First‐order reversal curve (FORC) diagrams of natural and cultured biogenic magnetic particles

Abstract

[1] First-order reversal curve (FORC) diagrams are rapidly becoming a standard tool for characterizing magnetic particles because they simultaneously incorporate information regarding magnetostatic interaction and domain states. The simplest interpretation of FORC diagrams of single-domain (SD) particles is based on the Neel interpretation of Preisach theory, which predicts that the FORC function is the product of a coercivity and an interaction field distribution. Although the underlying assumptions of this interpretation are not correct, a strictly quantitative model of weakly interacting SD grains proves that the distributions of coercivities and interaction fields can be retrieved from a FORC diagram. To test this model, we present the possibility of a quantitative interpretation of FORC diagrams, and we present measurements of samples containing magnetosomes from cultures of magnetotactic bacteria and from a lake sediment. Two samples are investigated under the electron microscope to characterize the geometrical arrangement of the particles. We find that the clustering of otherwise similar particles has a strong influence on FORC diagrams. We also obtained a crude estimate of packing densities form the FORC diagrams, which were consistent with transmission electron microscopy observations and measurements of the anhysteretic remanent magnetization.

Enhanced primary productivity and magnetotactic bacterial production in response to middle Eocene warming in the Neo-Tethys Ocean

“…The magnetofossil component is correlated to the medium-coercivity component 2 (Table 1) with coercivity values around 40 mT and a small DP (~0.3), similar to the IRM signal of magnetotactic bacteria found in other studies (e.g. Egli, 2004;Chen et al, 2007;Jovane et al, 2012;Roberts et al, 2012 and 52%. The low-coercivity component varies from 5% to 17%.…”

Searching for single domain magnetite in the “pseudo‐single‐domain” sedimentary haystack: Implications of biogenic magnetite preservation for sediment magnetism and relative paleointensity determinations

“…Such spreading has been documented in FORC diagrams (cf. Figure 4) for samples that contain disrupted magnetosome chains [ Chen et al , 2007; Egli et al , 2010; Kind et al , 2011]. Regardless, these data indicate that the studied ‘PSD’‐like clay‐rich sediments (Figure 3) contain significant concentrations of previously unrecognized magnetite magnetofossils (Figure 12).…”

“…The FORC distribution peaks consistently occur at ∼20 mT and are vertically spread, but they also narrow at higher coercivities. This horizontal ‘tear‐drop’ distribution could indicate significant disruption of magnetofossil chains, with a smaller contribution from non‐disrupted chains [ Chen et al , 2007; Egli et al , 2010]. However, it could also result from detrital SD magnetite from extrusive rocks of the McMurdo Volcanic Group [ Verosub et al , 2000].…”

Uncultivated Magnetotactic Cocci from Yuandadu Park in Beijing, China

“…The FORC diagram is able to reflect the chain-chain interactions, the intrachain interactions (particle-particle interaction within a chain), and cell-cell interactions (7,33). The vertical spread of FORC contours (H b1/2 ϭ 5.3 mT) in the present study is comparable to that of multiple-chain MTB reported previously (H b1/2 ϭ 6.3 mT) (33) but more prominent than that of single-chain AMB-1 (H b1/2 ϭ 2.4 mT) (21).…”