二维伊辛模型的畴壁动力学

表面可以改变纳米磁性薄膜的结构和相变温度,畴壁动力学由此成为研究的重点.本文采用动力学蒙特卡罗模拟方法,对二维Ising模型磁畴界面的非平衡动力学展开数值研究.系统初态设为半正半负,即由完全有序但自旋取向完全相反的两部分组成,其间的磁畴壁随时间生长.通过对磁化标度形式的分析,发现畴壁内外的动力学标度形式虽然相同,但临界...     

磁畴和畴壁的实验演示

利用透射光法进行磁畴和畴壁的实验演示，在磁性材料中的薄膜中，若无外加磁场时，可以观察到很多蜿蜒曲折的条状磁畴，当垂直薄膜面上加一个磁场（磁化场）时，这些条状磁畴就会产生变化，凡与外加磁场方向一致的磁畴体积扩大，与加外磁场方向相反的磁畴体积会缩小。     

二层Ising模型的短时临界动力学性质

采用动力学Monte Carlo 方法研究了二层Ising模型的临界性质及早期动力学标度行为.结果表明层间耦合不为零时也存在临界点;计算了早期动力学临界指数θ;估计了传统的临界指数1/νz.其结果支持临界线存在的猜想,并表明此模型很可能是一种弱普适模型.关键词：     

磁畴壁拓扑结构研究进展

拓扑磁性斯格明子作为信息载体单元具备高可靠性、高集成度、低能耗等优势,有望提高数据读写精度、降低功耗,从而研发新型拓扑自旋电子学材料与原理型器件,为信息技术、5G通信和大数据等的高速发展提供材料与技术支持.但磁性斯格明子同时存在需要磁场稳定以及电流驱动下斯格明子霍尔效应引起偏转等缺点,严重阻碍了其在实际器件中的应用,因此探索新型拓扑磁畴结构和适宜应用的材料体系成为研究的关键.本文将重点介绍自2013年理论预言磁畴壁斯格明子以来,利用高分辨率洛伦兹透射电子显微镜原位实空间发现并研究磁畴壁拓扑麦纫和磁畴壁斯格明子的实验工作.首次在范德瓦耳斯Fe_5–xGeTe₂二维磁性材料中发现温度诱发的180°磁畴壁转变为拓扑麦韧链,研究了磁畴壁麦纫态在外界电场、磁场作用下的集体运动行为,揭示了基于自旋重取向、磁畴壁限域效应以及弱相互作用下生成磁畴壁拓扑态的机制.在该机制指导下,设计制备了具有自旋重取向的GdFeCo非晶亚铁磁薄膜,不仅获得了磁畴壁麦纫,验证了生成机制的普适性,还成功实现了畴壁麦韧对到畴壁斯格明子的可逆拓扑转变,开辟了基于磁畴壁等内禀限域效应开展...     

振荡磁场驱动亚铁磁畴壁动力学研究

亚铁磁材料在角动量补偿点附近具有类比于反铁磁的超快动力学,且存在非零净自旋密度,其磁结构可以被传统磁性手段探测和调控,有望应用于新一代高性能自旋电子器件.有效调控亚铁磁畴壁动力学是当前自旋电子学领域的重要课题.本工作使用微磁学模拟研究了正弦波和方波振荡磁场驱动亚铁磁畴壁,从理论上揭示不同的振荡磁场会诱导出不同方式的畴壁运动.研究表明,具有非零净自旋角动量的畴壁面随振荡磁场振荡,正弦波磁场驱动亚铁磁畴壁的位移随时间单调增加,而方波磁场驱动畴壁位移随时间曲折增大.本工作系统探讨了亚铁磁畴壁速度与外部磁场和材料内部参数的关联,表明了同强度下的正弦波磁场具有更高的驱动效率,并揭示了相关物理机制,可以为未来的实验和自旋器件设计提供参考.     

微磁动力学的新进展

本文介绍微磁动力学领域的一个最新进展,我们的研究发现在磁场驱动下且保持畴结构不变地沿着纳米磁线运动的磁畴壁,其运动源于能量耗散,磁畴壁运动速度正比于能量耗散率。与此同时,我们根据能量守恒原则,给出了磁畴壁速度的一个合理定义,该定义适用于任意的磁畴壁结构。在此定义下,即使磁畴壁没有做刚性运动,我们也能得到磁畴壁运动的瞬时速度和平均速度。我们的结果不仅能重复低磁场下的沃克（Walker）解,还能反映出当磁场高于沃克极限（Walker limit）时速度{磁场的依赖关系,该结果跟数值模拟和实验数据都符合得很好。我们根据微磁动力学研究的这一新进展,最终澄清了一个事实即“磁畴壁质量”这个概念是错误的。     

电流驱动下磁畴壁快速运动的研究

在2011年第7期出版的Physics Today杂志上刊登了一篇关于自旋极化电流驱动磁畴壁运动的文章,介绍了该领域的最新进展.现对该文主要内容摘译如下:磁体具有净磁矩,但这并不意味着所有的非配对自旋指向同一方向.通常情况下,多个具有不同磁矩方向的磁畴同     

消磁场对纳米铁磁线磁畴壁动力学行为的影响

为了实现基于磁畴壁运动的自旋电子学装置, 掌握磁畴壁动力学行为是重要争论之一.研究了在外磁场驱动下L-型纳米铁磁线磁畴壁的动力学行为. 通过微磁学模拟,在各种外磁场的驱动下考察了纳米铁磁线磁畴壁的动力学特性; 在较强外磁场的驱动下, 在不同厚度纳米线上考察了纳米线表面消磁场对磁畴壁动力学行为的影响. 为了进一步证实消磁场对磁畴壁动力学的影响, 在垂直于纳米线表面的外磁场辅助下分析了磁畴壁的动力学行为变化. 结果表明, 随着纳米线厚度和外驱动磁场强度的增加, 增强了纳米线表面的消磁场的形成, 使得磁畴壁内部自旋结构发生周期性变化, 导致磁畴壁在纳米线上传播时出现Walker崩溃现象. 在垂直于纳米线表面的外磁场辅助下, 发现辅助磁场可以调节消磁场的强度和方向. 这意味着利用辅助磁场可以有效地控制纳米铁磁线磁畴壁的动力学行为.     

二维伊辛模型相变临界点温度的模拟计算

用计算模拟方法计算了二维伊辛模型的相变临界点温度,其结果接近严格解,明显布喇格－威廉斯近似和贝特近似的结果。     

磁性金属纳米结构的畴壁特性与磁逻辑电路构筑

自旋电子学由于其丰富的物理内涵和广泛的应用前景受到学术界和工业界的高度重视,成为近年来凝聚态物理和信息技术领域关注的焦点。本文介绍了利用磁性金属纳米结构实现作为自旋电子器件基础的自旋注入的方法,特别涉及利用铁磁金属纳米点接触结构钉扎磁畴的特点,研究自旋极化电流与磁畴壁的相互作用规律, 理解纳米结构中畴壁的动力学行为,并以此为基础构筑结构简单、性能优异的全金属磁逻辑电路,从而实现了由电信号驱动,通过电信号检测,并与CMOS技术兼容的目的。     

Energy of Long-Lifetime Configurations in Zero-Temperature Dynamics

Using Monte Carlo method with zero-temperature dynamics, we investigate energy evolution of Ising spin configuration on a square lattice. The energies of some configurations exhibit long duration before those configurations reach the final state -- ground state or frozen stripe state. For ground-state dynamical realization, the duration occurs when the energy per spin is 4/L, where L is the lattice size. For stripe-state dynamical realization, the energy is slightly higher than 2/L when the duration appears in the last evolution stage. In addition, it is found that the average energy per spin in final state is approximately 2/3L.     

The stochastic models method applied to the critical behavior of Ising lattices

The stochastic models (SM) computer simulation method for treating manybody systems in thermodynamic equilibrium is investigated. The SM method, unlike the commonly used Metropolis Monte Carlo method, is not of a relaxation type. Thus an equilibrium configuration is constructed at once by adding particles to an initiallyempty volume with the help of a model stochastic process. The probability of the equilibrium configurations is known and this permits one to estimate the entropy directly. In the present work we greatly improve the accuracy of the SM method for the two and three-dimensional Ising lattices and extend its scope to calculate fluctuations, and hence specific heat and magnetic susceptibility, in addition to average thermodynamic quantities like energy, entropy, and magnetization. The method is found to be advantageous near the critical temperature. Of special interest are the results at the critical temperature itself, where the Metropolis method seems to be impractical. At this temperature, the average thermodynamic quantities agree well with theoretical values, for both the two and three-dimensional lattices. For the two-dimensional lattice the specific heat exhibits the expected logarithmic dependence on lattice size; the dependence of the susceptibility on lattice size is also satisfactory, leading to a ratio of critical exponents/=1.85 ±0.08. For the three-dimensional lattice the dependence of the specific heat, long-range order, and susceptibility on lattice size leads to similarly satisfactory exponents:=0.12 ±0.03,=0.30 ±0.03, and=1.32 ±0.05 (assuming =2/3).     

Minor magnetization loops in two-dimensional dipolar Ising model

The two-dimensional dipolar Ising model is investigated for the relaxation and dynamics of minor magnetization loops. Monte Carlo simulations show that in a stripe phase an exponential decrease can be found for the magnetization maxima of the loops, M∼exp(−αN_l) where N_l is the number of loops. We discuss the limits of this behavior and its relation to the equilibrium phase diagram of the model.     

Phase transitions and thermodynamic properties of antiferromagnetic Ising model with next-nearest-neighbor interactions on the Kagomé lattice

We study phase transitions and thermodynamic properties in the two-dimensional antiferromagnetic Ising model with next-nearest-neighbor interaction on a Kagomé lattice by Monte Carlo simulations. A histogram data analysis shows that a second-order transition occurs in the model. From the analysis of obtained data, we can assume that next-nearest-neighbor ferromagnetic interactions in two-dimensional antiferromagnetic Ising model on a Kagomé lattice excite the occurrence of a second-order transition and unusual behavior of thermodynamic properties on the temperature dependence.     

Monte Carlo simulation of Ising models by multispin coding on a vector computer

Rebbi's efficient multispin coding algorithm for Ising models is combined with the use of the vector computer CDC Cyber 205. A speed of 21.2 million updates per second is reached. This is comparable to that obtained by special- purpose computers.     

A fast algorithm for the Cyber 205 to simulate the 3D Ising model

We describe a computer program that performs the Metropolis algorithm for the 3D Ising model at a peak speed of 98 million spin updates per second on a 2-pipe CDC Cyber 205. This speed is achieved using the special vector capabilities of the Cyber 205 and multispin coding techniques.     

Critical dynamics of the three-dimensional Ising model: A Monte Carlo study

Extensive Monte Carlo simulations have been performed to analyze the dynamical behavior of the three-dimensional Ising model with local dynamics. We have studied the equilibrium correlation functions and the power spectral densities of odd and even observables. The exponential relaxation times have been calculated in the asymptotic one-exponential time region. We find that the critical exponentz=2.09 ±0.02 characterizes the algebraic divergence with lattice size for all observables. The influence of scaling corrections has been analyzed. We have determined integrated relaxation times as well. Their dynamical exponentz _int agrees withz for correlations of the magnetization and its absolute value, but it is different for energy correlations. We have applied a scaling method to analyze the behavior of the correlation functions. This method verifies excellent scaling behavior and yields a dynamical exponentz _scal which perfectly agrees withz.