Carbon Trends (Jul 2021)
Twist-angle-disorder and sulfur-induced annihilation of percolative magnetism in exfoliated lamellae of highly oriented pyrolytic graphite
Abstract
The existence of low temperature ferromagnetism in twisted graphene systems and locally twisted-sublattices of highly-oriented-pyrolytic-graphite (HOPG) has recently attracted significant attention. Despite important progress, additional investigations are needed to evaluate the role of twist-angle-disorder on the magnetism arising from those locally-twisted-interfaces. Doping these systems with sulfur has been previously shown to induce a direct modification of their magnetic properties together with formation of competing ferromagnetic/superconductive ordering. Being interested in investigating the role of doping-temperature on carbon-sulfur interactions, we performed a novel investigation on the structural-magnetic-relationships of these systems. TEM of undoped-HOPG-lamellae revealed locally-twisted-sublattices, with hexagonal-moiré-superlattices exhibiting periodicities corresponding to θrot ~ 2.35°, 1.7°, 1.6°, 1.5°, 1.45°, 1.05°, 0.408°, 0.38°, 0.35° and 0.34°. Significant variation in the period of the moiré-superlattices was found, thus indicating the existence of twist-angle-gradients ∇θ. Sulfur-doping was achieved at T ~ 550 , ~ 900 and ~ 990 °C. The characteristics of the doped-samples were investigated by Raman-spectroscopy. The appearance of broad bands in the region of 1200 and 1400 cm−1 of the Raman-spectrum is shown. SQUID ZFC/FC magnetic curves acquired from 2 to 300 K together with T-dependent magnetization vs field signals revealed an almost-complete sulfur-induced annihilation of the ferromagnetic-ordering. Significant differences were further found between exfoliated- and unexfoliated-samples. Particularly, we analysed the evolution of the ZFC and FC curves in the region of T ~ 25 K. where a magnetic transition possibly arising from the locally twisted moiré superlattices is present. Furthermore, an unusual increase of the diamagnetic-component with the decrease of the temperature is discussed.
