Fundamental magnetic parameters from pure synthetic greigite (Fe3S4)

Abstract

Pure ferrimagnetic greigite (Fe3S4) has been synthesized by reacting ferric chloride with thiourea and formic acid at 1702*C. Sample purity was confirmed by X-ray diffraction, neutron diffraction and Mössbauer spectroscopy, coupled with magnetic measurements. Scannig electron microscope observations indicate clear cubo-octahedral and polyhedral crystal morphologies. The grain sizes are as large as 44 μm. Detailed low- and high-temperature magnetic measurements document the previously poorly known magnetic properties of greigite. The synthetic greigite samples are dominated by pseudo-single-domain and multi-domain behavior. The saturation magnetization (Ms) at room temperature is ∼59 Am2 kg-1 (3.13 μB per formula unit), which is higher than any value previously reported for greigite in the literature largely because of the high purity of this sample compared to others. No low-temperature magnetic transition has been detected; however, a local coercivity minimum is observed at around 130 K, which is probably associated with domain walls present in the studied samples. The high-temperature magnetic properties of greigite are dominated by chemical decomposition above around 250°C, which precludes determination of the Curie temperature, but our evidence indicates that it must exceed 350°C. On the basis of the Bloch spin wave expansion, the spin wave stiffness of greigite was determined for the first time as ∼193 meV.Å2 from low-temperature Ms measurements, with the corresponding exchange constant JAB of ∼1.03 meV. Show more