Synthesis of Fe2O3@CNT and Measurement of Linear & non-Linear Ac- Susceptibility.

Abstract

Certain anti-ferromagnets are known as weak ferromagnets (WFMs). These magnetic crystals display a weak net magnetization which originates from Dzyaloshinskii-Moriya interactions (DMIs) that causes canting of the magnetic moments. Examples include hematite (α-Fe2O3) and RCO3 (R = Mn, Co), which exhibit unusual magnetization dynamics, as observed in remanence measurements using superconducting quantum interference device (SQUID) magnetometry. Prior work has shown existence of a quasistatic remanence with a counterintuitive magnetic field dependence. These measurements also bring forward an ultra-slow magnetization dynamic in these samples. Encapsulation of hematite inside multiwall carbon nanotubes (CNT) enhances the magnitude of this quasi-static remanence, as observed using SQUID magnetometry. It is non-trivial to stabilize hematite inside carbon nanotubes (Fe2O3@CNT) and scale-up well characterized samples of Fe2O3@CNT prepared in different batches. Furthermore, AC susceptibility, ac-χ, measurements are yet not reported on such hybrids of Fe & Fe2O3@CNT. Here ac-χ can be a more sensitive probe for understanding the unusual magnetization dynamics, especially through frequency dependence. This project pertains to synthesis of α-Fe2O3@CNT, characterization by x-ray diffraction (XRD), Raman, scanning electron microscopy (SEM), and ac susceptibility measurements. The work first requires growth of Fe@CNT via chemical vapour deposition, followed by suitable annealing to convert it to α-Fe2O3@CNT. While the condition of growth using a single zone furnace and a two quartz-tubes based synthesis chamber is routinely done in the lab, during this project, Fe@CNT was prepared using a single quartz-tube. Slight variations in experimental condition also led to significant improvement in the yield. Conversion of Fe@CNT led to a well characterized Fe2O3@CNT. In addition, a few samples with systematically varying fraction of Fe@CNT in Fe2O3@CNT were also prepared and characterized. Three samples with systematically varying mass fraction of Fe2O3@CNT samples were probed using AC–χ measurements to study both linear and nonlinear AC susceptibilities as functions of field, frequency, and temperature. A reference sample of bare hematite crystals in the form of hex-plates were also probed as a reference sample. The real part of the first harmonic susceptibility exhibits a systematic variation in the magnitude of the signal as a function of ferromagnetic content in Fe2O3@CNT and shows signature of the Morin transition associated with the canted-AFM phase. The frequency dependence of the real part of susceptibility shows the time-dependent magnetization dynamics in these Fe2O3@CNT hybrids.