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

Iron oxide (Fe3O4, gamma-Fe2O3 and alpha-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 similar to 300Oe. X-ray diffraction (XRD) results, of spin coated thin films, confirm the formation of phase pure gamma-Fe2O3, Fe3O4 and alpha-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 gamma-Fe2O3, Fe3O4 and alpha-Fe2O3 is also confirmed using Raman spectroscopy. High saturation magnetization of 224.09-287.29 emu/cm(3), 467.5-490.42 emu/cm(3) and 18.31-20.61 emu/cm(3) is obtained for gamma-Fe2O3 (P/S = 0.08-0.24), Fe3O4 (P/S = 0.48, 0.56) and alpha-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 similar to 115 K while no such transition is observed in case of gamma-Fe2O3 thin films. FC/ZFC curves for alpha-Fe2O3 thin films exhibit spontaneous magnetization at low temperatures.