Fundamental magnetic parameters from pure synthetic greigite (Fe₃S₄)

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 (M_s) at room temperature is ∼59 Am² kg^-1 (3.13 μ_B 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.Ų from low-temperature M_s measurements, with the corresponding exchange constant J_AB of ∼1.03 meV.