Sud, Aakanksha;
(2021)
Magnetization dynamics study on low-symmetry magnetic multi-layers.
Doctoral thesis (Ph.D), UCL (University College London).
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Abstract
In this thesis we study magnetization dynamics in different systems comprising of single-layer, multi-layer ferromagnetic and synthetic-antiferromagnetic (SyAF) thin films using techniques of ferromagnetic resonance (FMR) spectroscopy and spin transfer torque (STT) based FMR. In the first part, magnetic relaxation processes following the dynamical excitation of the spin systems are investigated using half-Heusler NiMnSb, a ferromagnet with the zinc-blende (inversion broken) crystal structure, as a prototype system. The experiments, performed at several different microwave frequencies and static magnetic field directions, enabled to observe a strong in-plane anisotropy of the linewidth. We attribute the linewidth anisotropy contribution to intrinsic Gilbert damping, extrinsic inhomogeneous broadening and two-magnon scattering process, supporting this by calculations of possible linewidth broadening mechanisms. In the second part magnon-magnon coupling is studied in SyAFs and synthetic ferrimagnets (SyFs). We demonstrate the control of coupling in SyAFs via intrinsic and extrinsic symmetry breaking (SB) in symmetrical and asymmetrical SyAF structures. In symmetrical SyAF there is an apparent mode crossing resulting from degeneracy of two non-interacting modes which is protected by a two-fold rotational symmetry. By tilting the external field we break this symmetry and an anticrossing gap appears demonstrating the extrinsic SB. The coupling strength can be controlled by interlayer exchange field strength modification through growth and is demonstrated experimentally. We also study asymmetrical SyAF structures referred to as SyFs. An inherent SB exists in the SyF system which opens a gap even for an in-plane field condition. We generalize a theoretical model in the macro-spin limit to describe the microwave absorption spectroscopy for the SB induced coupling in SyAFs and SyFs. In the last part we use technique of STT-FMR to study dynamics in SyAF and multi-stack structures of Ni/Pt. Due to opposite polarization in two FM layers we observe change in parity of two spin wave modes in SyAF. Spin orbit torques are quantified in multi-stack structures of Ni/Pt and we show linewidth modulation with current in these multi-stack structures.
| Type: | Thesis (Doctoral) |
|---|---|
| Qualification: | Ph.D |
| Title: | Magnetization dynamics study on low-symmetry magnetic multi-layers |
| Event: | University College London |
| Open access status: | An open access version is available from UCL Discovery |
| Language: | English |
| Additional information: | Copyright © The Author 2021. Original content in this thesis is licensed under the terms of the Creative Commons Attribution-NonCommercial 4.0 International (CC BY-NC 4.0) Licence (https://creativecommons.org/licenses/by-nc/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 Engineering Science UCL > Provost and Vice Provost Offices > UCL BEAMS > Faculty of Engineering Science > Dept of Med Phys and Biomedical Eng |
| URI: | https://discovery.ucl.ac.uk/id/eprint/10138608 |
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