Synthesis and Characterizations of Magnetic Nanomaterials

Abstract

The work presented in this PhD dissertation deals with the synthesis and characterizations of magnetic nanostructures using chemical methods. These nanostructures include nanoparticles, core/shell nanoparticles and core/shell nanowires which were synthesized via chemical coprecipitation method, solution evaporation method and sol-gel route followed by electrodeposition respectively. In this research, structural and magnetic properties of various ferrites (Spinel, Perovskite and Hexa-ferrites) with different dopant and chemical composition have been explored. Core/shell structure for nanoparticles and nanowires has also been studied in detail. Subsequently, the crystallinity and the phase purity of nanostructures were studied by X-Ray Diffraction (XRD) and Fourier Transform Infrared Spectroscopy (FTIR). Surface morphology, grain size, and chemical composition was analyzed by Scanning Electron Microscope (SEM), Transmission Electron Microscope (TEM), High Resolution Transmission Electron Microscope (HRTEM) and Energy Dispersive X-ray Spectroscopy (EDX) techniques respectively. Vibrating sample magnetometer (VSM) was used for room temperature magnetic measurements, however, low temperature characterizations were performed with the help of physical property measurement system (PPMS). We have reported magnetic properties of La1-xCoxMnO3 nanostructures synthesized by hydrothermal route. The crystal structure has been characterized by X-ray diffraction (XRD) technique, which shows rhombohedral perovskite structure at room temperature. Scanning electron microscope (SEM) and energy dispersive X-ray spectroscopy (EDS) have been used to analyse morphology and chemical composition of prepared nanoparticles. Magnetic hysteresis loops of all the samples exhibit ferromagnetic behaviour at 10 K. Inverse susceptibility graphs as a function of temperature represent deviation from Curie Weiss law. The indication for short range ferromagnetic clusters well above Curie temperature is observed due to the Griffiths Phase (GP). It is proposed that the presence of GP arises from induced size effects of La and Co ions. BiFe1-xCoxO3 (0≤x≤0.1) nanoparticles prepared by using solution evaporation method endorsed the formation of rhombohedral Perovskite crystal structure with R3c space group. Structural parameters show decreasing behavior of lattice constants and increasing behavior of X-ray density by increase in doping (Co) content in BiFeO3. XRD vi and TEM claimed average particle size of 39 nm. Room temperature magnetic results shows increase in Hc and Ms of nanoparticles up to doping content of 7.5%. On the other hand, low temperature magnetic measurements showed increasing trend of magnetic parameters with decreasing temperature. Variation in coercivity with temperature was followed theoretically by using Kneller’s law while, saturation magnetization followed the modified Bloch’s law in temperature range of 5-300K. Thirdly, a series of hexaferrite nanoparticles, with general formula Ba2Co2-xMnxFe12O22 (0≤x≤1) was synthesized by chemical co-precipitation method. It was found that with increasing Mn concentration grain size of hexaferrite nanoparticles increased from few nanometers to micrometer range. Furthermore, magnetic analyzes revealed that with increasing Mn concentration at octahedral and tetrahedral sites, the coercivity and squareness were found to increase from 455Oe to 2550Oe, and 0.23 to 0.47, respectively. Theoretical approach was also used to calculate saturation magnetization of synthesized samples. Along with ferrite nanoparticles, core/shell nanoparticles of ferrite material were prepared. Bimagnetic monodisperse CoFe2O4/Fe3O4 core-shell nanoparticles were prepared by solution evaporation route. Preferential coating of iron oxide onto the surface of ferrite nanoparticles was confirmed from XRD and HRTEM analyzes. The average core size was about 18 nm with thickness of the shell 3 nm, which corroborates with TEM study. We have observed large coercivity 15.8kOe at T=5K, whereas maximum saturation magnetization (125 emu/g) is attained at T=100K for CoFe2O4/Fe3O4 core-shell nanoparticles. Saturation magnetization decreases due to structural distortions at the surface of shell below 100K. ZFC-FC plots show that synthesized nanoparticles are ferromagnetic up to room temperature and it has been noticed that core/shell sample possess high blocking temperature than Cobalt Ferrite. Presence of iron oxide shell significantly increases magnetic parameters as compared to the simple cobalt ferrite. A comprehensive study of magnetization properties for M-CoO (M=Co, Ni, Fe) core/shell nanowires has been presented. Ferromagnetic nanowires arrays layered with Cobalt oxide shell are fabricated by sol gel route followed by DC electrodeposition into AAO templates. Structure and morphology of fabricated nanowires reveals the formation of nanowires without existence of any impurity phase. Magnetic analysis briefly describes magnetization reversal mechanisms of nanowires by measurement of angular dependent coercivity for core and core-shell nanowires. It is found that two main vii mechanisms; curling and coherent rotation reversal modes describe the magnetization reversal mechanism at room temperature. Furthermore, angular dependence of squareness shows that easy axis of Co core and core shell nanowires lies along perpendicular to the wire axis whereas, for Ni and Fe nanowires easy axis is along parallel to the wire axis.