Abstract
Recently published structural analysis and galvanomagnetic studies of a large number of different bulk and mesoscopic graphite samples of high quality and purity reveal that the common picture assuming graphite samples as a semimetal with a homogeneous carrier density of conduction electrons is misleading. These new studies indicate that the main electrical conduction path occurs within 2D interfaces embedded in semiconducting Bernal and/or rhombohedral stacking regions. This new knowledge incites us to revise experimentally and theoretically the diamagnetism of graphite samples. We found that the c-axis susceptibility of highly pure oriented graphite samples is not really constant, but can vary several tens of percent for bulk samples with thickness t ≳ 30 μm, whereas by a much larger factor for samples with a smaller thickness. The observed decrease of the susceptibility with sample thickness qualitatively resembles the one reported for the electrical conductivity and indicates that the main part of the c-axis diamagnetic signal is not intrinsic to the ideal graphite structure, but it is due to the highly conducting 2D interfaces. The interpretation of the main diamagnetic signal of graphite agrees with the reported description of its galvanomagnetic properties and provides a hint to understand some magnetic peculiarities of thin graphite samples.
Highlights
The c-axis diamagnetic susceptibility of graphite is very large and anisotropic [1,2,3]
The decrease of the absolute value of the susceptibility decreasing the thickness of the ordered graphite samples does not appear to be related to extra defects one may introduce through handling or sample preparation
The influence of localized spins at the graphene edges [41] on the total susceptibility of our graphite samples should be negligible, since the lateral surface of the samples is much smaller than their volume, and the magnitude of the total susceptibility does not correlate with the lateral surface of the samples
Summary
The c-axis diamagnetic susceptibility of graphite is very large and anisotropic [1,2,3]. According to the literature of the last 50 years, there is consent to interpret this large diamagnetism as due to the Landau diamagnetic contribution of a certain density of free conduction electrons within the graphene planes of graphite. The relatively low density of conduction electrons in graphite arises from the overlap of the 2pz electronic orbitals, normal to the graphene planes; whereas the overlap between those orbitals from the carbon atoms at neighboring graphene layers, in both Bernal and rhombohedral stacking orders, remains very weak, i.e., van der Waals coupling, as the huge anisotropy of the resistivity and magnetization indicates. The calculations of the conduction-electrons magnetic susceptibility have been done in the past taking into account an electronic band structure inferred from electric transport and magnetic measurements [4,5,6]; in particular, using the quantum oscillations in the electrical resistance, the Hall effect, and magnetization, i.e., the Shubnikov–de Haas (SdH) and de Haas–van Alphen effects. One may doubt the accuracy of the used models in particular because no electron-electron or spin-orbit coupling interactions were included explicitly, plus the difficulties those calculations have in modeling the van der Waals interactions between the graphene layers
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