我們由Landau-Lifshitz-Gilbert方程式推導出磁壁的運動方程式,利用此
方程式可以分析磁壁在任何方向以及任意大小的橫向場(垂直於異向性軸
方向的外加磁場)下的運動行為.當平行於異向性軸方向的外加磁場(驅動
場),大(小)於Walker的臨界場時,磁壁為震盪(穩定)態運動.我們的方程
式,無論磁壁是在震盪或穩定態運動皆可適用.近年來根據自旋波(spin
wave)理論發現磁性材料的阻尼與自旋波的波向量有關,於是Bar'yakhtar
基於Landau-Lifshitz方程式導出一個更廣義的方程式,其結果更能符合自
旋波理論.Bar'yakhtar方程式與Landau-Lifshitz方程式不同之處除了考
慮空間色散引起的阻尼外,局部的磁化量不守恆也是不可避免的效應.我們
根據Bar'yakhtar的方程式導出磁壁的運動方程式,研究顯示磁化量不守恆
也會造阻尼效應.進一步推導磁壁的Mobility,發現由Mobility得到的阻
尼,與由鐵磁共振的實驗得到的阻尼,理論上並不相同.而且透過這兩種阻
尼的關係式可看出,兩種阻尼的值差距越大,表示由於磁化量不守恆引起的
阻尼效應越顯著.換句話說,若兩種阻尼的值很接近,那麼用Landau-
Lifshitz-Gilbert方程式得到的結果,與Bar'yakhtar方程式得到的結果相
差不大.我們還可以用此關係式推算磁性材料的磁化係數,對於那些異向性
場大於飽和磁化量的iron garnets,我們得到的磁化係數與自旋波理論預
測的結果非常接近.關於外加磁場作用的分析,發現驅動場對磁化量不守恆
的影響有放大的作用,而橫向場卻有壓抑的作用.

The generalized Slonczewski equations have been applied to study
the influenceof the field normal to the anisotropy axis on the
Walker critical field, critical velocity and the maximum
velocity of the steady-state domain wall motion. It is shown
that the maximum value of the steady-state velocity of the
domain wall is the Schlomann limiting velocity which is drive
field dependent. The dependences of the Walker critical field
and velocity as well as Schlomann limiting velocity on the field
normal to the anisotropy axis have been obtained. The new
equations take into account the relaxational dynamics of
magnetization modulus first introduced into the Landau-Lifshitz
equation by Bar'yakhtar. In the derivation of linear mobility, a
new expression of a relaxation parameter is obtained. It
reaveals a relation between ferromagnetic resonance(FMR) line
width and the relaxation parameter obtained from mobility
measurement. Based on this relation, it is found that the
nonconservation of magnetization modulus gives rise to a larger
contribution to the domain wall drag in ferromagnets with a
narrower FMR line width than in ones with a wider line width.
The description of the steady state domain wall motion in the
traditional Landau-Lifshitz-Gilbert equation may give
essentially the same dependency upon the drive field and
transverse field provided if the phenomenological relaxation
constant is deduced directly from experimental data on the
domain wall mobility, instead of from a ferromagnet with a wide
resonance line width. It is also found that the drag force due
to nonconservation of magnetization modulus is enhanced by drive
field but depressed by transeverse field.

Cover
Abstract(in Chinese)
Abstract
Contents
1
1.1 A Brief Review of Domain Theory
1.2 A Brief History of Domain Wall Dynamics
1.3 Magnetization Relaxation Terms
1.4 Slonczewski's Approach and Quality Factor
1.5 Summary of This Dissertation
2 Domain Wall Dynamics with Conservation of Modulus of Magnetization
2.1 Energy Density and Domain Wall Structure
2.2 Equations of the Domain Wall Motion
2.3 Oscillatory State of the Domain Wall Motion
2.4 The Critical Parameters of Domain Wall Motion
2.5 Dependences of the Domain Wall VElocity and Mobility on the Drive Field
2.6 Further Remarks
2.7 Summary
3 Domain Wall Dynamics with Nonconservation of Modulus of Magnetization
3.1 Introduction
3.2 Analysis
3.3 infuence of the Nonconservation of the Magnetization Modulus on the Domain Wall Motion
3.4 Numerical Results and Discussions
3.5 Conclusions
Appendix A
Appendix B
Appendix C
Bibliography