Date of Award
Doctor of Philosophy (PhD)
Professor J.E. Greedan
The magnetic properties of polycrystalline samples of pyrochlore antiferromagnets were investigated. The metal atoms in pyrochlores form a 3 dimensional network of comer sharing tetrahedra. Antiferromagnetic ordering is frustrated on such a lattice in the sense that all nearest neighbor microscopic interactions cannot be simultaneously satisfied. A number of experimental (Neutron diffraction, magnetic susceptibility and heat capacity) and theoretical techniques (mean field theory and Monte Carlo simulations) have been used to investigate the magnetic propenies of this interesting lattice.
Considerable experimental evidence was consistent with spin glass behavior which is unusual for chemically ordered materials. All of the compounds investigated showed dramatic irreversibilities in the magnetic susceptibility data. A few of the materials (FeF₃, Nd₂Mo₂O₇ and Mn₂Sb₂O₇) were seen to exhibit long range order, which was complex in nature. Tb₂Mo₂O₇ and Y₂Mn₂O₇ showed no signs of magnetic ordering down to 7K but neutron scattering data indicated that strong short range correlations were present over a wide temperature range. Inelastic scattering for Tb₂Mo₂O₇ was consistent with some degree of spin freezing at 25K.
Mean field theory calculations predicted that no long range order would exist in pyrochlore antiferromagnets with only nearest neighbor interactions. Further neighbor interactions would in most cases stabilize complex long range magnetic order. Monte Carlo simulations showed that thermal fluctuations were insufficient to stabilize long range order in the absence of further neighbor interactions. Extensive simulations for a model with further neighbor interactions and long range order gave strong evidence for the existence of a new universality class in pyrochlore antiferromagnets. This result was supported by neutron scattering results for FeF₃.
Reimers, Jan Naess, "Magnetic Properties of Frustrated Pyrochlore Antiferromagnets" (1990). Open Access Dissertations and Theses. Paper 2676.