PNMI · From the Planetary to the Nanoscale: Magnetism at the Interface

From the Planetary to the Nanoscale: Magnetism at the Interface

Project title and number PNMI—From the Planetary to the Nanoscale: Magnetism at the Interface, #237671 Call (part) identifier and Funding scheme FP7-PEOPLE-IEF-2008, Marie Curie Actions—Intra-European Fellowships (IEF) Aeromagnetic surveys record spatial and intensity variations in the Earth´s magnetic field. These variations are related to the type, spatial distribution and relative abundance of magnetic minerals in the crust. Thus the interpretation of large scale geophysical data requires generally a thorough understanding of the magnetic properties of minerals down to the atomic scale under variable pressure-temperature conditions. This is the objective of the PNMI (Planetary to Nanoscale: Magnetism at the Interface) project. In contrast to the common assumption that magnetite (Fe3O4) is the main magnetic carrier in the crust the remanent magnetization of many rocks is related to rhombohedral oxides in the system hematite (Fe2O3) – ilmenite (FeTiO3). This project justifies that the magnetic properties of igneous and metamorphic rocks containing ilmenite with hematite exsolution lamellae (hemo-ilmenite) and hematite with ilmenite exsolution lamellae (ilmeno-hematite) result from an interface phenomenon called “lamellar magnetism”. The strong and stable remanent magnetization is explained by the uncompensated spins provided by magnetically interacting atomic contact layers on both sides of lens-shaped nanometre-scale exsolution lamellae. The PNMI project extensively focused on the chemical and mineralogical characteristics of natural and synthetic rhombohedral oxides in the hematite – ilmenite system using electron microprobe analysis (EMPA), analytical transmission electron microscopy (TEM), electron-backscatter diffraction (EBSD) on a scanning electron microscope (SEM), and X-ray diffraction experiments. Synthetic oxides were annealed in a high-pressure piston cylinder apparatus to understand the temperature dependence of magnetic properties. All these investigations were carried out at the Bayerisches Geoinstitut, University of Bayreuth, Germany, and were complemented by magnetic measurements at the researcher´s home institute, Norwegian Geological Survey, Trondheim, Norway. The results of these investigations are mainly summarized in a series of three papers (Fabian et al. 2011, Robinson et al. 2012a and b), which develop a consistent interpretation of the physical and chemical processes responsible for self reversal and magnetic exchange bias in the haematite-ilmenite solid solution series. Quenched ferri-ilmenite X FeTiO3 + (1 – X) Fe2O3 with X ≈ 0.60 was studied to uncover the chemical and magnetic structures important for understanding the unusual self-reversal and exchange bias in this system. Samples were first annealed at 1055°C, above the Fe-Ti ordering temperature, and then quenched. The magnetic hysteresis and large negative magnetic exchange bias suggest the presence of two separate interface-coupled phases (Fabian et al. 2011). Amplitude contrast imaging on the TEM reveals the dominance of a Fe-Ti disordered antiferromagnetic phase containing small lenses of an ordered ferrimagnetic phase. Annealing experiments at temperatures of 500°C, 700°C, 750°C, and 790°C, followed by cooling in a 1 T field, produced positive room-temperature magnetic exchange bias, only for the latter two runs. These properties suggest the growth of ordered regions at the expense of the disordered regions, creating a self-organized structure essential for magnetic self-reversal and magnetic exchange bias. The microstructural and chemical evolution of annealed ferri-ilmenite was studied in detail by Robinson et al. (2012a). A major outcome of this study is the discovery of antiphase domains with ordered and anti-ordered domains that are first randomly distributed as small exsolutions. Ordering advances by the growth and impingement of these domains. Prolonged annealing results in the migration of antiphase boundaries, coupled with Fe enrichment of shrinking domains and Fe depletion of growing domains. These microstructural features provide the atomic basis for self-reversed thermoremanent magnetization and room-temperature magnetic exchange bias presented by Robinson et al. (2012b). This paper demonstrates that the dominant antiferromagnetic interactions between (0001) cation layers result in opposed net magnetic moments of ferrimagnetic ordered phases across chemical antiphase domain boundaries. The magnetic consequences of these interactions are a strong coupling across abundant antiphase boundaries which provides the probable configuration for self-reversed thermoremanent magnetization. Taking the self-reversed state into strong positive fields provides a probable mechanism for room-temperature magnetic exchange bias. Fabian, K., Miyajima, N., Peter Robinson, McEnroe, S.A., Boffa Ballaran, T. and Burton, B. P. (2011): Chemical and magnetic properties of rapidly cooled metastable ferri-ilmenite solid solutions – implications for magnetic self-reversal and exchange bias: I. Fe-Ti order transition in quenched synthetic Ilm 61, Geophysical Journal International 186, 997-1014. Robinson, P., Harrison, R. J., Miyajima, N., McEnroe, S.A. and Fabian, K. (2012a): Chemical and magnetic properties of rapidly cooled metastable ferri-ilmenite solid solutions - implications for magnetic self-reversal and exchange bias: II. Chemical changes during quench and annealing, Geophysical Journal International 188, 447-472. Robinson, P., Harrison, R. J., Fabian, K., and McEnroe, S.A. (2012b): Chemical and magnetic properties of rapidly cooled metastable ferri-ilmenite solid solutions: implications for magnetic self-reversal and exchange bias - III. Magnetic interactions in samples produced by Fe-Ti ordering, Geophysical Journal International. 191, 1025-1047.

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