基于电子全息技术的纳米磁性材料微观磁结构和性能的研究

发布时间：2018-01-21 00:49

本文关键词： 纳米颗粒洛伦兹透射电子显微镜离轴电子全息磁结构磁性能原位磁化　出处：《四川师范大学》2015年硕士论文　论文类型：学位论文

【摘要】：磁性纳米颗粒具有广泛的应用前景,如磁流体、化学反应催化剂、生物医学、磁数据存储等方面。对于纳米材料的磁性能表征我们一般侧重于对集合体的表征,如磁滞回线等,测量的对象是一个集合体,测量结果是平均效应,颗粒的形状、尺寸、大小分布、结晶构造、成分等都会引起颗粒磁性能的变化,很难通过集合体磁性能测量确定实验现象背后的物理本质。洛伦兹透射电子显微镜以及电子全息技术因其超高的空间分辨率能为纳米颗粒的磁性能评价能提供更多信息,如颗粒的单多磁畴状态、颗粒之间的静磁相互作用。如果能够直接观测到颗粒的微观磁畴结构,并讨论颗粒间交换耦合、静磁相互作用,既对纳米颗粒的磁学基础研究,又对磁性纳米颗粒的应用将有非常重要的意义。本论文中利用洛伦兹透射电子显微镜以及电子全息技术完成对磁性纳米颗粒的微观磁结构表征调试工作,在纳米尺度内成功定量地得到了磁性纳米颗粒、纳米线内部磁结构；然后利用此技术对磁性纳米颗粒、纳米线样品进行观测。在对约15纳米直径钴纳米线进行了微观磁结构的表征证明了其强的铁磁性。通过使用离轴电子全息技术,准确的表示出了钴铜多层纳米线中可能存在的磁状态,平行或反平行,表明此技术对于磁性纳米线磁化状态、磁化反转机制的研究具有确实的可行性。通过利用电子全息技术对钴镍合金微球进行表征,解释了表面结构对微波吸收性能的影响：刺和花瓣作为偶极子在电磁场的作用下会产生杂散磁场,所有偶极子共同与入射电磁波作用,电磁波能量被转换为其它形式的能量提高微波吸收能力。用电子全息直观的表征出了表面结构之间的偶极作用,说明电子全息技术可以用来表征吸波材料的微波吸收性能。本文还通过对不同形状Fe304磁性纳米颗粒微观磁结构分析,说明磁性纳米颗粒的微观磁结构状态由颗粒的形貌、晶体结构、颗粒相互作用共同决定,此状态的总自由能最小。通过对Fe304纳米盘进行了磁结构分析,解释了Fe304纳米盘的比吸收率高于各向同性纳米颗粒的原因：由纳米盘独特的形状各向异性使纳米盘随着外场方向改变时发生平行外场的旋转。本论文最后对磁性纳米纤维原位磁化的电子全息实验做了尝试,表明洛伦兹透射电镜在磁学材料表征的应用前景：利用电子全息实现磁场作用下的原位观察研究纳米磁性材料磁化机制。
[Abstract]:Magnetic nanoparticles have a wide range of application prospects, such as magnetic fluid, chemical reaction catalyst, biomedical, magnetic data storage, etc. For the characterization of magnetic properties of nanomaterials, we generally focus on the characterization of aggregates. For example, hysteresis loop, the object of measurement is an aggregate, the result of measurement is average effect, particle shape, size, size distribution, crystal structure, composition and so on will cause the change of particle magnetic properties. It is difficult to determine the physical nature of the experimental phenomena by measuring the magnetic properties of aggregates. Lorentz transmission electron microscope and electron holography can provide a better way to evaluate the magnetic properties of nanocrystalline particles because of their super-high spatial resolution. More information. For example, the single multi-domain state of particles, the magnetostatic interaction between particles. If the microscopic magnetic domain structure of particles can be observed directly, the exchange coupling and magnetostatic interaction between particles can be discussed. That is, the basic magnetic properties of nanoparticles. In this thesis, Lorentz transmission electron microscope and electron holography are used to test the microstructure of magnetic nanoparticles. The magnetic nanoparticles and the magnetic structure of nanowires were obtained quantitatively in nanoscale. Then the magnetic nanoparticles were prepared by this technique. About 15 nanometer-diameter cobalt nanowires were characterized by micromagnetic structure to prove their strong ferromagnetism. Off-axis electron holography was used. The magnetic state, parallel or anti-parallel, which may exist in cobalt-copper multilayer nanowires is accurately represented, which indicates that this technique is suitable for magnetization of magnetic nanowires. The study of magnetization inversion mechanism is feasible. The cobalt and nickel alloy microspheres are characterized by electron holography. The effect of surface structure on microwave absorption is explained: spines and petals acting as dipoles will produce stray magnetic fields under the action of electromagnetic field, and all dipoles interact with incident electromagnetic waves. The electromagnetic wave energy is converted into other forms of energy to improve the microwave absorption capacity. The dipole interaction between the surface structures is characterized directly by the electron holography. The results show that electron holography can be used to characterize the microwave absorption properties of microwave absorbing materials. The microstructure of Fe304 magnetic nanoparticles with different shapes is also analyzed in this paper. The results show that the microstructure of magnetic nanoparticles is determined by the morphology, crystal structure and interaction of particles, and the total free energy of the state is the least. The magnetic structure of Fe304 nanodisk is analyzed. The specific absorptivity of Fe304 nanoparticles is higher than that of isotropic nanoparticles. Due to the unique anisotropy of the nano-disk, the nano-disk rotates parallel to the external field when it changes with the direction of the external field. In the end of this thesis, the experiment of in-situ magnetization of magnetic nanofibers is attempted. The results show that Lorentz transmission electron microscope can be applied to the characterization of magnetic materials. The magnetization mechanism of nanomagnetic materials is studied by in situ observation under the action of magnetic field by electron holography.
【学位授予单位】：四川师范大学
【学位级别】：硕士
【学位授予年份】：2015
【分类号】：TB383.1

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