Processes affecting remanence in oxy-exsolved titanomagnetite: the role of pseudo-single domain grains and interaction coupling
Abstract
While single-domain and multidomain remanence are mature theories and are well supported by experiment, natural mineral samples may be influenced by processes that are not incorporated in these models. Particles exhibiting pseudo-single domain (PSD) behavior are frequently inferred using measurements such as Day et al. plots and first-order reversal curve diagrams of igneous and sedimentary rocks containing magnetite or greigite, yet an analogous model for PSD remanence is lacking, limiting the use of such samples as paleomagnetic recorders as well as quantitative characterisation of the magnetic grains. The role of interactions-magnetostatic or exchange-is similarly poorly understood in all but the simplest systems. This study presents direct observations of remanence states in mineral intergrowths which will be used to discuss the nature of the PSD state and systems with interacting particles and phases: questions of relevance both to paleomagnetists and the wider physics community. This study applies electron holography to synthetic titanomagnetite samples that reproduce grains that have undergone high-temperature oxidation, commonly observed in igneous rocks. The oxy-exsolved samples are dominated by submicron titanomagnetite grains intergrown with lamellae of titanohematite, which may be either paramagnetic, permitting the study of PSD behavior and magnetostatic interactions, or antiferromagnetic or ferrimagnetic, where exchange coupling can be a significant influence. Holography of magnetite and paramagnetic titanohematite shows that the PSD state can vary widely as a function of the applied field and the geometry of phases, including uniform magnetization and vortex states predicted by micromagnetic simulations. Particles that are uniformly magnetized may show strong magnetostatic interactions and can show a variety of switching mechanisms, including non-uniform rotation and domain walls spanning large regions of interacting particles. These observations will inform a future theory of PSD remanence. Holography of the intergrowth of ferrimagnetic titanomagnetite and ferrimagnetic titanohematite shows interesting phenomena that may be related to a strong negative coupling between the two phases. We present a simple magnetic energy model and show that some of the experimental observations can be explained in terms of negative coupling, which in this model takes the form of an exchange interaction, although the physical origin of the coupling may be either exchange or magnetostatic. The nature of the energy surface is explored and it is shown that the experimental observations can be explained using physically reasonable values of the anisotropy and interaction parameters. A number of unresolved questions remain, however, which will require the application of a more sophisticated micromagnetic model combined with simulation of electron holographic phase shift.
- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2011
- Bibcode:
- 2011AGUFMGP33A1111C
- Keywords:
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- 1540 GEOMAGNETISM AND PALEOMAGNETISM / Rock and mineral magnetism;
- 1594 GEOMAGNETISM AND PALEOMAGNETISM / Instruments and techniques