Magnetic properties of CNT arrays synthesized by the injection CVD method at various catalyst concentrations

Abstract

The arrays of multi-wall carbon nanotubes (CNTs) filled with ferromagnetic nanoparticles (MFCNTs) have been obtained by the high temperature pyrolysis of fluid hydrocarbon (o-xylene) in a mixture with the volatile source of catalyst (ferrocene) using Ar as the gas-carrier. The influence of the catalyst concentration cx (0.5%, 5%, and 10%) in the feeding solution on the composition, crystalline structure, morphology and, accordingly, magnetic properties of MFCNT arrays in a wide temperature range was investigated. The X-ray diffraction, SEM and TEM methods revealed that CNT arrays are filled by Fe3C and Fe phases and that the higher is the catalyst concentration in the feeding solution, the higher is Fe3C and Fe content in CNT arrays. Temperature dependence of the specific magnetization σ(T) shows that σ increases with the increasing of ferrocene concentration in a whole temperature range under investigation (78 ≤T < 600 K). It is shown that σ(T) follows the Bloch law in the temperature range 80-300 K with Bloch constant B=1.65.10-5 K-3/2 and the Stoner law at 300-480 K for samples with cx=10% and, correspondingly, 80-450 K with Bloch constant B=6.1.10-5 K-3/2 and 450-480 K for samples with cx=5%. The lower value of Bloch constant, which characterizes the exchange interaction, in the case of cx=10% might be attributed both to dimensional effects and the decrease of the effective magnetic momentum of Fe phases atoms. The hysteresis loops demonstrate that coercivity Hc(cx=5%) decreases, but Hc(cx=10%) is constant or even slightly increases with increasing the temperature. This phenomenon is explained by the increase both the saturation magnetization and shape anisotropy.