Analysis of magnetization in nanocomposite Nd4.5(Fe,Cr)77B18.5 by electron holography and simulation

Magnetization distribution in nanocomposite Nd4.5(Fe,Cr)77B18.5 was studied by electron holography and computer simulation. In order to understand the detailed magnetization distribution, the magnetic flux distribution was calculated taking into account the magnetic charge or the stray field on the basis of magnetization models consisting of small magnetic dipoles and was compared with that in reconstructed phase images experimentally observed. Through the comparison, the characteristic feature in the distribution of the magnetization distribution in the nanocomposite magnetic materials was clarified, and the distribution was found to well correspond to their magnetic properties. It is pointed out that for understanding magnetization, the interpretation of reconstructed phase images should be done through computer simulation just as the analysis of high-resolution electron microscope images. Eventually, it was demonstrated that electron holography with computer simulation is quite useful to analyze detailed magnetization distribution in nanocrystalline magnetic materials at the nanometer scale.     

计算全息法产生涡旋光束的实验

基于计算全息法,通过计算和模拟得到了涡旋光束与平面光束干涉所产生的位错条纹,用胶片记录位错条纹并制作成位错光栅,在自行设计搭建的光路中进行实验获得1~4 阶的涡旋光束。实验结果表明,各阶涡旋光束的空心半径随着阶数的增大而逐渐增大,与理论分析相符合;实验进一步观察到各阶光栅的二级甚至三级衍射光束,当入射光束中心与光栅位错中心重合时,能够产生光强分布对称的涡旋光束,当光束中心和光栅中心不重合时,产生的涡旋光束的光强分布不对称。这为后续以涡旋光束为捕获光束的光镊实验的进一步拓展及应用提供了理论和技术支持。     

Electron holography on remanent magnetization distribution of melt-spun Nd-Fe-B magnets

Microstructures and magnetic domain structures of melt-spun Nd-Fe-B permanent magnets were investigated in detail by analytical electron microscopy and electron holography. While the crystal orientation of matrix Nd2Fe14B grains was analyzed by nanobeam electron diffraction, precipitates of a few tens of nanometers at grain boundaries were identified to be alpha-Fe by energy-dispersive X-ray spectroscopy. The detailed magnetization distribution in Nd2Fe14B grains and at their boundaries was visualized by electron holography. Ex situ experimentation with an electromagnet revealed that the domain walls in the demagnetized state and remanent states were pinned at grain boundaries, and Fe precipitates at the grain boundary were situated at the center of the closure domain.     

In situ observations of domain structures and magnetic flux distributions in Mn-Zn and Ni-Zn ferrites by Lorentz microscopy and electron holography

Domain structures and magnetic flux distributions in Mn-Zn and Ni-Zn ferrites are investigated by in situ observations with Lorentz microscopy and electron holography. In situ Lorentz microscopic observation with the magnetic field applied reveals that the domain walls in Mn-Zn ferrite move easily across the grain boundary. On the other hand, each grain of Ni-Zn ferrite is magnetized by domain wall motion inside the grain. By taking a series of holograms with adjustment of the optical axis and astigmatism while the magnetic field is applied, we succeeded in observing the change in magnetic flux distribution quantitatively. Eventually, it is clarified that magnetization rotation does not take place in the magnetization process of Ni-Zn ferrite. The domain wall widths delta in Mn-Zn and Ni-Zn ferrites are evaluated to be 73 and 58 nm, respectively. Furthermore, through direct observation of the domain structure in Ni-Cu-Zn ferrite with Lorentz microscopy, it is found that the grains with size below 1.5 microm diameter are single domain.     

Analytical electron microscopy and electron holography on microstructures and magnetic domain structures of Sm-Co 2:17 magnets

Microstructures and magnetic domain structures of precipitation-hardened Sm-Co permanent magnets were systematically investigated by analytical electron microscopy and electron holography. By an elemental mapping method with energy-dispersive X-ray spectroscopy, the change in the local distribution of additive elements, i.e. Cu, Fe and Zr, in Sm-Co magnets with various heat treatments was visualized and the enrichment of Zr in the Z-phase with a width of approximately 1 nm was clarified directly. Detailed analysis with electron holography revealed that considerable fluctuation in the distribution of lines of magnetic flux in the step-aged magnet was due to the chemical partitioning of additives and resulted in magnetic hardening during the magnetization process.     

Computer-aided design of an axially symmetrical magnetic circuit and its application to electron-beam-focusing devices

This paper presents a method of computer simulation useful for analysis of an axially symmetrical magnetic circuit with exciting coils, magnetic poles, and permanent magnets. The vector magnetic potential is obtained by the use of the well-known successive over-relaxation method, so that devices having magnetic hysteresis characteristics can be analyzed easily and accurately. In this program, the magnetization intensity distribution within the magnet is taken into account. The new computer programming system is applied to a magnetic shielding hollow sphere, a bar magnet, a ring magnet, and a periodic permanent-magnet structure for traveling-wave tubes, and the flux distribution for these magnetic circuits is shown. The results are seen to be in good agreement with theoretical and experimental values.     

计算全息法获取高阶类贝塞尔光束的新设计

基于计算全息法（CGH）以及夫朗禾费衍射近似条件下的衍射理论,设计了获取各阶类贝塞尔空心光束的光栅,通过对光束在径向及其传播方向上的光强分布进行模拟分析,得到光场随着类贝塞尔光束的阶数、传播距离以及径向r的分布规律,得到的空心光束的暗斑尺寸、环状光束宽度和光束半径等表征空心光束特点的参数能更好满足光镊一光钳操作系统要求...     

Transmission electron microscopy on thin film of high density magnetic composite prepared by FIB method

A thin specimen of a high density magnetic composite (HDMC), which is a type of powder magnetic cores was prepared for transmission electron microscopy (TEM) using a focused ion beam (FIB) method. A homogeneous thin film containing an insulator boundary between the constituent Fe powders was obtained successfully. Using this thin film, detailed flow of magnetic flux was visualized by electron holography, and the magnetic flux density was estimated to be 1.73 +/- 0.09 T being consistent with that of a bulk HDMC (1.70 T). Moreover, through Lorentz microscopy, the characteristic magnetization process of HDMC was observed by applying the magnetic field up to approximately 8 kA/m.     

Cluster and Thickness Dependence of Ferromagnetism in Nickel In Situ-Doped Amorphous AlN Thin Films

We report on magneto-optical investigations of Ni-doped amorphous AlN (a-AlN) thin films. The a-AlN was grown by radiofrequency (rf) sputtering on Si (0001) substrates at low temperature and doped with Ni at fixed concentrations with different a-AlN layer thicknesses. As-grown a-AlN:Ni layers were annealed up to 900°C for 5 min and 15 min time duration in nitrogen ambient at atmospheric pressure. Each sample was characterized by the magneto-optical Kerr effect (MOKE) in both polar and longitudinal geometries. Only the 65-nm-thick a-AlN:Ni layers showed linear enhancement of magnetization after thermal treatment up to 900°C, indicating the presence of a critical a-AlN:Ni layer thickness supporting the formation of magnetic domains. No measurable MOKE signal was observed in the longitudinal geometry for any tested samples with different thicknesses. This observation confirms that the easy magnetization axis in a-AlN:Ni layers is out of plane due to the strong magnetic anisotropy observed in polar MOKE geometry. The morphology of as-grown and annealed a-AlN:Ni films was characterized by atomic force microscopy (AFM), magnetic force microscopy (MFM), and scanning electron microscopy (SEM) and revealed the existence of nanoclusters. The size distribution of nanoclusters was studied as a function of annealing time and temperature, and the results correlate well with those obtained from the MOKE measurements.     

Magnetic Nanoparticle Chains in Gelatin Ferrogels: Bioinspiration from Magnetotactic Bacteria

Inspired by chains of ferrimagnetic nanocrystals (NCs) in magnetotactic bacteria (MTB), the synthesis and detailed characterization of ferrimagnetic magnetite NC chain‐like assemblies is reported. An easy green synthesis route in a thermoreversible gelatin hydrogel matrix is used. The structure of these magnetite chains prepared with and without gelatin is characterized by means of transmission electron microscopy, including electron tomography (ET). These structures indeed bear resemblance to the magnetite assemblies found in MTB, known for their mechanical flexibility and outstanding magnetic properties and known to crystallographically align their magnetite NCs along the strongest <111> magnetization easy axis. Using electron holography (EH) and angular dependent magnetic measurements, the magnetic interaction between the NCs and the generation of a magnetically anisotropic material can be shown. The electro‐ and magnetostatic modeling demonstrates that in order to precisely determine the magnetization (by means of EH) inside chain‐like NCs assemblies, their exact shape, arrangement and stray‐fields have to be considered (ideally obtained using ET).