Interplay of edge/screw dislocations and enhanced magnetism in exfoliated pyrolytic graphite with distorted hexagonal moire superlattices

The recent observation of enhanced magnetic signals in post-exfoliated lamellae of highly oriented pyrolytic graphite (HOPG) has attracted an important attention. It has been suggested that such a magnetization enhancement may originate from exfoliation-induced structural transitions, possibly involving rhombohedral domain and/or topological disorder formation. Further studies are therefore needed in order to elucidate the origin of such magnetic effects. Here we report a novel investigation on the structural and magnetic properties of multiple lamellae exfoliated from the inner regions of a main HOPG sample. HRTEM revealed moire-superlattices with coexisting-periodicity-gradients del D similar to 13.1 nm to 16.2 nm, D similar to 2.6 nm in lamella-1, del D similar to 30 nm to 36 nm in lamella-2 and del D similar to 10.4 nm to similar to 12.0 nm, 3.1 nm in lamella-3. Exfoliation-induced distortion effects of the moire-areas were investigated by employing HRTEM and Fourier transform analyses. We describe this effect on the basis of the Burgers-Bragg-Read-Shockley model with contributions arising from both I) edge dislocations (tilting angle theta(c) not equal 0, altered by the tape-exfoliation) and II) screw dislocations (twisting angle theta(twist) not equal 0). Extended temperature dependent electron paramagnetic resonance (T-EPR) investigations revealed important shifts in the position of the pi-electron differential absorption feature. The reported results are possibly an indicator of defectinduced electron correlation effects. Superconducting quantum interference device (SQUID) magnetometry measurements revealed an unusual magnetization-enhancement interpreted as a consequence of the exfoliation process. (c) 2021 The Author(s). Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/)

Automated summary

The study investigates the structural and magnetic properties of exfoliated lamellae from HOPG, revealing distortion effects in the moire-superlattices. The results suggest contributions from edge and screw dislocations in the Burgers-Bragg-Read-Shockley model, and electron paramagnetic resonance investigations show shifts in the pi-electron absorption feature, possibly indicating defect-induced electron correlation effects. Magnetometry measurements indicate an unusual magnetization enhancement as a result of the exfoliation process.