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Title: Spin Ice : a wonderful world
Author: Sala, Gabriele
ISNI:       0000 0004 8497 8128
Awarding Body: Royal Holloway, University of London
Current Institution: Royal Holloway, University of London
Date of Award: 2014
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Among the huge variety of compounds that Nature offers us, two crystals exhibit extraordinary features at low temperatures: Dy2Ti2O7 and Ho2Ti2O7. Their properties, modelled on the basis of water ice, have earned them the name of Spin Ice crystals. The competition between the crystal electric field and the ferromagnetic interactions in their pyrochlore lattice, results in a geometrically frustrated system whose topological defects share properties similar to Dirac's magnetic monopoles. Modelling the dynamics of these defects, as well as studying their physical properties, is a challenging problem. The analysis of the internal field distribution showed that there is considerable local structure between high and low fields, mainly due to the symmetry of the pyrochlore lattice. However, the presence of monopoles alters this equilibrium and breaks the symmetry of the system. We studied two kinds of structural defects: oxygen vacancies and non magnetic Yttrium impurities. In the former case the diffuse neutron scattering analysis at low temperatures showed that the rare earth ions move away from the oxygen vacancy, due to Coulomb repulsion. The presence of neighbouring oxygen vacancies affects the crystal electric field of the rare earth; the single ion anisotropy changes from easy axis to easy plane, the magnitude of the magnetic moments is suppressed and, therefore, the magnetisation of the compound is reduced. Moreover AC susceptibility measurements showed that vacancies can slow down the long-time monopole dynamics at sub-Kelvin temperatures, trapping the monopole. Doping the crystal with non magnetic Yttrium impurities leaves the lattice unchanged, since the atomic radii of the Dy and Y are similar. The magnetisation of the system decreases linearly with the doping level, but the ice rules are still preserved at low temperatures. Non magnetic impurities are likely to affect the correlations between spins, as a result the internal field distribution at low and high temperatures for high doping levels look very similar.
Supervisor: Not available Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available