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

Abstract

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 μB), with a net magnetic moment of ∼3.0 μB 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 3.0μB and 3.25 μB 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. Copyright 2009 by the American Geophysical Union.