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

ε-iron oxide (ε-Fe 2 O 3 ) has drawn attention from the viewpoints of high-density magnetic recording and high-frequency millimeter wave absorption. To date, chemical conversion from Fe 3 O 4 (magnetite) to ε-Fe 2 O 3 under wet process conditions have been difficult. Herein, we report that ε-Fe 2 O 3 could be obtained from Fe 3 O 4 using a nanosize wet process. In the present method, 10 or 16 nm sized Fe 3 O 4 nanocrystals are used as the precursor. Fe 3 O 4 nanocrystals are embedded in a silica matrix and subsequently sintered around 1000 °C in air, resulting in the chemical conversion from Fe 3 O 4 to ε-Fe 2 O 3 being confirmed. In the case of 10 nm sized Fe 3 O 4 precursors, the sample consists of 16% ε-Fe 2 O 3 and 84% γ-Fe 2 O 3, whereas in the case of 16 nm sized Fe 3 O 4 precursors, the sample consists of 24% ε-Fe 2 O 3 and 76% γ-Fe 2 O 3 . The magnetic hysteresis loops of the samples are theoretically predicted using the large hysteresis loop of ε-Fe 2 O 3 and the magnetization curve of super-paramagnetic γ-Fe 2 O 3 . The experimental and predicted hysteresis loops agree well. First-principles calculations suggest that Fe 3 O 4 nanocrystals between 8 and 43 nm transform directly to ε-Fe 2 O 3 . Due to the strict size condition, the chemical conversion from Fe 3 O 4 to ε-Fe 2 O 3 is the first to be observed by a wet process. The nanosize wet process from Fe 3 O 4 to ε-Fe 2 O 3 should accelerate the development of highly functional hard magnetic ferrite ε-Fe 2 O 3 .