三类硬磁畴的形成及静态特性

本文提出畴壁中含有大量VBL的石榴石磁泡材料的三类硬磁畴的分类标准,描述了它们的形成方法和静态特性,并示出典型照片。从形成条件可以合理地推论,畴壁中VBL数目的增加导致硬磁畴静态特性的质的变化。 关键词：     

面内场和静态偏磁场作用下条畴和泡畴的稳定性

实验研究面内场H_in和静态偏磁场H_b作用下,(111)面磁泡膜内条畴的消失过程。保持H_b恒定,增加H_in,测量条畴消失场H_s^*和泡畴消失场H_k^*与面内场方向β的变化关系。计及立方磁晶各向异性的影响,建立H_in和H_b共同存在时的条畴稳定性理论。定性解释了实验的主要特点。导出黑、白条畴同时消失时的角度 β_n=1/3(2nπ±arc cos│3/(2^1/2)(M_sH_b)/K₁│)(n=0,±1,±2,…)与实验基本符合。 关键词：     

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。     

石榴石磁泡膜中3类硬磁畴的形成方法

研究石榴石磁泡膜中硬磁畴畴壁中垂直布洛赫线的稳定性可为研制布洛赫线存储器提供有益的帮助 .3类硬磁畴的形成是研究硬磁畴稳定性的前提 .本文综述了在石榴石磁泡膜上形成硬磁畴的 2类方法———“脉冲偏场法”和“低直流偏场法” .结合文献中的典型样品 ,对用“脉冲偏场法”和“低直流偏场法”形成 3类硬磁畴的过程进行了简单介绍 .     

脉冲偏场作用下石榴石磁泡薄膜中布洛赫线的形成

实验研究了石榴石磁泡薄膜中不同脉冲宽度下产生的硬磁畴的动态特性.结果表明：由零偏场下产生的枝状畴收缩而成的哑铃畴均逆时针转动,与产生枝状畴时所用的脉冲偏场的脉冲宽度无关.而固定直流偏场下由软畴段硬化而成的哑铃畴的转动状态,则与所用的硬化脉冲偏场的脉冲宽度有关.低脉冲宽度下硬化成的哑铃畴均逆时针转动,随硬化脉冲宽度的升高,出现混合转动向顺时针转动的过渡.由此揭示了直流偏场和脉冲偏场在石榴石磁泡薄膜中形成正、负垂直布洛赫线的作用. 关键词： 磁畴 磁泡 垂直布洛赫线     

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     

磁性材料的磁结构、磁畴结构和拓扑磁结构

首先简要地介绍了磁性材料中磁结构、磁畴结构和拓扑磁结构以及相互之间的关系. 一方面, 磁畴结构由材料的磁结构、内禀磁性和微结构因素决定; 另一方面, 磁畴结构决定了材料磁化和退磁化过程以及技术磁性. 拓扑学与材料物理、材料性能的联系越来越紧密. 最近的研究兴趣集中在一些拓扑磁性组态, 如涡旋、磁泡、麦纫、斯格米子等. 研究发现这些拓扑磁结构的拓扑性质与磁性能密切相关. 然后从尺寸效应、缺陷、晶界三个方面介绍国际学术界在磁结构、磁畴结构和拓扑磁结构方面的进展. 最后介绍了在稀土永磁薄膜材料的微观结构、磁畴结构和磁性能关系、交换耦合纳米盘中的拓扑磁结构及其动力学行为方面的工作. 通过对文献的评述, 得到以下结论: 开展各向异性纳米复合稀土永磁材料的研究对更好地利用稀土资源具有重要的意义. 可以有目的地改变材料的微结构, 可控地进行磁性材料的磁畴工程, 最终获得优秀的磁性能. 拓扑学的概念正在应用于越来越多的学科领域, 在越来越多的材料中发现拓扑学的贡献. 研究磁畴结构、拓扑磁性基态或者激发态的形成规律以及动力学行为对理解量子拓扑相变以及其他与拓扑相关的物理效应是十分重要的. 也会帮助理解不同拓扑学态之间相互作用的物理机制及其与磁性能之间的关系, 同时拓展拓扑学在新型磁性材料中的应用.     

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...     

具有条纹磁畴结构的NiFe薄膜的制备与磁各向异性研究

具有条纹磁畴结构的磁性薄膜表现出面内转动磁各向异性,对于解决高频电子器件的方向性问题起着至关重要的作用.本文采用射频磁控溅射的方法,研究了NiFe薄膜的厚度、溅射功率密度、溅射气压等制备工艺参数对条纹磁畴结构、面内静态磁各向异性、面内转动磁各向异性、垂直磁各向异性的影响规律.研究发现,在功率密度15.6 W/cm~2与溅射气压2 mTorr(1 Torr=1.33322×102Pa)下生长的NiFe薄膜,表现出条纹磁畴的临界厚度在250 nm到300 nm之间.厚度为300 nm的薄膜比250 nm薄膜的垂直磁各向异性场增大近一倍,从而磁矩偏离膜面形成条纹磁畴结构,并表现出面内转动磁各向异性.高溅射功率密度可以降低薄膜出现条纹磁畴的临界厚度.在相同功率密度15.6 W/cm~2下生长300 nm的NiFe薄膜,随着溅射气压由2 mTorr增大到9 mTorr,NiFe薄膜的垂直磁各向异性场逐渐由1247.8 Oe(1 Oe=79.5775 A/m)增大到3248.0 Oe,面内转动磁各向异性场由72.5 Oe增大到141.9 Oe,条纹磁畴周期从0.53μm单调减小到0.24μm.NiFe薄膜的断面结构表明柱状晶的形成是表现出条纹磁畴结构的本质原因,高功率密度下低溅射气压有利于柱状晶结构的形成,表现出规整的条纹磁畴结构,高溅射气压会导致柱状晶纤细化,面内转动磁各向异性与面外垂直磁各向异性增强,条纹磁畴结构变得混乱.     

铁磁质磁畴特性的演示实验

磁畴是铁磁质重要的微观结构．在工科物理教学中，一般实验室不太容易做到对磁畴结构的直接观察，因此，很有必要采用定性的实验方法来阐明磁畴结构的存在及其特性．笔者在华中理工大学生产的热磁轮演示仪器基础上．用更简单的方法进行磁畴存在的演示，几年来教学效果都比较好，能够引起学生的兴趣和思考．现将实验装置、实验方法及简单说明介绍如下，以供切磋．     

Magneto-optic study of the behavior of magnetic domains walls in ferrimagnetic garnet films placed over samples with in-plane magnetization

Magneto-optic (MO) imaging is based on Faraday rotation of a linearly polarized incident light beam illuminating a sensitive MO layer placed in close contact to the sample. For in-plane magnetized layers of Lu_3−xBi_x Fe_5−yGa_yO₁₂ ferrimagnetic garnet films, zig-zag domain formation occurs whenever the sample stray parallel field component changes sign. In this work we study the behavior of zig-zag domain walls that appear when the garnet is placed over samples with in-plane magnetization like audio tapes recorded with different signals. We describe the zig-zag walls considering the anisotropy, exchange and magnetostatic energies in the Neel tails and the contribution of an applied magnetic field. Using different recorded signals we have been able to control the gradient of stray parallel field component on the garnet, changing the distance between domains and the size of zig-zag walls. We could even avoid the appearance of these zig-zag domain walls and obtain closed domains structures. We also study the behavior of the domain walls when an external magnetic field is applied parallel to the sample.     

Formation of three kinds of hard domains by low-bias-field method

The formation of three kinds of hard domains (HDs) at low bias field was studied experimentally. Low-bias-field method provides a simple way to produce the three kinds of HDs in different proportions. Also, the relationship between the shape of the multi-branched domains (MBDs) and the different kinds of domains was studied. Most branches of the MBDs correspond to the first kind of dumbbell domains or the second kind of dumbbell domains; in contrast, few branches of the MBDs correspond to the hard bubbles or soft bubbles.     

Analysis of the domain wall structures of the three kinds of hard domains in garnet bubble films

It was found experimentally that dumbbell domains of the second kind produced at high temperature can convert to dumbbell domains of the first kind at low temperature. And the ordinary hard bubbles produced at low temperature can convert to dumbbell domains of the first kind at high temperature. So we conclude that the domain wall structures of the three kinds of hard domains are the same. We also observe that the equilibrium separation between two neighboring vertical-Bloch lines is widened with the increasing temperature when the hard domains collapse under the effect of bias field.     

Modular properties of the hard hexagon model

The physical quantities (or powers thereof) in the hard-hexagon model that were computed exactly by Baxter are shown to be modular functions with respect to the number-theoretic group ₁[N]. This allows us to determine the analytic structure of, the partition function per site in the thermodynamic limit, and, the density, as functions of the activityz.     

Note on the fractal dimension of hard sphere trajectories

Using Monte Carlo molecular dynamics, a new, careful study is made of the approach of the trajectory of a typical particle in a hard sphere fluid to that of a Brownian particle, discussed before by Powles and Quirke and Rapaport. The apparent fractal dimension of the trajectory, as a function of reduced length scale,(), characterizes the transition from mechanical to Brownian motion and differs markedly from 2 in all present computer simulations.     

Simulating the effects of time-varying magnetic fields with a realistic simulated scanner

Transient magnetic fields induce changes in magnetic resonance (MR) images ranging from small, visually undetectable effects (caused, for instance, by neuronal currents) to more significant ones, such as those created by the gradient fields and eddy currents. Accurately simulating these effects may assist in correcting or optimising MR imaging for many applications (e.g., diffusion imaging, current density imaging, use of magnetic contrast agents, neuronal current imaging, etc.). Here we have extended an existing MR simulator (POSSUM) with a model for changing magnetic fields at a very high-resolution time-scale. This simulator captures a realistic range of scanner and physiological artifacts by modeling the scanner environment, pulse sequence details and subject properties (e.g., brain geometry and air-tissue boundaries).