Hollow-cone Foucault imaging method

A hollow-cone Foucault (HCF) imaging method using Lorentz microscopy was developed. Hollow-cone illumination was realized by using deflectors above the specimen and an inclined electron beam circulating with respect to the optical axis. The advantage of the HCF method, having the bright and dark-field modes, is that it can simultaneously visualize both magnetic domains and magnetic domain walls under the in-focus condition. Furthermore, schlieren images, obtained under the specific inclination angle of the illumination beam by adjusting the angle between the bright-field and dark-field modes, can qualitatively visualize the electromagnetic fields in spaces around the specimen.

I n conventional transmission electron microscopy (TEM), observation of biological materials and organic materials with high contrast is difficult because these materials are composed of light elements, resulting in weak scattering for electron beams. Electromagnetic structures in metals or semiconductors, for example, magnetization distributions and dielectric polarization distributions, are also difficult to observe for the same reason. To observe these structures, visualization of phase distributions by electron beams or waves is effective and practical. For this purpose the following observation methods have been developed: electron holography, [1][2][3][4] Lorentz microscopy, [5][6][7][8] phase microscopy, [9][10][11] focus-modulation microscopy, [12][13][14] diffractive microscopy, [15][16][17] and ptychography. [18][19][20] These methods, however, require additional equipment and/or techniques to conventional TEM and are, therefore, not suitable for wide and practical applications. Among these methods, Lorentz microscopy is the most effective and widely utilized. There are two methods in Lorentz microscopy: the Fresnel imaging method, in which focus conditions of the specimen images are varied to visualize domain walls, and the Foucault imaging method, in which some of the deflected electron beams are selected for imaging under the in-focus condition to realize high-contrast magnetic or dielectric domain observations. In the Fresnel method, high spatial resolution cannot be obtained because of defocused images. On the other hand, in the Foucault method asymmetric images can be formed because an angle-limiting aperture-an objective aperture in ordinary optics-is placed at the asymmetric position with respect to the optical axis, resulting in azimuthally dependent image contrasts. Furthermore, in Lorentz microscopy electromagnetic fields are not observed because no contrasts are obtained.
To overcome these difficulties in obtaining information of weak scattering objects, the hollow-cone Foucault (HCF) imaging method was developed, where an incident electron beam on the specimen was tilted with respect to the optical axis with an inclination angle and was circulated in all azimuths around the optical axis. 21,22) Both magnetic domains and domain walls were simultaneously visualized with sufficient contrasts under the in-focus condition. Furthermore, the schlieren mode allows observation of electromagnetic fields around specimens by appropriately adjusting the inclination angle at the position between the bright-field and dark-field modes. 23) Figure 1 shows a schematic diagram of the optical system for HCF imaging. The objective lens is switched off to obtain a magnetic field-free space around the specimen and the objective mini-lens is used to focus a crossover just on the selected area (SA) aperture, whose optical system is the same as that of a small-angle electron diffraction 24,25) and that of the Foucault imaging method. 26,27) The SA aperture works as an angle-limiting tool. The parallel electron beams having less than 10 −6 rad diffusion angle are irradiated on the specimen having a wide area of about 85 μm in diameter with the inclination angles in the X and Y directions controlled by using the beam deflector system placed above the specimen. The circulating electron beam is illuminated in all azimuthal directions around the optical axis. We note that the special condition of the magnetic field-free space above the specimen due to switching off the objective lens leads to realization of small-angle-tilted hollow-cone beams with an inclination angle as small as 10 −4 rad. Figure 2(a) shows composite trajectories of the zerothorder diffraction spots in concentric circles on the SA aperture plane for five inclination angles. Each trajectory ring was recorded by the hollow-cone beam with about eight turns in 10 s (corresponding to 0.8 Hz in the azimuthal rotation). The diameter of the trajectory rings corresponds to the inclination angle of the hollow-cone beam. Figure 2(b) shows the relation between the input amplitude value and the inclination angle, indicating that a linear and precise angle control was realized in the hollow-cone inclination. The experiment was performed using a 200 kV thermal field emission TEM (JEM-2100F, JEOL Ltd) having the optionally attached hollow-cone illumination system. The HCF images were recorded with a 14-bit charge-coupled device camera (2k × 2k pixels, Ultrascan camera, Gatan Inc.). Each image was recorded through about 12 turns in the illumination azimuthal rotation in 7.5 s, corresponding to 1.6 Hz in the azimuth rotation.
To test the availability and validity of the developed HCF imaging method, we applied it to observe magnetic domains and domain walls in an Fe 0.88 Ga 0.12 alloy (in at%), 28) which has large magnetostriction at room temperature. The thin film of 250 nm thickness with [001] orientation for TEM observation was prepared using a focused ion beam instrument (NB-5000, Hitachi High-Technologies Corp.).
To show the good performance of the HCF imaging method, magnetization structures were observed for comparison using conventional Lorentz microscopy, Fresnel and Foucault methods, under the same experimental conditions as those used in the HCF method.   Fig. 4 are all the observation results that conventional Lorentz microscopy can produce. Figure 5 shows the HCF images for different inclination angle conditions. The SA aperture with opening size 100 μm in diameter corresponding to 1.30 × 10 −3 rad was utilized as the angle-limiting tool. Figure 5(a) shows a hollow-cone image for 0 rad inclination angle; this image corresponds to the bright-field micrograph in Fig. 3(b). Figures 5(b) and 5(c) are bright-field  A special condition for schlieren imaging corresponds to the specific inclination angle of the illumination beam changed from the bright-field mode to dark-field mode. The schlieren imaging method 23) is known as a high-speed imaging method applicable to low refractive index media, such as air, and is also known as old-fashioned phase-contrast optical microscopy. It is one of the asymmetric imaging methods using only a half of the diffraction pattern controlled by an angle-limiting aperture. Figure 5(d) corresponds to the image by the schlieren optical system. Figure 6 shows its wider view results: Fig. 6(a) shows imaging of the inside of specimen and Fig. 6(b) shows imaging in the outside space around the specimen. The brightness fluctuation around the specimen qualitatively shows that magnetic field leaked out and spread out from the specimen over a wide range and to a far distance. This is the reason that in electron holography reference waves must be placed far away from specimens for electromagnetic observation. 29,30) In conclusion, we developed a HCF imaging method with which simultaneously observation of magnetic domains and magnetic domain walls was realized under the in-focus condition. By adjusting inclination angles, observations in the bright-field mode and in the dark-field mode became possible. When the mode changed from the bright-field mode to the dark-field mode, we obtained schlieren images with which electromagnetic fields in the space around the specimen were qualitatively visualized.
The developed HCF imaging method has the advantages of both the Fresnel and Foucault imaging methods, therefore the HCF method may be called the third Lorentz microscopy. We hope the HCF method will be widely used for analyzing electromagnetic properties in materials in the future.