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Title: Theoretical and experimental investigations of frustrated pyrochlore magnets
Author: Champion, John Dickon Mathison
ISNI:       0000 0001 3526 4549
Awarding Body: University of London
Current Institution: University College London (University of London)
Date of Award: 2001
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This thesis describes the investigation of frustrated magnetic systems based on the pyrochlore lattice of corner-sharing tetrahedra. Monte Carlo simulations and analytical calculations have been performed on a pyrochlore ferromagnet with local (111) easy-axis anisotropy related to the problem of 'spin ice'. The anisotropy-temperature-magnetic field phase diagram was determined. It contained a tricritical point as well as features related to some real ferroelectrics. A pyrochlore antiferromagnet with local (111) easy-plane anisotropy was studied by Monte Carlo simulation. A general expression for its degenerate ground states was discovered and normal-modes out of the ground states were calculated. Both systems are frustrated yet have a long-range ordered state at low temperature. The degeneracy lifting observed is discussed as well as the reasons for its presence. The rare-earth titanate series Ln2Ti2O7 (Ln = rare earth), crystallizes in the Fd3m space group, with the magnetic ions situated on the 16c sites which constitute the pyrochlore lattice. Crystal-field effects are known to play a significant role in the frustration observed in these compounds. Powder neutron diffraction was performed on gadolinium and erbium titanate. Both systems are frustrated antifer- romagnets yet show long-range magnetic order at ~ 1 K and ~ 1.2 K respectively. The magnetic structures of both these compounds have been determined by powder neutron diffraction techniques and related to other theoretical results as well as the theoretical results of the author. Further neutron scattering experiments on the 'spin ice' materials Ho2Ti2O7 and Dy2Ti2O7 are also described.
Supervisor: Not available Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available
Keywords: Solid-state physics