  We report magnetic measurements up to 1200 K on three different multi-walled
carbon nanotube mat samples using Quantum Design vibrating sample
magnetometers. Three different samples prepared from arc discharge or chemical
vapor deposition contain magnetic impurities ranging from about 100 ppm to
about 1.5%. Our precise magnetic data clearly show two superconducting
transitions, one at temperatures between 533 K and 700 K, and another at about
1200 K. The first transition temperature T_cJ, which coincides with the
transition temperature seen in the resistance data, depends very strongly on
the magnetic field, as expected from the onset of intergrain Josephson coupling
in granular superconductors. The strong field dependence of T_cJ also excludes
magnetic contaminants as the origin of the first transition. We also present
direct and inferred diamagnetic Meissner fractions of 2 and 14%, respectively.
The present results provide compelling evidence for superconductivity well
above room temperature in multi-walled carbon nanotubes.
