Asian Research Thesis Index

Abstract

Coprecipitation method was used to prepare pure and doped CoFe 2 O 4 nanoparticles. Ferric and cobalt salts were used as precursors while oleic acid was used as surfactant. X- ray Diffraction and Transmission Electron Microscopy analysis confirmed single phase of nanoparticles with particle size ~20 nm. Electron transport in CoFe 2 O 4 nanoparticles was investigated using impedance spectroscopy from 300 K to 400 K over wide frequency range (20 Hz - 2 MHz). Impedance spectroscopy of CoFe 2 O 4 nanoparticles revealed a semiconductor to metal transition at ~330 K. The semiconductor to metal transition was attributed to existence of mixed valance states of Fe cations, reverse cation distribution among octahedral and tetrahedral sites and various types of interactions between these cations. Variation of exponent “s” with temperature suggested that overlapping large polaron tunneling was the dominant conduction mechanism in cobalt ferrite nanoparticles. The Mössbauer spectroscopy demonstrated the mixed inverse spinel structure of the CoFe 2 O 4 nanoparticles. X-ray Photoelectron Spectroscopy analysis was carried out to study the oxidation states and environment of Fe and Co cations. Electrical properties of Sn 2+ and La 3+ doped CoFe 2 O 4 nanoparticles were studied in detail. The change in dielectric constant and ac conductivity of CoFe 2 O 4 nanoparticles were observed with dopant concentration. The temperature induced delocalization of charge carriers and metallic phase in Co 0.6 Sn 0.4 Fe 2 O 4 nanoparticles was explained using M(H) loops and impedance spectroscopy. Metallic nature of Co 0.6 Sn 0.4 Fe 2 O 4 nanoparticles above 360 K was attributed to dominancy of delocalized charge carriers Fe 3+ –Fe 2+ /Co 3+ – Co 2+ interactions over localized charge carriers Fe 3+ –O 2− –Fe 3+ /Co 2+ –O 2− –Co 2+ interactions. This was suggested that the wasp - waist magnetic hysteresis loop was due to simultaneous existence of ferromagnetic and antiferromagnetic domains in the system. The M(H) loops of Co 0.6 Sn 0.4 Fe 2 O 4 nanoparticles indicated that at lower temperatures the superexchange interaction was dominant as compared to double exchange interaction while at higher temperatures double exchange interaction becomes more strong. The open M(H) loops of Co 0.6 Sn 0.4 Fe 2 O 4 nanoparticles indicated the absence of magnetic saturation. The temperature dependent electrical behavior of the grain boundaries was reported and discussed in terms of depletion space-charge layer in the vicinity of grain boundaries.

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