Speaker
Description
Magnetism and superconductivity in carbon allotropes are at the center of intense research in physics and material science. Defect-induced magnetism can arise from vacancies or nonmagnetic ad-atoms. Magnetic order can also be induced by ion implantation, such as O, P, S, or B.
We provide here evidence for weak room-temperature ferromagnetism and its possible interfacial coexistence with a small superconducting phase in ion-implanted graphitic samples. The effect of alkanes’ intercalation was also considered. PPMS magneto-transport and magnetization studies were conducted on virgin (diamagnetic) and ion-implanted graphite fibers, highly oriented pyrolytic graphite, graphite foil, and graphene samples for temperatures from 1.9 K to 300 K and magnetic fields up to 9 T.
Sample and temperature dependent, magnetoresistance loops show either anomalous or normal hysteresis. Anomalous hysteresis, as in conventional and high temperature superconductors, is explained on the basis of a two-level critical-state model where pinned fluxons exist inside the Josephson-coupled superconducting grains and also between them. Bulk superconductors with pinned Abrikosov flux lines and ferromagnets show normal hysteresis. The temperature-dependent remanent magnetization has the behavior consistent with excitations of spin waves in a 2D Heisenberg model with weak uniaxial anisotropy. Sharp steps in the remanent magnetization suggest a magneto-structural (martensic) transition with volume change, leading to an antiferromagnetic-ferromagnetic first-order phase transition. Step-like features are due to the coupling of BCS superconductors to inhomogeneous ferromagnetic textures. Imbalance in the defect number for the graphite’s A and B sub-lattices results in the nucleation of ferromagnetic islands within the antiferromagnetic domains. Ferromagnetic M(H) loops and superconducting-like M(H) loops were found in ion-implanted samples. The large magnetocaloric effect associated with a first order phase transition in antiperovskite materials has applications to magnetic refrigeration.
Acknowledgements: The Air Force Office of Scientific Research (AFOSR), The Aerospace Systems Directorate (AFRL/RQ), and United Energy Systems (UES, Inc.)