PhD Defense by Ursula B. Hansen – Niels Bohr Institute - University of Copenhagen

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PhD Defense by Ursula B. Hansen

The results of this thesis can be divided into two parts, one concerning neutron scattering studies of low dimensional magnetic systems and one concerning neutron optics for the European Spallation Source (ESS).

In the part concerning low dimensional magnetic systems, three aspects of the dy- namics of CoCl2 · 2 D2O have been investigated with neutron scattering experiments.
CoCl2 · 2 D2O can be considered a quasi one dimensional Ising system. This means, that it is a near ideal model material for investigating low dimensional magnetic phenomena.

The excitation spectrum of CoCl2 · 2 D2O has been investigated at low temperatures and in a longitudinal magnetic field using neutron spectroscopy. Here we observe the hybridisation of the magnon bound states, inherent to the low dimensional nature of
CoCl2 ·2D2O.

At higher temperature, signatures which can be attributed to Magnetic Bloch Oscilla- tions is observed in the excitation spectrum. In this picture the dynamics of the domain walls can be considered a magnetic analog to Electronic Bloch oscillations, where electrons in a periodic lattice will undergo oscillations when subjected to a constant electric field. The existence of Magnetic Bloch oscillations have been predicted for a long time, but no evidence of their existence had been found so far.

Futhermore, I have studied the phase ordering kinetics between the antiferromagnetic phase and the ferrimagnetic phase in CoCl2 · 2 D2O. It is possible to follow the domain coarsening of the antiferromagnetic order close to the phase boundary. The transition from short range to long range order is both temperature and field dependent and the corresponding timescales will vary from seconds to hours.

For the second part of the thesis, I have focused on the development of specialised neutron optics for the European Spallation Source. I have been using the ray tracing soft- ware McStas to investigate both the effect of waviness in neutron guides and a specialised focusing guide system.

The simulation of waviness was motivated by the fact that the current waviness im- plementation in McStas yielded unphysical results. Here, I present a new algorithm for modelling waviness in McStas and discuss its effects on a simple neutron guide system.

ESS is expected to have specialised neutron optics tailor made for the specific require- ments of each instrument. One common feature for several of the instruments is that smaller samples and more complex sample environment such as pressure cells or high field magnets, becomes much more important. These conditions calls for optics guarantying a very low background at the sample position. A possible solution for this problem is based on the focusing guide concept SELENE, which have been studied in detail here.