Annihilation of percolative correlation signals in sulfur doped highly oriented pyrolytic graphite with hexagonal Moiré superlattices

Abstract

We report a novel study on the magnetic properties of exfoliated HOPG lamellae exhibiting hexagonal Moiré superlattices with periodicity of ~40 nm, ~26 nm, ~23 nm and ~7.7 nm corresponding to rotational angles θrot of ~0.35°, ~0.55o, ~0.62° and ~1.8o. SQUID magnetometry revealed presence of percolative ferromagnetic contributions arising from topologically disordered areas and absence of superconductive transitions. Field-dependent zero field cooled (ZFC) and field cooled (FC) measurements of the magnetization evidenced presence of magnetic irreversibilities, arising from percolative ferromagnetism. Similar magnetic features could be detected in other exfoliated lamellae and in natural graphite. Experiments involving doping of the exfoliated lamellae with sulfur, evidenced further an unusual demagnetization effect; ZFC and FC magnetic curves of doped samples revealed a partial annihilation of the ferromagnetic signals as a consequence of interactions between the sulfur species and the topologically disordered regions. Raman spectroscopy highlighted the appearance of unknown broad bands in the regions of ~500 cm−1, 1200 cm−1 and 1400 cm−1 resulting from sulfur-defect interactions.