Unusual re-entrant spin-glass-behavior and enhanced diamagnetism in sp3-rich sulfur/oxygen doped highly oriented pyrolytic graphite

Abstract

The recent observations of unusual ferromagnetic and superconductive phenomena in sulfur-doped amorphous carbon and highly oriented pyrolytic graphite (HOPG) have attracted significant attention. In this work we present a novel in-depth investigation on the relationship between magnetic-ordering and sulfur-doping-induced structural modifications in pyrolytic graphite. We demonstrate the nucleation of an unusual amorphous carbon film with sp3-rich characteristics as a result of chemical interactions between sulfur and HOPG. Investigations were performed at the controlled temperatures (T) of ~300, 350, 500 °C, with the residence time parameter varying from 1 min to 15 h. Magnetic characterization revealed a re-entrant spin-glass behavior, possibly originating from vacancy-defects created by sulfur migration. Noticeably, long-time air-exposure of the samples (3 weeks) was found to significantly enhance the diamagnetic component. Density functional theory (DFT) calculations applied to a sp3-rich amorphous‑carbon-system in presence of sulfur-bonding revealed a significant variation of the system-metallicity. We report the presence of insulating states up to sulfur concentrations of ~6.25% (atom %), which then vanish at ~7.5% (atom %) of sulfur-doping. Interestingly, inclusion of the sulfur-migration effect within the DFT calculations revealed significant contributions arising from magnetic-moments generated by electron localization at the vacancy sites. Total spin isosurface analyses (spin up - spin down) highlighted the presence of ferromagnetic contributions arising from carbon-atoms, due to dangling bonds generated by point defects (with the largest magnetic moment values being 0.42 μΒ and 0.379 μΒ respectively).