Variable defect structures cause the magnetic low-temperature transition in natural monoclinic pyrrhotite

Koulialias, Dimitrios; Kind, Jessica; Charilaou, Michalis; Weidler, Peter G.; Löffler, Jörg F.; Gehring, A. U.

doi:10.1093/gji/ggv498

Cited by 15 publications

(24 citation statements)

References 42 publications

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“…The coercivity ratio and M r / M 3T are typical of 4C pyrrhotite in a multidomain state, generally with 180° domain walls (Ds) normal to the c ‐axis of the pyrrhotite crystal (Halgedahl & Fuller, ; O'Reilly et al, ). The M 3T is close to magnetization at saturation estimated for stoichiometric Fe 7 S 8 pyrrhotite (Néel, ) and measured in natural samples (Armstrong et al, ; Halgedahl & Fuller, ), but it is clearly higher than in a pyrrhotite crystal that contains epitaxially intergrown poly‐type, monoclinic superstructures (Koulialias et al, ). For our pyrrhotite crystal a magnetic moment per formula unit Fe 6.97 S 8 of 2.34 μ B was calculated.…”

Section: Resultssupporting

confidence: 82%

“…The shoulder is symmetrical with respect to the ascending and descending branches of the hysteresis loop and shifts to higher fields, with 1 T at 100 K and 1.3 T at 40 K (Figure d). A similar noncollective way to reach saturation as indicated by peaks in d M /d B was reported in the literature (Koulialias et al, ; Volk et al, ). Based on XRD and magnetic data, Koulialias et al () attributed this behavior to two different anisotropy systems stemmed from a 4C and an additional polymorph superstructure termed as 5C*.…”

Section: Resultssupporting

confidence: 81%

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On the Magnetism Behind the Besnus Transition in Monoclinic Pyrrhotite

et al. 2018

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In monoclinic 4C pyrrhotite (ideal formula Fe7S8), ordered vacancy distribution forms a superstructure with strong ferrimagnetism that makes this mineral a major remanent magnetization carrier in the Earth's crust. The pronounced decrease in isothermal remanence magnetization at about 32 K, known as Besnus transition, is a characteristic trait that marks the low‐temperature transition in 4C pyrrhotite, and it is used as a key to identify this mineral phase in rock samples. Here we take a nearly pure single pyrrhotite crystal (Fe6.97S8) from the Swiss Alps to study its Besnus transition in a broad mineral‐magnetic approach that combines detailed structural analysis with static and dynamic magnetization experiments. All the magnetic properties inferred from the experimental data are discussed in the context of a recent model that explains interacting anisotropy fields in the 4C pyrrhotite caused by the vacancy arrangement as the origin of the Besnus transition.

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Section: Resultssupporting

confidence: 82%

Section: Resultssupporting

confidence: 81%

On the Magnetism Behind the Besnus Transition in Monoclinic Pyrrhotite

et al. 2018

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“…No crystallographic change was detected in monoclinic pyrrhotite across its low-temperature transition from neutron powder diffraction analysis (Powell et al, 2004), whereas based on detailed single crystal neutron diffraction analyses and anisotropy measurements across the transition, Wolfers et al (2011) proposed that the room temperature monoclinic structure transforms into a low-temperature triclinic structure due to distortion of the shape of Fe octahedra. In contrast, Koulialias et al (2016) suggested that the low-temperature transition in pyrrhotite is due to stronger antiferromagnetic coupling between different monoclinic magnetic superstructures (4C and 5C*) below the transition temperature that form a single anisotropy system with higher magnetization and coercivity than above the transition. In contrast, Koulialias et al (2016) suggested that the low-temperature transition in pyrrhotite is due to stronger antiferromagnetic coupling between different monoclinic magnetic superstructures (4C and 5C*) below the transition temperature that form a single anisotropy system with higher magnetization and coercivity than above the transition.…”

Section: Introductionmentioning

confidence: 98%

The Low‐Temperature Besnus Magnetic Transition: Signals Due to Monoclinic and Hexagonal Pyrrhotite

Horng

Roberts

2018

Geochem Geophys Geosyst

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The low‐temperature magnetic properties of the many pyrrhotite varieties have not been studied extensively. Monoclinic pyrrhotite (Fe7S8) goes through the Besnus transition at ~30–34 K, which is used widely to diagnose its presence in bulk samples. Other pyrrhotite polytypes are assumed to be antiferromagnetic, although it has been suggested occasionally that some may also have remanence‐carrying capabilities. Here we compare the magnetic properties of monoclinic (4M) and hexagonal (3T) pyrrhotite at low temperatures. The 4M pyrrhotite records a Besnus transition consistently. Despite not recording a Besnus transition, 3T pyrrhotite has a magnetic remanence at room temperature and has distinctive room‐ and low‐temperature magnetic properties that cannot be explained by known or unidentified impurities (with abundances <0.1%). Some 3T‐pyrrhotite samples have exceptionally high (>700 mT) and stable coercivities below 50 K. The importance of this mineral in fossil or active gas hydrate and methane venting environments makes it important to develop a more detailed understanding of its occurrences and magnetic properties.

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“…In fact, the observed behavior of ac and dc susceptibilities of samples with iron excess below 20 K either suggests the onset of the antiferromagnetic order or anomalous magnetic behavior due [28,29]. Further studies are evidently necessary to discriminate between the two possible scenarios of anomalous magnetic behavior at low temperatures observed in the off-stoichiometric samples.…”

Section: Magnetic Propertiesmentioning

confidence: 96%

Structure, magnetic susceptibility, and specific heat of the spin-orbital-liquid candidate FeSc2S4 : Influence of Fe off-stoichiometry

et al. 2017

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We report structural, susceptibility and specific heat studies of stoichiometric and offstoichiometric poly-and single crystals of the A-site spinel compound FeSc 2 S 4 . In stoichiometric samples no long-range magnetic order is found down to 1.8 K. The magnetic susceptibility of these samples is field independent in the temperature range 10 -400 K and does not show irreversible effects at low temperatures. In contrast, the magnetic susceptibility of samples with iron excess shows substantial field dependence at high temperatures and manifests a pronounced magnetic irreversibility at low temperatures with a difference between ZFC and FC susceptibilities and a maximum at 10 K reminiscent of a magnetic transition. Single crystal x-ray diffraction of the stoichiometric samples revealed a single phase spinel structure without site inversion. In single crystalline samples with Fe excess besides the main spinel phase a second ordered single-crystal phase was detected with the diffraction pattern of a vacancy-ordered superstructure of iron sulfide, close to the 5C polytype Fe 9 S 10 . Specific heat studies reveal a broad anomaly, which evolves below 20 K in both stoichiometric and off-stoichiometric crystals. We show that the low-temperature specific heat can be well described by considering the low-lying spin-orbital electronic levels of Fe 2+ions. Our results demonstrate significant influence of excess Fe ions on intrinsic magnetic behavior of FeSc 2 S 4 and provide support for the spin-orbital liquid scenario proposed in earlier studies for the stoichiometric compound.

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Variable defect structures cause the magnetic low-temperature transition in natural monoclinic pyrrhotite

Cited by 15 publications

References 42 publications

On the Magnetism Behind the Besnus Transition in Monoclinic Pyrrhotite

On the Magnetism Behind the Besnus Transition in Monoclinic Pyrrhotite

The Low‐Temperature Besnus Magnetic Transition: Signals Due to Monoclinic and Hexagonal Pyrrhotite

Structure, magnetic susceptibility, and specific heat of the spin-orbital-liquid candidate FeSc2S4 : Influence of Fe off-stoichiometry

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