Investigation of geometric frustration in magnetic oxides

This thesis describes the experimental investigations of geometric frustration in \nmagnetic oxides. The rare earth double perovskites Ba2HoSbO6 and Ba2ErSbO6 crystallise \ninto the Fm3m cubic space group with the rare earth ions forming a face centred \ncubic arrangement of edge sharing tetrahedra. This arrangement is expected to result \nin geometric magnetic frustration. Previous studies have revealed no long range order \nor spin glass behaviour down to 1.5 K. In this work, low temperature neutron scattering \nmeasurements were carried out to investigate the magnetic behaviour below 1.5 K. \nThe crystalline electric field was found to dominate the magnetic behaviour. Using experimental \nresults from inelastic neutron scattering the crystal field level scheme was \nsolved for Ba2HoSbO6 and Ba2ErSbO6. These results were used to successfully predict \nthe observed behaviour of both systems, showing that they can be considered to behave \nas single ion systems down to the lowest temperature investigated of 0.06 K. As such \nexchange interactions and any effects of frustration are not evident at the temperatures \ninvestigated. \nAs a further step to investigate frustration in magnetic oxides LuCuGaO4 was considered. \nThis has triangular bilayers of magnetic Cu2+ and non-magnetic Ga3+ that are \nexpected to lead to two dimensional geometric magnetic frustration of the Cu2+ ions. \nThe presence of Ga3+ on the same lattice site as the Cu2+ lead to charge frustration. \nPolarised neutron analysis, inelastic neutron scattering and \\muSR build up a coherent \npicture of the low temperature behaviour of the system which questions the previous \nbelief in the literature of a spin glass transition. Instead what is found is a spin liquid \nstate. \nFinally, the problem of interpreting the subtle features and signatures of frustration \nis considered with an alternative \\muSR technique. \\muSR allows local interactions to be \ninvestigated, however the problem of interpreting the results can lead to ambiguity. It \nis shown that it is possible to successfully implant muons outside the sample of interest \nand accurately measure the sample’s magnetic dipolar field. In this way \\muSR can be \nused as a bulk magnetometer with the same frequency response as standard \\muSR and it \nis shown that this can be useful in the investigation of frustrated materials with reference \nto results on Tb2Sn2O7.