GP23A-0161 1340h
Archaeomagnetic studies in Mesoamerica using non-conventional materials
For the first time results of an archaeomagnetic study of mural paintings and unburned lime-plasters from Mesoamerica are presented. The magnetic measurements show that at least four murals (sites: Cacaxtla, Cholula and Templo Mayor) retain a remanent magnetization carried by a mixture of hematite and magnetite grains. In most specimens, a characteristic magnetization is successfully isolated by alternating field demagnetization. The mean directions are reasonably well determined for each murals and within the range of secular variation during the last centuries. Studied Mesoamerican murals apparently retain the direction of the magnetic field at the time they were painted and are therefore an invaluable source of information concerning its secular variation. Lime-plaster samples were selected from two archaeological excavation projects in the Teopancazco residential compound of Teotihuacan and the large multi-stage structure of Templo Mayor in Tenochtitlan, where chronological information is available. The intensity of remanent magnetization and low-field susceptibility are weak reflecting low relative content of magnetic minerals. NRM directions are well grouped and alternating field demagnetization shows single or two-component magnetizations. Rock-magnetic experiments point to fine-grained titanomagnetites with pseudo-single domain behavior. Anisotropy of magnetic susceptibility measurements document a depositional fabric, with normal to free-surface minimum AMS axes. Characteristic mean site directions were correlated to the paleosecular variation curve for Mesoamerica. Our results suggest that archaeomagnetic dating can be applied to mural paintings and lime-plasters, which are materials widely employed in Mesoamerica.
GP23A-0162 1340h
Magnetic Properties of Santorini Volcanics and Archaeological Material From Northern Greece
Absolute palaeointensity determination is a long-winded and complicated procedure. The Thellier-Thellier technique, in particular, requires multiple heating and cooling steps. Magnetic minerals are often chemically unstable, and multiple heatings may cause oxidation, hence aiding the subsequent neo-formation of remanence carriers during the procedure. Samples from recent rhyodacitic lava flows on Santorini (Southern Greece)and archaeological material (burnt clay) from two Hellenistic pottery kilns at Katerini (Northern Greece) have been tested for their thermal stability. Small specimens were used to study the temperature dependence of magnetic low-field susceptibility, using a Thellier-Thellier like measurement sequence. The remanence properties were tested on palaeomagnetic standard sized samples by detailed alternating field (AF) demagnetisation of isothermal remanent magnetisation (IRM) and IRM acquisition at different annealing temperatures. The coercivity content was analysed using the method of Egli (2003, 2004). Two low coercivity ($<100$ $mT$) components were found in the lava samples, the individual contribution to the total magnetisation being unchanged after annealing up to $500$ $^{o}C$. The temperature dependence of susceptibility indicates also two components, which are both thermally stable. Heating and cooling cycles are almost undistinguishable up to $500$ $^{o}C$. In comparison to the lava, the burnt clay from the archaeological site seems to be slightly less resistant against multiple heatings as indicated by susceptibility and remanence behaviour. This is possibly due to the formation of chemical weathering products after the last burning. Temperature dependence of magnetic susceptibility in combination with coercivity spectra analysis at different annealing temperatures gives a reliable assessment about magnetic mineral alterations.
GP23A-0163 1340h
A phenomenological model of multidomain behavior during Thellier paleointensity experiments
The most commonly used method for paleointensity determination, the Thellier method, requires the absence of multidomain magnetic behavior. The influence of multidomain remanence on paleointensity determination and its recognition during the experiment are, however, only partially understood. Here, we present a phenomenological model of TRM acquisition and its application to the Thellier experiment. This model is used to investigate, beside other multidomain characteristics, the effect of pTRM$^*$-tails. Furthermore, the appropriateness of typically applied checks during paleointensity determination like alteration checks, pTRM$^*$-tail checks and additivity checks is scrutinized. It is shown that pTRM$^*$-tails significantly affect the shape of the Arai diagram. Concave curvature of the Arai plot for samples containing multidomain particles is related to remanences with unblocking temperatures below their respective blocking temperatures. pTRM$^*$-tails, however, which affect unblocking temperatures above the respective blocking temperatures, lead to a convex shape. An overlap of both convex and concave characteristics can produce s-shaped Arai diagrams. The effect of pTRM$^*$-tails is strongly dependent on the angle and intensity difference between applied field and NRM of the sample. The commonly used in-field alteration check is biased by pTRM$^*$-tails and shows deviations from the expected value even if alteration is absent. pTRM$^*$-tail checks show the presence of tails and thus give evidence for multidomain behavior, only if the laboratory field is applied in a different direction than the NRM. The larger the angle between NRM and laboratory field, the larger the value of the pTRM$^*$-tail check. A new method for calculation of the pTRM$^*$-tail check is proposed which accounts for such angular difference, as well as intensity differences. Conducting the Thellier experiment with alteration checks, pTRM$^*$-tail checks, and additivity checks at an angle between NRM and laboratory field $> 0$, provides the possibility to unequivocally identify multidomain bias.
GP23A-0164 1340h
Magnetic Properties of the Magnetite-Ulvoespinel Solid Solution: Magnetic Susceptibility Measurements
Temperature-dependent magnetic susceptibility measurements ($\chi$(T)-curves) are very useful to estimate the chemical composition of magnetic minerals, especially in case of titanomagnetite (Tmt; solid solution between magnetite and ulvoespinel) which is, together with ilmenite-hematite solid solutions (Ilm$_{ss}$), the major carrier of rock magnetism in basaltic rocks. Magnetic susceptibility measurements are rapid and straightforward, non-destructive and can be performed on multi-phase materials. The method is especially useful for crystals that are not measurable with the electron microprobe, e.g. very small or skeletal grains or Tmt crystals crowded with fine lamellae of ilmenite or of other spinel phases. However, there are still uncertainties in the interpretation of some features of the $\chi$(T)-curves, especially at low temperatures and we do not know of any systematic study on compositionally and crystal-chemically well-defined synthetic samples over the whole Tmt solid solution. We have synthesized Tmt-Ilm$_{ss}$ assemblages, single phase Tmt as well as Tmt-wuestite (Wue) samples in sub-solidus conditions in the Fe-Ti-O system at 1 bar, $1100\deg$C and $1300\deg$C. Oxygen fugacities (fO$_{2}$) were fixed with CO/CO$_{2}$ gas mixtures or by solid oxygen buffers placed together with the sample in evacuated silica-glass tubes. Runs lasted $>$24 h at $1300\deg$C, up to 160 h at $1100\deg$C and were terminated by drop-quenching into water. The samples were characterized with the EMP and SEM. The alternating low-field magnetic susceptibility was measured in the temperature range 77 up to 970 K using a KLY-2 Kappabridge and between 5 and 300 K using a MPMS instrument. As expected, the Curie temperature (T$_{C}$) of Tmt linearly decreases with increasing mole fraction of ulvoespinel (X$_{Usp}$). But in the products of syntheses at $1300\deg$C, Tmt in equilibrium with Ilm$_{ss}$ have up to 30 K higher T$_{C}$s than Tmt of the same compositions in equilibrium with Wue. Tmt synthesized with Ilm$_{ss}$ at $1100\deg$C display intermediate T$_{C}$s. In accord with previous results (e.g. Hauptman, 1974; Rahman and Parry, 1978) T$_{C}$ seems to positively correlate with the concentration of cation vacancies in Tmt. We have recognized three main types of $\chi$(T)-curves for Tmt. At high X$_{Usp}$ there is a sharp symmetric peak, at intermediate X$_{Usp}$ an asymmetric peak and at low X$_{Usp}$ a plateau-like curve with nearly constant susceptibility between the Verwey transition temperature and T$_{C}$. $\chi$(T)-curves of chemically inhomogeneous single-phase Tmt show stepwise drops. Several T$_{C}$s can be retrieved from these curves, which seem to mirror the different chemical compositions. Many high-temperature $\chi$(T)-curves show a distinct irreversibility. The difference in T$_{C}$ determined from the heating vs. cooling curve is largest (up to about $70\deg$C) for Tmt with intermediate compositions, and for a given composition it is larger in samples synthesized at $1300\deg$C than at $1100\deg$C. The irreversibility of $\chi$(T)-curves may reflect nonreversible changes in cation order or composition (e.g. Harrison and Putnis, 1996). Harrison, R.,J. and Putnis, A., Am. Mineral., 81, 375-384, 1996. Hauptman, Z., Geophys. J. R. astr. Soc., 38, 29-47, 1974. Rahman A., A. and Parry L., G., Phys. Earth Planet. Inter., 16, 232-239, 1978.
GP23A-0165 1340h
Titanomaghemite inversion and associated changes of remanence in oceanic basalts
Oceanic basalts older than 10 Ma often show a complex remanence behavior during stepwise thermal demagnetization with maximum unblocking temperatures above 500$^{\circ}$C. At the same time, strong field thermomagnetic measurements reveal Curie temperatures of 300-360$^{\circ}$C for these basalts corresponding to a highly oxidized Ti-rich titanomaghemite (TMgh). In the past, this apparently paradoxical fact has been taken as evidence that the large TMgh grains, although dominating strong field thermomagnetic measurements, are in the MD grain size range and are not the principal remanence carriers. Instead, it has been proposed that small SD sized Ti-poor titanomagnetites are carrying the primary NRM. However, the Ti-rich TMgh phase dominating thermomagnetic curves is unstable and prone to inversion when heated above 300$^{\circ}$C. The result of this inversion is also Ti-poor titanomagnetite. Thus, the high unblocking temperatures might as well be an artifact of this inversion process. In order to resolve this ambiguity we conducted strong field thermomagnetic measurements as well as continuous thermal demagnetization experiments. By comparing the results we can demonstrate that the initial maximum unblocking temperature is well below 400$^{\circ}$C being in good agreement with the initial Curie temperature of the Ti-rich TMgh. Ti-poor titanomagnetite as the remanence carrier is not present during thermal demagnetisation runs up to 400$^{\circ}$C. Only during subsequent heating to higher temperatures causing the inversion of the TMgh, the maximum unblocking temperature is increasing to its final value above 500$^{\circ}$C. Surprisingly, the remanence direction remains constant during this process. This implies that the primary remanence is carried by the oxidized TMgh and is inherited by the Ti-poor titanomagnetite formed by inversion.
GP23A-0166 1340h
Magnetic and Electron Microscopic Investigation on Rock Samples from the PACMANUS Hydrothermal Vent Field in Papua New Guinea
The PACMANUS hydrothermal vent field in the Eastern Manus back-arc basin, Papua New Guinea, is considered as a modern-day analog of massive volcanogenic sulfide deposits within felsic volcanic sequence. This active vent field was drilling in November-December 2003 by Ocean Drilling Program Leg 193. The recovery was generally low with less than 15% due to fragility of rocks. Paleomagnetic measurements and scanning electron microscope observations were performed on samples from three major sites (Sites 1188, 1189 and 1191). Site 1188, a low-temperature diffused venting region, was drilled to 370 mbsf. Site 1189, a black smoker region, was drilled to a depth of 200 mbsf using RCB. The recovered rock samples have inclination close to the present-day Earth field ($-7\deg$), but those near the seafloor have much steeper inclination of up to $-25\deg$. The upper 35 m of the sites consists of fresh to moderately altered dacite-rhyodacite, which exhibits moderately high natural remanent magnetization ($<$ 6 A/m). The region below this extrusive layer largely comprises of pervasively altered rocks with little evidence of sulfide deposit and as a whole exhibits a low magnetization intensity. However, two intervals with extremely high remanent magnetization were discovered below the upper extrusive layer at Site 1188 (135-211 mbsf and 280-370 mbsf) and one interval at Site 1189 (137-190 mbsf). In particular, the samples between 135-211-mbsf interval at Site 1188 have extremely high remanence with intensities ranging up to 300-500 A/m. Although pockets of magnetite are not uncommon in the ancient hydrothermal ore bodies, they have seldom been documented in modern-day system, and little is known about the physical and chemical condition that allows the magnetite to form in hydrothermal vent systems. Two possibilities of magnetite formation are explored: one that these magnetites precipitated from magnetite-rich fluid as it cooled from above the Curie temperature (TRM) and the other that magnetization was acquired by the growth of magnetite grains below the Curie temperature (CRM).
GP23A-0167 1340h
Magnetic Mineralogy in Oxidized Lava Flows
Oxidized basaltic lava flows are common in terrestrial environment and magnetic mineralogy ranges from rhombohedral to spinel phases contributing to a complex overall magnetic behavior. Similar rock types are responsable for very intense magnetic anomalies on Earth and may be an equivalent to those on Mars. Therefore this research may identify the magnetic techniques that should be employed as part of instrumentation on board of remote control rover on surface of Mars. We obtained a large number of magnetic and mineralogic tests on a basaltic profile across a tree mold from Kilauea volcano, Hawaii, where the oxygen fugacity was presumably modified by the presence of combustible organic material within the lava flow. The lava flow cools down rapidly as it approaches much cooler organic material. This process results in contrasting contents of titanium in magnetic phases and different Fe-Ti oxide textures, which is important for the overall rock magnetic properties. We attempted to identify the contribution of all the magnetic phases within these rocks using different remanence and susceptibility measurement at very low-, low- and high-temperature as well as optical and electron microscope investigations. The phases within these rocks are titanohematite (self reversing composition), ferrian ilmenite and titanomagnetite, which contribute to the induced as well as to the remanent component of the magnetization.
GP23A-0168 1340h
Paleomagnetic and rock magnetic investigation of the high magnetic remanence in fault pseudotachylites
Frictional heating during coseismic deformation may melt fault rocks and form pseudotachylite (PSDT) vein networks if slip is important. Limited previous work on PSDT suggests that their remanence properties are similar to those of lightning struck rocks, with anomalously high magnetization, implying that large electric pulses were involved in magnetization acquisition. We are testing the hypothesis that remanence anomalies in PSDT are typical and that coseismic electric currents are responsible for these anomalies. We have sampled young PSDT and immediately adjacent host rocks from three seismically active fault zones (Eastern Peninsular Ranges [SR, Santa Rosa area, 62-56 Ma tonalite host rock], California, Uchinoura shear zone [UC, 14 Ma granodiorite host rock], Kyushu, Japan, and Val Gilba [VG, 37 Ma 3 Gpa gneiss host rock], Dora Maira, Western Alps). All materials collected are oriented; specimen preparation involves the making slabs, cut perpendicular to PSDT vein networks, which are then cut into oriented cubes (about 1 cc). Notably, magnetic properties of PSDT differ considerably from host rock. For SR samples, typical NRM intensities for PSDT range from 2.0 to 7.0 A/m, with the NRM of single component character (median destructive fields are typically about 40 mT and 80 percent of laboratory unblocking temperature spectra between 500 and 580C). NRM intensities of host rock typically decrease away from PSDT and range from 2.0 to 0.01 A/m, host rock adjacent to veins yields magnetizations similar to those in the veins. The characteristics of UC samples are considerably different from those of the SR locality; PSDT has NRM intensities of about 0.08 A/m with host rock of similar NRM intensities. Directions of magnetizations isolated in PSDT and adjacent host rock are similar. Modified Lowrie-Fuller tests suggest that PSDT at both of these localities contains abundant fine, single-domain magnetite particles. VG PSDT have relatively low NRM intensities (2.0 to 7.0 mA/m) and a more complicated demagnetization response. Host rock gneiss is comparable in intensity, regardless of distance from PSDT veins and has no coherent magnetization, even adjacent to veins. A tentative explanation of the VG results involves PSDT formation at considerably deeper levels (about 10km) than those from the other localities. Ultimately, our results will be compared with those from artificially generated PSDT to obtain a better understanding of coseismic electric phenomena.
GP23A-0169 1340h
Modeling of 6-line Ferrihydrite Low-Temperature Magnetic Properties for Environmental Change Studies
Ferrihydrite (5Fe$_{2}$O$_{3}$.9H$_{2}$O) is a poorly crystalline, highly reactive mineral that occurs in a wide variety of natural settings and has been identified as a key element in the biogeochemical cycle of iron in the environment. Magnetically, it is considered as an antiferromagnetic mineral with a weak magnetic moment possibly due to uncompensated spins. In our presentation, we will focus on the modeling and interpretation of low-temperature ($<$300K) magnetic data obtained on three samples of 6-line ferrihydrite nanoparticles that have been synthesized by precipitation from homogeneous solution. The three samples, which possess similar composition and crystallinity (probed by x-ray diffraction and extended x-ray absorption fine structure), are characterized by three different particle size distributions with average diameters between 3 and 5.5 nm. Using the particle size distributions obtained from transmission electron microscopy images, we performed numerical fits of the high-field (0-5T) induced magnetization data for several temperatures above the unblocking temperatures of the samples. The magnetization was modeled as the superposition of an antiferromagnetic signal and a magnetization due to uncompensated moments (which are assumed coupled to the antiferromagnetic moments). Fits of the low-field magnetic susceptibility data as a function of temperature were also obtained, using the low-field approximation of the Langevin function. Combined with a detailed study of the thermal evolution of the Mössbauer spectra, our modeling of the magnetic data allowed us to quantify the sensitivity of the magnetic properties of 6-line ferrihydrite to temperature and particle size, useful as environmental signatures.
GP23A-0170 1340h
Detrital and Authigenic Magnetic Micro- and Nanoparticles in Pelagic Sediments of the Equatorial Atlantic
Magnetic paleofield and paleoenvironmental information of marine sediments is mostly carried by submicron magnetic particles from various sources. Most existing studies make plausible, but largely unconfirmed assumptions about the origin, mineralogy and grain size of the magnetic mineral assemblages of pelagic sediments. This study intends to provide a detailed characterization of magnetic micro- and nanoparticles in oxic to mildly suboxic sedimentary environments of the Equatorial Atlantic and compares three sites (Cear$\'{a}$ Rise, Mid Atlantic Ridge (MAR), Sierra Leone Rise) along a W-E transect. This region offers magnetic particle sources such as continental dust, fluvial discharge and weathering of ocean ridge basalts. Remanence, hysteresis, low- and high-temperature rock magnetic investigations were performed on bulk sediments, magnetic extracts and heavy liquid separates and were combined with analytic scanning (SEM) and transmission (TEM) electron microscopy. Curie temperatures between 580 and 600$\deg$C indicate oxidized magnetite as the major low coercivity component in all samples. The Verwey transition ($\sim$110 K) is weakly expressed in the samples from the Cear$\'{a}$ Rise and the MAR and disappears at the Sierra Leone Rise. SEM studies on the magnetic extracts show that the quantitative main components are detrital titanomagnetite particles with increasing Ti-content throughout the transect towards the East. Magnetite particles with very low to zero Ti-content provide about one third of the detrital component. They often show shrinking cracks indicating external maghemitization. Further components are octahedral titanomagnetite crystals, silicates with (titano-) magnetite inclusions and spherules with low Ti-content. An important high coercive component, most likely goethite, is unsaturated at 2.5 T and missing in the magnetic extracts. It is manifested by a large discrepancy of the slopes in field cooling and the zero field cooling low-temperature curves, which disappear after heating to 340$\deg$C. Shape, grain size, roundness as well as the degree of preservation of the magnetic grains give insight into the transport mechanisms and regional provenances of the various components. The close coupling of ARM and IRM signals hints at a single source mechanism, eolian dust input, for micro- and nanoparticle fractions in the East of the transect. Therefore, the ARM/IRM ratio indicates mainly grain size variations related to varying wind intensity. In the western Equatorial Atlantic, the accumulation rates of the micro- and nanoparticles are much more weakly coupled. The coarser fraction is reflecting sea level controlled Amazon discharge while the fine fraction is yet to be identified by TEM. In contrary to the eastern part, the ARM/IRM signal of this region expresses rather a grain size mixing than sorting regime.
GP23A-0171 1340h
Bioturbation-assisted Polarity Reversal in Pleistocene Marine Sediments; a Case of Chemical Remagnetisation Rather Than Physical Grain Re-alignment
High-fidelity paleomagnetic records retained in marine sediments are generally attributed to pDRM. This process requires a zone below the water-sediment interphase where a gradual lock-in of detrital magnetic particles takes place. Within this lock-in zone, bioturbation probably represents an important factor controlling this lock-in process. There are, however, few direct observations (if any!) of how bioturbation actually affects grain re-alignment. A Reversed-Normal polarity reversal record in Pleistocene marine sediments from the island of Rhodes, Greece (E-Mediterranean), has been studied in detail by retrieving 240 sub-samples (6.4cc) from a sediment-block (12x12x20cm) covering a previously determined polarity reversal. Demagnetisation (af and thermal) reveals an abrupt polarity reversal, coinciding with large variations in NRM, ARM, SIRM-intensities and susceptibility. X-ray graphs of sediment slabs obtained prior to sub-sampling show a dramatic increase in burrows at the polarity boundary. This is not reflected by the almost identical, bimodal grain-size distributions (sedigraph; 1 to 64m) throughout the block. Ore microscopy of magnetic extracts shows a stronger degree of low-temperature alterations in the N-polarity, bioturbated section. This is reflected by thermomagnetic curves run in air and Argon. The N-polarity zone also carries magnetic minerals with significantly higher coercivities (MDFSIRM, MDFARM, Hcr,). Magnetic fabric derived from AMS is completely overprinted by an tectonic overprint suggesting NNE-SSW extension. The significant changes in magnetic mineral composition occurring at the R-N level suggests that bioturbation in the present case acts as a proxy for changes in nutrition/oxygen conditions at the sediment/water inter-phase.
GP23A-0172 1340h
Discrete element modeling of post-depositional remanent magnetization acquisition: numerical principles.
A discrete element model (DEM) is presented that simulates the processes considered to influence the formation of post-depositional remanent magnetization (PDRM) in marine and limnic sediments. The premise of the model is a collection of deformable particles held within a two-dimensional container and their behaviour in response to the influence of external forces. The considered external forces are gravity, magnetic torque and particle-particle plasto-elastic collisions. The calculation of the change in a given particle's velocity and position is performed according to Newton's laws of motion over extremely small time steps (fractions of a second). Under such conditions, the motion of any particle can be considered to only affect its nearest neighbours. This approach substantially increases the calculation efficiency of the DEM model but limits the time span which can be represented during one simulation. Interactions between touching particles can also included in the simulation and are represented in terms of contact forces with both normal and shear components. To simulate the short term processes of sedimentation and PDRM formation, we construct a model suspension of initially randomly distributed magnetic and non-magnetic particles settling under the influence of gravity in the presence of an external magnetic field. It is possible to modify the distributions of particle size, shape and magnetization in order to represent PDRM formation in different sedimentary settings. Additional forces can be assigned to the particles to represent naturally occurring phenomena such as van der Waals interactions, rotational Brownian motion, compaction by overlying sediment and bacterial scale bioturbation. The outcome of the individual DEM runs is evaluated by calculations of porosity, PDRM intensity, PDRM inclination and the numbers of free and locked particles within the sedimentary matrix. A series of models simulating different depositional conditions show that particle shape as well as compaction and the external magnetic field have a dominant influence on the acquisition of PDRM, whilst van der Waals forces control the formation of pore spaces within the sediment. Numerical details of the underlying physical processes and the numerical evaluations will be presented.
GP23A-0173 1340h
Trajectories of Unsaturated Magnetic Properties on Day Plots
The Day plot was initially developed to delineate the domain states of magnetite grains based on saturated magnetic properties and has since become a standard method in rock magnetic studies. Because most natural magnetic systems are "dirty", meaning they may contain a combination of non-stoiciometric magnetite, partially oxidized magnetite, other magnetic minerals, and/or complicated grain-size distributions, Day plots can be difficult to interpret and may not even be applicable to these systems. Furthermore there is a tendency for many natural systems to have similar magnetic parameters, and thus fall in the same field of a Day plot, even though they have distinctly different magnetic compositions and grain sizes. We aim to overcome some of these limitations by using the trajectories of unsaturated magnetic properties in a Day plot. Below saturation, rock magnetic parameters are highly variable with respect to the maximum applied field, B$_{max}$, though surprisingly little work has been done to utilize this variability as a rock magnetic tool. The magnetic moment, the magnetic remanance, the coercivity, and the coercvity of remanance were measured as a function of B$_{max}$, from 10 mT (unsaturated) to 1400 mT (saturated for magnetite). These four parameters were then plotted on a Day plot, which gave a trajectory of points as a function of B$_{max}$, instead of a single point. These trajectories are useful for discriminating grain-size distributions and compositions as illustrated for several pure and mixed synthetic samples of magnetite and hematite, all diluted in vacuum grease to minimize interactions. The technique was also applied to NIST standards 1633 (Fly Ash), 1649 (Urban Dust), and 1650 (Diesel PM) and revealed that all three samples contained at least two magnetic components, each of which has different coercivities and grain-size distributions. This information would not have been discernable with most conventional magnetic measurements.
GP23A-0174 1340h
Predicting Hysteresis With Preisach Models
Rock magnetists characterize magnetic systems using first order reversal curve (FORC) functions. These are functions of two fields derived from a particular set of magnetization curves called FORCs. FORC functions borrow the formalism of classical Preisach theory, which has also been used to predict hysteresis in nonlinear systems. The theory has two criteria that the hysteresis must satisfy for the model to work: "congruency" and "wiping out". If these criteria are satisfied then a function called the Preisach function can be derived from hysteresis measurements in the same way that the FORC function is derived. The Preisach function can be integrated to predict the hysteresis, or predictions can be made directly from the FORCs. It is shown that if the criteria for the classical Preisach model are not met, there are two FORC functions. One is derived from FORCs that start at positive saturation, another from FORCs that start at negative saturation. Predictions of classical Preisach theory for single-domain particles with uniaxial anisotropy are compared with direct predictions using Stoner-Wohlfarth theory. If FORC functions are used the predictions are entirely unlike the true hysteresis. Piecing together FORC curves works better, but the predictions accumulate errors as the field oscillates. As a result, remanence predictions are also incorrect. There are many varieties of Preisach theory besides classical Preisach theory. Nonlinear Preisach theory uses a function of three fields and is known to correctly predict hysteresis for Stoner-Wohlfarth systems. However, it is shown that nonlinear Preisach theory cannot predict the hysteresis of more complex magnetic particles. Preisach theories may still be able to predict hysteresis in multi-particle systems if certain statistical properties hold.
GP23A-0175 1340h
Magnetostatic interaction fields in First-order-reversal-curve (FORC) diagrams.
From experiments it is known that magnetostatic interactions between grains strongly affect the magnetic hysteresis behaviour of samples, however, because of the difficulty in predicting the non-linear behaviour our knowledge of the contribution of interactions to hysteresis and in particular first-order-reversal-curve (FORC) diagrams is limited. Previous FORC simulations of three-dimensional structures have been considered only the effect of interactions on the FORC diagram itself. To better understand the relationship between FORC diagrams and interactions, in this new study we examine the shape of the interaction field distribution (IFD) and how it changes during a FORC simulation for assemblages of ideal single domain (SD) grains. The IFDs tend to be more Gaussian on average than Cauchian as predicted analytically for disordered systems, due to ordering during FORC diagram determination. The spreading of the FORC distribution in the vertical direction of the FORC diagram, is shown to be directly related to the mean standard deviation of the IFD during the FORC measurement, with a small offset related to the smoothing factor. We use for the first time in our FORC simulations a three-dimensional dynamic micromagnetic algorithm that solves for the Landau-Liftshitz-Gilbert equation.
GP23A-0176 1340h
Rules of Thumb for Producing FORC Diagrams
A large number of partial hysteresis curves called first order reversal curves (FORCs) are commonly measured to determine the FORC distribution of a sample. These distributions, as represented in FORC diagrams, provide information about micro coercivities and magnetic particle interactions in samples, which ultimately aid in deciphering the composition and grain sizes of the magnetic minerals. Because a variable number of FORCs are used in making FORC diagrams, with each curve potentially representing hundreds of data points, the time it takes to produce a single FORC diagram varies from only a few minutes when only a few curves are measured to several hours when over a hundred curves are measured. Using more curves in building a FORC diagram generally gives a more detailed look at the coercivity distribution and interactions between magnetic particles, but it also increases the measurement time. As might be expected, there is a point at which there is no significant gain in resolution no matter how many additional curves are measured. Our goal is to provide some "rules of thumb" for producing FORC diagrams for natural materials that fully resolve the FORC distributions with as few curves as possible. Such information is extremely valuable for studies involving large numbers of samples or for studies where measurement time is limited, as might be the case for researchers visiting other laboratories. Even though the answer depends somewhat on the magnetic properties of the samples, experiments with a range of rock and mineral types provide useful measurement strategies. For example, experiments with oceanic basalt samples indicate that, when using fewer than 40 curves, the FORC distribution is overly smoothed. In contrast, FORC diagrams produced with more than 100 curves reveal no new features relative to FORC diagrams produced with around 80 curves. At some point, variations between densely spaced data are more indicative of instrument noise than magnetic properties. Besides the number of curves measured, factors such as the size of the space examined in the FORC diagram and the amount of smoothing done in processing the raw data are important. These factors mean that we cannot make a single set of definitive rules governing how FORC diagrams should be produced, but we can provide some general rules of thumb for the efficient production of FORC diagrams that sufficiently resolve the underlying FORC distributions.