GP22A-01 INVITED 10:20h
Magnetite morphology and micromagnetic modelling: Characteristics of magnetotactic bacteria.
Bacteria have evolved a means of navigating using the earth's magnetic field by producing chains of magnetic particles that act like a compass needle. There are distinct magnetic characteristics of such chains that will optimise their efficiency as compass needles, and these will be examined in this talk. A three-dimensional finite element micromagnetic model will be used to examine different geometries of magnetite chains. The finite element approach allows modelling of much more complex magnetite geometries than can be achieved using finite difference modelling on regular grids. Furthermore, stable magnetic structures are obtained using a combination of both minimum energy and a dynamic solution of the Landau-Lifshitz-Gilbert equation. This achieves robust solutions which can follow the magnetization structure through the switching process at a constant external field. The algorithm is also capable of including thermal fluctuations in a much more rigorous way than can be obtained using minimum energy methods alone. The model will examine the role of grain geometry, magnetostatic interactions fields and domain states on the effectiveness of magnetite chains as compass needles. A comparison will be made between the most effective geometries predicted by micromagnetic modelling with those achieved by genetic optimisation in magnetosomes.
GP22A-02 10:35h
Viscous behaviour of multidomain magnetite
Magnetic materials acquire viscous remanent magnetisation (VRM) by exposure to ambient fields over long time-scales. This VRM is usually identified in rocks by a component of magnetisation parallel to the direction of the Earth's magnetic field at the site. VRM masks the original palaeomagnetic signal in the rock and must be removed. The expectation in palaeomagnetic studies is that this VRM component will demagnetise between ambient temperatures and a maximum temperature that is not likely to exceed 150-200 \deg C. However, Dunlop and \"{O}zdemir (2000) have demonstrated that multidomain (MD) VRM acquired at 200 \deg C in crushed and sized natural crystals of magnetite persists on thermal demagnetisation up to the Curie temperature, that is, there is a MD VRM component which is metastable. This goes against the classic MD theory of N?el which predicts that domain walls which move at low-temperatures in the earth's field, are easily re-organised by small increases in temperatures. Which means that any VRM acquired by domain walls at 200 \deg C will not persist to the Curie temperature. Clearly, there are large gaps in our understanding of MD VRM behaviour and MD remanence theory in general. In this paper we report the study of MD magnetic viscosity using high-temperature controlled-field magnetometers. To examine the contribution of disaccomodation to viscous behaviour we measured the viscosity both before and after high-temperature thermal equilibration. In such a study it is very important to have well-characterised samples. So, to conduct these experiments we have examined a synthetic and selected natural multidomain magnetite samples. In particular we have grown a new set of sized (mean grain sizes between 1-100 $\mu$m) MD magnetites by hydrothermal recrystallisation. In addition to these samples, we have considered sized MD synthetic magnetites of commercial origin, and large natural individual crystals of magnetite.
GP22A-03 10:50h
Thermoviscous Magnetization in Planetary Lithospheres
The nature of magnetic anomaly sources in the deep crust or uppermost mantle remains poorly constrained. I investigate the rock magnetic character of potential source rocks and the relative importance of induced, thermoviscous, and remanent magnetization, starting from first principles. Neel single-domain theory gives a useful picture of thermoviscous effects. It can be adapted to describe multidomain grains provided self-demagnetization is taken into account. Lower crustal rocks from deep drilling or uplifted crustal blocks have multidomain magnetite as their principal magnetic phase. Measured at room temperature, both remanent and induced magnetizations are in the range 0.1-0.5 A/m, but at high T in the lower crust, induced magnetization remains constant or increases while remanence decreases by a factor 2-4. Thermoviscous magnetization produced over the Brunhes chron likely causes a 30-60% increase compared to short-term induced magnetization produced by the present Earth's field. Data on thermoviscous effects at high T are mainly for synthetic magnetites but one set of data for serpentinized peridotites implies that these rocks in situ in the uppermost oceanic mantle could be sources of both broad-scale induced magnetic anomalies and of lineated remanent anomalies. Mars is a special setting because although most of its surface is magnetically barren, the remaining areas have magnetic anomalies more intense than any on Earth. Mars lacks a present-day field so that remanent sources magnetized in an ancient field before 4 Ga and occupying a substantial fraction of the crust must be responsible for the anomalies. Many minerals are possible, including magnetite, hematite, titanohematite and pyrrhotite. The brief (100-200 Ma) existence of a dynamo field combined with slow cooling prior to 4 Ga result in a narrow T window for TRM acquisition in the juvenile crust. Only limited ranges of crustal depths likely acquired a stable TRM. On the other hand, subsequent cooling of only 20-25 C stabilized this TRM for the rest of Mars' history.
GP22A-04 11:05h
Directional Behavior of Remanent Magnetization at Low Temperatures Measured With a New SQUID Magnetometer Probe
Many magnetic minerals undergo magnetic transitions at low temperatures. In magnetite, such a transition (the Verwey transition) occurs at $\sim$120 K. A loss of remanence at the Verwey transition is thought to be preferentially associated with large multidomain grains. Because these grains often carry secondary magnetizations, low temperature treatments are gaining popularity as an additional means of magnetic cleaning, used to isolate primary magnetizations. Smaller grains also lose some remanence, but part of this remanence (known as the low-temperature ``magnetic memory'') restores after warming. However, our understanding of the magnetic memory and other processes in magnetic minerals at low temperatures is based almost entirely on total moment measurements (rather than directional data). The assumption has been that these total moment data adequately mimic the behavior of directions. We examined this assumption using a prototype of a new device enabling the measurement of remanence directions at low temperatures using a 2G Enterprises 3-component SQUID magnetometer. Surprisingly, we found that remanence directions carried by magnetite after cycling to low temperatures (in a zero field environment) do not always coincide with the original magnetization. We discuss possible mechanisms which may result in such a deviation, as well as continuing efforts to develop the low temperature probe.
GP22A-05 INVITED 11:20h
A new three-axis vibrating sample magnetometer for continuous high-temperature magnetization measurements: Applications to paleo- and archeointensity determinations
We have developed a new three-axis vibrating sample magnetometer (Triaxe) which allows continuous high-temperature magnetization measurements of individual cylindrical 0.75 cm3 samples up to 650°C, and the acquisition of thermoremanent magnetization (TRM) in any direction with a field of up to 200 microT. This equipment offers many possibilities for investigating rock magnetic properties at high temperature. As a first application, we propose a fast (2 hours) automated experimental procedure based on a modified version of the Thellier and Thellier (1959) method revised by Coe (1967) which provides continuous intensity determinations over a large (typically 300°C) temperature interval for each sample. This procedure takes into account both the cooling rate dependence of the TRM acquisition and the anisotropy of TRM. Analyses of numerous pottery and baked brick fragments from Mesopotamia demonstrate the quality and the reliability of the data, and illustrate the potential of this new instrument for paleo- and archeomagnetism.
GP22A-06 11:35h
Toward an optimal geomagnetic field intensity determination technique: Testing the IZZI protocol
Paleointensity determinations based on double heating techniques (Aitken, Coe, and Thellier) are generally considered to be functionally interchangeable, producing equally reliable paleointensity estimates. To investigate this premise, we have developed a simple mathematical model. We find that both the zero-field first and in-field first methods have a strong angular dependence on the laboratory field (parallel, orthogonal, and anti-parallel) while the two in-field steps method is independent of the direction of the laboratory-produced field. Contrary to common practice, each method yields quite different outcomes if the condition of reciprocity of blocking and unblocking temperatures is not met, even with marginal (10%) tails of partial thermoremanence. By far the best approach, however, is to alternatethe infield-zerofield (IZ) steps with zerofield-infield (ZI) steps. We tested the IZZI method associated with the partial thermoremanent magnetization (pTRM) tail check. As predicted, the IZZI method shows a strong angular dependence, resulting from the undemagnetized portions of pTRM tails. We also tested two fundamental assumptions embedded in Thellier experiments, the initial state dependence and the effect of multi-cycle heat treatment. We observed that the magnitude of pTRMs with initial state of thermal demagnetization was larger than that of pTRMs in Thellier analysis. A multi-cycle Thellier analysis on coarse-grained magnetites progressively produces more intense pTRMs and progressively erases more pTRM tails. Both pre-history and multi-cycle dependence will likely to enhance the non-linear features of the Arai plot for coarse-grained magnetites.
GP22A-07 11:50h
The FORC Method: Where we are and where we are going.
In the seven years since its development, the FORC method has evolved into a powerful new technique for probing the microscopic mechanisms of magnetic behavior. The original motivation for development of the method was to obtain more detailed information about the magnetic mineralogy and magnetic grain-size distribution of natural geologic samples consisting of magnetite, maghemite and/or hematite. In recent years, the method has also proven to be useful for identifying other magnetic minerals, such as greigite, goethite, and siderite, as well as for unraveling more complex magnetic systems, such as intergrowths of magnetic exsolution lamellae. The FORC method has also been successfully used in studies of the progressive transformation of magnetic minerals at both low temperatures, i.e. pedogenesis and sulfate reduction, and at high temperatures, i.e. hydrothermal and geothermal alteration. Additional advances can be expected with the availability of instruments that can acquire FORC data at both low and high temperatures. For example, the ability to acquire FORC data at high temperatures holds the promise of a major advancement in the both the rate and reliability with which absolute paleointensity determinations can be made. Progress has also been made in developing new theoretical models for the interpretation of FORC diagrams and in establishing a better understanding of the relationship between FORC diagrams and Preisach diagrams. The FORC method is also being used to study the magnetic properties of advanced magnetic media, condensed matter systems, magnetic nanostructures and atmospheric aerosols. In turn, these applications are leading to new methods for representing FORC diagrams and for quantifying and interpreting the information that they contain. The development and availability of standardized computer programs for processing, displaying and analyzing FORC data is further contributing to development of the method.
GP22A-08 12:05h
How is a FORC diagram different from a Preisach diagram?
First-order reversal curve (FORC) diagrams are a new method of characterizing magnetic minerals in natural samples by measuring the microcoercivity distribution $f(H_c)$ and the interaction field distribution $g(H_u)$. The equivalence between FORC and more traditional Preisach diagrams was tested by measuring both for samples of synthetic single-domain (SD) magnetite, synthetic elongated SD maghemite and natural submarine basalts. Even though the resolution of a Preisach diagram is about one order of magnitude less than that of a FORC diagram, there is good agreement in all cases between the two diagrams. The main results (coercivity and spreading of the distribution peak) are very consistent. However, patterns in the low coercivity region are problematic for Preisach diagrams. In examples where some important features of the FORC distribution were at low coercivities, the Preisach diagram failed to image these features and showed closed contours instead. Similarly, samples containing an important PSD or MD fraction are not suitable for a good result on Preisach diagrams, because their magnetization is dominated by induced rather than remanent magnetization. Therefore, FORC diagrams are preferable to Preisach diagrams.