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The discontinuous change in magnetic properties at a temperature of about 32 K, denoted as the Besnus transition, is a diagnostic feature for detecting 4C pyrrhotite in geological systems. The transition is grain size dependent, and it has been assumed that the single‐domain (SD) to multidomain (MD) threshold is in the micron size range. Here we present a combined crystallographic and magnetic study of a pure phase, SD 4C pyrrhotite that was produced by ball milling of a MD precursor. The mechanical treatment generated varying grain sizes of less than 20 μm, which are solely in a SD magnetic state. It also decreases the coherent scattering domains of the grains from 216 nm in the MD sample to 30 nm, which is taken to set limits for the MD/SD threshold in 4C pyrrhotite grains. The alternating current susceptibility response associated with the Besnus transition is diagnostic for distinguishing SD and MD 4C pyrrhotite. The unambiguous identification of SD 4C pyrrhotite provides a better insight into its occurrence in geological systems.

Magnetic stability of iron mineral phases is a key for their use as paleomagnetic information carrier and their applications in nanotechnology, and it critically depends on the size of the particles and their texture. Ferrimagnetic greigite (Fe3S4) in nature and synthesized in the laboratory forms almost exclusively polycrystalline particles. Textural effects of inter-grown, nano-sized crystallites on the macroscopic magnetization remain unresolved because their experimental detection is challenging. Here, we use ferromagnetic resonance (FMR) spectroscopy and static magnetization measurements in concert with micromagnetic simulations to detect and explain textural effects on the magnetic stability in synthetic, polycrystalline greigite flakes. We demonstrate that these effects stem from inter-grown crystallites with mean coherence length (MCL) of about 20 nm in single-domain magnetic state, which generate modifiable coherent magnetization volume (CMV) configurations in the flakes. At room temperature, the instability of the CVM configuration is exhibited by the angular dependence of the FMR spectra in fields of less than 100 mT and its reset by stronger fields. This finding highlights the magnetic manipulation of polycrystalline greigite, which is a novel trait to detect this mineral phase in Earth systems and to assess its fidelity as paleomagnetic information carrier. Additionally, our magneto-spectroscopic approach to analyse instable CMV opens the door for a new more rigorous magnetic assessment and interpretation of polycrystalline nano-materials.

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<p>Greigite (Fe<sub>3</sub>S<sub>4</sub>) is a ferrimagnetic mineral widespread in sedimentary environments, commonly found in lacustrine and marine sediments that records ancient geomagnetic field variations and environmental processes. However, its magnetic properties are not yet well understood due to the lack of a single crystal greigite suitable for magnetic measurements. In particular, the dependency of its magnetic properties with respect to structural and morphological properties remains uncertain.</p><p>In the present study, we analyzed the structural and magnetic properties of synthetic, polycrystalline greigite formed by controlled colloidal synthesis [Rhodes et al. 2017]. X-ray diffractometry and transition electron microscopy reveal that greigite forms flakes of about 100 nm that consist of epitaxial intergrown nanoparticles with a mean coherence length of 19 nm. Therefore, our synthetic greigite can be considered as polycrystalline flakes with a nanotexture.</p><p>The saturation magnetization (M<sub>s</sub>) of the nanotextured greigite is 32.7 Am<sup>2</sup>kg<sup>-1 </sup>and the coercivity is B<sub>c</sub> = 41 mT. The M<sub>s</sub> is about 45% below the value for relatively large, synthetic crystal and this in turn is probably caused by the nanotexture, e.g., interfaces between nanocrystallites. The ratios M<sub>r.</sub>/M<sub>S</sub> = 0.54 and B<sub>ar</sub>/B<sub>Sc</sub> = 1.33 indicate single-domain (SD) particles with pre-dominant uniaxial anisotropy [Roberts 1995]. The FORC diagram at room temperature shows an oval contour plot supporting that the flakes are nanotextured with interacting SD particles. The hysteresis parameters B<sub>c</sub> and M<sub>S</sub> continuously increase upon cooling to 10 K.</p><p>Low-temperature cycling of the magnetization between 300 and 10 K in fields between 10 mT and 1000 mT shows the expected behavior for ferrimagnets with the superposition of the cooling and warming curves at fields B ³ 500 mT. At weaker fields a slight magnetic induction upon warming is found and the relative increase in magnetization is field dependent. This irreversibility most likely stems from the magnetization of the nanoparticle interfaces and their interactions in the flakes.</p><p>Ferromagnetic resonance spectroscopy (FMR) at room temperature shows a resonance field B<sub>res</sub>= 213 mT and linewidth DB = 160 mT. Upon cooling the B<sub>res</sub> decreases continuously down to 50 K followed by a pronounced shift to lower values down to 10 K. The shift goes along with markedly linewidth broadening. The discontinuity of the spectral parameters at T < 50 K points to a change in the effective anisotropy of the flakes most likely due to changes of the magnetocrystalline and the interaction anisotropies in the nanotexture, because the shape anisotropy of the polycrystalline flakes undergoes no significant change. </p><p>In summary, the magnetic properties of greigite can be critically affected by the nanotexture. The response of the nanotexture to the magnetization and anisotropy properties can be taken to identify and characterize greigite nanoparticles in natural environments and to critically evaluate their use for paleomagnetic studies.</p><p>Rhodes, Jordan M., et al. "Phase-controlled colloidal syntheses of iron sulfide nanocrystals via sulfur precursor reactivity and direct pyrite precipitation." Chemistry of Materials 29.19 (2017): 8521-8530.</p><p>Roberts, Andrew P. "Magnetic properties of sedimentary greigite (Fe3S4)." Earth and Planetary Science Letters 134.3-4 (1995): 227-236.</p>