Local structure and magnetic studies of orbitally degenerate spinel

In the AB2O4 spinel systems the interplay between the orbital, electron and lattice degrees of freedoms in an intricate way determines the low temperature structural and magnetic properties. A small changes brought about by doping to the structure can sometimes have a great impact on the whole system due to strong spin-lattice coupling. For this reason spinels offer an excellent platform from which to study the relationship between the physical properties and the underlying interactions. In this thesis a series of vanadium spinel compounds with the chemical composition of Cd 1-xZnxV2O 4 are studied to investigate the effects of ion substitution on the local structure and magnetic dynamics. The pure(undoped) compounds undergo a structural transition from a cubic to a tetragonal symmetry. The structural transition partially relieves the geometrical frustration in the end members and enables the subsequent spin order at a lower temperature. On the other hand in the doped compounds both the structural and magnetic order are absent. It is generally assumed that the ionic substitution on the A-site has little of any effect on the crystal structure. This is inferred from the fact that the doped compounds exhibit the same cubic symmetry over a wide range of substitutions. However by utilizing the pair distribution function analysis technique it is found that the local and global symmetries are quite distinct with important implications on the magnetic properties. Due to the difference in the size of the Zn and Cd ions, the long range uniform arrangement of the oxygen octahedra is destroyed and no cooperative orbital order can take place. The local analysis shows that the local environment around the Zn and Cd ions of the doped compounds is similar to the ones in the pure compounds. Local orbital order is therefore possible. Another spinel system ZnMn2O4 with different orbital configuration is also studied as a comparison to the vanadium system. The structural and magnetic properties of this manganese spinel are studied and discussed in the hope to give a complete picture of the orbital physics in spinels.