Unusual butterfly-shaped hysteresis and defect-induced ferromagnetic transitions in highly oriented pyrolytic graphite with variable Bernal ordering

We report a novel investigation on the structural-, thermal- and magnetic-properties of highly-oriented-pyrolytic-graphite (HOPG) characterized by a variable level of Bernal-stacking. T-XRD and Rietveld-refinement were employed to extract the variation of the c-axis and thermal-expansion alpha-factor as a function of the temperature from 90K to 673K. Presence of a structural transition at similar to 200 K is shown. Electron-spin-resonance (ESR) and Raman-Spectroscopy together with superconducting-quantum-interference-device (SQUID)-magnetometry revealed the presence of magnetization-signals compatible with defect-induced-magnetic-ordering. Presence of unusual butterfly-shaped hysteresis-signals was also found in other internal-lamellae. The field-dependence-magnetization-measurements and rocking-curve-characteristics of this type of lamellae indicate presence of intrinsic ferrimagneti-cordering. (C) 2019 Elsevier Ltd. All rights reserved.