There are recent reports of weak ferromagnetism in graphite1,2 and synthetic carbon materials3 such as rhombohedral C60 (ref. 4), as well as a theoretical prediction of a ferromagnetic instability in graphene sheets5. With very small ferromagnetic signals, it is difficult to be certain that the origin is intrinsic, rather than due to minute concentrations of iron-rich impurities. Here we take a different experimental approach to study ferromagnetism in graphitic materials, by making use of meteoritic graphite, which is strongly ferromagnetic at room temperature. We examined ten samples of extraterrestrial graphite from a nodule in the Canyon Diablo meteorite. Graphite is the major phase in every sample, but there are minor amounts of magnetite, kamacite, akaganéite, and other phases. By analysing the phase composition of a series of samples, we find that these iron-rich minerals can only account for about two-thirds of the observed magnetization. The remainder is somehow associated with graphite, corresponding to an average magnetization of 0.05 Bohr magnetons per carbon atom. The magnetic ordering temperature is near 570 K. We suggest that the ferromagnetism is a magnetic proximity effect induced at the interface with magnetite or kamacite inclusions.
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The authors declare that they have no competing financial interests.
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We thank M. Viret, F. Guinea and S. Sanvito for discussions. This work was supported by Science Foundation Ireland.
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