The Study of Magnetic Nanostructures in relation to its Atomic ordering And Oxidation State

Abstract

The present work includes the detail study of magnetic nanostructures in relation to its atomic ordering, oxidation state, morphology and its influence on magnetic properties. Here, Cobalt (Co) and Iron (Fe) are the element of our research interest and the synthesis of nanostructures is being carried out using high temperature thermal plasma route. It has been observed from previous research carried out on various materials, that the physical as well as chemical properties of materials are enhanced when the dimension of material reaches nanoscale. Also the synthesis route plays an important role in the formation of various nanostructures. The high temperature thermal plasma is one of the physical routes sof synthesis of nanoparticles wherein its high heat flux might have direct impact on the ordering or disordering as well as on the properties of as synthesized nanostructures which can vary from its bulk counterpart. Hence, we have adopted this route of synthesizing magnetic nanostructures and investigate the changes in morphology and magnetic properties of Co and Fe. The high temperature thermal plasma route involves arcing between two metal electrodes, herein our case graphite is used as cathode and commercially available Co and Fe powder as anode. The steel chamber is double walled in order to achieve steep temperature gradient. When these metal electrodes struck, high heat flux is generated resulting into the evaporation of the anode metal. Due to evaporation, the atomic flux of anode metal moves out of the plasma zone. Here, the process of homogenous nucleation is initiated. This process involves the formation of cluster of atoms. The growth of these clusters start taking place in order to reduce the Gibbs free energy of the system (metallic cluster). This growth is achieved by outgoing atomic flux from the anode metal. The whole of above described phenomenon take place in the vapour phase only. Also the chamber being filled up with chilled water, a steep temperature gradient is generated resulting into faster condensation of the nanoparticles on the wall of the chamber. These are scraped and further used for characterization. There are different variable parameters involved in the experiments which affect the formation of nanopowders. These involve arc current, ambient (air, helium, argon), base pressure set during the experiment and external electric fields. The characterization techniques used are X-Ray Diffraction (XRD), Transmission Electron Microscopy (TEM), X-Ray Absorption Spectroscopy (XAS), Vibrating Sample Magnetometry (VSM) & Ac Susceptibility. From XRD technique, structural identification of Co and Fe nanopowders is obtained showing that high arc currents involve different Co based and Fe based nanostructures such as Co, CoO, Co3O4 and Foe, Fe2O3, Fe3O4, respectively. The detail

morphological study of nanopowders can be achieved using TEM. For both Co and Fe wherein it is observed that higher arc currents show various shapes of nanostructures which involve spherical, rods and hexagons with broad size distribution as compared to low arc current. EDAX allow us to probe the percentage elemental composition present in the nanopowder. The atomic ordering and or disordering can be probe using XAS. XAS involves two regimes: XANES and EXAFS. XANES give us information about the oxidation state for Co for two different arc current 50 A and 80 A. While the EXAFS region of the spectra probe the local structure. Here it has been found that higher arc current (herein 80 A, in case of Co) leads to the high heat flux which tends to produce large amount of thermal stress resulting into higher local structural disorder as compared to low arc currents (50 A). The magnetic properties of the Co and Fe nanostructures are probed using VSM and Ac susceptibility. These all result also depend on base pressure and ambient atmosphere set in the chamber during the experiment.