GP24A-01 INVITED
Aging in Ferrian Ilmenite: the Result of a Spin Glass Ordered State or Not?
The work presented focuses on titanium rich (Ti mole % > 65) single phase compositions of the hematite- ilmenite solid solution series. As titanium enters hematite's crystal structure two Fe3+ atoms are replaced by one Fe2+ and one Ti4+ atom. Ferrous iron and titanium atoms either order on alternate sublattices or enter randomly if quenched from a temperature above the cation order–disorder phase transition. The distribution of ferrous iron and titanium atoms coupled to the concentration of ferric iron atoms dictate the strength and sign of the superexchange interactions which in turn fundamentally controls the magnetic behaviour. The spin glass magnetic state is an ordered magnetic state which persists in a non-ground state configuration due to competing superexchange interactions. The quest of a spin glass to reach a state at equilibrium is an extremely slow relaxation process which has been described in the physics literature as a thermally activated random walk in a set of traps with a wide distribution of trapping times. Spin glasses never do reach an equilibrium state which is thought to be the origin of experimental observation of experimentally aging, memory and rejuvenation. Both ZFC and TRM aging experiments were conducted at variable aging times and temperatures. Large and narrow frequency dependent peaks in the out-of-phase component of AC magnetic susceptibility (χ") are observed at around 40 K in all samples and are thought to be associated with the transition into the spin glass magnetic state. Ilm90Hem10 sample quenched from 1300°C and Ilm70Hem30 quenched from 900°C both convincingly exhibit aging below the frequency dependent peaks in χ" as well as above. Ilm80Hem20 quenched from 1300°C show no aging at all measuring temperatures. Lastly, Ilm70Hem30 quenched from 1050°C and 1300°C both show no aging. The experimental data suggest that the observed aging and frequency dependent peaks in χ" do not result from the same physical mechanism; that aging may not be restricted to materials magnetically ordered in a spin glass state; and that cation distribution plays a key role in whether aging is observed or not in ferrian ilmenite.
GP24A-02 INVITED
Understanding the Superparmagnetism of Random Assemblages of Fine Particles: a Journey Between Insufficient Order and Insufficient Disorder
Fine magnetic particles are the most important recorders of the Earth's field. First- principles theories to explain the magnetism of such particles have been established more than 50 years ago, however, active research is still ongoing to gain a full understanding of a variety of processes related to interactions and non-uniform magnetization patterns. A great deal of complexity is added by unavoidable alteration processes occurring over geological time, such as low-temperature oxidation. Superparamagnetic particles (SP) provide an ideal material to investigate such processes, since chemical alteration can be reproduced in laboratory time, and magnetic blocking/unblocking phenomena can be accessed at low temperatures. However, the interpretation of such experiments is challenged by the intrinsic non-uniqueness of magnetic quantities such as remanence or susceptibility. This was recently shown in an attempt to reconstruct the grain size distribution of SP particles in a soil (J. Nie, The IRM Quarterly vol. 17, 2008), where two theoretically well founded methods led to totally different results. A partial solution to this problem is provided by measurements of the complex susceptibility as a function of temperature and frequency. The scope of my presentation is to review the large amount of work done in modeling such measurements for technical applications, which is mostly unknown in our scientific community, and to present an inversion technique I developed to analyze such measurements without a-priori assumptions about any material property. Examples of this new inversion method on a variety of materials ranging from ferritin to acicular magnetite in the Tiva Canyon tuff show how it is possible to effectively identify and characterize distinct phenomena, such as magnetic interactions, the competition between surface and bulk anisotropies, and non- homogeneous magnetization patterns.
GP24A-03 INVITED
Magnetism - a key to understanding evolutionary aspects of magnetotactic microorganisms
Magnetic bacteria represent an obvious model system for studying adaptation of organisms to the geomagnetic field. Generally, the intracellular bacterial compass is magnetically optimized in terms of magnetic material, crystal dimensions and arrangement. The question rises as to whether the magnetic optimization rule applies to multicellular magnetotactic prokaryotes (MMP) as well. An MMP consists of some 20 cells, which are radially arranged about an acellular space and each of which contains chains of magnetosomes. We conducted in-vivo magnetic remanence measurements on individual MMPs and found ratios of natural remanence to saturation remanence to be close to unity. This indicates that the principle of magnetic optimization even holds at the level of the cellular arrangement, that is, the cells are arranged such that their chain moments all add up in a coherent fashion. We show that the high degree of magnetic optimization is unlikely to be achieved by mere aggregation of individual magnetotactic cells into an MMP and discuss possible evolutionary scenarios which may have brought about MMPs.
GP24A-04 INVITED
Nature's nanostructures: Use of magnetism in tracking environmental change
Three factors have recently conspired to create excitement about the importance of the weak but finite magnetization of Hematite and other antiferromagnets like Goethite, Lepidocrocite and Ferrihydrite. These factors are: (1) Interest in soils and lake sediments as environmental change recorders, (2) Discovery of "the New Magnetism" of solids of nanophase (1-~10 nm) and (3) Developments in materials chemistry to synthesize such oxides and oxyhydroxides with narrow size distributions (monodisperse). We are particularly fortunate that extant theories of ultrafine particle magnetism (superparamagnetism, superferromagnetism) constitute a powerful approach to study critical properties of such nanophase "iron oxides" (sensu lato) such as size dependence, texture variation ("clumped" vs. discrete), role of natural impurities (adsorbed vs. incorporated) and most importantly, iron reductivity and oxidativity as responses to environmental change. We will use data form our laboratory and others' to claim that within experimental errors a combination of low temperature magnetism and in-field Mössbauer spectral analysis can help determine (from only ~10 mg aliquots)the composition, adsorbed vs. incorporated impurity dopants (e.g., Al, As, ...) and the clumped vs. discrete textures. Much remains to be done, but current data and models can help interpret more correctly the"Enviromagnetic Proxies" we now use to characterize modern soils and lake sediments, and aid in contamination remediation theories and in reconstruction of paleoenvironments.