Facile and Low-Temperature Synthesis of γ-Fe₂O₃ Nanoparticles with Thermally Stable Ferrimagnetism for Use in Magnetic Recording Tapes

Maghemite, γ-Fe2O3, has regained importance as a magnetic tape storage material for the big data era due to its low cost and long-term durability. However, the conventional synthesis of γ-Fe2O3, which involves dehydration, reduction, and oxidation processes, is complicated and requires improvement. We synthesized γ-Fe2O3 directly via dehydration from the high-pressure phase of iron hydroxide, ϵ-FeOOH, using a low-temperature solution method. Bulk ϵ-FeOOH prepared from α-FeOOH at 8.2 GPa and 773 K transformed into γ-Fe2O3 in a 2-phenoxyethanol and LiOH·H2O mixture at 498 K, which is the lowest temperature among similar dehydration reactions. 57Fe Mössbauer, Raman, and Fourier transform infrared spectroscopy analyses revealed that the obtained γ-Fe2O3 particles were covered with amorphous lithium phases, which stabilized their ferrimagnetism with a magnetization of 61.9 emu g–1 up to 1273 K, by converting to ferrimagnetic Li0.5Fe2.5O4. Furthermore, we reduced the size of γ-Fe2O3 to an average of 0.5 μm and a minimum of approximately 100 nm by applying ϵ-FeOOH nanoparticles as a precursor, which were prepared under higher-pressure (10 GPa) and lower-temperature (673 K) conditions. Although synchrotron X-ray diffraction measurements found that the γ-Fe2O3 nanoparticles were contaminated with ion-exchanged α-LiFeO2 at 80 wt %, a facile and low-temperature method to synthesize γ-Fe2O3 was established.