Magnetic anomalies associated with slowly cooled igneous and metamorphic rocks are commonly attributed to the presence of the mineral magnetite. Although the intermediate members of the ilmenite–haematite mineral series can also carry a strong ferrimagnetic remanence1, it is preserved only in rapidly cooled volcanic rocks, where formation of intergrowths of weakly magnetic haematite and paramagnetic ilmenite is suppressed. But the occurrence of unusually large and stable magnetic remanence in rocks containing such intergrowths has been known for decades2,3,4,5, and has recently been the subject of intense investigation6,7,8,9,10. These unmixed oxide phases have been shown to contain pervasive exsolution lamellae with thickness from 100 µm down to about 1 nm (one unit cell). These rocks, many of which contain only a few per cent of such oxides, show natural remanent magnetizations up to 30 A m-1—too strong to be explained even by pure haematite in an unsaturated state11,12. Here we propose a new ferrimagnetic substructure created by ferrous–ferric ‘contact layers’ that reduce charge imbalance along lamellar contacts between antiferromagnetic haematite and paramagnetic ilmenite. We estimate that such a lamellar magnetic material can have a saturation magnetization up to 55 kA m-1—22 times stronger than pure haematite—while retaining the high coercivity and thermal properties of single-domain haematite.
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TEM imaging and TEM analytical studies at Institut für Mineralogie, Universitat Münster, were done in cooperation with A. Putnis and U. Golla, and atomic simulations were carried out in cooperation with U. Becker. Electron microprobe studies at Univ. Massachusetts were completed with assistance from P.T. Panish and M.J. Jercinovic. S.A.M. and P.R. were supported by the strategic research fund of the Geological Survey of Norway and Bayerisches Geo-Institut. The work of S.A.M. on Swedish granulites was initiated during a Fellowship from the American-Scandinavian Foundation and from the Institute of Rock Magnetism, and her work in the Adirondacks was supported by the NSF. R.J.H. carried out his studies while a research fellow at the Institut für Mineralogie, Universitat Münster.
The authors declare that they have no competing financial interests.
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Robinson, P., Harrison, R., McEnroe, S. et al. Lamellar magnetism in the haematite–ilmenite series as an explanation for strong remanent magnetization. Nature 418, 517–520 (2002). https://doi.org/10.1038/nature00942
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