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Title: Dzyaloshinskii-Moriya interaction and domain wall motion in polycrystalline and epitaxial magnetic multilayers
Author: Shahbazi, Kowsar
ISNI:       0000 0004 7654 8857
Awarding Body: University of Leeds
Current Institution: University of Leeds
Date of Award: 2018
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The domain wall (DW) dynamics of epitaxial and polycrystalline multilayers with perpendicular magnetic anisotropy have been investigated. DC magnetron sputtering was used to deposit different multilayers. The saturation magnetization, anisotropy energies, exchanges stiffness constant, and DW width and energy density were calculated using hard-axis hysteresis loops and change of magnetization with temperature. The evaluated magnetic parameters were used in study of asymmetrical bubble expansion and Brillouin light spectroscopy (BLS) spectra to derive the strength of interfacial Dzyaloshinskii-Moriya interaction (DMI) in the films. Pt\Co\Au_xPt_(1−x) (x = 0, 0.5, 1) epitaxial trilayers with increasing broken inversion symmetry were deposited by sputtering. Zero DMI and no visible current-induced DW motion proven the expected fully symmetrical layers and interfaces in the case of x = 0. Field-induced DW motion (FIDWM) was used to calculate the depinning field, temperature, and velocity. All these material dependent parameters increased with x. They were used to estimate the value of Gilbert damping constant, which was compared to the same value evaluated from full micromagnetic calculations performed in collaboration with Simone Moretti. Larger values of Gilbert damping from FIDWM were attributed to other dissipation mechanisms that happen solely during DW motion. It was shown that DMI and SHE in the multilayers rise with concentration of Au on the top layer. The effect of inserting Ir in the top interface of polycrystalline Pt\Co\Ta films was also investigated through DMI and DW behaviour. It was shown that motion of the left-handed DWs in Pt\Co\Ir(t_Ir)\Ta multilayers (t_Ir = 0.0-2.0 nm) perfectly follows the universal creep and depinning regimes. The depinning related parameters and evaluated DMI did not change significantly for Ir of more than two monolayers. Evaluation of DMI showed that insertion of Ir layer reduces the net DMI of the multilayer, suggesting a negative DMI constant for Ir\Co interfaces. This opposes previous theoretical calculations and experimental studies. The observed anomalies in asymmetrical bubble expansion results could not be explained with the previous simple creep model of DW motion. Including changes of depinning field with in-plane applied field into the dynamics, micromagnetic simulation by Joo-Von Kim could successfully reproduce the experimental data. Finally, to study the DMI of Hf layers with different thicknesses, multilayers of Pt\Co\Hf(t_Hf)\Ta (tHf = 0.0-2.0 nm) were deposited. The magnetic characterization of films showed an approximately 0.7 nm thick magnetic dead-layer in the multilayers. The DMI in these films was again investigated using BLS and asymmetric bubble expansion. The results showed that Hf enhances DMI in the stack, suggesting a positive sign of DMI for the Hf\Co interface, which is opposite to DMI of Pt\Co interfaces. Changing the thickness did not show a significant effect on DMI factor.
Supervisor: Marrows, Christopher H. ; Moore, Thomas A. Sponsor: European Community
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available