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Neutron scattering investigations of frustated magnets

Fennell, Tom; (2003) Neutron scattering investigations of frustated magnets. Doctoral thesis (Ph.D), UCL (University College London). Green open access

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This thesis describes the experimental investigation of frustrated magnetic systems based on the pyrochlore lattice of corner-sharing tetrahedra. Ho₂Ti₂0₇ and Dy₂Ti₂0₇ are examples of spin ices, in which the manifold of disordered magnetic groundstates maps onto that of the proton positions in ice. Using single crystal neutron scattering to measure Bragg and diffuse scattering, the effect of applying magnetic fields along different directions in the crystal was investigated. Different schemes of degeneracy removal were observed for different directions. Long and short range order, and the coexistence of both could be observed by this technique.The field and temperature dependence of magnetic ordering was studied in Ho₂Ti₂0₇ and Dy₂Ti₂0₇. Ho₂Ti₂0₇ has been more extensively investigated. The field was applied on [00l], [hh0], [hhh] and [hh2h]. Dy₂Ti₂0₇ was studied with the field applied on [00l] and [hh0] but more detailed information about the evolution of the scattering pattern across a large area of reciprocal space was obtained.With the field applied on [00l] both materials showed complete degeneracy removal. A long range ordered structure was formed. Any magnetic diffuse scattering vanished and was entirely replaced by strong magnetic Bragg scattering. At T =0.05 K both materials show unusual magnetization curves, with a prominent step and hysteresis. This was attributed to the extremely slow dynamics of spin ice materials at this temperature.Both materials were studied in greatest detail with the field applied on [hh0]. The coexistence of long and short range order was observed when the field was raised at T = 0.05 K. The application of a field in this direction separated the spin system into two populations. One could be ordered by the field, and one remained disordered. However, via spin-spin interactions, the field restricted the degeneracy of the disordered spin population. The neutron scattering pattern of Dy₂Ti₂0₇ shows that the spin system was separated into two populations of spin chains, one set ordered and the other only partly so. Cycling the field induced dynamics in these chains, again via spin-spin interactions, as the field acted on the ordered si)in chains. These field regulated dynamics were particularly noted in Ho₂Ti₂0₇ where a full field cycle was executed. Raising the temperature in an applied field also activated the dynamics of the partially ordered spin chains. The continued evolution of the spin system toward a more ordered state, when dynamics can be induced, suggested that a spin ice does indeed have an energetic groundstate.The remaining two directions probed in Ho₂Ti₂0₇ both have two populations of spins with different Zeeman energies. The competition of the field and the spin- spin interactions was used to investigate the onset of the ice rules regime (field on [hh2h] and the breaking of the ice rules by a strong field (field on [hhh]). It was shown that the behavior of Ho₂Ti₂0₇ with field on [hhh] was consistent with the "kagome ice" hypothesis.

Type: Thesis (Doctoral)
Qualification: Ph.D
Title: Neutron scattering investigations of frustated magnets
Open access status: An open access version is available from UCL Discovery
Language: English
Additional information: Thesis digitised by ProQuest.
Keywords: Pure sciences; Neutron scattering
URI: https://discovery.ucl.ac.uk/id/eprint/10099752
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