Calder, S.; (2010) Investigation of geometric frustration in magnetic oxides. Doctoral thesis , UCL (University College London).

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Abstract

This thesis describes the experimental investigations of geometric frustration in magnetic oxides. The rare earth double perovskites Ba2HoSbO6 and Ba2ErSbO6 crystallise into the Fm3m cubic space group with the rare earth ions forming a face centred cubic arrangement of edge sharing tetrahedra. This arrangement is expected to result in geometric magnetic frustration. Previous studies have revealed no long range order or spin glass behaviour down to 1.5 K. In this work, low temperature neutron scattering measurements were carried out to investigate the magnetic behaviour below 1.5 K. The crystalline electric field was found to dominate the magnetic behaviour. Using experimental results from inelastic neutron scattering the crystal field level scheme was solved for Ba2HoSbO6 and Ba2ErSbO6. These results were used to successfully predict the observed behaviour of both systems, showing that they can be considered to behave as single ion systems down to the lowest temperature investigated of 0.06 K. As such exchange interactions and any effects of frustration are not evident at the temperatures investigated. As a further step to investigate frustration in magnetic oxides LuCuGaO4 was considered. This has triangular bilayers of magnetic Cu2+ and non-magnetic Ga3+ that are expected to lead to two dimensional geometric magnetic frustration of the Cu2+ ions. The presence of Ga3+ on the same lattice site as the Cu2+ lead to charge frustration. Polarised neutron analysis, inelastic neutron scattering and \muSR build up a coherent picture of the low temperature behaviour of the system which questions the previous belief in the literature of a spin glass transition. Instead what is found is a spin liquid state. Finally, the problem of interpreting the subtle features and signatures of frustration is considered with an alternative \muSR technique. \muSR allows local interactions to be investigated, however the problem of interpreting the results can lead to ambiguity. It is shown that it is possible to successfully implant muons outside the sample of interest and accurately measure the sample’s magnetic dipolar field. In this way \muSR can be used as a bulk magnetometer with the same frequency response as standard \muSR and it is shown that this can be useful in the investigation of frustrated materials with reference to results on Tb2Sn2O7.

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