Neutron scattering studies of low dimensional magnetic systems

Publikation: Bog/antologi/afhandling/rapportPh.d.-afhandling

  • Ursula Bengård Hansen
The results of this thesis can be divided into two parts, one concerning neutron scatteringstudies of low dimensional magnetic systems and one concerning neutron optics for theEuropean Spallation Source (ESS).In the part concerning low dimensional magnetic systems, three aspects of the dynamicsof CoCl· 2D2O have been investigated with neutron scattering experiments.CoCl· 2D2O can be considered a quasi one dimensional Ising system. This means, thatit is a near ideal model material for investigating low dimensional magnetic phenomena.The excitation spectrum of CoCl· 2D2O has been investigated at low temperaturesand in a longitudinal magnetic eld using neutron spectroscopy. Here we observe thehybridisation of the magnon bound states, inherent to the low dimensional nature ofCoCl· 2D2O.At higher temperature, signatures which can be attributed to Magnetic Bloch Oscillationsis observed in the excitation spectrum. In this picture the dynamics of the domainwalls can be considered a magnetic analog to Electronic Bloch oscillations, where electronsin a periodic lattice will undergo oscillations when subjected to a constant electric eld.The existence of Magnetic Bloch oscillations have been predicted for a long time, but noevidence of their existence had been found so far.Futhermore, I have studied the phase ordering kinetics between the antiferromagneticphase and the ferrimagnetic phase in CoCl2 2D2O. It is possible to follow the domaincoarsening of the antiferromagnetic order close to the phase boundary. The transitionfrom short range to long range order is both temperature and eld dependent and thecorresponding timescales will vary from seconds to hours.For the second part of the thesis, I have focused on the development of specialisedneutron optics for the European Spallation Source. I have been using the ray tracing softwareMcStas to investigate both the eect of waviness in neutron guides and a specialisedfocusing guide system.The simulation of waviness was motivated by the fact that the current waviness implementationin McStas yielded unphysical results. Here, I present a new algorithm formodelling waviness in McStas and discuss its eects on a simple neutron guide system.ESS is expected to have specialised neutron optics tailor made for the specic requirementsof each instrument. One common feature for several of the instruments is thatsmaller samples and more complex sample environment such as pressure cells or high eldmagnets, becomes much more important. These conditions calls for optics guarantying avery low background at the sample position. A possible solution for this problem is basedon the focusing guide concept SELENE, which have been studied in detail here.
OriginalsprogEngelsk
ForlagThe Niels Bohr Institute, Faculty of Science, University of Copenhagen
Antal sider211
StatusUdgivet - 2017

ID: 184293678