微磁学中的边界条件及动态磁化翻转的研究 Alternative Title Boundary conditions in micromagnetics and the study of dynamic magnetization reversal 王伟伟 Thesis Advisor 王建波 2011-05-29 Degree Grantor 兰州大学 Place of Conferral 兰州 Degree Name 硕士 Keyword 微磁学模拟 二维周期性边界条件 一维无限边界条件 Abstract 周期性的边界条件在物理学中的很多领域中多有涉及,在微磁学中可以用来模拟高度规则的周期性的磁性结构,然而在微磁学中却没有那么容易实现。因为微磁学中退磁场起源为偶极相互作用,而偶极相互作用又为长程相互作用。K.M.Lebecki 等在 2008 年实现了一维周期性边界条件,我们在其工作的基础上给出了二维边界条件的实现方法:离散的无限退磁因子求和被连续的积分代替。退磁场通过重新定义的退磁因子进行计算,这种方法的好处是依然能够使用快速傅里叶变化加速退磁场的计算。最后在微磁学模拟软件 OOMMF 中实现了二维周期性边界条件,方法是在实空间中用重新定义的退磁因子替换普通的退磁因子,这些重新定义的退磁因子只需在初始的时候计算一遍。 近年来在微磁学领域对畴壁在纳米线中的运动规律研究越来越多因为有潜在的应用价值。但是在实际的模拟计算过程中很多时候我们想忽略有限长效应对畴壁运动带来的影响,为了实现这一目标,我们实现了实现了一维无限边界条件。 虽然单轴各向异性的样品在外场作用下的运动规律被研究的很透彻,但是椭圆形薄膜在外场的作用下的运动规律却不是很容易得到因为椭圆形薄膜引入的易面各向异性破坏了旋转对称性。为了简单起见,椭圆形薄膜被等效为同时具有易面各向异性和单轴各向异性的宏观磁矩。我们使用微扰的办法从理论上计算出了使薄膜翻转的最小理论磁场,并且给出了这种最有效外场的形式。最后研究了该体系在微波场作用下的运动,说明了对于这种椭圆形薄膜的铁磁共振频率会随着外场的增加而降低的规律。 Other Abstract Periodic boundary conditions (PBCs) are a useful concept in many areas of physics. In micromagnetic simulations they allow efficient modelling of highly regular periodic magnetic structures. However, it' not easy to implement PBC in micromagnetic because the demagnetization interaction have a long-range character and it's not reasonable to introduce a cutoff radius due to its quite slow decay. Therefore, Lebecki et al. described PBC for one-dimensional case in detail. A method based on K.M.Lebecki's work was put forward here for the situation of two-dimensional periodic boundary conditions (2D PBC): discrete infinity summation of demagnetization tensors was approached by a continuous integration without any range cutoff. The demagnetization field for 2D PBC was calculated by recombining the demagnetization tensors, for which fast Fourier Transform (FFT) algorithm can be applied to accelerate the computation. The method was implemented in the publicly full micromagnetic package (OOMMF) by replacing the normal demagnetization tensor in real space with the assembled demagnetization tensors that have to be evaluated only once at the beginning of the simulations. Magnetic domain wall motion in nanowire (such as induced by spin-polarized electric currents) has attracted significant interests recently due to its potential applications. However, we want to neglect the effect caused by the finite length of the nanowire in most cases when the domain wall dynamics are studied using the micromagnetic simulations. Thus, we implement the one-dimensional infinite boundary conditions (IBC) in OOMMF. Although the dynamics of the magnetic sample only with the uniaxial anisotropy under the applied field have been well studied, the elliptical thin film magnetic sample haven't because the easy-plane anisotropy breaks the rotational symmetry which will cause the magnetization motion are complex. In order to simple the analysis the elliptical thin film magnetic sample were considered with a strong easy-plane anisotropy and a weaker uniaxial anisotropy. The the perturbation method can be performed to chase the minimal switching field, and the trajectory of fastest magnetization switching was given. The system under circularly-polarized microwave was also researched, which showed the result that the frequency of FMR will decrease with the increasing of amplitude of microwave. URL 查看原文 Language 中文 Document Type 学位论文 Identifier https://ir.lzu.edu.cn/handle/262010/229229 Collection 物理科学与技术学院 Recommended CitationGB/T 7714 王伟伟. 微磁学中的边界条件及动态磁化翻转的研究[D]. 兰州. 兰州大学,2011.
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