Remanent Magnetization Measurements Research Articles

Magnetic particle characterization in the Seine river system: Implications for the determination of natural versus anthropogenic input

This study presents an innovative application of well‐established environmental magnetic proxy parameters on fluvial sediments, with the aim to trace geological and human‐induced processes in the complex Seine river drainage system in northern France. We seek to identify environmental fingerprints of mechanical and chemical weathering processes, the regional distribution of the suspended material, and changes in the balance of natural input versus anthropogenic pollution. In order to reach these goals, we applied a combination of rock magnetic and advanced scanning electron microscopic techniques on a large number of sediment trap samples. Generally, we observe an increase in magnetic concentration coupled with a coarsening in magnetic grain size downstream of the Seine river system. Furthermore, the dominant magnetomineralogy changes from high‐coercivity minerals upstream in more rural areas to magnetite‐dominated assemblages downstream. Each river (segment) shows its specific trend line depending on regional initial input, weathering conditions, drainage area, and potential pollution sources. One major outcome of this study is the observed significant correlation between anthropogenic antimony‐rich iron oxide particles and the magnetic concentration. This shows the potential of magnetic remanence measurements as proxy parameters for specific heavy metal pollution concentrations. Hence, the environmental magnetic study presented herein serves to identify major trends as well as local particularities and leads to quantitative analyses of the contributions of individual tributaries in the Seine river system.

Measuring remanence anisotropy of hematite in red beds: anisotropy of high-field isothermal remanence magnetization (hf-AIR)

SUMMARY The potential of using high-field anisotropy of isothermal remanence magnetization (hf-AIR) measurements for determining the origin of natural remanent magnetization in red beds and for identifying and correcting possible red-bed inclination shallowing was investigated for specimens of the Carboniferous Shepody Formation of New Brunswick and Nova Scotia, Canada. The technique makes it possible for a typical paleomagnetic laboratory to measure the remanence anisotropy of high-coercivity hematite. High-field (hf) AIR was used in conjunction with 100 mT alternating field (af) and 120 °C thermal demagnetization to separate the contribution of hematite to the remanence anisotropy from that of magnetite/maghemite and goethite, respectively. A 5-T impulse DC magnetic field was used for the hf-AIR to reset the magnetic moment of high-coercivity hematite so that demagnetization between AIR orientations was not necessary. The ability of a 5-T field to reset the magnetization was tested by generating an isothermal remanent magnetization acquisition curve for hematite by using impulse DC magnetic fields up to 5 T in one orientation and followed by applying a field in the opposite direction at each step. Each field application was treated by 120 °C heating and 100 mT af demagnetization before measurement. At 5 T, the difference between the magnetizations applied in opposite directions disappeared indicating that no magnetic memory persisted at this field strength. We performed a validity and reproducibility test of our hf-AIR measurement technique by measuring three specimens multiple times along two orthogonal coordinate systems. The method yielded highly reproducible results and, on rotating the specimen's coordinates, the fabric rotated by 90° as expected, showing that it is not an artifact of the technique. We also measured hf-AIR on samples that had previously been chemically demagnetized in 3N HCl to remove the secondary, chemically grown pigmentary hematite. The hf-AIR fabric of leached samples is similar to that of untreated samples, but shows a better-defined magnetic lineation and imbrication. We interpret the fabric observed for the Shepody Formation to be a compactional fabric that has been reoriented by strain during folding following a flexural-slip model.

Evaluation and application of biomagnetic monitoring of traffic-derived particulate pollution

Inhalation of particulate pollutants below 10 μm in size (PM 10) is associated with adverse health effects. Here we use magnetic remanence measurements of roadside tree leaves to examine levels of vehicle-derived PM around Lancaster, UK. Leaf saturation remanence (SIRM) values exhibit strong correlation with both the SIRM and particulate mass of co-located, pumped-air samples, indicating that these leaf magnetic values are an effective proxy for ambient PM 10 concentrations. Biomagnetic monitoring using tree leaves can thus provide high spatial resolution data sets for assessment of particulate pollution levels at pedestrian-relevant heights. Leaf SIRM values not only increase with proximity to roads with higher traffic volumes, but are also ∼100% higher at 0.3 m than at ∼1.5–2 m height. Magnetic and SEM data indicate that the particle populations are dominated by spherical, iron-rich particles ∼0.1–1 μm in diameter, with fewer larger, more angular, silica-rich particles. Comparison of the roadside leaf-calculated PM 10 concentrations with PM 10 concentrations predicted by a widely-used atmospheric dispersion model indicates some agreement between them. However, the model under-predicts PM 10 concentrations at ‘urban hotspots’ such as major–minor road junctions and traffic lights. Conversely, the model over-predicts PM 10 concentrations with distance from the road wherever one tree is screened by another, indicating the filtering/protective effect of roadside trees in leaf.

Gilbert-Gauss geomagnetic reversal recorded in Pliocene volcanic sequences from Georgia (Lesser Caucasus): revisited

Abstract We carried out a detailed paleomagnetic, rock-magnetic, and Thellier paleointensity study of an ~3.6 My Pliocene lava flow succession in southern Georgia. An earlier study (Camps et al., 1996) revealed that several consecutive lava flows record an intermediate polarity direction at the base of the section followed by a thick reverse polarity zone. The transitional field was interpreted as an excursion within chron 2Ar or an upper Cochiti-Gilbert reversal. New paleomagnetic data reported here have been obtained from nearby lava successions. In total, about 170 standard paleomagnetic cores belonging to 22 lava flows were collected during the 2005 sample collection campaign. Rock-magnetic experiments showed that the remanence is carried by Ti-poor titanomagnetite in most of the samples. The fraction of grains with multidomain magnetic structure does not seem to be important. Characteristic remanent magnetization was successfully determined on all samples. The direct correlation with the original (Thoki) sequence, previous preliminary measurements of natural remanent magnetization (Sologashvili, 1986), and field observations allowed us to establish a new magnetic stratigraphy. The lower part of section is characterized by intermediate magnetic polarity followed by thick reversely magnetized lavas. The upper sequence, represented by 18 consecutive flows yielded normal magnetic polarity. The mean paleointensity of the intermediate field is drastically reduced with respect to the post-transitional field strength. Based on all available radiometric ages and new paleomagnetic data, it may be speculated that Gilbert-Gauss (R-N) reversal was recorded at the upper part of sequence. Lower intermediate polarity flows possibly represent a form of precursor of this reversal that is similar to the Matuyama-Brunhes geomagnetic transition.

Nonmagnetic high pressure cell for magnetic remanence measurements up to 1.5 GPa in a superconducting quantum interference device magnetometer

We describe here a compact nonmagnetic composite high pressure cell of piston-cylinder type with inner diameter of 6 mm equipped with manganin pressure sensor. This cell was developed for room temperature measurements of magnetic remanence of relatively large rock samples (up to 5.8 mm in diameter and 15 mm long cylinders) under hydrostatic pressure up to 1.5 GPa (the operating pressure limit) in the 2G Enterprises superconducting quantum interference device magnetometer. Its design was focused on minimizing the remanent magnetic moment m(r) of the cell (m(r)=3 x 10(-8) A m(2)) that allowed direct measurements of remanent magnetic moment M(r) under pressure for weakly magnetic materials-rock samples (M(r) epsilon[5 x 10(-7),10(-4)] A m(2)). The inner part of this composite cell is made of hard "Russian alloy" (Ni(57)Cr(40)Al(3)) whereas the envelope of the cell corps is made of less magnetic titanium alloy. This design solution permitted to reduce the total remanent magnetic moment of the whole cell and represents the main device feature. We describe here the choice of materials for pressure cell based on their magnetic and mechanical properties, the choice of the pressure transmitting medium (polyethilsiloxane liquid) providing perfectly hydrostatic conditions for the sample as well as the cell geometry. The cell performance is illustrated by results of pressure demagnetization experiments on rocks and minerals.

Microstructure and switching mechanism of stacked CoCrPt–SiO 2 perpendicular recording media

Perpendicular magnetic recording media consisting of two stacked CoCrPt–SiO 2 recording layers incorporating different O 2 content in each layer were studied. The samples used Ru intermediate layers. With increasing O 2 levels in the lower recording layer, the samples exhibited marginal change in the crystallographic texture, and superior overall magnetic properties such as higher coercivity and improved exchange decoupling. However, the media grain size remained unchanged. Investigations into the switching mechanism by means of virgin and isothermal remanence magnetization measurements showed the predominance of Stoner–Wohlfarth coherent reversal modes in all samples. The thermal stability dropped slightly. A model that describes the evolution of the microstructure in stacked perpendicular recording media is presented to understand these observations.

Diagenetic alteration of natural Fe–Ti oxides identified by energy dispersive spectroscopy and low-temperature magnetic remanence and hysteresis measurements

Low-temperature (LT) magnetic remanence and hysteresis measurements, in the range 300–5 K, were combined with energy dispersive spectroscopy (EDS) in order to characterize the magnetic inventory of strongly diagenetically altered sediments originating from the Niger deep-sea fan. We demonstrate the possibility of distinguishing between different compositions of members of the magnetite–ulvöspinel and ilmenite–hematite solid solution series on a set of five representative samples, two from the upper suboxic and three from the lower sulfidic anoxic zone of gravity core GeoB 4901. Highly sensitive LT magnetic measurements were performed on magnetic extracts resulting in large differences in the magnetic behavior between samples from the different layers. This emphasizes that both Fe–Ti oxide phases occur in different proportions in the two geochemical environments. Most prominent are variations in the coercivity sensitive parameter coercive field ( B C). At room-temperature (RT) hysteresis loops for all extracts are narrow and yield low coercivities (6–13 mT). With decreasing temperature the loops become more pronounced and wider. At 5 K an approximately 5-fold increase in B C for the suboxic samples contrasts a 20–25-fold increase for the samples from the anoxic zone. We demonstrate that this distinct increase in B C at LT corresponds to the increasing proportion of the Ti-rich hemoilmenite phase, while Fe-rich (titano-)magnetite dominates the magnetic signal at RT. This trend is also seen in the room-temperature saturation isothermal remanent magnetization (RT-SIRM) cycles: suboxic samples show remanence curves dominated by Fe-rich mineral phases while anoxic samples display curves clearly dominated by Ti-rich particles. We show that the EDS intensity ratios of the characteristic Fe K α and Ti K α lines of the Fe–Ti oxides may be used to differentiate between members of the magnetite–ulvöspinel and ilmenite–hematite solid solution series. Furthermore it is possible to calculate an approximate composition for each grain if the intensity ratios of natural particles are linked to well-known standards. Thus, element spectra with high Fe/Ti intensity ratios were found to be rather typical of titanomagnetite while low Fe/Ti ratios are indicative of hemoilmenite. The EDS analyses confirm the LT magnetic results, Fe-rich magnetic phases dominate in the upper suboxic environment whereas Ti-rich magnetic phases comprise the majority of particles in the lower anoxic domain: The mineral assemblage of the upper suboxic environments is composed of magnetite (∼19%), titanomagnetite (∼62%), hemoilmenite (∼17%) and ∼2% other particles. In the lower anoxic sediments, reductive diagenetic alteration has resulted in more extensive depletion of the (titano-)magnetite phase, resulting in a relative enrichment of the hemoilmenite phase (∼66%). In these strongly anoxic sediments stoichiometric magnetite is barely preserved and only ∼5% titanomagnetite was detected. The remaining ∼28% comprises Ti-rich particles such as pseudobrookite or rutile.

Thermal stability and the magnetization process in CoCrPt–SiO 2 perpendicular recording media

The correspondence between the crystallographic texture and intergranular exchange coupling interactions, with the switching mechanism and the thermal response of the magnetization in CoCrPt–SiO 2 perpendicular recording media was investigated. Virgin hysteresis and isothermal remanence magnetization measurements both showed a three-stage process, which was interpreted to indicate that the Stoner–Wohlfarth coherent reversal mode is the dominant switching mechanism irrespective of the texture. For media samples with similar degree of texture, improvement in exchange decoupling of the media grains caused an increase in the onset field for the virgin magnetization process. The thermal decay of the magnetization, evaluated via the field-dependent viscosity coefficient peaked near the nucleation field, and the peak value showed a strong dependence on the strength of the exchange coupling interactions. A model establishing the role of the texture and exchange interactions in perpendicular recording media is put forth.

Observation of spin-glass behavior in antiperovskite Mn3GaN

The dc magnetization, ac susceptibility, and isothermal remanent magnetization measurements are carried out for antiperovskite Mn3GaN. Strong indication for a frozen state with freeing temperature of Tf at ∼133K is found, determined from the difference between field-cooled and zero-field-cooled magnetizations. The ac susceptibility measurements show a peak around Tf, with the peak position shifting as a function of the driving frequency f and magnetic field H, respectively. Relaxation effects are observed after switching the external magnetic field below Tf. These findings consistently demonstrate that Mn3GaN exhibits a canonical spin-glass state at low temperature.

Ruthenium Interlayer as Diffusion Barrier Under Carbon Overcoat in Magnetic Recording Media

A Magnetic film of ternary CoCrTa alloy was deposited onto textured NiP/Al substrate. Then, an atomic (8 A) layer of ruthenium (Ru) film was sputtered before a layer of hydrogenated amorphous carbon (a-C:H) was deposited by hot filament Plasma Enhanced Chemical Vapor Deposition method. Dynamic remanent magnetization and thickness product (MrT) measurement showed that samples without Ru interlayer were susceptible to plasma damage during carbon deposition compared to samples with Ru interlayer. The impact was more obvious as the substrate bias was increased during carbon deposition. Having an intermediate layer of ruthenium film in between magnetic layer and carbon overcoat made magnetic performance of the film more stable. After annealed for 30 minutes under 1 atm of N2 environment, MrT values dropped drastically in samples without Ru interlayer but varied only a little in samples with the interlayer. Water contact angle measurement showed little difference in surface energy with or without the interlayer. XPS studies indicated that Tantalum carbide formed in samples without Ru interlayer. Formation of metal carbide was not found in samples with Ru interlayer.

Tectonic fabric revealed by AARM of the proterozoic mafic dike swarm in the Salvador city (Bahia State): São Francisco Craton, NE Brazil

Magnetic fabric and rock magnetism studies were performed on 25 unmetamorphosed mafic dikes of the Meso-Late Proterozoic (∼1.02 Ga) dike swarm from Salvador (Bahia State, NE Brazil). This area lies in the north-eastern part of the São Francisco Craton, which was dominantly formed/reworked during the Transamazonian orogeny (2.14–1.94 Ga). The dikes crop out along the beaches and in quarries around Salvador city, and cut across both amphibolite dikes and granulites. Their widths range from a few centimeters up to 30 m with an average of ∼4 m, and show two main trends N140–190 and N100–120 with vertical dips. Magnetic fabrics were determined using both anisotropy of low-field magnetic susceptibility (AMS) and anisotropy of anhysteretic remanent magnetization (AARM). The magnetic mineralogy was investigated by many experiments including remanent magnetization measurements at variable low temperatures (10–300 K), Mössbauer spectroscopy, high temperature magnetization curves (25–700 °C) and scanning electron microscopy (SEM). The rock magnetism study suggests pseudo-single-domain magnetite grains carrying the bulk magnetic susceptibility and AARM fabrics. The magnetite grains found in these dikes are large and we discard the presence of single-domain grains. Its composition is close to stoichiometric with low Ti substitution, and its Verwey transition occurs around 120 K. The main AMS fabric recognized in the swarm is so-called normal, in which the K max– K int plane is parallel to the dike plane and the magnetic foliation pole ( K min) is perpendicular to it. This fabric is interpreted as due to magma flow, and analysis of the K max inclination permitted to infer that approximately 80% of the dikes were fed by horizontal or sub-horizontal flows ( K max < 30°). This interpretation is supported by structural field evidence found in five dikes. In addition, based on the plunge of K max, two mantle sources could be inferred; one of them which fed about 80% of the swarm would be located in the southern part of the region, and the other underlied the Valéria quarry. However, for all dikes the AARM tensors are not coaxial with AMS fabrics and show a magnetic lineation (AARM max) oriented to N30–60E, suggesting that magnetite grains were rotated clockwise from dike plane. The orientation of AARM lineation is similar to the orientation of a system of faults in which the Salvador normal fault is the most important. These faults were formed during Cretaceous rifting in the Recôncavo-Tucano-Jatobá assemblage that corresponds to an aborted intra-continental rift formed during the opening of the South Atlantic. Therefore, the AARM fabric found for the Salvador dikes is probably tectonic in origin and suggests that the dike swarm was affected by the important tectonic event responsible for the break-up of the Gondwanaland.

Mineral magnetic signatures in a long core from Lake Qarun, Middle Egypt

The analysis and interpretation of changes in mineral magnetic signatures from a long (ca. 8.2 m) sedimentary sequence recovered from Lake Qarun, Middle Egypt in 2003 spanning a timescale of approximately the last 2,000 years is reported. A suite of mass specific susceptibility and magnetic remanence measurements were made at irregular intervals downcore on 39 samples. These samples were selected on the basis of trends and abrupt changes in whole-core magnetic susceptibility measured using a Bartington® MS2E sensor and were analysed for low and high temperature loss on ignition and their particle size distribution. Trends in all mineral magnetic concentration parameters are remarkably similar and were initially used to divide the core into three magnetically distinct zones. The upper and lower sections of the core (0–119 cm and 445–822 cm depth) are characterised by low values for all magnetic concentration parameters. Between 153 and 380 cm depth, concentration parameters are considerably higher, although somewhat variable. The S ratio and percentage loss of remanence after 24 h (IRMloss) follow a different trend and are inversely related to each other. A low S ratio ( 4%. On the basis of these parameters, the core can be divided into four zones, and differences in magnetic mineralogy between these four zones were confirmed by measurement of IRM acquisition curves. The major difference between concentration parameters and ratios or percentage loss of IRM lies in the identification of an additional zone below 619 cm depth where the S ratio is high and IRMloss is low. There is little evidence to suggest that the magnetic signatures are controlled by particle size or by trends in organic matter and/or carbonate content. The signatures appear to be predominantly detrital and show little evidence of post-depositional alteration through dissolution or authigenic addition of bacterial magnetite or greigite. Analysis of Saharan dust deposition rates in Northern Egypt suggests that atmospheric fallout is likely to make only a very minor contribution (<1%) to sedimentation rates in Lake Qarun. The downcore trends therefore appear to reflect major changes in fluvial sediment sources over the ca. 2,000 year time period spanned by this ∼8 m core. Preliminary mineral magnetic characterisations of potential local sources suggest that these cannot account for the range of signatures recorded in the Qarun sediments and it is hypothesised that these sediments are derived from Nile river floods.

Strain induced magnetoelectric coupling between magnetite and BaTiO3

Magnetite (Fe3O4) thin films have been grown on ferroelectric BaTiO3 (BTO) single crystal substrates by pulsed laser deposition. Transmission-electron microscope observations demonstrate the orientation relationship between Fe3O4 and BTO as [400]Fe3O4∥[200]BTO and [004]Fe3O4∥[002]BTO. Experimental measurements of magnetization, coercivity and remanent magnetization of the films show abrupt jumps at around phase transition temperatures of BTO and opposite jump signs are observed for the in-plane and out-of-plane measurements. The magnetization jumps can be suppressed by a strong external magnetic field. These results were discussed in terms of the interface strain induced changes of magnetic domain structure in the Fe3O4 film. This work demonstrates the presence of strong magnetoelectric coupling between Fe3O4 and BTO.

Application of anisotropy of magnetic susceptibility (AMS) for the determination of paleo-wind directions and paleo-environment during the accumulation period of Bag Tephra, Hungary

This study provides the first application of anisotropy of magnetic susceptibility (AMS) measurements for the study of Pleistocene-wind directions and the environment during the deposition of dust and lithifaction of loess in the Carpathian Basin. A total of 67 samples were taken from seven loess–paleosol outcrops to test this method on loess–paleosol sequences. The samples were collected from the loess overlying the Bag Tephra, a characteristic marker horizon of the middle Pleistocene age in the Hungarian loess sequences. Statistical analysis of AMS is a good tool to distinguish between redeposited and the undisturbed loess. The first measurements indicated a N/NE–S/SW direction dust transport. In two outcrops, when the principal susceptibilities were placed into a stereographic projection, the effect of local slope proved to be important in determining the magnetic fabric. The AMS results were completed by isothermal remanent magnetization (IRM) measurements to determine the main magnetic mineral components of the samples. The dominant magnetic mineral which determines the AMS character of the sample was magnetite (or maghemite).

First‐order reversal curve (FORC) diagrams of natural and cultured biogenic magnetic particles

First‐order reversal curve (FORC) diagrams are rapidly becoming a standard tool for characterizing magnetic particles because they simultaneously incorporate information regarding magnetostatic interaction and domain states. The simplest interpretation of FORC diagrams of single‐domain (SD) particles is based on the Neel interpretation of Preisach theory, which predicts that the FORC function is the product of a coercivity and an interaction field distribution. Although the underlying assumptions of this interpretation are not correct, a strictly quantitative model of weakly interacting SD grains proves that the distributions of coercivities and interaction fields can be retrieved from a FORC diagram. To test this model, we present the possibility of a quantitative interpretation of FORC diagrams, and we present measurements of samples containing magnetosomes from cultures of magnetotactic bacteria and from a lake sediment. Two samples are investigated under the electron microscope to characterize the geometrical arrangement of the particles. We find that the clustering of otherwise similar particles has a strong influence on FORC diagrams. We also obtained a crude estimate of packing densities form the FORC diagrams, which were consistent with transmission electron microscopy observations and measurements of the anhysteretic remanent magnetization.

Influences of urban fabric on pyroclastic density currents at Pompeii (Italy): 2. Temperature of the deposits and hazard implications

During the A.D. 79 eruption of Vesuvius, Italy, the Roman town of Pompeii was covered by 2.5 m of pyroclastic fall pumice and then partially destroyed by pyroclastic density currents (PDCs). Thermal remanent magnetization measurements performed on the lithic and roof tile fragments embedded in the PDC deposits allow us to quantify the variations in the temperature (Tdep) of the deposits within and around Pompeii. These results reveal that the presence of buildings strongly influenced the deposition temperature of the erupted products. The first two currents, which entered Pompeii at a temperature around 300–360°C, show drastic decreases in the Tdep, with minima of 100–140°C, found in the deposits within the town. We interpret these decreases in temperature as being the result of localized interactions between the PDCs and the city structures, which were only able to affect the lower part of the currents. Down flow of Pompeii, the lowermost portion of the PDCs regained its original physical characteristics, emplacing hot deposits once more. The final, dilute PDCs entered a town that was already partially destroyed by the previous currents. These PDCs left thin ash deposits, which mantled the previous ones. The lack of interaction with the urban fabric is indicated by their uniform temperature everywhere. However, the relatively high temperature of the deposits, between 140 and 300°C, indicates that even these distal, thin ash layers, capped by their accretionary lapilli bed, were associated with PDCs that were still hot enough to cause problems for unsheltered people.

Magnetic anomalies in the spin-chain compoundSr3CuRhO6: Griffiths-phase-like behavior of magnetic susceptibility

We report the results of ac and dc magnetic susceptibility $(\ensuremath{\chi})$, heat-capacity $(C)$, isothermal magnetization, and isothermal remanent magnetization measurements on the compound ${\mathrm{Sr}}_{3}\mathrm{Cu}\mathrm{Rh}{\mathrm{O}}_{6}$ crystallizing in a ${\mathrm{K}}_{4}\mathrm{Cd}{\mathrm{Cl}}_{6}$-derived monoclinic structure. The magnetization data reveal distinct magnetic anomalies near 6 and $12\phantom{\rule{0.3em}{0ex}}\mathrm{K}$ with decreasing temperature $(T)$. While the transition below $({T}_{0}=)$ $6\phantom{\rule{0.3em}{0ex}}\mathrm{K}$ appears to be of a spin-glass type as inferred from all the data, the one at $12\phantom{\rule{0.3em}{0ex}}\mathrm{K}$ is not typical of bulk ferromagnetism, in contrast to an earlier proposal. In the range of $6--12\phantom{\rule{0.3em}{0ex}}\mathrm{K}$, the dc $\ensuremath{\chi}$ obeys the form ${\ensuremath{\chi}}^{\ensuremath{-}1}$ $\ensuremath{\alpha}\phantom{\rule{0.3em}{0ex}}{(T\ensuremath{-}{T}_{0})}^{1\ensuremath{-}\ensuremath{\lambda}}$ $(0l\ensuremath{\lambda}l1)$ for very low dc fields, and the values of $\ensuremath{\chi}$ decrease gradually with the application of higher magnetic fields, mimicking the behavior of Griffiths phases.

Establishing fire incidence in temperate soils using magnetic measurements

Natural, unburnt topsoil samples from the Cotswolds region of England and samples from archaeological contexts in the UK and Hungary, some of which are known to have been affected by burning, have been used to identify a distinctive magnetic ‘signature’ linked to the effects of fire on magnetic properties. A combination of bulk, low field ( χ lf) and frequency dependent ( χ fd) magnetic susceptibility and normalized anhysteretic remanent magnetization ( χ ARM) measurements have been used on both unburnt and experimentally burnt samples from the Cotswolds and on the archaeological samples. Complementary determinations were made for both sets of samples of the maximum potential susceptibility ( χ max) and, hence, percentage fractional conversion ( χ conv), which provides independent evidence of burning. The results show that the ferrimagnetic mineral assemblages produced as a result of fire have a significantly finer grain size than those arising from weathering and soil formation alone. Burning thus gives rise to a distinctive envelope of values on a bilogarithmic plot of χ ARM/ χ fd versus χ ARM/ χ lf. Such a diagram provides a convenient basis for identifying magnetic signatures in soils, archaeological materials and palaeosols that have arisen mainly through burning.

Unconventional relaxation in antiferromagneticCoRh2O4nanoparticles

Magnetic relaxation in antiferromagnetic CoRh$_2$O$_4$ nanoparticles is investigated at 2 K by cooling the sample from a temperature (70 K) well above the antiferromagnetic ordering temperature at 27 K, following zero field cooled (ZFC) and field cooled (FC) process. In ZFC process, the sample at 2 K is subsequently followed by magnetic field on and off sequences, whereas in FC process the cooling field is made off during measurement of remanent magnetization as a function of time. The experiments suggest an unconventional relaxation behaviour in the system, as an effect of increasing surface exchange anisotropy with decreasing the size of antiferromagnetic nanoparticles.

Diagenetic modification of magnetic properties observed in a piston core (MD01–2407) from the Oki Ridge, Japan Sea

We investigated the magnetic properties of u-channel samples from a giant piston core (IMAGES MD01–2407), in order to obtain high-resolution records of past geomagnetic field and paleoenvironmental changes in the Japan Sea. Measurements of natural remanent magnetization (NRM) show occurrence of anomalous magnetic directions at several horizons, which may represent geomagnetic polarity excursions in the Brunhes Chron. However, most of them occur close to the lithological boundary including tephra layers, suggesting that these features might have been caused by deformation of core sediments. Intensity of NRM normalized by artificially induced remanence shows no significant correlation with a standard paleointensity record. This is probably caused by a variation of lithology including volcanic ash layers and effect of the diagenetic changes of magnetic minerals. Down-core variations of magnetic properties in the topmost part depict in detail the process of dissolution and alteration of magnetic minerals during reductive diagenesis, which is to be expected for hemipelagic sediments characterized by a high sedimentation rate and a high organic matter content. Magnetic concentration parameters including low-field susceptibility, anhysteretic remanent magnetization and isothermal remanent magnetization (IRM) indicate that a loss of fine-grained magnetite occurs from 0.6 to 1.2 mbsf. While remanence intensities below the level of the early diagenesis are one or two orders of magnitude lower than those in the topmost part, most of the thinly-laminated (TL) dark layers showed rather enhanced magnetic intensities. Typically, TL2 shows a marked peak at the top with a gradual decrease towards the bottom. Variation of S-ratio, which gives the proportion of a low-coercivity component in saturation IRM, suggests that low-coercivity and fine-grained ferromagnetic minerals are dominant. These data suggest new magnetic minerals formed during deposition of the TL layers, which may be analogous to the diagenetic magnetic enhancement known in sapropels of the Mediterranean Sea.