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

We report the physical properties of α-FeO (hematite), synthesized by dry-heating (350-1,000°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 α-FeO was studied with X-ray diffraction, scanning and transmission electron microscopy, and magnetic techniques. α-FeO synthesized at 350°C consisted of single-nanocrystal particles (length × width 20 ± 6 nm (L) × 15 ± 4 nm (W)), which at room temperature exhibited very narrow hysteresis loops of low coercivities (<300 Oe). However, α-FeO synthesized at higher temperatures (1,000°C) was composed of larger nanocrystalline particle aggregates (352 ± 109 nm (L) × 277 ± 103 nm (W)) that also showed wide-open hysteresis loops of high magnetic coercivities (∼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.