Ferromagnetism of a graphite nodule from the Canyon Diablo meteorite

TL;DR

This study demonstrates room-temperature ferromagnetism in meteoritic graphite, likely caused by magnetic proximity effects at interfaces with iron-rich mineral inclusions, providing evidence for intrinsic magnetic properties in carbon materials.

Contribution

It provides experimental evidence of ferromagnetism in extraterrestrial graphite and suggests a magnetic proximity effect as the origin, which is a novel insight compared to prior weak or impurity-driven reports.

Findings

Ferromagnetism observed at room temperature in meteoritic graphite.

Magnetization cannot be fully explained by iron-rich mineral impurities.

Magnetic ordering temperature near 570 K.

Abstract

There have recently been various reports of weak ferromagnetism in graphite (1,2) and synthetic carbon materials (3) such as rhombohedral C60 (4), as well as a theoretical prediction of a ferromagnetic instability in graphene sheets (5). 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, akaganeite, 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 23 Am2kg-1, or 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.