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Title: Spin correlations in frustrated magnets with orbital ordering
Author: Magee, Andrea
Awarding Body: Royal Holloway, University of London
Current Institution: Royal Holloway, University of London
Date of Award: 2012
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The magnetism of two model systems that undergo orbital ordering transitions, MnV204 and GdV03, has been studied using neutron scattering and synchrotron x-rays. Mn V 204 comprises a network of corner-sharing tetrahedra with the vanadium ions forming a pyrochlore lattice. This structure is typical of systems that are geometrically frustrated in three dimensions. In the case of MnV204, the V3+ 3d electrons occupy two out of three t2g orbitals and, therefore, the orbital degrees of freedom come into play. The presence of the magnetic Mn2+ ions leads to ferrimagnetic structures, allowing the possibility to control the orbital ordering. The low temperature Mn V 204 magnetic structure and phase diagram were de- termined using single-crystal neutron diffraction. The magnetic excitations in the low temperature phases were studied using inelastic neutron scattering. The rotation method was employed on MAPS to obtain four-dimensional volumes of reciprocal space. Unidirectional polarization analysis was employed on IN20 to 3 Abstract focus on details of the magnetic dispersion with great sensitivity. The previously published magnetic exchange model was unable to account for our data and a new model is proposed. This magnetic exchange model distinguishes between the two candidate orbital ordering models. The perovskite orthovanadates, RV03 (R = rare earth or V), display a vari- ety of commensurate magnetic structures. Until now the magnetic structure of GdV03 had not been determined by neutron diffraction due to strong absorp- tion at thermal wavelengths. Magnetisation measurements for GdV03 reveal an unusual magnetic memory effect and a series of magnetic-field-induced phase tran- sitions at low temperature. We have studied the complex magnetic ordering in GdV03 using hot neutrons on D9 at ILL and resonant x-ray scattering on ID20 at ESRF. We have deter- mined new magnetic structures in the rich magnetic phase diagram of GdV03 at low temperature, which we find to be comprised of incommensurate orderings of gadolinium moments. 4
Supervisor: Goff, Jon Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available