单根磁性纳米管和低维螺旋磁体的磁性实验研究

Recently, ferromagnetic nanotubes and low dimension helimagnets have attracted great interests among researchers, because of their potential applications in spin-related applications and high density magnetic storage of information. Common magnetization measuring techniques, even the advanced SQUID vibrating sample magnetometer, require an ensemble of nano-objects. Since their sensitivity is not good enough to detect signals from a single nano-object. A general method for studying magnetizations of an individual nanowire/nanotube is measuring their magneto resistance, by which only limited information of magnetic states can be inferred. To understand fundamental magnetic properties and magnetization processes in nanowires, nanotubes and nanodisks, it is crucial to have a direct and sensitive measuring of their magnetizations individually. This project aims to investigate magnetic states and magnetization processes of ferromagnetic nanotubes and low dimensional helimagnets, such as nanowires and nanodisks, by torque magnetometry using ultra-sensitive cantilevers. We will shape bulk helimagnetic materials into designed sizes and structures, such as nanowires and nanodisks by focus ion beam (FIB), and cut ferromagnetic nanotubes into expected lengths. Then a single nanotube, nanowire or nanodisk will be transferred onto an ultra-sensitive cantilever. Varies configurations of attaching nanotubes, nanowires and nanodisks onto the cantilever will be studied with torque magnetometry. Based on these magnetic measurements of individual nano-structures, we will reveal how the shape and size of ferromagnetic nanotubes influence their magnetic states and stability of these magnetic states. This project will also investigate the formations and stabilizing mechanism of Skymions phases in low dimension helimagnets.

由于在自旋器件和高密度磁存储领域的潜在应用，铁磁纳米管和低维螺旋磁体最近特别受到关注。在实验研究方面，目前对于纳米线、纳米颗粒等纳米磁性材料的测量通常基于系综测量，实验测量得到的是很多个纳米结构的平均磁化。从这样的实验数据很难得到单个纳米结构的微观磁结构和磁化过程。而另一种常用的磁阻测量方法，虽然可以观测单根纳米线、纳米管磁结构的变化，但是只能间接见证有限的纳米磁结构改变。本研究项目计划利用微纳加工方法把铁磁纳米管和螺旋磁体加工成特定几何形状和尺寸，利用灵敏悬臂梁开展力矩磁化分析，研究单根的纳米管、纳米线和单个纳米盘的磁结构和磁化过程。具体地说，通过对铁磁纳米管、纳米线单体的磁化曲线的测量，理解铁磁纳米管的直径，管壁厚度和长度对其磁结构和稳定性的影响；通过对不同形状和尺寸螺旋磁体的力矩磁化分析，直接实验观测，确定空间受限效应对Skyrmions相在低维螺旋磁体中形成和稳定机制的影响。

低维纳米磁性材料在自旋器件和高密度磁存储领域具有潜在应用。其中一类具有拓扑磁结构的斯格明子材料是低能耗赛道存储器件理想的物理载体。要应用斯格明子作信息存储，我们需要了解其磁结构拓扑稳定的物理机制和其磁结构的调控机制。..本研究项目开发了纳米样品的微纳加工工艺和样品制备技术，制备出了高质量的纳米磁性材料试样。通过微纳悬臂梁力矩磁化分析技术，实验测量了纳米线、纳米盘和纳米薄膜斯格明子材料的磁化过程。主要结果有：（a）对一维锰硅纳米线的研究表明来源于纳米线形状的单轴各向异性可以拓展纳米线中形成稳定斯格明子的磁场和温度区间。我们发现在垂直磁场下锰硅纳米线中斯格明子相与块材中的斯格明子相一样只存在于居里温度附近一个非常小的磁场-温度区域内。在平行磁场中斯格明子相在锰硅纳米线稳定存在其存在温区从 T = 29 K 降到至少 T = 0.4 K。(b) 通过对铁钴氧纳米金字塔样品单体磁化过程的测量和研究，证实其在室温下即具有单磁畴结构，有望作为比特材料应用在磁存储方面。（c）微透镜光纤干涉仪纳米线振动模式光学测量的理论和实验研究。实现了纳米线在其垂直方向上矢量弯曲振动的光学测量。自下而上生长的纳米线具有比自上而下加工的微纳悬臂梁更好的力学性质，还可以实现垂直与纳米线方向的矢量力矩磁化分析。这些研究可以帮助我们进一步提高纳米磁性样品的力学测量的灵敏度和能力。（d）在实验研究开展过程中，我们发现力矩磁化分析技术的灵敏度（可测单个纳米样品）具有竞争力，其测量能力（对样品形状没有任何要求）也很有特色。因此我们申请了三个相关专利：一种使用微透镜的光纤干涉仪，一种使用保偏光纤和双微透镜测量纳米线位移的装置和一种激光干涉法信号提取系统。这些专利将为我们未来精密磁性测量仪器的商业开发打下基础。