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Title: Models of spin torque using self-consistent solutions of the magnetisation and spin accumulation
Author: Chureemart, Phanwadee
ISNI:       0000 0004 2744 7187
Awarding Body: University of York
Current Institution: University of York
Date of Award: 2013
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A model of spin accumulation (m) is proposed to develop theoretical approaches to calculate the m in any arbitrary magnetic structure. The model is based on generalising the approach of Zhang, Levy and Fert (PRL 88, 236601, 2002). The calculation involves the layer-wise discretisation of the structure and the development of semi-analytical approaches to solve for the equilibrium m throughout the structure. Interestingly, the layer discretisation allows the treatment of diff�use interfaces using a gradual variation of the magnetic and transport properties across the interface. The e�ffect of the interfaces between a ferromagnet and a nonmagnet and between two ferromagnets on spin injection is investigated. The formalism for calculating the m is first generalised by taking m as the di�fference of spin-up and spin-down density of states, which is necessary for treating the interface between diff�erent ferromagnets. Then, the e�ffect of atomic species interdiffusion at the interface is included by using Ficks's law. It is shown that the discontinuity of the m at the interface depends strongly on the degree of interface mixing. Subsequently, current-induced domain wall (DW) motion in a ferromagnetic thin fi�lm driven by a spin-polarised current is investigated using an atomistic model coupled with a standard Landau-Lifshitz-Gilbert equation. The inclusion of the spin-transfer torque is represented as an additional �field. The m is calculated self-consistently and naturally includes the adiabatic and non-adiabatic contributions depending on the rate of change of magnetisation relative to the spin di�ffusion length. In this work, it is importantly found that the constants �x and �x used in the standard micromagnetic model do not provide a good description of the spin torque phenomenon due to the non-physical behaviour. Therefore, it is suggested to describe the spin-transfer torque directly from the m. Finally, the evolution of the magnetisation and m are demonstrated by introducing a spin-polarised current into a material containing a DW whose width is varied by changing the anisotropy constant. It is found that the adiabatic spin torque tends to develop in the direction of the magnetisation whereas the non-adiabatic spin torque arising from the mistracking of conduction electrons and local magnetisation results in out-of-plane magnetisation components. However, the adiabatic spin torque signifi�cantly dominates the dynamics of magnetisation. The total spin torque acting on the magnetisation increases with anisotropy constant due to the increasing magnetisation gradient. This leads to increasing DW displacement.
Supervisor: Chantrell, Roy Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available