TEM Studies on magnetic microstructures of Permanent Magnets

초록

Observing the magnetic microstructures on a nanometer scale is very important for understanding material property and its applications. Among the so-called “magnetic imaging” methods, imaging techniques in transmission electron microscope (TEM) are Lorentz microscopy and electron holography, which can be implemented in a standard TEM. These techniques have been widely utilized to image magnetic microstructures and the magnetization process in situ. In the Fresnel (out-of-focus) method of Lorentz microscopy, domain walls (DWs) are imaged, and these place a lower limit of tens of nanometers on the requirement for spatial resolution. Electron holography, using the wave nature of electrons, provides opportunities for directly detecting and visualizing the phase shifts φ of electron waves due to electromagnetic fields. In this talk, I introduce recent studies on permanent magnets such as sintered Nd2Fe14B magnets using Lorentz microscopy and electron holography. Among the permanent magnets currently available, sintered Nd2Fe14B magnets show the best magnetic properties (Ku ~ 4.5 MJ m-3, Hc ~ 1.1 MA/m, ?0Ms ~ 1.6 T); that is, they show the largest value of maximum-energy product [(BH)max > 400 kJm-3]. In order to further improve both coercivity (Hc) and maximum-energy product (BH)max, the magnetization process and the magnetism at the ultrathin intergranular boundary regions will be discussed