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postgraduate thesis: Investigation of magnetization dynamics in nanostructures
Title  Investigation of magnetization dynamics in nanostructures 

Authors  
Issue Date  2015 
Publisher  The University of Hong Kong (Pokfulam, Hong Kong) 
Citation  Chen, J. [陳健]. (2015). Investigation of magnetization dynamics in nanostructures. (Thesis). University of Hong Kong, Pokfulam, Hong Kong SAR. Retrieved from http://dx.doi.org/10.5353/th_b5699917 
Abstract  This thesis investigated magnetization dynamics in nanostructures. Magnetization can be considered as an ensemble of large number of spins. In a macrospin model, magnetization is assumed to be spatially uniform so that it can be treated as a classical vector. Typically a LandauLifshitzGilbert (LLG) equation is used to described the dynamics of magnetization. The LLG equation is usually derived phenomenologically and the parameters of the equation needs to be tuned in order to comply with the experiments. Study of the magnetization dynamics from first principles is still insufficient.
In this thesis, by using the time dependent Green’s function theory, magnetization dynamics is studied in two systems: quantum dot with normal leads, and quantum dot with ferromagnetic leads. Expressions of intrinsic Gilbert damping tensor as well as fluctuating torque are derived in terms of Green’s function. Our expression of Gilbert damping tensor resembles the one derived from scattering matrix theory in the limit of low temperatures and wide band approximation, but is suitable for general case. Spin continuity equation of the system is also discussed, which shows how spin current is included in the equation of magnetization dynamics.
Recently spin torque effect due to the presence of Rashba spinorbit coupling (RSOC) opens the possibility of a new mechanism to manipulate magnetization. Currently most of the studies focused on infinite two dimensional electron gas (2DEG) system where current is driven by an external electric field. A semiclassical Boltzmann (SCB) transport equation was used and the nonequilibrium spin density was found to be linearly proportional to the charge current density. However, systematic investigation of such effect in a mesoscopic system beyond the semiclassical Boltzmann description has not been reported. It is purpose of this thesis to fill this gap.
In this thesis, magnetization dynamics is investigated in a finite 2DEG system where current is driven by bias instead of electric field. Rashba spin orbit coupling (RSOC) in 2D ferromagnetic materials generates spin polarization in 2DEG, thus a spin torque is induced. Magnetization dynamics of the ferromagnetic 2DEG is investigated in a tight binding model, which shows similar magnetic field assist switching effect as in the experiment. Our formalism is feasible for the first principle calculation of magnetization dynamics. 
Degree  Doctor of Philosophy 
Subject  Nanostructures Electromagnetism 
Dept/Program  Physics 
Persistent Identifier  http://hdl.handle.net/10722/223009 
HKU Library Item ID  b5699917 
DC Field  Value  Language 

dc.contributor.author  Chen, Jian   
dc.contributor.author  陳健   
dc.date.accessioned  20160217T23:14:30Z   
dc.date.available  20160217T23:14:30Z   
dc.date.issued  2015   
dc.identifier.citation  Chen, J. [陳健]. (2015). Investigation of magnetization dynamics in nanostructures. (Thesis). University of Hong Kong, Pokfulam, Hong Kong SAR. Retrieved from http://dx.doi.org/10.5353/th_b5699917   
dc.identifier.uri  http://hdl.handle.net/10722/223009   
dc.description.abstract  This thesis investigated magnetization dynamics in nanostructures. Magnetization can be considered as an ensemble of large number of spins. In a macrospin model, magnetization is assumed to be spatially uniform so that it can be treated as a classical vector. Typically a LandauLifshitzGilbert (LLG) equation is used to described the dynamics of magnetization. The LLG equation is usually derived phenomenologically and the parameters of the equation needs to be tuned in order to comply with the experiments. Study of the magnetization dynamics from first principles is still insufficient. In this thesis, by using the time dependent Green’s function theory, magnetization dynamics is studied in two systems: quantum dot with normal leads, and quantum dot with ferromagnetic leads. Expressions of intrinsic Gilbert damping tensor as well as fluctuating torque are derived in terms of Green’s function. Our expression of Gilbert damping tensor resembles the one derived from scattering matrix theory in the limit of low temperatures and wide band approximation, but is suitable for general case. Spin continuity equation of the system is also discussed, which shows how spin current is included in the equation of magnetization dynamics. Recently spin torque effect due to the presence of Rashba spinorbit coupling (RSOC) opens the possibility of a new mechanism to manipulate magnetization. Currently most of the studies focused on infinite two dimensional electron gas (2DEG) system where current is driven by an external electric field. A semiclassical Boltzmann (SCB) transport equation was used and the nonequilibrium spin density was found to be linearly proportional to the charge current density. However, systematic investigation of such effect in a mesoscopic system beyond the semiclassical Boltzmann description has not been reported. It is purpose of this thesis to fill this gap. In this thesis, magnetization dynamics is investigated in a finite 2DEG system where current is driven by bias instead of electric field. Rashba spin orbit coupling (RSOC) in 2D ferromagnetic materials generates spin polarization in 2DEG, thus a spin torque is induced. Magnetization dynamics of the ferromagnetic 2DEG is investigated in a tight binding model, which shows similar magnetic field assist switching effect as in the experiment. Our formalism is feasible for the first principle calculation of magnetization dynamics.   
dc.language  eng   
dc.publisher  The University of Hong Kong (Pokfulam, Hong Kong)   
dc.relation.ispartof  HKU Theses Online (HKUTO)   
dc.rights  This work is licensed under a Creative Commons AttributionNonCommercialNoDerivatives 4.0 International License.   
dc.rights  The author retains all proprietary rights, (such as patent rights) and the right to use in future works.   
dc.subject.lcsh  Nanostructures   
dc.subject.lcsh  Electromagnetism   
dc.title  Investigation of magnetization dynamics in nanostructures   
dc.type  PG_Thesis   
dc.identifier.hkul  b5699917   
dc.description.thesisname  Doctor of Philosophy   
dc.description.thesislevel  Doctoral   
dc.description.thesisdiscipline  Physics   
dc.description.nature  published_or_final_version   