Electronic and Magnetic Structure of Few Strongly Correlated Spinel Oxides

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A detailed Density Functional theory calculations and experimental studies have been reported on few novel spinel cobaltites. A substantial increase in the size of the unit cell along with the collapse of long-range antiferromagnetic (AFM) ordering with spin-orbit compensation effect among the key features noticed as the octahedral Co site is diluted with non-magnetic cations. The bandstructure calculations from DFT+U were compared with the experimental observations and it was observed that the increase of the size of the dilutants triggers a drastic decrease in the energy band gap. The normal spinel Co2RuO4 exhibits AFM transition at Néel temperature (TN) ~ 15.2 K, along with a spin-glass state below TN mainly governed by ordering of the spins of Co2+ ions occupying the A-site. We found that the spin-glass phase was mainly originated due to the exchange interaction between the Co2+ ions on the A-site and randomly distributed Ru3+ on the B-site. We also report a detailed Field-Temperature (H-T) phase diagram of Co2RuO4 system for different H for T < TN. Further, using the low temperature (1.6K-300K) neutron diffraction technique, a detailed structural, magnetic and spin dynamics of single/poly crystals of Co2TiO4 and Co2SnO4 were investigated. Both these compounds exhibit highest magnetic intensity from the (111)M reflection due to Ferrimagnetic ordering, which sets in below TC = 48.6 K and 41 K for Co2TiO4 and Co2SnO4, respectively. Finally, using the first-principles methods the electronic structure and magnetic properties of Ge diluted Co2TiO4 spinels (Co2Ti1-xGexO4) were extensively studied and we proposed a new approach of tailoring Pyrochlore geometry that led to remarkable change in their electronic and magnetic structure by tuning the cationic disorder.
Supervisors: Thota, Subhash and Mishra, Pankaj Kumar
Density of States, Spin-Glass, Magnetism, Neutron Diffraction