Low-field Magnetic Susceptibility Research Articles

Investigation of electronic properties of alloyed double metal ring

The present work is a theoretical study of the electronic properties of alloyed double metal ring (ADMR) threaded by magnetic flux. The studied system is consisting of two rings connected parallel, with two different types of atomic sites. This work contains two parts. First is an analytical study of calculation of the energy spectrum, persistent current, Drude weight and low-field magnetic susceptibility of ADMR by the tight-binding method. Second is a computation study of electronic properties of the alloyed double metal ring by density functional theory (DFT). The latter part is considered as a complementary study of what was studied in the first part in order to determine the best materials which will be used to design alloyed double metal ring. Our results show that the on-site energies for both types of atoms, inter-ring coupling strength and hopping strengths play an interesting role in controlling the electronic properties.

Preparation, Crystal Chemistry, and Hidden Magnetic Order in the Family of Trigonal Layered Tellurates A2Mn(4+)TeO6 (A = Li, Na, Ag, or Tl).

We report the first four magnetic representatives of the trigonal layered A2M(4+)TeO6 (here, M = Mn) family. Na2MnTeO6 was synthesized from NaMnO2, NaNO3, and TeO2 at 650-720 °C, but analogues for which A = Li and K could not be obtained by direct synthesis. However, those for which A = Li, Ag, and Tl (but not K) were prepared by exchange reactions between Na2MnTeO6 and the corresponding molten nitrates. The oxygen content was verified by redox titration. According to the X-ray diffraction Rietveld analysis, the four new compounds are isostructural with Na2GeTeO6, trigonal ( P3̅1 c), based on ilmenite-like layers of edge-shared oxygen octahedra occupied by Mn(4+) and Te(6+) in an ordered manner. These layers are separated by cations A, also in a distorted octahedral coordination. However, off-center displacement of Tl+ is so strong, due to the lone-pair effect, that its coordination is better described as trigonal pyramid. Each MnO6 octahedron shares two opposite faces with AO6 octahedra, whereas TeO6 octahedra avoid sharing faces. Besides this double-layered structure, Na2MnTeO6 was often accompanied by a transient triple-layered rhombohedral polytype. However, it could not be prepared as a single phase and disappeared on annealing at 700-720 °C. All A2MnTeO6 samples (A = Ag, Li, Na, or Tl) revealed the unusual phenomenon of hidden magnetic order. Low-field magnetic susceptibility data exhibit a Curie-Weiss type behavior for all samples under study and do not show any sign of the establishment of long-range magnetic order down to 2 K. In contrast, both the magnetic susceptibility in sufficiently high external magnetic fields and the zero-field specific heat unambiguously revealed an onset of antiferromagnetic order at low temperatures. The frustration index f = Θ/ TN takes values larger than the classical values for three-dimensional antiferromagnets and implies moderate frustration on the triangular lattice.

NEOGENE TECTONIC ROTATIONS IN THE VICINITY OF THE NORTH AEGEAN TROUGH: NEW PALAEOMAGNETIC EVIDENCE FROM ATHOS AND SAMOTHRAKI (GREECE)

The present study focuses on two post-orogenic plutons, the Athos (Grigoriou) and Samothraki granites, as well as the Samothraki volcanics, located in the vicinity of the North Aegean Trough. A detailed palaeomagnetic study was carried out, with the aim of constraining the age and mechanism of tectonic rotations. In addition, anisotropy of low field magnetic susceptibility (AMS) was studied and isothermal remanent magnetization (IRM) and thermomagnetic analyses were performed. Finally, a radiometric age for the Athos granite was obtained (43.3 ± 1.0 Ma K/Ar biotite). The measured declinations indicate clockwise rotations of the Athos (16.6°) and Samothraki (36.3°) blocks. The age of rotation is constrained to be <18 Ma at Samothraki, whereas the much smaller rotation of the Athos block can only be dated as younger than Eocene. Comparing the new palaeomagnetic data to the published dataset for Northern Greece, we suggest that the palaeomagnetically determined rotations in the vicinity of the North Aegean Trough are dominantly of post-Early Miocene age, and are controlled by major strike-slip faults and distributed "small" or minor faults.

Rock Magnetic Approaches Used on Deep-sea Sediments in the Northeastern Equatorial Pacific

Rock magnetic properties of unconsolidated sediments from the Korea Deep Ocean Study area of the northeastern equatorial Pacific were analyzed to trace the time-dependent variations of sedimentary environments. For upper Pleistocene sediments, light-brown sediments predominate, whereas the lower sediments deposited in the late Pliocene are dark brown. Rock magnetic properties also clearly differ among the two distinct color environments. Values of anhysteretic remanent magnetization and low-field magnetic susceptibility are highly responsive to changes in the color of the sediment. For example, comparatively low values denote a dominance of coarse-grained magnetic minerals as observed in the lower dark-brown layers. With respect to the content of magnetic minerals, coarse magnetic grains are highly concentrated in the darker-colored sediment layers. However, both the magnetic mineral dependent parameter (S ratio) and results of the scanning electron microscope observation indicate that magnetic mineral changes did not occur along the core depth, which means that there was no apparent source change. Without distinctive source changes, the variations in the rock magnetic properties likely reflect a process by which the magnetic grains were primarily transported by enhanced wind and bottom currents and affected by diagenetic dissolution as a function of both the time from burial and the extremely low sedimentation rates after deposition.

Enhanced moments of Eu in single crystals of the metallic helical antiferromagnetEuCo2−yAs2

The compound EuCo{2-y}As2 with the tetragonal ThCr2Si2 structure is known to contain Eu{+2} ions with spin S = 7/2 that order below a temperature TN = 47 K into an antiferromagnetic (AFM) proper helical structure with the ordered moments aligned in the tetragonal ab plane, perpendicular to the helix axis along the c axis, with no contribution from the Co atoms. Here we carry out a detailed investigation of the properties of single crystals. Enhanced ordered and effective moments of the Eu spins are found in most of our crystals. Electronic structure calculations indicate that the enhanced moments arise from polarization of the d bands, as occurs in ferromagnetic Gd metal. Electrical resistivity measurements indicate metallic behavior. The low-field in-plane magnetic susceptibilities chi{ab}(T < TN) for several crystals are reported that are fitted well by unified molecular field theory (MFT), and the Eu-Eu exchange interactions Jij are extracted from the fits. High-field magnetization M data for magnetic fields H||ab reveal what appears to be a first-order spin-flop transition followed at higher field by a second-order metamagnetic transition of unknown origin, and then by another second-order transition to the paramagnetic (PM) state. For H||c, the magnetization shows only a second-order transition from the canted AFM to the PM state, as expected. The critical fields for the AFM to PM transition are in approximate agreement with the predictions of MFT. Heat capacity Cp measurements in zero and high H are reported. Phase diagrams for H||c and H||ab versus T are constructed from the high-field M(H,T) and Cp(H,T) measurements. The magnetic part Cmag(T, H = 0) of Cp(T, H = 0) is extracted and is fitted rather well below TN by MFT, although dynamic short-range AFM order is apparent in Cmag(T) up to about 70 K, where the molar entropy attains its high-T limit of R ln8.

The magnetic state in the binary DyxGe1−x (x ≤ 0.02) alloy semiconductor

Diluted bulk magnetic alloy semiconductors DyxGe1−x (x ≤ 0.02) were prepared by a two-step fabrication procedure and studied experimentally by XRD, EDX and magnetic methods. The polycrystalline materials had the ccp (cubic closed-packed) structure of the host Germanium up to ≈2 at.% of Dysprosium. They exhibited different low-temperature and high-temperature behavior. Low-field dc magnetic susceptibility data showed sharp peaks and irreversibility between zero-field cooled and field-cooled states for x < 0.02. A stable antiferromagnetic phase with temperature of the antiferromagnetic to paramagnetic transition TN = 25 K was observed at x = 2 at.% of Dysprosium. The solubility range of the binary DyxGe1−x alloy system is limited to ∼2 at.% due to the large atomic radius of the solute. The low-temperature magnetic phase is characterized as spin-glass below 2 at.%. The magnetic relaxation and non-linear susceptibility χnl of Dy0.01Ge0.99 were analyzed, and the spin-glass phase was defined with the critical exponents of the phase transition β = 0.52 ± 0.10, γ = 2.85 ± 0.10 and δ = 6.49 ± 0.02. From the Curie-Weiss behavior of the susceptibility at high temperatures, we determined an average effective Dy-Dy exchange constant J1 = −6.90 K and effective magnetic moment per Dy ion µeff = 10.08 µB. The rare-earth Dy atoms behave as isolated in Ge matrix at high-temperatures.

Environmental magnetic records of PETM in the southwestern Sichuan Basin

Collision between the Indian and Eurasian plates produced concomitant uplift of the Tibetan Plateau and its basin-ridge geomorphological systems. As the world’s largest and highest plateau, Tibetan Plateau surface relief has significant dynamic and thermal effects on atmospheric circulation and on regional and global climate. Large numbers of Cenozoic sedimentary basins that developed concomitantly in and around the Tibetan Plateau are attributed to intracontinental deformation and Tibetan Plateau uplift due to India-Eurasia collision, such as the Sichuan Basin which is located in the eastern margin of the Tibetan Plateau. During the Cenozoic, the Sichuan Basin has received thick sediments which provide us an excellent opportunity to reconstruct the tectonic and climatic evolution of the southeastern Tibetan Plateau areas. In this study, we present results from a long sedimentary sequence (thick of 742 m) at Shiyang section (102°50′E, 30°0′N) from the southwestern Sichuan Basin. Magnetostratigraphic sequences were constructed for Shiyang section which dates the Mingshan Formation to ~65–42 Ma. Mass-specific low field magnetic susceptibility ( χ ), anhysteretic remanent magnetization (ARM) and isothermal remanent magnetization (IRM) sequences of 761 bulk samples were measured at the Paleomagnetism Laboratory of the Institute of Geology and Geophysics, Chinese Academy of Sciences, Beijing. Thus, χ arm and S -ratio can be calculated. These environmental magnetism sequences display that there is a distinct peak in the stratigraphic interval 340–305 m. Eight samples were selected for detailed rock magnetism including temperature-dependent susceptibility ( κ - T ) curves and hysteresis loops. Rock magnetism results display that samples from the 340–305 m interval are dominated magnetically by magnetite and maghemite particles. However, magnetic minerals are dominated by hematite particles for samples outside of the 340–305 m interval. Inorganic carbon isotope results of the selected 13 samples display that there exist some distinct negative shifts of carbon isotope values in the 340–305 m interval. The maximum negative shift can reach −10‰. Combining results of geochronology, environmental magnetism and carbon isotope of Cenozoic deposits from the Shiyang section, we can define that the PETM (Paleogene-Eocene Thermal Maximum) palaeoclimatic event was recorded in the 340–305 m interval at Shiyang section. Discovery of the PETM in the Sichuan Basin enriches the PETM records in the world’s terrestrial sediments and provides a new view to reveal its processes and mechanism. The various curves of high-resolution environmental magnetism in the 340–305 m interval are comparable to results of the reported carbon isotope from other terrestrial and marine sediments, such as the Nanyang Basin in China, the Bighorn Basin in USA, and the ODP 690B core in Antarctica Ocean. This indicates that environmental magnetism sequences in this study can be used as reliable palaeoclimatic proxies to unveil the PETM. After learning more geoscience records and the paleoenvironment conditions they reveal in different areas in the world during the PETM, we suppose that the peak values of the environmental magnetism sequences in this study were attributed to increased palaeoprecipitation during the PETM period. Firstly, increased precipitation led to chemical weathering enhancement and facilitated the decomposition of silicate minerals and separation of Fe2+ which could quickly transfer into ferromagnetic minerals. Secondly, increased precipitation also contributed to enhanced hydrodynamic force of rivers. This accelerated the depositional processes of the clastic materials and shortened the oxidization time of magnetic debris. It may be another reason for the increased content of ferromagnetic minerals in the stratigraphic interval of PETM. Therefore, changes in environmental magnetic curves could be linked with the detailed process of the PETM.

The magnetic method as a tool to investigate the Werenskioldbreen environment (south-west Spitsbergen, Arctic Norway)

We used a novel approach of applying magnetic analyses to investigate the material released from the receding glacier Werenskioldbreen on Spitsbergen, Svalbard, Arctic Norway. Surface sediments were taken from the bay Nottinghambukta and the Werenskioldbreen foreland, along two main proglacial streams. Magnetic analyses, namely the low-field mass magnetic susceptibility, anhysteretic susceptibility mass normalized and hysteresis parameters, served to determine magnetic properties and identify the magnetic composition of the study material. We selected two distinct types of sediments. The first group, consisting of magnetite and pyrrhotite, has more single-domain grains in comparison to the second one, containing only magnetite. In the second group, multi-domain particles dominate. Deposits from the north stream, glacier river and an area close to the estuary of Nottinghambukta include magnetite and pyrrhotite. Magnetite was found in the south stream and in the outside part of the bay. Magnetic composition reflects different source rocks of sediments. This study demonstrates the utility of the magnetic method in analysing the current state of glacier environments.

Influence of classical anisotropy fields on the properties of Heisenberg antiferromagnets within unified molecular field theory

A comprehensive study of the influence of classical anisotropy fields on the magnetic properties of Heisenberg antiferromagnets within unified molecular field theory versus temperature T, magnetic field H, and anisotropy field parameter hA1 is presented for systems comprised of identical crystallographically-equivalent local moments. The anisotropy field for collinear z-axis antiferromagnetic (AFM) ordering is constructed so that it is aligned in the direction of each ordered and/or field-induced thermal-average moment with a magnitude proportional to the moment, whereas that for XY anisotropy is defined to be in the direction of the projection of the moment onto the xy plane, again with a magnitude proportional to the moment. Properties studied include the zero-field Neel temperature TN, ordered moment, heat capacity and anisotropic magnetic susceptibility of the AFM phase versus T with moments aligned either along the z axis or in the xy plane. Also determined are the high-field magnetization perpendicular to the axis or plane of collinear or planar noncollinear AFM ordering, the high-field magnetization along the z axis of a collinear z-axis AFM, spin-flop (SF), and paramagnetic (PM) phases, and the free energies of these phases versus T, H, and hA1. Phase diagrams at T = 0 in the Hz-hA1 plane and at T > 0 in the Hz-T plane are constructed for spins S = 1/2. For hA1 = 0 the SF phase is stable at low field and the PM phase at high field with no AFM phase present. As hA1 increases, the phase diagram contains the AFM, SF and PM phases. Further increases in hA1 lead to the disappearance of the SF phase and the appearance of a tricritical point on the AFM-PM transition curve. Applications of the theory to extract hA1 from experimental low-field magnetic susceptibility data and high-field magnetization versus field isotherms for single crystals of AFMs are discussed.

Impact of an iron mine and a nickel smelter at the Norwegian/Russian border close to the Barents Sea on surface soil magnetic susceptibility and content of potentially toxic elements

An important problem in soil magnetometry is unraveling the soil contamination signal in areas with multiple emitters. Here, geophysical and geochemical measurements were performed at four sites on a north - south transect along the Pasvik River in the Barents Region (northern Norway). These sites are influenced by depositions from the Bjørnevatn iron mine and a Ni-Cu smelter in Nikel, Russia. To relate the degree and type of pollution from these sources to the corresponding magnetic signal, the topsoil concentrations of 12 Potentially Toxic Elements (PTEs) (As, Cd, Co, Cr, Cu, Fe, Mo, Ni, Pb, Se, Ti, Zn), were determined, magnetic hysteresis parameters and thermomagnetic properties were measured. In situ magnetic low-field susceptibility decreases from north to south with increasing distance from the iron mine. Relatively large magnetic multidomain grains of magnetite and/or titanomagnetite are responsible for the strong magnetic signal from the topsoil close to Bjørnevatn. These particles are related to increased enrichment factors of As, Mo and Cu, yielding high positive correlation coefficients with susceptibility values. At a site furthest away from the iron mine and located 7 km from the Ni-Cu smelter magnetic susceptibility values are much lower but significant positive correlations on the level of p < .1 with 8 PTEs (Ni, Cu, Co, Se, As, Zn, Cd, Cr) have been observed. The magnetic signal in this area is due to fine-grained primary sulphides and secondary fine-grained magnetite and/or maghemite.

An AMS study of different silicic units from the southern Paraná-Etendeka Magmatic Province in Brazil: Implications for the identification of flow directions and local sources

In the Southern portion of the Paraná-Etendeka Magmatic Province in Brazil, extensive silicic (dacite-rhyolite) deposits occur at the top of a sequence of low-Ti pahoehoe to rubbly basalts. The internal architecture of the silicic deposits and their eruptive style, as well as the location of their sources are still unsatisfactorily known. In an attempt to provide independent evidence for flow directions in deposits previously characterized as effusive, and test the hypothesis of local sources, we carried out anisotropy of magnetic susceptibility (AMS) studies on the two main silicic units (Caxias do Sul dacites and Santa Maria Rhyolites) with the best exposures in an area previously mapped in detail.Magnetic anisotropies were determined on oriented cylindrical specimens from a total of 28 sites. Rock magnetism properties indicate that “pseudo-single-domain” magnetite carries the fabrics and the remanence. Magnetic fabrics were determined by applying anisotropy of low-field magnetic susceptibility (AMS) and anisotropy of anhysteretic remanent magnetization (AARM). Both AMS and AARM tensors are coaxial, indicating that the AMS fabric is not affected by the effect of magnetite single-domain grains.Magnetic data from several dacitic coulées (Caxias do Sul unit) indicate flows from SE to NW. The location and spatial distribution of these lavas support the hypothesis of local sources, aligned along a NE-SW trend. These data are in agreement with the alignments of structures (dome-shaped hills) observed in field work and DEM images. On the other hand, magnetic data obtained in Santa Maria rhyolites indicate that flow directions in two different areas are distinct (towards NW/NE and W), suggesting that they derived from different emission centers.So, regarding the silicic volcanism in the studied region, our data do not support the model which classifies the entire silicic volcanism of the province as extensive rheomorphic pyroclastic deposits released from a central conduit. In contrast, we propose the occurrence of local volcanic events, implying in the existence of different sources, possibly characterized by local emission centers.

Quasi-two-dimensional fluctuations in the magnetization of La1.9Ca1.1Cu2O6+δ superconductors

We report the results of magnetization measurements with the magnetic field applied along the $c$ axis on superconducting $\mathrm{L}{\mathrm{a}}_{1.9}\mathrm{C}{\mathrm{a}}_{1.1}\mathrm{C}{\mathrm{u}}_{2}{\mathrm{O}}_{6+\ensuremath{\delta}}$ single crystals processed under ultrahigh oxygen pressure. Strong fluctuation effects were found in both low- and high-field regimes. Scaling analysis of the high-field magnetization data near the critical temperature $({T}_{c}=53.5\phantom{\rule{0.16em}{0ex}}\mathrm{K})$ region reveals the characteristics of critical fluctuation behavior of quasi-two-dimensional (2D) superconductivity, described by Ginzburg-Landau theory using the lowest Landau level approximation. Low-field magnetic susceptibility data can be successfully explained by the Lawrence-Doniach model for a quasi-2D superconductor, from which we obtained the $ab$ plane Ginzburg-Landau coherence length of this system, ${\ensuremath{\xi}}_{ab}(0)=11.8\ifmmode\pm\else\textpm\fi{}0.9\phantom{\rule{0.16em}{0ex}}\AA{}$. The coherence length along the $c$ axis, ${\ensuremath{\xi}}_{c}(0)$, is estimated to be about 1.65 \AA{}, which is in between those of 2D cuprate systems, such as $\mathrm{B}{\mathrm{i}}_{2}\mathrm{S}{\mathrm{r}}_{2}\mathrm{C}{\mathrm{a}}_{2}\mathrm{C}{\mathrm{u}}_{3}{\mathrm{O}}_{10}$ and $\mathrm{B}{\mathrm{i}}_{2}\mathrm{S}{\mathrm{r}}_{2}\mathrm{CaC}{\mathrm{u}}_{2}{\mathrm{O}}_{8}$, and quasi-three-dimensional (3D) cuprate systems, such as overdoped $\mathrm{L}{\mathrm{a}}_{2\ensuremath{-}x}\mathrm{S}{\mathrm{r}}_{x}\mathrm{Cu}{\mathrm{O}}_{4}$ and $\mathrm{YB}{\mathrm{a}}_{2}\mathrm{C}{\mathrm{u}}_{3}{\mathrm{O}}_{7\ensuremath{-}\ensuremath{\delta}}$. Our studies suggest a strong interplay among the fluctuation effects, dimensionalities, and the ratios of the interlayer Cu-O plane spacing, $s$, to the c-axis coherence lengths. A high $s/{\ensuremath{\xi}}_{c}(0)$ was observed in the high-pressure oxygenated $\mathrm{L}{\mathrm{a}}_{1.9}\mathrm{C}{\mathrm{a}}_{1.1}\mathrm{C}{\mathrm{u}}_{2}{\mathrm{O}}_{6+\ensuremath{\delta}}$, and that apparently drives this system to behave more like a quasi-2D superconductor.

A conceptual magnetic fabric development model for the Paks loess in Hungary

Abstract We describe magnetic fabric and depositional environments of aeolian (loess) deposits from Paks, Hungary, and develop a novel, complex conceptual sedimentation model based on grain size and low-field magnetic susceptibility anisotropy data. A plot of shape factor (magnetic fabric parameter) and dry deposition velocity estimated from grain-size reveals primary and secondary depositional processes during the sedimentation of loess. Primary ones are driven by gravity, with poorly oriented MF for fine grain materials, and by tangential stress, with flow-aligned or flow-transverse fabric for coarser grain sediments. The fabric developed by a primary process is called depositional magnetic fabric. Secondary processes develop in unconsolidated sediments, beginning right after deposition and terminating before the start of diagenesis. Under slow sedimentation conditions, deposited materials are likely to be exposed near the surface for longer periods. Therefore, relatively strong winds with a stable direction can alter the fabric of non-buried surficial sediments. As a result, grain orientations may change from scattered, non-flow oriented fabric to flow-oriented fabric. This type of fabric, developed by a secondary process, is called transformed magnetic fabric, and is characterized by relatively well-defined grain orientation, which allows us to estimate a dominant wind direction.

Magnetic fabrics of the Cretaceous dike swarms from São Paulo coastline (SE Brazil): Its relationship with South Atlantic Ocean opening

Magnetic fabric and rock magnetism studies were performed on 91 dikes from Cretaceous diabase and lamprophyre dike swarms that outcrop side by side on the beaches of NE São Paulo State coastline. The dikes crosscut Archean and Proterozoic poly-metamorphic rocks of the Costeiro Complex. Their thicknesses range from a few centimeters to 2m for lamprophyre and up to about 10m for the diabase. They trend predominantly N30°–60°E with vertical dip. Magnetic fabrics were determined using anisotropy of low-field magnetic susceptibility (AMS) and anisotropy of anhysteretic remanent magnetization (AARM). Rock-magnetism measurements reveal that magnetite grains in the range of 2–5μm are the magnetic mineral of both swarms. For most dikes, these grains are the carriers of bulk magnetic susceptibility but, surprisingly, are not responsible for the AMS which is carried by Fe-bearing minerals as shown by AARM. The main AMS fabric recognized in the swarms is due to magma flow, in which the Kmax-Kint plane is parallel to the dike's plane, and the magnetic foliation pole (Kmin) is perpendicular to it. The analysis of the Kmax inclination showed that the dikes were fed by horizontal to vertical flows. However, for the majority of the dikes the AMS and AARM tensors are not coaxial. The AARM lineation (AARMmax) is oriented N30–60W, approximately perpendicular to AMS lineation (Kmax) suggesting that magnetite grains were rotated approximately 90° anticlockwise from the dike plane. The AARMmax orientation is similar to the direction of a fault system mainly in the Santos marginal basin which was formed in the Cretaceous rifting during the South Atlantic opening. Therefore the AARM fabric is tectonic in origin, and the comparison of AMS and AARM fabrics suggests that lamprophyre and diabase dikes were emplaced in three distinct events in the earliest stages of the South Atlantic opening.

Magnetic susceptibility as a proxy for coal ash pollution within riverbed sediments in a watershed with complex geology (southeastern USA)

A study of near surface sediments from the Dan River (southeastern USA) was conducted to assess the use of magnetic properties as proxies of coal ash after a recent spill. The watershed geology is diverse and potentially contributes magnetic minerals to riverbed sediment from diabase dikes in the Dan River Triassic Basin and from granitic gneiss outside the basin. Coal ash is heterogeneous, including aluminosilicate spheres, amorphous particles and carbonaceous rods and lacy particles. The magnetic fraction of ash from the failed storage pond is up to 17 wt% and is mostly composed of black spheres with maghemite and magnetite. Ash was detected in riverbed sediment from quiet water settings such as inside of meander bends, the confluence of tributary streams and near islands between the spill site and 20 miles downstream in the Schoolfield Reservoir, Danville, VA. The strong magnetic signal is detected above background in riverbed samples and is strongly positively correlated with total ash; elevated low field magnetic susceptibility (χ LF) is evident in samples with ≥ 12% ash content. Anhysteretic remanent magnetization and hysteresis parameters delineate native sediment, ash-bearing sediment, and diabase dikes. Between 20 and 70 miles downstream of the spill site, ash concentrations were either buried or too low due to dilution with native sediment to be detected with χ LF in riverbed samples.

Magnetic structure and magnetization of helical antiferromagnets in high magnetic fields perpendicular to the helix axis at zero temperature

The zero-temperature angles of magnetic moments in a helix or sinusoidal fan confined to the xy plane, with respect to an in-plane magnetic field Hx applied perpendicular to the z axis of a helix or fan, are calculated for commensurate helices and fans with field-independent turn angles kd between moments in adjacent layers of the helix or fan using the classical J0-J1-J2 Heisenberg model. For 0 < kd < $4\pi/9$, first-order transitions from helix to a fan structure occur at fields Ht as previously inferred, where the fan is found to be approximately sinusoidal. However, for $4 \pi/9$ < kd < $\pi$, different behaviors are found depending on the value of kd and these properties vary nonmonotonically with kd. In this kd range, the change from helix to fanlike structure is usually a crossover with no phase transition between them, although first-order and second order transitions are found. We also calculated the average x-axis moment per spin $\mu_{x ave}$ versus Hx for helices and fans with crossovers and phase transitions between them. When smooth helix to fanlike crossovers occur in the range $4\pi/9$ < kd < $\pi$, $\mu_{x ave}$ exhibits an S-shape behavior with increasing Hx. This predicted behavior is consistent with $\mu_{x ave}$(Hx) data previously reported by Sangeetha, et al. [Phys. Rev. B 94, 014422 (2016)] for single-crystal EuCo2P2 possessing a helix ground state with kd = $0.85\pi$. The low-field magnetic susceptibility and the ratio Ht/Hc are calculated analytically or numerically versus kd for helices.

Microstructure and Soft Magnetic Properties of Fe-Zr-(Pt)-Nb-Cu-B Amorphous Alloys

AbstractThis paper presents the results of investigations into the microstructure and magnetic properties of Fe86Zr7Nb1Cu1B5, Fe82Zr7Nb2Cu1B8and Fe81Pt5Zr7Nb1Cu1B5alloys. The alloys were investigated in their as-quenched state, in the form of thin ribbons with approximate dimensions as follows: width 3 mm and thickness 20 μm.The investigations were performed utilizing Mössbauer spectrometry and X-ray diffractometry. Also, an evaluation of the low-field magnetic susceptibility and measurements of the magnetization versus temperature and magnetizing field were performed.

Stratigraphic correlations in mid- to late-Proterozoic carbonates of the Democratic Republic of Congo using magnetic susceptibility

In this paper, we have tested the application of magnetic susceptibility measurements in Cu-Ag-Zn-Pb-(Fe)-mineralized carbonates of the BIe subgroup (Democratic Republic of Congo) as an efficient tool for regional and global high-resolution stratigraphic correlations in the Neoproterozoic marine carbonates. To achieve this goal, we integrate the low-field magnetic susceptibility (XLF) data with facies analyses, geochemistry and isotope stratigraphy. The microfacies analyses of two cores, Tshinyama#S70 and Kafuku#15, drilled in the early Neoproterozoic carbonates of the Mbuji-Mayi Supergroup reveal a deep carbonate ramp setting associated with a microbial/stromatolitic mid-ramp environment. High-resolution stratigraphic correlations using magnetic susceptibility and C-isotope curves established for both cores, 190km apart, suggest a sedimentary hiatus at the base of the Tshinyama#S70 succession. C-O and Sr isotopes and Sr/Ca and Fe abundances show that a diagenetic meteoric overprint affected the series of the Tshinyama#S70 core and a thermal effect related to mineralizing fluids affected the Kafuku#15 core carbonates.

New constraints on geomagnetic field intensity variations in the Balkans during the Early Byzantine period from ceramics unearthed at Thasos and Delphi, Greece

We report on five new archeomagnetic field intensity data obtained in Greece from groups of pottery fragments precisely dated to between the middle of the fourth century and the beginning of the seventh century CE. These potsherds were unearthed on the islands of Thasos (Northern Greece) and Delphi (Central Greece). Their dating is primarily ensured by typo-morphological arguments, combined with archeological and historical constraints. Archeointensity measurements were performed using the Triaxe protocol, which involves continuous magnetization measurements at high temperatures and which allows us to overcome the thermoremanent magnetization anisotropy and cooling rate effects. Magnetic mineralogy measurements such as low-field magnetic susceptibility versus temperature and thermal demagnetization of three orthogonal IRM components have identified magnetite with possible impurities as the main carrier of the magnetization. The new data range from 52.0μT to 61.5μT after reduction to Thessaloniki and show an increase in geomagnetic field intensity in Greece during the Early Byzantine period. They appear in good agreement with previous intensity results satisfying a set of quality criteria and obtained in a region of 700km around Thessaloniki, therefore incorporating data from Bulgaria, Greece and South Italy. This study is part of an ongoing effort to better constrain the evolution in geomagnetic field intensity in the Balkans over the past few millennia, with potential use for dating in archeology. The rapid intensity variations documented here during the Early Byzantine period are clearly of interest in this respect.

Magnetic signature of river sediments drained into the southern and eastern part of the South China Sea (Malay Peninsula, Sumatra, Borneo, Luzon and Taiwan)

Magnetic properties of 22 river samples collected in the Malay Peninsula, Sumatra, Borneo, Luzon and Taiwan have been investigated in order to magnetically characterize the sediments drained and deposited into the South China Sea. The geological formations as well as the present climatic conditions are different from one region to another. Laboratory analyses include low-field magnetic susceptibility, anhysteretic (ARM) and isothermal (IRM) remanent magnetizations acquisition and decay, back-field acquisition, thermal demagnetization of three-axes IRM, hysteresis cycles and low-temperature magnetic measurements. The magnetic properties indicate that the sediments are a mixture of hematite, magnetite and pyrrhotite in different proportions depending on the region. Combined with results previously reported for the three main Asian rivers (Pearl, Red and Mekong rivers), the new data indicate that, in general, hematite-rich sediments are delivered to the southern basin of the South China Sea while the northern basin is fed with magnetite and pyrrhotite-rich sediments. In addition to this general picture, some variability is observed at smaller geographic scales. Indeed, the magnetic assemblages are closely related to the geology of the various catchments while clay minerals, previously reported for the same samples, are more representative of the climatic conditions under which the parent rocks have evolved within each catchment. The magnetic fraction, now well characterized in the main river sediments drained into the South China Sea, can be used as a tracer for changes in precipitation on land and in oceanic water mass transport and exchange.