Magnetic studies of multi-walled carbon nanotube mats: Evidence for the paramagnetic Meissner effect

摘要

We report magnetic measurements up to 1200 K on multi-walled carbon nanotubemats using Quantum Design vibrating sample magnetometer. Extensive magneticdata consistently show two ferrromagnetic-like transitions at about 1000 K and1275 K, respectively. The lower transition at about 1000 K is associated withan Fe impurity phase and its saturation magnetization is in quantitativeagreement with the Fe concentration measured by an inductively coupled plasmamass spectrometer. On the other hand, the saturation magnetization for thehigher transition phase ($\geq$1.0 emu/g) is about four orders of magnitudelarger than that expected from the measured concentration of Co or CoFe, whichhas a high enough Curie temperature to explain this high transition. We showthat this transition at about 1275 K is not consistent with a magneticproximity effect of Fe-carbon systems and ferromagnetism of any carbon-basedmaterials or magnetic impurities but with the paramagnetic Meissner effect dueto the existence of $\pi$ Josephson junctions in a granular superconductor.