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[1] Identification of the mineral remains of magnetotactic bacteria (MTB), known as magnetofossils, is of particular interest because their occurrence can be used for environmental and climatic reconstructions. Single-domain magnetite particles, which are biomineralized in the cell body of MTB, have characteristic properties that can be used to detect their fossil remains. Acquisition of anhysteretic and isothermal remanent magnetization (ARM and IRM), first-order reversal curve (FORC) diagrams, and ferromagnetic resonance (FMR) spectra were used to detect the magnetic mineral inventory in Holocene lake sediments. A comparative analysis in terms of the discriminatory power of these methods is presented. The FORC diagrams contain two distinct features: a sharp horizontal ridge centered on the horizontal axis B c and a feature with symmetric spread along the vertical B b axis. The coercivity spectra derived from the central ridge coincides with that derived from ARM and IRM acquisition curves and is compatible with the presence of noninteracting linear chains of single-domain magnetite. The second feature on FORC diagrams is indicative of interacting particles in clusters. In the FMR spectra from bulk sediment, two populations are separated empirically based on the FORC information. An asymmetric signal is taken to describe the population, which contains single-domain particles in clusters. Empirical spectral separation of this contribution results in FMR spectra that are similar to those of intact MTB, which strongly suggests that a fraction of linear magnetosome chains is present. Combination of FMR and FORC results demonstrates the strong potential of these methods for identifying magnetofossils, based on alignment and interaction patterns of magnetic particles.

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Monoclinic 4C pyrrhotite (Fe 7 S 8 ) is ferrimagnetic due to an ordered defect structure with alternating vacancy and vacancy-free sublattices. Its low-temperature magnetic transition near 35 K is characterized by the distinct increase in coercivity and remanent magnetization. The increase of these parameters has been attributed to changes in the domain wall structure. We present static and dynamic magnetization data of a powder sample to study the domain-wall dynamics across the low-temperature transition. The amplitude-dependent ac susceptibility and the ferromagnetic resonance spectroscopy indicate that the hardening of the domain-wall pinning at the transition occurs simultaneously with the decrease in initial saturation remanent magnetization. These two effects are explained by the enhanced inhomogeneity of the bulk material caused by the persistency of the ordered vacancies and by newly formed defects due to localized distortion of Fe(II) sites in the vacancy-free sublattice. The generated localized defects are the link between the domain wall dynamics and the low-temperature transition in 4C pyrrhotite.

Magnetotactic bacteria (MTB) build magnetic nanoparticles in chain configuration to generate a permanent dipole in their cells as a tool to sense the Earth's magnetic field for navigation toward favorable habitats. The majority of known MTB align their nanoparticles along the magnetic easy axes so that the directions of the uniaxial symmetry and of the magnetocrystalline anisotropy coincide. Desulfovibrio magneticus sp. strain RS-1 forms bullet-shaped magnetite nanoparticles aligned along their (100) magnetocrystalline hard axis, a configuration energetically unfavorable for formation of strong dipoles. We used ferromagnetic resonance spectroscopy to quantitatively determine the magnetocrystalline and uniaxial anisotropy fields of the magnetic assemblies as indicators for a cellular dipole with stable direction in strain RS-1. Experimental and simulated ferromagnetic resonance spectral data indicate that the negative effect of the configuration is balanced by the bullet-shaped morphology of the nanoparticles, which generates a pronounced uniaxial anisotropy field in each magnetosome. The quantitative comparison with anisotropy fields of Magnetospirillum gryphiswaldense, a model MTB with equidimensional magnetite particles aligned along their (111) magnetic easy axes in well-organized chain assemblies, shows that the effectiveness of the dipole is similar to that in RS-1. From a physical perspective, this could be a reason for the persistency of bullet-shaped magnetosomes during the evolutionary development of magnetotaxis in MTB.

The magnetic anisotropy of linear chains of spherical magnetite nanocrystals was investigated by means of angle-resolved ferromagnetic resonance spectroscopy, in order to determine the different anisotropy contributions. The linear assembly of nanocrystals generates an interaction-induced uniaxial anisotropy, which is nearly an order of magnitude stronger than the intrinsic magnetocrystalline anisotropy of magnetite, and can only exist in magnetic nano-chains, where the easy axes of the nanocrystals are collinear.

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