Temperature selectivity for single phase hydrothermal synthesis of PEG-400 coated magnetite nanoparticles

Herein, we have presented a detailed investigation of the temperature effect on hydrothermal synthesis of Fe₃O₄ magnetic nanoparticles (MNPs). The appearance of single-phase cubic spinel Fe₃O₄ at and above critical temperature provides a clear indication that temperature plays a crucial role in the single-phase synthesis of the Fe₃O₄ MNPs. A detailed investigation of the structural, magnetic and spin dynamic properties of PEG-400 coated Fe₃O₄ MNPs synthesized by a facile hydrothermal method at different temperatures (120 °C, 140 °C, 160 °C and 180 °C for 16 hours) has been presented. The single-phase cubic magnetite structure with high crystallinity was found in the samples synthesized at 160 and 180 °C and confirmed from XRD results, whereas samples prepared at 120 and 140 °C are of mixed phase (α-Fe₂O₃ and Fe₃O₄). The magnetic hysteresis curves reveal that saturation magnetization and coercivity of MNPs enhanced systematically with the increase in the reaction temperature from 120 °C to 180 °C. Maximum saturation magnetization (88.98 emu g^-1) and coercivity (134.16 Oe) were found for the sample synthesized at 180 °C. Furthermore, ferromagnetic resonance (FMR) spectra obtained for samples synthesised at higher temperatures indicate a lower value of the line width due to the high magnetic ordering in the samples. Also, the resonance field decreased, and the g-value increased due to enhancement in magnetization for the single-phase samples synthesized at higher reaction temperatures. The spin resonance properties obtained from fitting the FMR data clearly indicate that a large spin-orbit coupling was observed for the single phase Fe₃O₄ MNPs and excellent magnetic properties were obtained from the static magnetic measurements.