Magnetic properties, texture and microstructure of cohenite grains from Morasko iron meteorite have been investigated using electron backscattered diffraction, Bitter pattern technique, magneto-optical imaging method and magnetic force microscopy. Cohenite shows much stronger magnetic contrast compared to kamacite because it is magnetically harder than the Fe-Ni alloy, and thus causes higher stray fields. A surprising result is the high stability and reversibility of the global stripe-like magnetic domain structure in cohenite when applying high magnetic fields up to 1.5 T, and exposing it to high temperatures above the Curie temperature of about 220 °C. Heating up to 700 °C under atmosphere conditions has shown that cohenite remains stable and that the global magnetic domain structures mainly recover to its preheating state. This observation suggests that magnetic domains are strongly controlled by the crystal anisotropy of cohenite. Branching magnetic domain structures at the grain boundary to kamacite can be annealed, which indicates that they are very sensitive to record deformation. EBSD observations clearly demonstrate that increasing deviation from the easy  crystallographic axis and stress localization are the main factors controlling the distortion of Bitter patterns, and suggest a high sensitivity of the cohenite magnetic domain structure to local microstructural heterogeneities. The results of this study substantiate the theory that cohenite can be a good recorder of magnetic fields in planetary core material.
Bibliographical noteFunding Information:
This work was funded by the priority program Planetary Magnetism of the German Research Foundation (DFG) (project KO1514/8: ?Mineral magnetism of shocked ferrimagnetic minerals?). We are grateful to W. Luecke for kindly providing the meteorite sample. We thank D. Litvinov for conducting the EBSD investigations. B.R. thanks the members of the Institute for Rock Magnetism for a fellowship that was given in February 2016 and for the large support during the time of the stay. This article profited significantly from the constructive comments of an anonymous reviewer.
- Bitter patterns
- Electron backscatter diffraction (EBSD)
- Magnetic domains
- Magnetic force microscopy (MFM)
- Morasko meteorite