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Title: Scattering studies of magnetism in non-centrosymmetric systems
Author: Pásztorová, Jana
ISNI:       0000 0004 8497 7512
Awarding Body: University of Edinburgh
Current Institution: University of Edinburgh
Date of Award: 2019
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With there being a strong connection between the underlying structure of materials and their properties, understanding this inherent coupling will lead to more effcient materials. This thesis studies three non-centrosymmetric compounds MnSb2O6, Cu3Nb2O8 and CeRhSi3 with the goal of understanding the coupling between the structure and the underlying magnetic and electronic properties. All three materials lack an inversion centre and this thesis investigates how this constraint influences the underlying interactions and therefore properties of these compounds. MnSb2O6 is predicted to be multiferroic - a group of materials that has drawn considerable attention these days. One of the most interesting aspects of multiferroics is the so-called magnetoelectric coupling. Whereas in ferroics, one can tune magnetization with magnetic field, polarization with electric field, elasticity with stress, the idea behind multiferroics is that one can tune the electric polarization by the magnetic field (or vice versa). In terms of applications, the idea of electric-field control of magnetism is particularly exciting and it could lead to smaller and more effective devices. In the first part of this thesis, I discuss single crystal preparation based on three different methods in an attempt to obtain high quality single crystals for neutron inelastic scattering studies. Improvement in the size of single crystals was achieved using the flux method. This turned out to be an important step for inelastic experiment with the total mass of 1.3 g single crystals (approximately 150 sealed ampoules). Moreover, polycrystalline sample of MnSb2O6 has been prepared successfully with the hydrothermal method. To the best of our knowledge, this process of preparation has not been published before. The second part of the MnSb2O6 study is focused on neutron experiments and following analysis. The experiment on triple axis spectrometer RITA-II mapped behaviour of spins in horizontal magnetic field. Critical exponent β was estimated with discussion on properties of this system. Obtained results were compared with complementary experiment on BT4 spectrometer with same results. The magnetic and structural chirality was tested with polarized neutrons on instrument D3. Whereas, magnetic chirality in compound has a dominant domain, we were not able to confirm the same for the structural chirality. The last part of this section is discussing another interesting aspect of MnSb2O6 - it's noncollinear magnetic structure within triangular lattice is making it an ideal candidate to study so called multimagnon processes. The classical picture of magnon as non-interacting and long-lived excitation has been questioned by recent studies of classical systems with measured multimagnon excitations. Whereas existing spin wave theory can explain these excitations for quantum systems (S = 1/2), it doesn't provide an answer for their existence in classical systems. Cu3Nb2O8 compound belongs to type-II multiferroics, which means that electric polarization emerges at a magnetic order temperature. I prepared a polycrystalline sample that was measured on the time-of-flight instrument MARI. Experiments revealed excitations around 30 meV that it is not expected for system with spin S = 1/2. We close this study with the discussion about origin of these excitations - seems like they do result from localised clusters of spins (coupled triplet) rather than individual Cu2+ ions. Whereas for elements and some alloys, magnetism and superconductivity are in an antagonistic relation, several non-centrosymmetric heavy fermion compounds have been reported, where superconductivity and magnetism do coexist together. This is also the case of CeRhSi3 (superconducting at high pressures). For these systems, it is not unusual to see transitions between localized and itinerant (delocalized) magnetism. We describe neutron inelastic experiment focused on study of magnetic fluctuations in the normal state at ambient conditions. This has been followed by the experiment in magnetic field, in respect to define their antiferromagnetic or ferromagnetic character. The study contains also a heuristic description and therefore possible models that can be used as an interpretation of our measured data. This thesis also provides an example of two different types of magnetism - Mn atoms in MnSb2O6 are d elements with characteristic localized magnetism and LS coupling, whereas Ce atoms in CeRhSi3 are f elements which typically displays itinerant magnetism and j-j coupling. I discuss the framework on how to describe these two types of magnetism and also discuss various implications to 4d and 5d transition metal ions.
Supervisor: Stock, Christopher ; Attfield, John Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available
Keywords: neutrons ; magnetism ; crystal field theory ; crystal growth