On the magnetocrystalline anisotropy of greigite (Fe₃S₄)

Abstract

The ferrimagnetic mineral greigite (cubic Fe 3 S 4 ) 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. K 1 is in the range 215 . . . 223 J/m 3 (<111>) or 110 . . . 115 kJ/m 3 (<100>), yielding upper limits of 44 or 34 nm for the superparamagnetic grain size, respectively.

Magnetic properties of uncultivated magnetotactic bacteria and their contribution to a stratified estuary iron cycle

“…4c). Such an asymmetry can be expected for systems dominated by a negative cubic magnetocrystalline anisotropy (o1114 easy axis), which has been suggested as a probable case for greigite with aspect ratio 4B0.75 (if elongated along o1114) 54 . In terms of magnetofossil detection, although the low-field absorption peaks remain a useful FMR trait in recognizing magnetite-producing MTB, our results from the water column, sediment trap and the surface sediment suggest that deviations of FMR empirical parameters from those of intact magnetite-producing MTB cultures characterized to date should not be inevitably taken to signify lower magnetofossil concentrations nor diagenetic alteration of MTB chains.…”

Micromagnetic simulations of first-order reversal curve (FORC) diagrams of framboidal greigite

“…For greigite, the magnetocrystalline anisotropy energy (φ anisotropy ) is dominated by the first term (K 1 ) at room temperature (Winklhofer et al, 2014), and the anisotropy energy can be written in terms of the reduced magnetisation:…”

“…(3) K 1 = −1.7 × 10 4 J/m 3 (Winklhofer et al, 2014). To model nonuniform structures it is sufficient that the spatial discretisation in the model is always smaller than the exchange length l exch = 2A/µ 0 M 2 S (Hubert and Schäfer, 2000), which for greigite is l exch ≈ 6.6 nm; a maximum element size of 5 nm was chosen here for all meshes.…”

Magnetic detection and characterization of biogenic magnetic minerals: A comparison of ferromagnetic resonance and first‐order reversal curve diagrams

“…The shortcomings of this approach are apparent from its application to analysis of MTB samples: for strain MV‐1, model components have a g factor of 2.2 or greater [ Kopp et al ., ], which significantly exceeds the values for magnetite (2.12 [ Bickford , ]) or maghemite (1.97 [ Valstyn et al ., ]). Likewise, for relatively strong anisotropy field B an relative to the applied DC field B app , approximate evaluation of the resonance condition to first order in B an / B app yields a distorted FMR spectrum compared to that produced by exact evaluation of the resonance condition [ Winklhofer et al ., ]. For MV‐1, B an was obtained as 170 mT [ Kopp et al ., ], which is not small compared to B app under typical X‐band magnetic fields of 300 mT.…”