Role of precursor to solvent ratio in tuning the magnetization of iron oxide thin films – A sol-gel approach

Iron oxide (Fe₃O₄, γ-Fe₂O₃ and α-Fe₂O₃) is a versatile material that can be utilized in various spintronic devices. However, there is a need to explore preparation methods that can result in three important iron oxide phases using single route. For this purpose, we here report low cost sol-gel method to optimize different phases of iron oxide with iron nitrate as precursor. Precursor to solvent (water and ethylene glycol) (P/S) ratio is varied in the range of 0.08–0.8 (interval 0.08). Magnetic properties of iron oxide sols show S-type ferromagnetic behavior with coercivity of ∼300Oe. X-ray diffraction (XRD) results, of spin coated thin films, confirm the formation of phase pure γ-Fe₂O₃, Fe₃O₄ and α-Fe₂O₃ at P/S ratio of 0.08–0.24, 0.48–0.56 and 0.72–0.80, respectively. Whereas, P/S ratios 0.32, 0.40 and 0.64 result in mixed iron oxide phases. Formation of phase pure γ-Fe₂O₃, Fe₃O₄ and α-Fe₂O₃ is also confirmed using Raman spectroscopy. High saturation magnetization of 224.09–287.29 emu/cm³, 467.5–490.42 emu/cm³ and 18.31–20.61 emu/cm³ is obtained for γ-Fe₂O₃ (P/S = 0.08–0.24), Fe₃O₄ (P/S = 0.48, 0.56) and α-Fe₂O₃ (P/S = 0.72–0.8), respectively. Increase in saturation magnetization and decrease in coercivity at P/S of 0.08–0.24, 0.48–0.56, 0.72–0.8 is correlated with decrease in magnetocrystalline anisotropy. FC/ZFC curves indicate Verwey transition in Fe₃O₄ thin films at ∼115 K while no such transition is observed in case of γ-Fe₂O₃ thin films. FC/ZFC curves for α-Fe₂O₃ thin films exhibit spontaneous magnetization at low temperatures.