Investigations on magnetic and semiconducting nanomaterials prepared by sol-gel and electrospinning routes
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1-d undoped, ferromagnetic Co doped and non-magnetic Mg doped ZnO nanowires consisting of interconnected nanometer sized beads have been prepared by electrospinning method. Annealed as-spun undoped 1-d ZnO exhibited the highest ever achieved TC of 885 K. Maximum magnetization of 0.039 emu/g was observed for 500 C annealed sample at an applied field of 10 kOe. The observed room temperature ferromagnetism (RTFM) is due to singly ionized oxygen vacancies in undoped 1-d ZnO. Although no magnetization was observed in Mg doped 1-d ZnO, considerable enhancement in band gap energy was observed upon Mg2+ doping. Mg doping quenched the oxygen defects in 1-d ZnO. However, Co doped 1-d ZnO exhibited RTFM. Defects analysis revealed that exchange interaction between local spin-polarized electrons and conduction electrons of Co is responsible for RTFM in these nanostructures. Moreover, Zn vacancies present in the samples decreased upon increasing Co doping. 0-d and 1-d CaFe2O4 nanostructures have been prepared by sol-gel and electrospinning routes, respectively. Cubic CaFe2O4 nanoparticles slowly transformed to orthorhombic phase when annealed above 300 C and the transformation completes at 1100 C. Superparamagnetic behavior is associated with the cubic phase and weak ferrimagnetic behaviour was found in the orthorhombic phase. 1100 C annealed orthorhombic 0-d CaFe2O4 exhibited Néel temperature at ~175 K. Co substitution in Ca site in 0-d CaFe2O4 improved its magnetic properties. Phase transformation from cubic to orthorhombic phase was also observed in 0-d Ca0.9Co0.1Fe2O4. 1-d cubic CaFe2O4 also slowly transformed to orthorhombic phase when annealed above 500 C. Apart from clarifying the mechanisms responsible for RTFM in ZnO nanostructures, this work also revealed several new applications for these novel ZnO and CaFe2O4 nanostructures.
Supervisor: A. Srinivasan