Superparamagnetism and metamagnetic transition in Fe3O4 nanoparticles synthesized via co-precipitation method at different pH

In the present work, Fe3O4 nanoparticles have been synthesized via low temperature co-precipitation method at different pH (7.0, 11.0 and 12.4) with the aim to study the variation of pH on the structural, optical and magnetic properties of samples. Further, the sample synthesized at pH ~12.4 has been annealed at 230 °C for 10 h to study the effect of annealing on structural, optical and magnetic properties. X-ray diffraction (XRD) results reveal the formation of pure spinel phase with the space group Fd-3m. Further, XRD, FESEM and TEM results confirm the nanocrystalline nature of the as synthesized samples, and the particle size of the samples decreases as the pH increases and increases after annealing at 230 °C. FTIR analysis indicates that the sample synthesized at pH ~12.4 and the same sample annealed at 230 °C are pure spinel Fe3O4, whereas the samples synthesized at pH ~7.0 and 11.0 have small content of α-Fe2O3. The optical measurements of the as synthesized samples show two band gaps in all synthesized samples. Field dependent magnetization measurements (M-H) reveal superparamagnetic nature of all the synthesized samples at room temperature and ferromagnetic behavior at low temperature (~5 K). Furthermore, M-H plots measured at 5 K show presence of metamagnetic transition in all samples. The metamagnetic transition along with ferromagnetic behavior at low temperature in Fe3O4 nanoparticles are observed first time in the present work to the best of our knowledge. Further the value of magnetization decreases with decreasing particle size at both temperatures. The fitting of the field cooled (FC) temperature dependent magnetization (M-T) measurements data with modified Bloch-spin wave model with additional surface disorder term and mixed magnetic phases indicates surface spin disorder and mixed magnetic phases in the as synthesized samples, which may be the possible reason for the existence of metamagnetic transition in the samples. The correlation between the observed magnetic properties and structural characteristics of the samples with the synthesizing parameters (pH value and annealing effect) has been described and discussed in this paper.