temperatures and the system shows superparamagnetic behavior above the blocking temperature. Saturation magnetization values (at low temperatures) are low compared to the expected ones, according to the Néel model of collinear spins, this can be attributed to canting effects or the presence of a second antiferromagnetic phase, specifically in the sample treated at 500 ^°C. No significant differences were observed in the magnetic behavior of the samples. Semiconducting characteristics of the ferrites were confirmed by optical absorption measurements, obtaining an energy gap value close to 2.23 eV at room temperature.