Nanoscale magnetic information by HR-EELS and role of native defects on the origin of ferromagnetism in CoO and Co/Mn doped ZnO/

Abstract

There are two different aspects in tliis thesis. The first one is to develop a nano and atomic scale magnetometry technique based on electron energy loss spectroscopy (EELS) in a transmission electron microscope (TEM). The second aspect deals with the role o f native defects on the ferromagnetic response in Co doped ZnO and CoO epitaxial thin fdms. In the first part (Chapters 2-3), HR-EELS technique in combination with first principle calculations were employed to obtain spatially resolved (few square nanometers) quantitative magnetic order information in CuCr2S4 nanocrystals. The technique was able to quantitatively detennine the magnetic order from the core and surface regions o f the nanocrystals, shed light on the role of capping layers and provided an explanation on the much smaller magnetization observed in this system by bulk magnetometry technique. This was possible with the development of a soft EELS based technique which could give spatial resolution down to one atomic plane. The technique allowed to record high quality spectra without beam damage. The technique was further extended to probe the surface spin canting in Fc3 0 4 and CoFe: 0 4 nanocrystals and offered an alternative to polarized neutron based technique with advantages in temis of superior spatial resolution, atomic site selectivity and without any special sample preparation method. The primary driving force for the development o f the above techniques are in tune with the miniaturization o f electronic and novel functional devices at nano and atomic scales as well as investigation o f fundamental science. In the second part (Chapters 4-7) o f the thesis, the role of native point defects was revealed on the observation of giant coercivity in Co doped ZnO and room temperature ferromagnetism with various coexisting electronic phases in CoO epitaxial thin film. Associated results based on Mn doped ZnO and growth of Co metal epitaxial film from Co:ZnO target were also described. A future perspective was also provided based on the results presented in the thesis.