Temperature dependent switching of magnetoresistance in multiwall carbon nanotube-polypyrrole composite fibrils

Carbon nanotubes (CNT) are considered one of the most significant materials in nanoelectronic device applications because they can be used in the fabrication of both CNT-inorganic hybrid structures and CNT-organic composite materials. Also, the study of the electrical properties of these materials has its own fundamental and technological significance. Here, we report on low temperature charge transport characteristics (down to 4.2 K in the magnetic fields up to 11 T) of multiwall carbon nanotube (MWCNT)-polypyrrole (PPy) coaxial composite fibrils synthesized by a facile electrochemical polymerization method. Two types of samples were synthesized by carrying out electrochemical polymerization at room temperature (RT) for different durations of 90 and 45 min, respectively. Scanning electron microscopy studies indicated that the diameters of as-prepared MWCNT-PPy fibril samples were ∼1.5 μm and 0.5 μm, respectively. The dc electrical resistance of the two samples was ∼10³ and 10² ω at RT and exhibited a pronounced temperature dependence, which is indicative of the hopping process being dominant. Furthermore, a large positive magnetoresistance (MR) of ∼29% and ∼18% is displayed at 4.2 K, which switched to negative MR with a maximum magnitude of ∼11% and ∼15% at 10 K for the two samples, respectively. The switching of MR as a function of temperature showed the dominance of two important competing phenomena, namely, wave function shrinkage and forward interference of electron waves.