Use this URL to cite or link to this record in EThOS: http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.639936
Title: Neutron scattering from low-dimensional quantum magnets
Author: Wheeler, Elisa Maria da Silva
Awarding Body: University of Oxford
Current Institution: University of Oxford
Date of Award: 2007
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Abstract:
Neutron scattering measurements were used to investigate the magnetic and crystal structure and magnetic excitations of three compounds characterized as low-dimensional quantum magnets. The materials are frustrated systems with low spin quantum number. The first was a powder sample of AgNiO2. The Ni ions form a triangular lattice antiferromagnet in which, according to the published crystal structure, both the orbital order and magnetic couplings are frustrated. However, it is shown here that there was a small distortion of the crystal structure at 365 K, which is proposed to result from charge disproportionation and this relieves the orbital frustration. The magnetic structure was investigated and, below 20 K, the triangular lattice of electron-rich Ni sites was observed to order into antiferromagnetic stripes. Investigations of the magnetic excitations showed that the main dispersions were within the triangular plane, indicating a strong two-dimensionality. The dispersion was larger along the stripes than between the stripes of collinear spins. The second material investigated was CoNb2O6, a quasi Ising-like ferromagnet. It was studied with a magnetic field applied transverse to the Ising direction. The magnetic field introduced quantum fluctuations which drove a phase transition at a field comparable to the main exchange interaction. The phase diagram of the magnetic order was mapped outs and a transition from an ordered phase to a paramagnetic phase was identified at high field. This low-temperature high-field phase transition was further investigated by inelastic neutron scattering measurements to observe the change in the energy gap and magnetic excitation spectrum on either side of the transition. The spectrum had two components in the ordered phase and had sharp magnon modes in the paramagnetic phase. The third material was the spin-half layered antiferromagnet CuSb2O6. It has a square lattice of Cu2+ ions in which the main interaction is across only one diagonal of the square. The magnetic structure was studied by neutron scattering with a field applied along the direction of the zero-field ordered moment. A spin-flop was observed at low field and there was evidence for a high-field transition. The magnetic excitation spectrum was unusual in that it had an intense resonance at 13 meV at the magnetic Brillouin zone boundary.
Supervisor: Boothroyd, A. T.; Coldea, R.; Enderle, M. Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID: uk.bl.ethos.639936  DOI: Not available
Keywords: Condensed Matter Physics ; magnetism ; neutron scattering ; transition metal oxides ; strongly correlated electron systems
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