Synthesis and Characterization of Constrained Magnetism in Niobates

Abstract

This thesis contains the results of the extensive study into the synthesis of nickel niobate (NiNb2O6) including the formation of what was a previously unreported polymorph of the material, as well as the magnetic properties of both cobalt niobate (CoNb2O6) and nickel niobate using techniques including SQUID magnetometry, powder and single crystal x-ray scattering, powder and single crystal neutron scattering and muon spin rotation/relaxation. In cobalt niobate we found extremely long relaxation times in the heat capacity which showed up strongly in muon spin rotation experiments but not in neutron measurements. Additionally, with field applied to the system we see the emergence of spin-wave like structures in the neutron scattering data. Within cobalt niobate the strongest interaction is ferromagnetic and along the chain. The chains themselves are laid out on a triangular fashion and interact, although far more weakly, in an antiferromagnetic manner. This triangular patterning as well as an antiferromagnetic interaction results in interchain frustration, which protects the quasi-1D nature of the system due to the difficulty generated in creating 3D order. In nickel niobate we found that growth conditions caused highly variable changes, and we were able to create two different polymorphs. One polymorph was in the same space group as cobalt niobate, which gave us an opportunity to explore the magnetic difference between a spin-½ and spin-1 magnetic system and in nickel niobate in the new space group we performed an ab initio characterization solving the unit cell structure, the magnetic structure with neutron scattering as well as a magnetic characterization with SQUID magnetometry and muon spin rotation, allowing us to contrast the significant crystallographic differences. For the new polymorph we were able to determine its magnetic structure, characterized by Ising-like spins arranged in frustrated tetrahedra with three of the four points lying in the same plane as the spin, and for both materials we were able to use zero-field μSR data to estimate behaviour near the critical point and determine a critical exponent near the magnetic transitions. In both polymorphs there is evidence of constrained magnetism or reduced dimensionality, although the evidence for low dimensionality is much stronger in the columbite polymorph.