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

TL;DR

This study investigates the magnetic properties of multi-walled carbon nanotube mats at high temperatures, revealing evidence for a paramagnetic Meissner effect likely caused by Josephson junctions in a granular superconductor.

Contribution

The paper provides the first magnetic evidence supporting the existence of a paramagnetic Meissner effect in carbon nanotube mats, suggesting superconducting behavior related to Josephson junctions.

Findings

Two magnetic transitions at ~1000 K and ~1275 K observed.

The high-temperature transition is inconsistent with impurity or proximity effects.

Evidence suggests the presence of a paramagnetic Meissner effect in the material.

Abstract

We report magnetic measurements up to 1200 K on multi-walled carbon nanotube mats using Quantum Design vibrating sample magnetometer. Extensive magnetic data consistently show two ferrromagnetic-like transitions at about 1000 K and 1275 K, respectively. The lower transition at about 1000 K is associated with an Fe impurity phase and its saturation magnetization is in quantitative agreement with the Fe concentration measured by an inductively coupled plasma mass spectrometer. On the other hand, the saturation magnetization for the higher transition phase ($\geq$1.0 emu/g) is about four orders of magnitude larger than that expected from the measured concentration of Co or CoFe, which has a high enough Curie temperature to explain this high transition. We show that this transition at about 1275 K is not consistent with a magnetic proximity effect of Fe-carbon systems and ferromagnetism of any carbon-based materials or magnetic impurities but with the paramagnetic Meissner effect due to the existence of $\pi$ Josephson junctions in a granular superconductor.