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Title: Magnetic properties of two geometrically frustrated compounds : SrHo2O4 and SrGd2O4
Author: Young, Olga
Awarding Body: University of Warwick
Current Institution: University of Warwick
Date of Award: 2013
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Frustrated magnetism is the study of competing interactions, and when these are incompatible with the geometry of the lattice, systems can find it hard to establish a unique ground state. Recently, a new family of frustrated rare earth compounds, with the formula SrLn2O4, have started to be investigated. The crystal structure of the SrLn2O4 systems allows for two crystallographically inequivalent sites of the rare-earth ions in a unit cell. These magnetic ions form zigzag chains that run along the c axis. The triangular ladders can be frustrated, and are magnetically equivalent to one-dimensional chains with first- and second-nearest-neighbour interactions. The chains of Ln3+ ions interconnect by forming a distorted honeycomb structure, a bipartite lattice made up of edge-sharing hexagons, in the a-b plane. An investigation into two members of the SrLn2O4 family of geometrically frustrated magnets, SrHo2O4 and SrGd2O4, is presented in this thesis. Even though both compounds share the same structure, very different magnetic behaviour is expected. In the ground state Gd has L = 0, and it is found to be a good realisation of a classical Heisenberg antiferromagnet. Conversely, SrHo2O4, shows pronounced Ising anisotropy at low temperatures, and more unusually, the spins on the two magnetic sites point along orthogonal crystallographic axes. Both powders and single crystals have been used to study the magnetic behaviour of SrHo2O4 and SrGd2O4, and the single crystals of SrGd2O4 have been grown for the first time. Bulk property measurements of magnetisation, susceptibility and specific heat have been performed to map out the complex magnetic phase diagram of SrGd2O4, and to investigate the highly anisotropic behaviour of SrHo2O4. Powder neutron diffraction has been used to determine the unusual coexistence of two magnetic phases in SrHo2O4, and to solve the magnetic structure. Further single crystal neutron diffraction data shows that all of the scattering from SrHo2O4 is short-range in nature, and one of the magnetic Ho3+ sites orders in a one-dimensional structure. The changes to the diffraction patterns upon the application of a magnetic field were also investigated. These measurements point to an new up-up-down spin phase being stabilised for certain values of the applied field along the b axis. Inelastic scattering experiments on powder samples have also been used to try and understand the origin of the highly anisotropic nature of this compound by studying the crystalline electric field. However, these have lead to new mysteries and the need for further analysis and investigation. Overall, both SrHo2O4 and SrGd2O4 exhibit a variety of magnetic phenomena, and their magnetic properties contribute to the understanding of the physics of the SrLn2O4 compounds.
Supervisor: Not available Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available
Keywords: QC Physics