Direct Observation of Chemical Conversion from Fe3O4 to ε-Fe2O3 by a Nanosize Wet Process

epsilon-iron oxide (epsilon-Fe2O3) has drawn attention from the viewpoints of high-density magnetic recording and high-frequency millimeter wave absorption. To date, chemical conversion from Fe2O3 (magnetite) to epsilon-Fe2O3 under wet process conditions have been difficult. Herein, we report that epsilon-Fe2O3 could be obtained from Fe2O3 using a nanosize wet process. In the present method, 10 or 16 nm sized Fe2O3 nanocrystals are used as the precursor. Fe2O3 nanocrystals are embedded in a silica matrix and subsequently sintered around 1000 degrees C in air, resulting in the chemical conversion from Fe2O3 to epsilon-Fe2O3 being confirmed. In the case of 10 nm sized Fe2O3 precursors, the sample consists of 16% epsilon-Fe2O3 and 84% gamma-Fe2O3, whereas in the case of 16 nm sized Fe2O3 precursors, the sample consists of 24% epsilon-Fe2O3 and 76% gamma-Fe2O3. The magnetic hysteresis loops of the samples are theoretically predicted using the large hysteresis loop of epsilon-Fe2O3 and the magnetization curve of super-paramagnetic gamma-Fe2O3. The experimental and predicted hysteresis loops agree well. First-principles calculations suggest that Fe3O4 nanocrystals between 8 and 43 nm transform directly to epsilon-Fe2O3. Due to the strict size condition, the chemical conversion from Fe2O3 to epsilon-Fe2O3 is the first to be observed by a wet process. The nanosize wet process from Fe2O3 to epsilon-Fe2O3 should accelerate the development of highly functional hard magnetic ferrite epsilon-Fe2O3.