Coherent Lorentz Imaging of Soft, Thin-Film Magnetic Materials
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where e is the electronic charge, v is the velocity of the electrons, and B is the magnetic induction. In the transmission electron microscope, the specimen is in the form of a thin film with the thickness restricted to approximately 100 nm when imaging with electrons of energy lower than or equal to 200 keV. In this article, we deal only with fixed-beam TEM and give no details of techniques in scanning TEM. The most commonly used magnetic imaging modes in TEM,
MRS BULLETIN/OCTOBER 1995
1 2 Fresnel and Foucault, - in principle can be implemented on any standard electron microscope. Following brief descriptions of these techniques, we then outline the recently developed quantitative TEM technique known as coherent Foucault (CF) imaging where the instrumental requirements are more demanding for CF experiments. Examples are given of the type of images obtained in soft magnetic materials along with a discussion of requirements needed to implement this mode. Finally, the ability to magnetize samples in situ is demonstrated and the results are discussed.
Lorentz Imaging Using TEM Figure la shows a parallel beam of electrons incident on a thin-film specimen in which magnetic domains are present. The domains have their magnetization in the plane of the film and are separated by narrow domain walls. The deflection of the electrons JSL by the Lorentz force as they pass through the thin film is given by the relation (2) where h is Planck's constant, A is the electron wavelength, and B±t represents the total induction that causes the deflection (i.e., the component perpendicular to the path integrated along the electron trajectory). For a typical thin film possessing the magnetic structure shown, the integrated induction other than at a domain wall becomes Bot where Bo is the saturation induction of the film and t is the film thickness. In the case of a 50-nm-thick film with saturation induction of 1.0 T in a 200 kV TEM for which
A = 2.5 pm, the deflection is —30 μ-rad. By contrast, the deflection for multilayer magneto-optical samples can be s i μ-rad. Schematically, the Fresnel and Foucault modes as practiced in a TEM microscope are shown in Figure la. Domain walls are revealed in the Fresnel mode by defocusing the image-forming lens by an amount Az. Depending on the orientation of the magnetization within the domains, the electrons deflected from adjacent domains will either converge or diverge in this object plane in the region of the domain walls. Therefore, domain walls appear as either light or dark narrow bands on a uniform background. An example of a Fresnel image obtained from a 60-nm-thick, small magnetic element, described later, is shown in Figure 2a, along with its schematic domain structure in Figure 2b. The inferred domain structure here shows that the magnetization stays parallel to the edges of the element with the 3 domains separated by 90° domain walls. (The angle indicates the angle of rotation of the magnetization across the domain wall.) In the case of the Foucault mode, the specimen plane and t
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