On the magnetocrystalline anisotropy of greigite (Fe₃S ₄)

The ferrimagnetic mineral greigite (cubic Fe₃S₄) is well known as an intracellular biomineralization product in magnetic bacteria and as a widely occurring authigenic mineral in anoxic sediments. Due to the lack of suitable single-crystal specimens, the magnetic anisotropy parameters of greigite have remained poorly constrained, to the point where not even the easy axis of magnetization is known. Here we report on an effort to determine the anisotropy parameters on the basis of ferromagnetic resonance (FMR) powder spectroscopy on hydrothermally synthesized, chemically pure greigite microcrystals dispersed in a nonmagnetic matrix. In terms of easy axis orientations, the FMR data are consistent with <111> or <100>, or less likely, a more general <uv0> type. With a g factor of 2.09, the anisotropy field is about 90 mT and in some samples may reach 125 mT, compared to 30 mT for cubic magnetite. This confirms the dominating role of cubic anisotropy on the magnetic properties of greigite, which we show to be responsible for large SIRM/k values. K₁ is in the range -15 ⋯ -23 J/m³ (<111>) or +10 ⋯ +15 kJ/m³ (<100>), yielding upper limits of 44 or 34 nm for the superparamagnetic grain size, respectively.