Magnetic structure of greigite (Fe₃S₄) probed by neutron powder diffraction and polarized neutron diffraction

We have investigated greigite (Fe₃S₄ using a combination of neutron powder diffraction and polarized neutron diffraction to give the first unambiguous assignment of its magnetic structure. Our results confirm that greigite has a collinear ferrimagnetic structure with antiferromagnetic coupling between the tetrahedral (A) and octahedral (B) sites. Our analysis also indicates that greigite does not have a significant vacancy concentration or spin canting. High-resolution neutron powder diffraction results enable determination of sublattice magnetizations of the A and B sites. At room temperature, the average magnetic moments on the two sites are almost the same (∼3.0 μ_B), with a net magnetic moment of ∼3.0 μ_B per formula unit (fu). The magnetic moment of the B sites decreases slightly between 10 K and room temperature, while the A site moment is relatively stable as a function of temperature; this indicates that greigite is probably an R-type ferrimagnet. At 10 K, the average magnetic moments of the A and B sites are 3.0μ_B and 3.25 μ_B respectively. Neutron diffraction measurements, coupled with magnetic measurements, on our pure synthetic greigite samples indicate that the saturation magnetization of greigite is lower than that of magnetite (Fe₃O₄). It is proposed that the lower magnetic moment in greigite (saturation magnetization is ∼59 A m kg^-1) compared to magnetite is probably caused by an increased degree of covalency between iron and sulfur compared to oxygen ligands or by greater derealization of the 3d electrons in greigite.