Master thesis defense by Magnus Aagren Jensen – Niels Bohr Institute - University of Copenhagen

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Master thesis defense by Magnus Aagren Jensen

In this project the phase diagram of TbPO 4 has been investigated using neutron scattering at both E4 at Helmholtz-Zentrum Berlin and at RITA-II at the Paul Scherrer Institute. Especially the presumed spin flop phase has been investigated in much more detail than previously. The reflection profiles of the structural peaks (020) and (220) were seen to split into respectively three and two peaks in this phase, which is inconsistent with a spin flop phase. The exact nature of the phase has not been determined, but an example of how the reciprocal room might look is given. The peak splitting, although large, was unfortunately not very successful as an order parameter, because the resolution did not allow for the peaks to be resolved close enough to the phase boundary. The transition temperature could however be determined quite well using the width of a Gaussian fit.

The phase boundary between the presumed spin flop phase and the canted antiferromagnetic phase was investigated using the antiferromagnetic (110) peak. The relatively low flux at E4 restricted us to measuring at discrete magnetic fields. The transition fields could therefore only be estimated by inspection of the intensity vs field plots, all the transitions seemed to take place between 0.5T and 0.6T or right around 0.6T for the lowest temperatures.

A high voltage stick was built at Helmholtz-Zentrum Berlin in order to test if a magnetic phase transition could be induced by applying a large electric field close to the phase boundary. This was also tested with the high voltage stick at the Paul Scherrer Institute. The largest achieved field before breakdown was around 2.5kV/mm at both experiments. It was not possible to induce a magnetic phase transition during either experiment. It was also tested if the transition field could not be moved by applying an electric field, this was not achieved either. It is considered unlikely that a phase transition can be induced like this in TbPO 4 , even if higher electric fields could be achieved.