Imaging Magnetic Structures Using TEM

4. Summary

We discussed the observation of micro- and nano-magnetic structures by TEM, focusing on Lorentz microscopy and electron holography. In general, Lorentz microscopy is advantageous for real time observation and disadvantageous for quantitative analysis comparing with electron holography, and vice versa. However, a number of techniques have been developed to overcome these disadvantages, which include Foucault mode, Lorentz phase microscopy, real time electron holography and multi wave interferometry. A formula for Lorentz phase microscopy, which coincides with that of TIE, was derived newly using a weak phase approximation. High-precise electron holography, phase-shifting holography and differential holography were shown with mathematical treatments and applications. The phase-shifting holography reconstructed the image of multilayers with better quality than the Fourier method. Two-dimensional differentiation of the electron phase is effective to reveal magnetic induction directly. Differential holography using an electron trapezoidal prism is also useful to reveal structures especially in the case where a well-defined reference wave is difficult to obtain. Since TEM method such as Lorentz microscopy and electron holography can observe inside the specimen, they are favorable for specimens whose inside structure might be different from the surface like magnetic structures. Improving techniques to image nano-magnetic structures should encourage the development of new magnetic materials and devices.