Structural and magnetic properties of iron in graphite

Pure and impure graphite powders, and two samples of highly oriented pyrolytic graphite (HOPG) have been studied by X-ray diffraction, electron microscopy, magnetometry and 57Fe Mössbauer spectrometry in order to identify the distribution of iron impurities and establish their role in the weak ferromagnetism of some of these materials. Pure graphite powder (99.9995%) shows straightforward diamagnetic behavior, with quantum oscillations at 4 K. The impure (>99%) powder gives a ferromagnetic signal exhibiting little temperature dependence in the temperature range 1.8–300 K, due to the presence of roughly isometric inclusions of metallic iron and cementite (Fe3C) about 100 nm in size. These inclusions are not superparamagnetic. The HOPG-ZYA and -ZYB samples show similar, but highly anisotropic ferromagnetic signals with a saturation magnetization of 1–5 × 10−3 A m2 kg−1, due to inclusions of thin Fe3C platelets in the graphite sheet with a diameter of ≈1 μm and diameter to thickness ratio >10. These platelets form Frisbee-shaped rings around primary lens-shaped inclusions of cubic (Ti, V)C. It is important to account quantitatively for the presence of these metallic, iron-rich inclusions before claiming that the origin of any weak ferromagnetic behavior of graphite is related to intrinsic lattice defects.