Enhanced magnetic coercivity of α-Fe2O3 obtained from carbonated 2-line ferrihydrite

We report the physical properties of alpha-Fe2O3 (hematite), synthesized by dry-heating (350-1,000 degrees C) of a new, poorly ordered iron oxyhydroxide precursor compound that we name carbonated 2-line ferrihydrite. This precursor was characterized by powder X-ray diffraction, Fourier transform infrared spectroscopy, electron microscopy, and thermogravimetric analysis, whereas the alpha-Fe2O3 was studied with X-ray diffraction, scanning and transmission electron microscopy, and magnetic techniques. alpha-Fe2O3 synthesized at 350 degrees C consisted of single-nanocrystal particles (length 9 width 20 +/- 6 nm (L) x 15 +/- 4 nm (W)), which at room temperature exhibited very narrow hysteresis loops of low coercivities (<300 Oe). However, alpha-Fe2O3 synthesized at higher temperatures (1,000 degrees C) was composed of larger nanocrystalline particle aggregates (352 +/- 109 nm (L) x 277 +/- 103 nm (W)) that also showed wide-open hysteresis loops of high magnetic coercivities (similar to 5 kOe). We suggest that these synthesis-temperature-dependent coercivity values are a consequence of the subparticle structure induced by the different particle and crystallite size growth rates at increasing annealing temperature.