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.     

不对称钴纳米环磁特性及涡旋态控制的蒙特卡罗模拟

利用Monte Carlo模拟方法研究不同外加磁场方向对不对称钴纳米环磁特性的影响.主要研究磁化反转过程中畴壁运动可控制机制、磁滞回线及涡旋态的形成.模拟结果表明:①不同的外加磁场方向对不对称钴纳米环在磁化反转过程中畴壁的运动和涡旋态的形成及稳定性有较大的影响;②可利用畴壁能的变化及外加磁场的方向控制纳米环畴壁的运动;③磁化反转过程中涡旋态的形成及稳定性存在明显的尺寸效应.     

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。     

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。     

磁畴壁手性和磁斯格明子的拓扑性表征及其调控

磁性斯格明子由于拓扑的保护性,具有很高的稳定性和较小的临界驱动电流,有望应用于未来的赛道存储器件中.而在中心对称体系,由于偶极作用的各向同性,磁泡的拓扑性和螺旋度都呈现出多样性的特征.其中非平庸的磁泡即等同于磁性斯格明子.我们通过近期实验结果,结合微磁学模拟的方法,发现在中心对称体系中磁斯格明子的拓扑性会受到体系垂直各向异性的调控.另外在加磁场的演变过程中,会很大程度上依赖于基态畴的畴壁特性.磁场的倾斜或者一定的面内各向异性也会改变磁斯格明子的形态.通过对材料的基态磁结构及磁各向异性的调节,辅助以面内分量的控制,可以对基态磁畴、进而对磁斯格明子的拓扑性实现调控.这对磁斯格明子在电流驱动存储器件中的应用具有重要意义.     

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...     

横向偏置场作用的非晶带巨磁阻抗效应理论

采用多畴结构模型,考虑非晶带具有180°畴壁的磁畴及其两面的偏置磁场方向的不同,根据自由能最小原理,Maxwell方程组及带阻尼项的Landau-Lifshitz方程,建立了非晶态合金带在横向偏置磁场作用下的巨磁阻抗效应的理论计算公式. 提出并采用四状态平均磁导率代替单畴模型获得的磁导率,得到了更符合实际的处于偏置场作用的阻抗随外磁场变化的理论结果. 关键词： 偏置磁场 四状态 多畴结构 巨磁阻抗     

金属镍的Barkhausen发射特性的研究及磁畴壁运动临界场值的计算

本文研究了金属镍的Barkhausen声发射(ABE)和电磁发射(EDE)的特性。作者利用能量密度图和修正的Kersten理论解释了镍对外加磁场的响应。文中所介绍的测量结果除证实ABE之源为非180°磁畴壁运动引起外,还进而演示了外加磁场变化,使镍从(退出)饱和到反向饱和时,三种磁畴壁运动的顺序依次为:71°,180°和109°.文章阐明了如何用所测到的Barkhausen信号去确定相应于这三种磁畴壁(不可逆)运动的临界磁场值大小。     

铁纳米线磁行为的微磁学模拟与研究

通过微磁学模拟的手段对用电化学沉积法制备出的呈圆柱形的铁纳米线在常温下的磁行为进行了系统的研究,结果表明不同形状参量的铁纳米线都表现出磁各向异性,矫顽力随纳米线直径变化近似呈现平方反比关系,而对每一固定的直径,矫顽力随纳米线长度的增加而增大,最后趋向定值.磁滞回线形状、静态磁矩分布和反转机制等都随纳米线的直径和长度的变化而变化,对相应的规律给出了明确的解释.模拟结果显示实验上尚不能制备出的直径为5nm的纳米线呈现一致反转机理,同时还发现当纳米线过渡为颗粒时表现出更为复杂的性质 关键词： 纳米线 微磁学     

Influence of exchange coupling on current-driven domain wall motion in a nanowire

In this study, the effect of exchange stiffness constant on current-driven domain wall motion in nanowires with in-plane magnetic anisotropy (IMA) and perpendicular magnetic anisotropy (PMA) has been investigated using micromagnetic simulation. The critical current density in a nanowire with IMA decreases as the exchange stiffness constant decreases because the domain wall width at the upper edge of the nanowire narrows according to the decrease of the exchange stiffness constant. On the other hand, the critical current density in a nanowire with PMA slightly decreases contrary to that of IMA although the domain wall width reasonably decreases as the exchange stiffness constant decreases. The slight reduction rate of the critical current density is due to the increase of the effective hard-axis anisotropy of PMA nanowire.     

SPIN-DEPENDENT TUNNELING EFFECTS ON MAGNETIC NANOSTRUCTURES

The contrast mechanism of spin-polarized scanning tunneling spectroscopy (SP-STS) is demontrated on in-plane magnetized Gd islands grown on W (110) using Fe-coated tips. The use of Gd-coated tips enables the imaging of the antiferromagnetic domain structure of Fe nanowires being perpendicularly magnetized grown on a stepped W(110) substrate. The influence of an external magnetic field on the nanowire domain structure is demonstrated. It is shown that the antiferromagnetic domain structure of the Fe nanowires strongly depends on the miscut of the W(110) substrate. While at high miscut the magnetization direction alternates between adjacent DL stripes it was found to alternate within single Fe DL stripes at low miscut. Nanoscale Fe islands with a height of two atomic layers were found to be single domain particles.     

磁场对模板法制备的Co纳米线结构和磁性的影响

在用二次氧化法制备的高度有序的氧化铝模板上通过交流电化学方法制备了Co纳米线阵列．研究了外加磁场及电解液pH值对纳米线生长的影响．在pH值为6.0和6.5的电解液中分别在不加磁场和沿纳米线轴向施加0.3 T磁场情况下制备了hcp结构的Co纳米线阵列．实验数据表明,沉积时外加磁场和调节pH值能有效影响纳米线中hcp结构的Co晶粒的易磁化轴沿纳米线长轴方向生长．由于晶粒的磁晶各向异性和纳米线沿长度方向的宏观形状各向异性叠加,制备的Co纳米线阵列具有高垂直各向异性,高矫顽力和较高矩形比． 关键词： Co纳米线阵列 织构 磁性     

Magnetic field control of 90°, 180°, and 360° domain wall resistance

In the present work, we have compared the resistance of the 90°, 180°, and 360° domain walls in the presence of external magnetic field. The calculations are based on the Boltzmann transport equation within the relaxation time approximation. One-dimensional Néel-type domain walls between two domains whose magnetization differs by angle of 90°, 180°, and 360° are considered. The results indicate that the resistance of the 360° DW is more considerable than that of the 90° and 180° DWs. It is also found that the domain wall resistance can be controlled by applying transverse magnetic field. Increasing the strength of the external magnetic field enhances the domain wall resistance. In providing spintronic devices based on magnetic nanomaterials, considering and controlling the effect of domain wall on resistivity are essential.     

面心立方单晶镍纳米线稳定性及磁性的第一性原理计算

本文采用第一性原理方法,研究了轴向为低指数晶向的面心立方(fcc)单晶镍纳米线的稳定性和磁性.计算表明,[110] 是fcc镍纳米线最容易出现的取向,[111] 取向次之,而 [001] 取向则很难出现,这一结果与实验事实符合.镍纳米线按照原子位置和磁性强弱的不同,可以分成简单的芯-壳结构,在纳米线芯部,原子的磁矩大小与块体基本一致.在纳米线表面,镍原子的磁矩比芯部原子有所增加.表面原子磁矩与轴向的取向相关,[110] 为轴向的纳米线表面原子磁矩最低,而[001] 为轴向的纳米线表面原子磁矩最高. 关键词： 镍 纳米线 第一性原理 原子磁矩     

Dynamics of magnetization in ferromagnet with spin-transfer torque

We review our recent works on dynamics of magnetization in ferromagnet with spin-transfer torque. Driven by constant spin-polarized current, the spin-transfer torque counteracts both the precession driven by the effective field and the Gilbert damping term different from the common understanding. When the spin current exceeds the critical value, the conjunctive action of Gilbert damping and spin-transfer torque leads naturally the novel screw-pitch effect characterized by the temporal oscillation of domain wall velocity and width. Driven by space- and time-dependent spin-polarized current and magnetic field, we expatiate the formation of domain wall velocity in ferromagnetic nanowire. We discuss the properties of dynamic magnetic soliton in uniaxial anisotropic ferromagnetic nanowire driven by spin-transfer torque, and analyze the modulation instability and dark soliton on the spin wave background, which shows the characteristic breather behavior of the soliton as it propagates along the ferromagnetic nanowire. With stronger breather character, we get the novel magnetic rogue wave and clarify its formation mechanism. The generation of magnetic rogue wave mainly arises from the accumulation of energy and magnons toward to its central part. We also observe that the spin-polarized current can control the exchange rate of magnons between the envelope soliton and the background, and the critical current condition is obtained analytically. At last, we have theoretically investigated the current-excited and frequency-adjusted ferromagnetic resonance in magnetic trilayers. A particular case of the perpendicular analyzer reveals that the ferromagnetic resonance curves, including the resonant location and the resonant linewidth, can be adjusted by changing the pinned magnetization direction and the direct current. Under the control of the current and external magnetic field, several magnetic states, such as quasi-parallel and quasi-antiparallel stable states, out-of-plane precession, and bistable states can be realized. Th     

Structural and magnetic properties of electrodeposited Cobalt nanowire arrays

Ordered magnetic nanowires have tremendous potential in future magnetic storage and high frequency magnetic logic devices. Here, we present the fabrication of ordered arrays of Cobalt nanowires by electrodeposition through porous polycarbonate membranes. Vertically and horizontally aligned nanowires were produced in presence of an external bias field during post deposition etching of the polycarbonate membrane. Structural and compositional analyses have been carried out to establish the material and structural purity. The magneto-optical Kerr effect was employed to measure the magnetic hysteresis loops for the nanowires assembled in the substrate plane. A good magneto-optical signal to noise ratio is observed with clean ferromagnetic hysteresis loops. The loops measured with external magnetic field applied parallel and perpendicular to the axis of the nanowires show a clear difference in the shape and the coercive field, indicating the effect of shape anisotropy in these samples. Micromagnetic simulations were performed to understand the experimental results and to obtain insight to the magnetization reversal mechanism in magnetic nanowires.