Magnetic fatigue of magnetite: effect of cyclic loading on the magnetic and microstructural behavior at elevated temperatures

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Magnetic properties of rocks are influenced by deformation processes within the Earth crust, ranging from slow processes in plate tectonics, through volcanic eruptions to the high dynamic seismic events. Up to now, modifications of magnetic properties are mainly investigated in relation to static deformation. However, it is generally accepted that tectonic stresses first accumulate non-uniformly, and then rapidly increase. Such a rapid stress increase may be especially dominant if the tectonic loading exhibits a cyclic or a fatigue character at elevated temperatures. This assumption raises the question whether magnetic properties of rocks are sensitive to cyclic deformation at elevated temperatures. Surprisingly, such information is unavailable in the geoscience literature. Our preliminary investigations, however, show clearly that cyclic deformation is causing a degradation of magnetic properties or a magnetic fatigue of a magnetite-bearing ore. Preliminary experiments at 500°C reveal a non-linear relationship between the increasing loading frequency and the variations in the Verwey transition temperature, magnetic remanence and coercively as well as a drastic non-linear drop of magnetic susceptibility and its anisotropy with increasing cyclic loading duration. Our project aims to test the magnetic fatigue hypothesis with a special consideration of irreversible (microcracks, vacancy formation, twinning) and reversible effects (magnetostriction, elastic strains). For this purpose, samples prepared from a magnetite ore, single crystals of magnetite and magnetite-bearing granite will be cyclically loaded under various conditions and subsequently studied by different magnetic and microstructural methods. On the basis of the obtained results, fatigue deformation maps related to the magnetic behaviour for magnetite will be constructed. We expect from this project basic knowledge on the magnetic field stress sensitivity, which is currently poorly constrained but might be useful for a sophisticated interpretation of magnetic signals from deformed rocks as they occur ubiquitous in the Earth crust.