Jessica Kind | ETH - Academia.edu (original) (raw)

Papers by Jessica Kind

Research paper thumbnail of Magneto-electronic coupling in modulated defect-structures of natural Fe<sub>1−</sub><sub><i>x</i></sub>S

Journal of Applied Physics, Aug 28, 2015

We provide compelling experimental evidence that the low-temperature transition in natural non-st... more We provide compelling experimental evidence that the low-temperature transition in natural non-stoichiometric Fe 7 S 8 , a major magnetic remanence carrier in the Earth's crust and in extraterrestrial materials, is a phenomenon caused by magnetic coupling between epitaxially intergrown superstructures. The two superstructures differ in their defect distribution, and consequently in their magnetic anisotropy. At T < 30 K, the magnetic moments of the superstructures become strongly coupled, resulting in a 12-fold anisotropy symmetry, which is reflected in the anisotropic magnetoresistance. V

Research paper thumbnail of Pyrrhotite domain dynamics

Research paper thumbnail of Variable defect structures cause the magnetic low-temperature transition in natural monoclinic pyrrhotite

Geophysical Journal International, Dec 15, 2015

Non-stoichiometric monoclinic 4C pyrrhotite (Fe 7 S 8) is a major magnetic remanence carrier in t... more Non-stoichiometric monoclinic 4C pyrrhotite (Fe 7 S 8) is a major magnetic remanence carrier in the Earth's crust and in extraterrestrial materials. Because of its low-temperature magnetic transition around 30 K also known as Besnus transition, which is considered to be an intrinsic property, this mineral phase is easily detectable in natural samples. Although the physical properties of pyrrhotite have intensively been studied, the mechanism behind the pronounced change in magnetization at the low-temperature transition is still debated. Here we report magnetization experiments on a pyrrhotite crystal (Fe 6.6 S 8) that consists of a 4C and an incommensurate 5C * superstructure that are different in their defect structure. The occurrence of two superstructures is magnetically confirmed by symmetric inflection points in hysteresis measurements above the transition at about 30 K. The disappearance of the inflection points and the associated change of the hysteresis parameters indicate that the two superstructures become strongly coupled to form a unitary magnetic anisotropy system at the transition. From this it follows that the Besnus transition in monoclinic pyrrhotite is an extrinsic magnetic phenomenon with respect to the 4C superstructure and therefore the physics behind it is in fact different from that of the well-known Verwey transition.

Research paper thumbnail of Anisotropy of Bullet-Shaped Magnetite Nanoparticles in the Magnetotactic Bacteria Desulfovibrio magneticus sp. Strain RS-1

Biophysical Journal, Mar 1, 2015

Magnetotactic bacteria (MTB) build magnetic nanoparticles in chain configuration to generate a pe... more 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.

Research paper thumbnail of Ferromagnetic response of a sediment record from Lake Soppensee

Research paper thumbnail of Rock magnetic techniques complemented by ferromagnetic resonance spectroscopy to analyse a sediment record

Geophysical Journal International, Aug 22, 2012

Environmental magnetism uses the spatial and temporal occurrence of magnetic carriers as diagnost... more Environmental magnetism uses the spatial and temporal occurrence of magnetic carriers as diagnostic tools to detect environmental changes. Concentration, composition, grain size and configuration of the carriers inferred from magnetic properties are key parameters, because they are indicative of the formation conditions of magnetic phases, and/or of processes such as diagenesis and weathering. We present a detailed ferromagnetic resonance (FMR) spectroscopy analysis in concert with routinely used rock magnetic measurements to determine these parameters in a sediment record that documents the development of Lake Soppensee (Central Switzerland) since latest Pleistocene. FMR spectroscopy monitors varying concentration of the predominant magnetite/maghemite by the spectral signal intensity, whereas the stable single domain and superparamagnetic states are determined by the signal shape at room and low temperature. Fitting and simulation of FMR spectra are successfully applied to samples with well-defined magnetite components in the sediment matrix. Clear evidence for the colonization of magnetotactic bacteria (MTB) in Lake Soppensee was possible by applying empirical spectral separation to measured FMR signals that yield two magnetite populations differing in their configuration, that is, dispersed and aligned in chains. Low temperature measurements showed that these MTB can be preserved as pure or oxidized magnetite. The FMR data set confirms and completes rock magnetic information obtained from the lacustrine sedimentary record. The advanced application of FMR spectroscopy in the presented study critically highlights the benefit of this rapid and non-destructive method for future analysis of magnetic properties in environmental studies.

Research paper thumbnail of A high-resolution paleointensity stack of the past 14 to 68 ka from Black Sea sediments

Earth and Planetary Science Letters, Dec 1, 2013

Detailed paleo-and mineral magnetic analyses of a sediment composite record from the southeastern... more Detailed paleo-and mineral magnetic analyses of a sediment composite record from the southeastern Black Sea yielded a high-resolution, well-dated paleointensity record. Though hampered by some larger hiatuses in some cores, and contaminated by diagenetically formed greigite, the paleomagnetic composite record obtained from the preserved primary detrital magnetite phase reflects a highly dynamic geomagnetic field during the last glacial period. Relative variations of paleointensity inferred from the sediments' magnetisations were converted into a record of the virtual axial dipole moment (VADM). Lowest VADM values are linked with the Laschamp (0.50 × 10 22 A m 2 at 41.0 ka), the Norwegian-Greenland-Sea (1.5 × 10 22 A m 2 at 64.5 ka), and the Mono Lake (3.0 × 10 22 A m 2 at 34.5 ka) geomagnetic excursions. The fully reversed field during the Laschamp excursion exhibits a VADM of 2.0 × 10 22 A m 2 which is more than 25% of the present day axial dipole moment (7.628 × 10 22 A m 2). Rates of change calculated from the Black Sea VADM record also give some information on how to assess the global decay of the present-day geomagnetic field, which is significantly enhanced in the area of the South Atlantic Anomaly. Comparison with provided 14 C and 10 Be records confirm, partly in the very detail, the nonlinear anti-correlation of geomagnetic field intensity and the production of cosmogenic radionuclides in the Earth's upper atmosphere. However, discrepancies in the timing of lows and highs in the compiled records points out that the combination of different data sets from different archives remains a challenge.

Research paper thumbnail of Dynamics of the Laschamp geomagnetic excursion from Black Sea sediments

Earth and Planetary Science Letters, Oct 1, 2012

Research paper thumbnail of The formation of the Namib Sand Sea inferred from the spatial pattern of magnetic rock fragments

Earth and Planetary Science Letters, Jun 1, 2014

The Namib Sand Sea on the west coast of Namibia is one of the world's oldest desert region and ba... more The Namib Sand Sea on the west coast of Namibia is one of the world's oldest desert region and based on cosmogenic dating it has likely existed since the earlier Pleistocene. Among the possible sand sources, geomorphological and petrographic evidence points towards the Orange River catchment as the most prominent one. Little is known about the dynamics of transport and mixing of the sand during the desert formation and this is because the information about the Namib Sand Sea generally rests upon study sites at its edges. Here, we present a statistical analysis of magnetic components in sand samples collected along a south to north transect through the desert and at two inland sites. The magnetic components are rock fragments mainly of basaltic origin. Their statistically uniform distribution in the Namib Sand Sea indicates no significant sand source other than the Orange River and thus a predominant northward direction of the sand transport. A northward transport and the absence of a magnetic trend along the transect suggests mixing of the sand prior to its deposition in the Namib Sand Sea, most likely during river transport and under high current conditions along the shoreline. Finally, the uniform magnetic pattern provides compelling evidence for a stable erosion regime in the Orange River catchment with a steady release of magnetic components at least since the Pleistocene.

Research paper thumbnail of Magnetic anisotropy of non-interacting collinear nanocrystal-chains

Applied Physics Letters, Mar 17, 2014

The magnetic anisotropy of linear chains of spherical magnetite nanocrystals was investigated by ... more 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.

Research paper thumbnail of The detection of magnetotactic bacteria and magnetofossils by means of magnetic anisotropy

Earth and Planetary Science Letters, Sep 1, 2011

Magnetospirillum gryphiswaldense lake sediments assembly of magnetosomes cellular dipole chain fr... more Magnetospirillum gryphiswaldense lake sediments assembly of magnetosomes cellular dipole chain fragments An important characteristic of magnetotactic bacteria (MTB) is the anisotropy of one-dimensionally aligned magnetite particles. This paper introduces the use of ferromagnetic resonance spectroscopy (FMR) at two different frequencies to compare the anisotropic properties of magnetite chains of cultured intact MTB with those of lake sediments of Holocene age in order to detect magnetofossils and to characterize their preservation in a geological system. Magnetite chains of intact MTB exhibit a predominantly uniaxial anisotropy. In the lake sediments, where diagenetic processes disintegrate the chains and diminish their uniaxiality, magnetite chains or chain fragments and dissociated bulk magnetite particles differ in their anisotropy properties. The two groups of assembly can be distinguished by empirical spectral separation of the FMR signal. This straightforward use of the characteristics of magnetic anisotropy provides a way to detect magnetofossils experimentally, thus allowing a better insight into microbial ecology during Earth's history.

Research paper thumbnail of S-band ferromagnetic resonance spectroscopy and the detection of magnetofossils

Journal of the Royal Society Interface, Mar 6, 2013

We report the use of S-band ferromagnetic resonance (FMR) spectroscopy to compare the anisotropic... more We report the use of S-band ferromagnetic resonance (FMR) spectroscopy to compare the anisotropic properties of magnetite particles in chains of cultured intact magnetotactic bacteria (MTB) between 300 and 15 K with those of sediment samples of Holocene age in order to infer the presence of magnetofossils and their preservation in a geological time frame. The spectrum of intact MTB at 300 K exhibits distinct uniaxial anisotropy because of the chain alignment of the cellular magnetite particles and their easy axes. This anisotropy becomes less pronounced upon cooling and below the Verwey transition (T V) it is nearly vanished mainly owing to the change of direction of the easy axes. In a natural sample, magnetofossils were detected by uniaxial anisotropy traits similar to those obtained from cultured MTB above T V. Our comparative study emphasizes that indispensable information can be obtained from S-band FMR spectra, which offers even a better resolution than X-band FMR for discovering magnetofossils, and this in turn can contribute towards strengthening our relatively sparse database for deciphering the microbial ecology during the Earth's history.

Research paper thumbnail of detection of magnetofossils S-band ferromagnetic resonance spectroscopy and the

Cite this article: Gehring AU, Kind J,Charilaou M, Garci´a-Rubio I. 2013 S-bandferromagnetic reso... more Cite this article: Gehring AU, Kind J,Charilaou M, Garci´a-Rubio I. 2013 S-bandferromagnetic resonance spectroscopy and thedetection of magnetofossils. J R Soc Interface10: 20120790.http://dx.doi.org/10.1098/rsif.2012.0790Received: 1 October 2012Accepted: 30 November 2012Subject Areas:biophysics, environmental science, biomaterialsKeywords:S-band ferromagnetic resonance spectroscopy,Magnetospirillum gryphiswaldense, cellulardipole, magnetofossils, chain fragments,lake sedimentsAuthor for correspondence:Andreas U. Gehringe-mail: agehring@erdw.ethz.ch

Research paper thumbnail of Variable defect structures cause the magnetic low-temperature transition in natural monoclinic pyrrhotite

Geophysical Journal International, 2015

Non-stoichiometric monoclinic 4C pyrrhotite (Fe 7 S 8) is a major magnetic remanence carrier in t... more Non-stoichiometric monoclinic 4C pyrrhotite (Fe 7 S 8) is a major magnetic remanence carrier in the Earth's crust and in extraterrestrial materials. Because of its low-temperature magnetic transition around 30 K also known as Besnus transition, which is considered to be an intrinsic property, this mineral phase is easily detectable in natural samples. Although the physical properties of pyrrhotite have intensively been studied, the mechanism behind the pronounced change in magnetization at the low-temperature transition is still debated. Here we report magnetization experiments on a pyrrhotite crystal (Fe 6.6 S 8) that consists of a 4C and an incommensurate 5C * superstructure that are different in their defect structure. The occurrence of two superstructures is magnetically confirmed by symmetric inflection points in hysteresis measurements above the transition at about 30 K. The disappearance of the inflection points and the associated change of the hysteresis parameters indicate that the two superstructures become strongly coupled to form a unitary magnetic anisotropy system at the transition. From this it follows that the Besnus transition in monoclinic pyrrhotite is an extrinsic magnetic phenomenon with respect to the 4C superstructure and therefore the physics behind it is in fact different from that of the well-known Verwey transition.

Research paper thumbnail of Magneto-electronic coupling in modulated defect-structures of natural Fe1−xS

Journal of Applied Physics, 2015

We provide compelling experimental evidence that the low-temperature transition in natural non-st... more We provide compelling experimental evidence that the low-temperature transition in natural non-stoichiometric Fe7S8, a major magnetic remanence carrier in the Earth's crust and in extraterrestrial materials, is a phenomenon caused by magnetic coupling between epitaxially intergrown superstructures. The two superstructures differ in their defect distribution, and consequently in their magnetic anisotropy. At T < 30 K, the magnetic moments of the superstructures become strongly coupled, resulting in a 12-fold anisotropy symmetry, which is reflected in the anisotropic magneto-resistance.

Research paper thumbnail of The unstable geomagnetic field during the last glacial

Detailed stratigraphic analyses of a sediment composite record from three different sites in the ... more Detailed stratigraphic analyses of a sediment composite record from three different sites in the southeastern Black Sea yielded a high-resolution, well-dated paleomagnetic record of the past 14 to 68 ka. Age constraints are provided by 16 AMS 14C ages, identification of the Campanian Ignimbrite tephra (39.28±0.11 ka), and by detailed tuning of sedimentologic parameters of the Black Sea sediments to the oxygen isotope record from the Greenland NGRIP ice core. Dansgaard-Oeschger events 3 through 18 are very well expressed in the Black Sea sedimentary records of Ca-content, oxygen isotopes as well as in records of ice-rafted detritus. Though hampered by some larger hiatusses at one site, and patchy contaminations by diagenetically formed greigite, the paleomagnetic composite record obtained from the preserved primary detrital magnetite phase reflects a highly dynamic geomagnetic field during the last glacial period. Relative variations of paleointensity inferred from the sediments'...

Research paper thumbnail of Anisotropy of Bullet-Shaped Magnetite Nanoparticles in the Magnetotactic Bacteria Desulfovibrio magneticus sp. Strain RS-1

Biophysical Journal, 2015

Research paper thumbnail of S-band ferromagnetic resonance spectroscopy and the detection of magnetofossils

Journal of The Royal Society Interface, 2012

We report the use of S-band ferromagnetic resonance (FMR) spectroscopy to compare the anisotropic... more We report the use of S-band ferromagnetic resonance (FMR) spectroscopy to compare the anisotropic properties of magnetite particles in chains of cultured intact magnetotactic bacteria (MTB) between 300 and 15 K with those of sediment samples of Holocene age in order to infer the presence of magnetofossils and their preservation in a geological time frame. The spectrum of intact MTB at 300 K exhibits distinct uniaxial anisotropy because of the chain alignment of the cellular magnetite particles and their easy axes. This anisotropy becomes less pronounced upon cooling and below the Verwey transition (TV) it is nearly vanished mainly owing to the change of direction of the easy axes. In a natural sample, magnetofossils were detected by uniaxial anisotropy traits similar to those obtained from cultured MTB aboveTV. Our comparative study emphasizes that indispensable information can be obtained from S-band FMR spectra, which offers even a better resolution than X-band FMR for discovering...

Research paper thumbnail of Rock magnetic techniques complemented by ferromagnetic resonance spectroscopy to analyse a sediment record

Geophysical Journal International, 2012

Environmental magnetism uses the spatial and temporal occurrence of magnetic carriers as diagnost... more Environmental magnetism uses the spatial and temporal occurrence of magnetic carriers as diagnostic tools to detect environmental changes. Concentration, composition, grain size and configuration of the carriers inferred from magnetic properties are key parameters, because they are indicative of the formation conditions of magnetic phases, and/or of processes such as diagenesis and weathering. We present a detailed ferromagnetic resonance (FMR) spectroscopy analysis in concert with routinely used rock magnetic measurements to determine these parameters in a sediment record that documents the development of Lake Soppensee (Central Switzerland) since latest Pleistocene. FMR spectroscopy monitors varying concentration of the predominant magnetite/maghemite by the spectral signal intensity, whereas the stable single domain and superparamagnetic states are determined by the signal shape at room and low temperature. Fitting and simulation of FMR spectra are successfully applied to samples with well-defined magnetite components in the sediment matrix. Clear evidence for the colonization of magnetotactic bacteria (MTB) in Lake Soppensee was possible by applying empirical spectral separation to measured FMR signals that yield two magnetite populations differing in their configuration, that is, dispersed and aligned in chains. Low temperature measurements showed that these MTB can be preserved as pure or oxidized magnetite. The FMR data set confirms and completes rock magnetic information obtained from the lacustrine sedimentary record. The advanced application of FMR spectroscopy in the presented study critically highlights the benefit of this rapid and non-destructive method for future analysis of magnetic properties in environmental studies.

Research paper thumbnail of Domain-wall dynamics in 4C pyrrhotite at low temperature

Geophysical Journal International, 2013

Monoclinic 4C pyrrhotite (Fe 7 S 8) is ferrimagnetic due to an ordered defect structure with alte... more 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.

Research paper thumbnail of Magneto-electronic coupling in modulated defect-structures of natural Fe<sub>1−</sub><sub><i>x</i></sub>S

Journal of Applied Physics, Aug 28, 2015

We provide compelling experimental evidence that the low-temperature transition in natural non-st... more We provide compelling experimental evidence that the low-temperature transition in natural non-stoichiometric Fe 7 S 8 , a major magnetic remanence carrier in the Earth's crust and in extraterrestrial materials, is a phenomenon caused by magnetic coupling between epitaxially intergrown superstructures. The two superstructures differ in their defect distribution, and consequently in their magnetic anisotropy. At T < 30 K, the magnetic moments of the superstructures become strongly coupled, resulting in a 12-fold anisotropy symmetry, which is reflected in the anisotropic magnetoresistance. V

Research paper thumbnail of Pyrrhotite domain dynamics

Research paper thumbnail of Variable defect structures cause the magnetic low-temperature transition in natural monoclinic pyrrhotite

Geophysical Journal International, Dec 15, 2015

Non-stoichiometric monoclinic 4C pyrrhotite (Fe 7 S 8) is a major magnetic remanence carrier in t... more Non-stoichiometric monoclinic 4C pyrrhotite (Fe 7 S 8) is a major magnetic remanence carrier in the Earth's crust and in extraterrestrial materials. Because of its low-temperature magnetic transition around 30 K also known as Besnus transition, which is considered to be an intrinsic property, this mineral phase is easily detectable in natural samples. Although the physical properties of pyrrhotite have intensively been studied, the mechanism behind the pronounced change in magnetization at the low-temperature transition is still debated. Here we report magnetization experiments on a pyrrhotite crystal (Fe 6.6 S 8) that consists of a 4C and an incommensurate 5C * superstructure that are different in their defect structure. The occurrence of two superstructures is magnetically confirmed by symmetric inflection points in hysteresis measurements above the transition at about 30 K. The disappearance of the inflection points and the associated change of the hysteresis parameters indicate that the two superstructures become strongly coupled to form a unitary magnetic anisotropy system at the transition. From this it follows that the Besnus transition in monoclinic pyrrhotite is an extrinsic magnetic phenomenon with respect to the 4C superstructure and therefore the physics behind it is in fact different from that of the well-known Verwey transition.

Research paper thumbnail of Anisotropy of Bullet-Shaped Magnetite Nanoparticles in the Magnetotactic Bacteria Desulfovibrio magneticus sp. Strain RS-1

Biophysical Journal, Mar 1, 2015

Magnetotactic bacteria (MTB) build magnetic nanoparticles in chain configuration to generate a pe... more 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.

Research paper thumbnail of Ferromagnetic response of a sediment record from Lake Soppensee

Research paper thumbnail of Rock magnetic techniques complemented by ferromagnetic resonance spectroscopy to analyse a sediment record

Geophysical Journal International, Aug 22, 2012

Environmental magnetism uses the spatial and temporal occurrence of magnetic carriers as diagnost... more Environmental magnetism uses the spatial and temporal occurrence of magnetic carriers as diagnostic tools to detect environmental changes. Concentration, composition, grain size and configuration of the carriers inferred from magnetic properties are key parameters, because they are indicative of the formation conditions of magnetic phases, and/or of processes such as diagenesis and weathering. We present a detailed ferromagnetic resonance (FMR) spectroscopy analysis in concert with routinely used rock magnetic measurements to determine these parameters in a sediment record that documents the development of Lake Soppensee (Central Switzerland) since latest Pleistocene. FMR spectroscopy monitors varying concentration of the predominant magnetite/maghemite by the spectral signal intensity, whereas the stable single domain and superparamagnetic states are determined by the signal shape at room and low temperature. Fitting and simulation of FMR spectra are successfully applied to samples with well-defined magnetite components in the sediment matrix. Clear evidence for the colonization of magnetotactic bacteria (MTB) in Lake Soppensee was possible by applying empirical spectral separation to measured FMR signals that yield two magnetite populations differing in their configuration, that is, dispersed and aligned in chains. Low temperature measurements showed that these MTB can be preserved as pure or oxidized magnetite. The FMR data set confirms and completes rock magnetic information obtained from the lacustrine sedimentary record. The advanced application of FMR spectroscopy in the presented study critically highlights the benefit of this rapid and non-destructive method for future analysis of magnetic properties in environmental studies.

Research paper thumbnail of A high-resolution paleointensity stack of the past 14 to 68 ka from Black Sea sediments

Earth and Planetary Science Letters, Dec 1, 2013

Detailed paleo-and mineral magnetic analyses of a sediment composite record from the southeastern... more Detailed paleo-and mineral magnetic analyses of a sediment composite record from the southeastern Black Sea yielded a high-resolution, well-dated paleointensity record. Though hampered by some larger hiatuses in some cores, and contaminated by diagenetically formed greigite, the paleomagnetic composite record obtained from the preserved primary detrital magnetite phase reflects a highly dynamic geomagnetic field during the last glacial period. Relative variations of paleointensity inferred from the sediments' magnetisations were converted into a record of the virtual axial dipole moment (VADM). Lowest VADM values are linked with the Laschamp (0.50 × 10 22 A m 2 at 41.0 ka), the Norwegian-Greenland-Sea (1.5 × 10 22 A m 2 at 64.5 ka), and the Mono Lake (3.0 × 10 22 A m 2 at 34.5 ka) geomagnetic excursions. The fully reversed field during the Laschamp excursion exhibits a VADM of 2.0 × 10 22 A m 2 which is more than 25% of the present day axial dipole moment (7.628 × 10 22 A m 2). Rates of change calculated from the Black Sea VADM record also give some information on how to assess the global decay of the present-day geomagnetic field, which is significantly enhanced in the area of the South Atlantic Anomaly. Comparison with provided 14 C and 10 Be records confirm, partly in the very detail, the nonlinear anti-correlation of geomagnetic field intensity and the production of cosmogenic radionuclides in the Earth's upper atmosphere. However, discrepancies in the timing of lows and highs in the compiled records points out that the combination of different data sets from different archives remains a challenge.

Research paper thumbnail of Dynamics of the Laschamp geomagnetic excursion from Black Sea sediments

Earth and Planetary Science Letters, Oct 1, 2012

Research paper thumbnail of The formation of the Namib Sand Sea inferred from the spatial pattern of magnetic rock fragments

Earth and Planetary Science Letters, Jun 1, 2014

The Namib Sand Sea on the west coast of Namibia is one of the world's oldest desert region and ba... more The Namib Sand Sea on the west coast of Namibia is one of the world's oldest desert region and based on cosmogenic dating it has likely existed since the earlier Pleistocene. Among the possible sand sources, geomorphological and petrographic evidence points towards the Orange River catchment as the most prominent one. Little is known about the dynamics of transport and mixing of the sand during the desert formation and this is because the information about the Namib Sand Sea generally rests upon study sites at its edges. Here, we present a statistical analysis of magnetic components in sand samples collected along a south to north transect through the desert and at two inland sites. The magnetic components are rock fragments mainly of basaltic origin. Their statistically uniform distribution in the Namib Sand Sea indicates no significant sand source other than the Orange River and thus a predominant northward direction of the sand transport. A northward transport and the absence of a magnetic trend along the transect suggests mixing of the sand prior to its deposition in the Namib Sand Sea, most likely during river transport and under high current conditions along the shoreline. Finally, the uniform magnetic pattern provides compelling evidence for a stable erosion regime in the Orange River catchment with a steady release of magnetic components at least since the Pleistocene.

Research paper thumbnail of Magnetic anisotropy of non-interacting collinear nanocrystal-chains

Applied Physics Letters, Mar 17, 2014

The magnetic anisotropy of linear chains of spherical magnetite nanocrystals was investigated by ... more 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.

Research paper thumbnail of The detection of magnetotactic bacteria and magnetofossils by means of magnetic anisotropy

Earth and Planetary Science Letters, Sep 1, 2011

Magnetospirillum gryphiswaldense lake sediments assembly of magnetosomes cellular dipole chain fr... more Magnetospirillum gryphiswaldense lake sediments assembly of magnetosomes cellular dipole chain fragments An important characteristic of magnetotactic bacteria (MTB) is the anisotropy of one-dimensionally aligned magnetite particles. This paper introduces the use of ferromagnetic resonance spectroscopy (FMR) at two different frequencies to compare the anisotropic properties of magnetite chains of cultured intact MTB with those of lake sediments of Holocene age in order to detect magnetofossils and to characterize their preservation in a geological system. Magnetite chains of intact MTB exhibit a predominantly uniaxial anisotropy. In the lake sediments, where diagenetic processes disintegrate the chains and diminish their uniaxiality, magnetite chains or chain fragments and dissociated bulk magnetite particles differ in their anisotropy properties. The two groups of assembly can be distinguished by empirical spectral separation of the FMR signal. This straightforward use of the characteristics of magnetic anisotropy provides a way to detect magnetofossils experimentally, thus allowing a better insight into microbial ecology during Earth's history.

Research paper thumbnail of S-band ferromagnetic resonance spectroscopy and the detection of magnetofossils

Journal of the Royal Society Interface, Mar 6, 2013

We report the use of S-band ferromagnetic resonance (FMR) spectroscopy to compare the anisotropic... more We report the use of S-band ferromagnetic resonance (FMR) spectroscopy to compare the anisotropic properties of magnetite particles in chains of cultured intact magnetotactic bacteria (MTB) between 300 and 15 K with those of sediment samples of Holocene age in order to infer the presence of magnetofossils and their preservation in a geological time frame. The spectrum of intact MTB at 300 K exhibits distinct uniaxial anisotropy because of the chain alignment of the cellular magnetite particles and their easy axes. This anisotropy becomes less pronounced upon cooling and below the Verwey transition (T V) it is nearly vanished mainly owing to the change of direction of the easy axes. In a natural sample, magnetofossils were detected by uniaxial anisotropy traits similar to those obtained from cultured MTB above T V. Our comparative study emphasizes that indispensable information can be obtained from S-band FMR spectra, which offers even a better resolution than X-band FMR for discovering magnetofossils, and this in turn can contribute towards strengthening our relatively sparse database for deciphering the microbial ecology during the Earth's history.

Research paper thumbnail of detection of magnetofossils S-band ferromagnetic resonance spectroscopy and the

Cite this article: Gehring AU, Kind J,Charilaou M, Garci´a-Rubio I. 2013 S-bandferromagnetic reso... more Cite this article: Gehring AU, Kind J,Charilaou M, Garci´a-Rubio I. 2013 S-bandferromagnetic resonance spectroscopy and thedetection of magnetofossils. J R Soc Interface10: 20120790.http://dx.doi.org/10.1098/rsif.2012.0790Received: 1 October 2012Accepted: 30 November 2012Subject Areas:biophysics, environmental science, biomaterialsKeywords:S-band ferromagnetic resonance spectroscopy,Magnetospirillum gryphiswaldense, cellulardipole, magnetofossils, chain fragments,lake sedimentsAuthor for correspondence:Andreas U. Gehringe-mail: agehring@erdw.ethz.ch

Research paper thumbnail of Variable defect structures cause the magnetic low-temperature transition in natural monoclinic pyrrhotite

Geophysical Journal International, 2015

Non-stoichiometric monoclinic 4C pyrrhotite (Fe 7 S 8) is a major magnetic remanence carrier in t... more Non-stoichiometric monoclinic 4C pyrrhotite (Fe 7 S 8) is a major magnetic remanence carrier in the Earth's crust and in extraterrestrial materials. Because of its low-temperature magnetic transition around 30 K also known as Besnus transition, which is considered to be an intrinsic property, this mineral phase is easily detectable in natural samples. Although the physical properties of pyrrhotite have intensively been studied, the mechanism behind the pronounced change in magnetization at the low-temperature transition is still debated. Here we report magnetization experiments on a pyrrhotite crystal (Fe 6.6 S 8) that consists of a 4C and an incommensurate 5C * superstructure that are different in their defect structure. The occurrence of two superstructures is magnetically confirmed by symmetric inflection points in hysteresis measurements above the transition at about 30 K. The disappearance of the inflection points and the associated change of the hysteresis parameters indicate that the two superstructures become strongly coupled to form a unitary magnetic anisotropy system at the transition. From this it follows that the Besnus transition in monoclinic pyrrhotite is an extrinsic magnetic phenomenon with respect to the 4C superstructure and therefore the physics behind it is in fact different from that of the well-known Verwey transition.

Research paper thumbnail of Magneto-electronic coupling in modulated defect-structures of natural Fe1−xS

Journal of Applied Physics, 2015

We provide compelling experimental evidence that the low-temperature transition in natural non-st... more We provide compelling experimental evidence that the low-temperature transition in natural non-stoichiometric Fe7S8, a major magnetic remanence carrier in the Earth's crust and in extraterrestrial materials, is a phenomenon caused by magnetic coupling between epitaxially intergrown superstructures. The two superstructures differ in their defect distribution, and consequently in their magnetic anisotropy. At T < 30 K, the magnetic moments of the superstructures become strongly coupled, resulting in a 12-fold anisotropy symmetry, which is reflected in the anisotropic magneto-resistance.

Research paper thumbnail of The unstable geomagnetic field during the last glacial

Detailed stratigraphic analyses of a sediment composite record from three different sites in the ... more Detailed stratigraphic analyses of a sediment composite record from three different sites in the southeastern Black Sea yielded a high-resolution, well-dated paleomagnetic record of the past 14 to 68 ka. Age constraints are provided by 16 AMS 14C ages, identification of the Campanian Ignimbrite tephra (39.28±0.11 ka), and by detailed tuning of sedimentologic parameters of the Black Sea sediments to the oxygen isotope record from the Greenland NGRIP ice core. Dansgaard-Oeschger events 3 through 18 are very well expressed in the Black Sea sedimentary records of Ca-content, oxygen isotopes as well as in records of ice-rafted detritus. Though hampered by some larger hiatusses at one site, and patchy contaminations by diagenetically formed greigite, the paleomagnetic composite record obtained from the preserved primary detrital magnetite phase reflects a highly dynamic geomagnetic field during the last glacial period. Relative variations of paleointensity inferred from the sediments'...

Research paper thumbnail of Anisotropy of Bullet-Shaped Magnetite Nanoparticles in the Magnetotactic Bacteria Desulfovibrio magneticus sp. Strain RS-1

Biophysical Journal, 2015

Research paper thumbnail of S-band ferromagnetic resonance spectroscopy and the detection of magnetofossils

Journal of The Royal Society Interface, 2012

We report the use of S-band ferromagnetic resonance (FMR) spectroscopy to compare the anisotropic... more We report the use of S-band ferromagnetic resonance (FMR) spectroscopy to compare the anisotropic properties of magnetite particles in chains of cultured intact magnetotactic bacteria (MTB) between 300 and 15 K with those of sediment samples of Holocene age in order to infer the presence of magnetofossils and their preservation in a geological time frame. The spectrum of intact MTB at 300 K exhibits distinct uniaxial anisotropy because of the chain alignment of the cellular magnetite particles and their easy axes. This anisotropy becomes less pronounced upon cooling and below the Verwey transition (TV) it is nearly vanished mainly owing to the change of direction of the easy axes. In a natural sample, magnetofossils were detected by uniaxial anisotropy traits similar to those obtained from cultured MTB aboveTV. Our comparative study emphasizes that indispensable information can be obtained from S-band FMR spectra, which offers even a better resolution than X-band FMR for discovering...

Research paper thumbnail of Rock magnetic techniques complemented by ferromagnetic resonance spectroscopy to analyse a sediment record

Geophysical Journal International, 2012

Environmental magnetism uses the spatial and temporal occurrence of magnetic carriers as diagnost... more Environmental magnetism uses the spatial and temporal occurrence of magnetic carriers as diagnostic tools to detect environmental changes. Concentration, composition, grain size and configuration of the carriers inferred from magnetic properties are key parameters, because they are indicative of the formation conditions of magnetic phases, and/or of processes such as diagenesis and weathering. We present a detailed ferromagnetic resonance (FMR) spectroscopy analysis in concert with routinely used rock magnetic measurements to determine these parameters in a sediment record that documents the development of Lake Soppensee (Central Switzerland) since latest Pleistocene. FMR spectroscopy monitors varying concentration of the predominant magnetite/maghemite by the spectral signal intensity, whereas the stable single domain and superparamagnetic states are determined by the signal shape at room and low temperature. Fitting and simulation of FMR spectra are successfully applied to samples with well-defined magnetite components in the sediment matrix. Clear evidence for the colonization of magnetotactic bacteria (MTB) in Lake Soppensee was possible by applying empirical spectral separation to measured FMR signals that yield two magnetite populations differing in their configuration, that is, dispersed and aligned in chains. Low temperature measurements showed that these MTB can be preserved as pure or oxidized magnetite. The FMR data set confirms and completes rock magnetic information obtained from the lacustrine sedimentary record. The advanced application of FMR spectroscopy in the presented study critically highlights the benefit of this rapid and non-destructive method for future analysis of magnetic properties in environmental studies.

Research paper thumbnail of Domain-wall dynamics in 4C pyrrhotite at low temperature

Geophysical Journal International, 2013

Monoclinic 4C pyrrhotite (Fe 7 S 8) is ferrimagnetic due to an ordered defect structure with alte... more 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.