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

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 sp(3)-rich characteristics as a result of chemical interactions between sulfur and HOPG. Investigations were performed at the controlled temperatures (T) of -300, 350, 500 degrees 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 sp(3)-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 mu B and 0.379 mu B respectively).

Automated summary

The recent observations of unusual ferromagnetic and superconductive phenomena in sulfur-doped amorphous carbon and highly oriented pyrolytic graphite have attracted significant attention. This study presents a novel investigation on the relationship between magnetic ordering and sulfur-doping-induced structural modifications in pyrolytic graphite. The results demonstrate the nucleation of an unusual amorphous carbon film with sp(3)-rich characteristics as a result of chemical interactions between sulfur and HOPG. Magnetic characterization reveals a re-entrant spin-glass behavior, possibly originating from vacancy-defects created by sulfur migration. Additionally, long-time air-exposure of the samples was found to significantly enhance the diamagnetic component.