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Study of spin dynamics in chiral and two-dimensional magnets

Khan, Safe-Ur-Rahman; (2020) Study of spin dynamics in chiral and two-dimensional magnets. Doctoral thesis (Ph.D), UCL (University College London). Green open access

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This thesis explores the spin dynamics in three magnetic systems comprising of a chiral magnet and the bulk van der Waals (vdWs) layered magnets by mainly utilising the novel experimental technique of ferromagnetic resonance (FMR) spectroscopy. The non-centrosymmetric chiral ferrimagnet, Cu2OSeO3, has recently seen a great deal of interest from the spintronics community, as it has been shown to host various different magnetic spin textures and it was the first insulating system to show the presence of the skyrmion lattice. Over the last couple of years, the research field of layered vdWs materials has grown rapidly since the discovery of intrinsic magnetic order in the two-dimensional (2D) limit, i.e. down to the atomically thin monolayer limit, prohibited otherwise due to thermal fluctuations at low dimensionality. In the first part of this thesis, the interaction of microwave cavity photons with magnons (a quanta of spin excitations in magnetically ordered systems) in different magnetic phases of Cu2OSeO3 is probed. A strong-coupling regime is realised in the field-polarised phase, creating a hybridised state of cavity-magnon-polariton. In the conical and helical phases, dispersive coupling and Purcell-effect regime is seen, respectively. In the skyrmion phase, an onset of anti-crossing feature is observed between photons and the counter-clockwise gyration mode, whereas high-order excitation mode are believed to be seen for the breathing resonance mode. The spin dynamics of layered Cr2Ge2Te6 bulk single crystal is investigated in the second study. The magnetisation dynamics experiment confirmed the presence of out-of-plane magnetic anisotropy, where the scaling of the uniaxial anisotropy constant and saturation magnetisation showed a deviation from the Callen-Callen power law. The obtained spectroscopic g factor exhibits anisotropic behaviour and a value higher than 2, suggesting the presence of orbital moment. The temperature dependence of the orbital moment is obtained via the X-ray magnetic circular dichroism (XMCD) experiment, which qualitatively compliments the results observed in the magnetic resonance experiments. Finally, the domain-mode resonance features are observed confirming the presence of multidomain structures in the system. In the final study, sodium (Na) ions are intercalated in the vdWs gaps of Cr2Ge2Te6 creating the Na(0.3)-Cr2Ge2Te6 samples. The intercalated samples show an enhancement in the ferromagnetic Curie temperature, where the FMR signal remains up to the temperature of 240 K. A change in the magnetic anisotropy is found compared to the unintercalated sample, and magnetic easy axis seems to lie along the in-plane orientation, whereas it is in the out-of-plane direction for Cr2Ge2Te6.

Type: Thesis (Doctoral)
Qualification: Ph.D
Title: Study of spin dynamics in chiral and two-dimensional magnets
Event: UCL (University College London)
Open access status: An open access version is available from UCL Discovery
Language: English
Additional information: Copyright © The Author 2020. Original content in this thesis is licensed under the terms of the Creative Commons Attribution 4.0 International (CC BY 4.0) Licence (https://creativecommons.org/licenses/by/4.0/). Any third-party copyright material present remains the property of its respective owner(s) and is licensed under its existing terms. Access may initially be restricted at the author’s request.
UCL classification: UCL
UCL > Provost and Vice Provost Offices > UCL BEAMS
UCL > Provost and Vice Provost Offices > UCL BEAMS > Faculty of Maths and Physical Sciences
UCL > Provost and Vice Provost Offices > UCL BEAMS > Faculty of Maths and Physical Sciences > London Centre for Nanotechnology
URI: https://discovery.ucl.ac.uk/id/eprint/10098057
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