Revealing common artifacts due to ferromagnetic inclusions in highly-oriented pyrolytic graphite

TL;DR

This study investigates the origin of room-temperature ferromagnetism in highly-oriented pyrolytic graphite, revealing that magnetic signals are caused by micron-sized Fe-rich inclusions rather than intrinsic properties.

Contribution

The paper demonstrates that ferromagnetism in HOPG is due to Fe-rich inclusions, highlighting contamination as the source rather than intrinsic magnetic properties.

Findings

Magnetic clusters (Fe) are present in ferromagnetic HOPG samples.

Ferromagnetic signals correlate with the presence of magnetic inclusions.

Samples without magnetic inclusions do not show ferromagnetism.

Abstract

We report on an extensive investigation to figure out the origin of room-temperature ferromagnetism that is commonly observed by SQUID magnetometry in highly-oriented pyrolytic graphite (HOPG). Electron backscattering and X-ray microanalysis revealed the presence of micron-size magnetic clusters (predominantly Fe) that are rare and would be difficult to detect without careful search in a scanning electron microscope in the backscattering mode. The clusters pin to crystal boundaries and their quantities match the amplitude of typical ferromagnetic signals. No ferromagnetic response is detected in samples where we could not find such magnetic inclusions. Our experiments show that the frequently reported ferromagnetism in pristine HOPG is most likely to originate from contamination with Fe-rich inclusions introduced presumably during crystal growth.