Local Magnetic Measurements of Trapped Flux Through a Permanent Current Path in Graphite
Temperature and field dependent measurements of the electrical resistance of different natural graphite samples, suggest the existence of superconductivity at room temperature in some regions of the samples. To verify whether dissipationless electrical currents are responsible for the trapped magnetic flux inferred from electrical resistance measurements, we localized them using magnetic force microscopy on a natural graphite sample in remanent state after applying a magnetic field. The obtained evidence indicates that at room temperature a permanent current flows at the border of the trapped flux region. The current path vanishes at the same transition temperature $T_c\approx370$~K as the one obtained from electrical resistance measurements on the same sample. This sudden decrease of the phase is different from what is expected for a ferromagnetic material. Time dependent measurements of the signal show the typical behavior of flux creep of a permanent current flowing in a superconductor. The overall results support the existence of room-temperature superconductivity at certain regions in the graphite structure and indicate that magnetic force microscopy is suitable to localize them. Magnetic coupling is excluded as origin of the observed phase signal.