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We developed a split-illumination electron holography that uses an electron biprism in the illuminating system and two biprisms (applicable to one biprism) in the imaging system, enabling holographic interference micrographs of regions far from the sample edge to be obtained. Using a condenser biprism, we split an electron wave into two coherent electron waves: one wave is to illuminate an observation area far from the sample edge in the sample plane and the other wave to pass through a vacuum space outside the sample. The split-illumination holography has the potential to greatly expand the breadth of applications of electron holography.

Extended abstract of a paper presented at Microscopy and Microanalysis 2011 in Nashville, Tennessee, USA, August 7–August 11, 2011.

Extended abstract of a paper presented at Microscopy and Microanalysis 2004 in Savannah, Georgia, USA, August 1–5, 2004.

We developed a 1-MV field-emission transmission electron microscope to help in further improving electron holography, Lorentz microscopy, and high-resolution electron microscopy. This microscope is characterized by an electron beam having the highest brightness ever, 2×1010 A/cm2, and by the highest lattice-resolution below 0.5 Å. These two features were attained by minimizing the mechanical vibration of the whole column and by improving the stability of both the electron beam and the high voltage. If the tiny electron source located at the top of the 7-m-high microscope moves by as little as a fraction of the source size, 50 Å in diameter relative to the column, due to mechanical vibration or beam deflection by the AC magnetic fields, the beam brightness will be greatly degraded. If the ripples ΔE of the high-voltage E exceed ΔE/E = 5 × 10−7 /min, then the inherent monochromatic feature of the beam is deteriorated by the increase in energy spread.Through the preliminary experiments testing the vibration and magnetic shielding of the acceleration tube as well as the high stability of the high voltage, and through the numerical simulations on the vibration modes of the whole column, we were led to the conclusion that the microscope must be separated into three parts that are connected by cables.

Recently out-of-focus, low angle electron diffraction and Foucault experiments have been carried out on superconducting specimens in a range of applied magnetic fields where the fluxons form a more or less regular lattice. Let us recall that in order to describe the effect on the electron beam in a transmission experiment of a fluxon in a tilted specimen, the fluxon itself has been approximated by a suitable bundle of straight flux tubes, relying on the important result that the phase shift of the flux tube can be calculated analytically even in the tilted specimen geometry. In this way, it is only necessary to convolute this phase shift with the chosen projected magnetic field distribution (in our case a London model with a phenomenological penetration depth of 50 nm) in order to obtain a fairly realistic model. Therefore, in order to interpret the main features of the experimental results

We have developed 100-kV, 200-kV, and 350-kV cold-field-emission transmission electron microscopes (FE-TEMs) successively up to this time. Using these instruments, we have been studying the magnetic structure of materials, high-resolution imaging by electron holography, and dynamic observation of the vortex in superconductors by Lorentz microscopy. To make more progress in our research, we need a better electron beam in terms of coherency, beam brightness, and penetration. Here, we report a new lMV-cold-field-emission transmission electron microscope we have developed. Historically, the pioneering projects on a lMV-field-emission scanning transmission electron microscope (FE-STEM) (Zeitler and Crewe, 1974) and a 1.6MV FE-STEM (Jouffrey et al., 1984) have been reported. In 1988, Maruse and Shimoyama obtained a lMV-field-emission beam using their 1.25MV-STEM connected to a field-emission gun. Since then, continuous improvements in beam brightness has been made.The target specifications of our 1 MV-cold-field-emission TEM (H-1000FT) are as follows: Acceleration voltage: 1MV, high-voltage stability

The development of increasingly coherent and penetrating electron beams in electron microscopy will hasten progress in such fields as high resolution imaging, electron holography, and material structure investigation. Now that we have completed the development of the 1MV-FE- TEM (H-1000FT), we report its performance from the viewpoint of coherent illumination.We observed thin films of gold in testing the high-resolution performance of H-1000FT.Crystal lattice fringes are formed from the interference between Bragg diffracted electron waves. In TEM observation of the films, the visibility of the fringes depends on the coherence of the electron wave and the overall stability of the microscope. Here, we calculated chromatic aberration-limited resolution d, given by ∼-(Δ ƛ)1/2, to be ∼ 0.6 Å (Δ:focus spread). We then slightly tilted the illumination to partially escape from the chromatic effect and to enable us to a shorter spacing lattice image. Figure 1 shows a lattice fringe image we obtained for a Au thin film, in which lattice fringes of 0.498 Å are clearly visible.

Bright and coherent electron beams have been opening new frontiers in science and technology. So far, we have developed several field-emission transmission electron microscopes (FE-TEM) with increasing accelerating voltages to provide higher beam brightness. By using a 200-kV FE-TEM and electron holography techniques, we directly confirmed the Aharonov-Bohm effect. A 350-kV FE-TEM equipped with a low-temperature specimen stage enabled us to observe moving vortices in superconductors.2 Most Recently, we have developed a new 1-MV FE-TEM with a newly designed FE gun to obtain an even brighter and more coherent electron beam.Electron beam brightness, Br, defined in Figure 1, is suitable for measuring the performance of electron guns, since both lens aberrations and mechanical/electrical vibrations contribute to a decrease in beam brightness, and beam coherency is proportional to (Br)1/2. Therefore, we optimized design of the illuminating system and its operation by maximizing the electron beam brightness.

Magnetic vortices in superconductors have become directly and dynamically observable[l] by Lorentz microscopy in our 300 kV field-emission TEM[2]. Since material defects can also be observed simultaneously though the images are defocused, the microscopic flux pinning mechanism can be directly observed. Using this technique, we observed when and how vortices trapped at artificial defects were depinned.Artificial point defects were produced by the irradiation of a focused Ga ion beam. Electron microscopy revealed that the defect consisted of a pit 300 Å in radius surrounded by entangled dislocations. The depth of a pit increased with the ion dose.We first investigated the pinning force of individual defect: by observing the depinning of vortices from defects produced with various ion-doses. When a force was exerted on vortices trapped at the defects by changing the magnetic field and increased, we found that the pinning force of defects increased with the ion doses.

The out-of-focus method has been successfully employed in the dynamical observation of superconducting fluxons. However, owing to the large defocus distance needed to image fluxons with enough contrast it turns out that their correlation with structural defects, better imaged at focus, is troublesome. Among the standard methods of Lorentz microscopy the Foucault technique is a good candidate for removing this drawback, since it generates phase contrast in the focused image by masking part of the transmitted beam by means of an aperture.Therefore, simulations have been carried out both for one-dimensional fluxon models and more realistic two dimensional ones, with the result that enough contrast can be generated in the focused image in order to detect them. However, although these theoretical results suggest the feasibility of Foucault experiments, filling the gap between theory and practice is not an easy task, especially when considering the small angular deflections involved, of the order of 10−5 rad.

The successful observation of superconducting flux lines (fluxons) in this specimens both in conventional and high T{sub c} superconducting flux lines (fluxons) in thin specimens both in conventional and high T{sub c} superconductors by means of Lorentz, and electron holography methods has presented several problems concerning the interpretation of the experimental results. The first approach has been to model the fluxon as a bundle of flux tubes perpendicular to the specimen surface (for which the electron optical phase shift has been found in analytical form) with a magnetic flux distribution given by the London model, which corresponds to a flux line having an infinitely small normal core. In addition to being described by an analytical expression, this model has the advantage that a single parameter, the London penetration depth, completely characterizes the superconducting fluxon. The obtained results have shown that the most relevant features of the experimental data are well interpreted by this model. However, Clem has proposed another more realistic model for the fluxon core that removes the unphysical limitation of the infinitely small normal core and has the advantage of being described by an analytical expression depending on two parameters (the coherence length and the London depth). Therefore,more » simulations with this improved and physically more appealing model have been carried out, with the conclusion that both in the Lorentz and holographic cases the predictions of the two models are very similar and the discrimination on the basis of the experimental data can be done reliably only using the holographic high resolution mode.« less

Tiny magnetic vortices hold the key in practical applications of superconductors, since the dynamic behaviors of vortices at pinning centers determine the critical current. These vortices have become observable directly and dynamically by using a “coherent” field-emission electron beam: The magnetic lines of force of vortices can be quantitatively observed as contour fringes in an electron-holographic interference micrograph. The change in the vortex radius could actually be measured when the temperature of a superconducting thin film changed. Vortices were also dynamically observed in a Lorentz micrograph. Both vortices and defects in the thin film can be observed simultaneously with this method, though the defects image is blurred.This paper reports the dynamic observation of vortices at defects when the applied magnetic field changes. A superconducting Nb thin film, set on a low-temperature stage, is tilted 45° to an incident beam of 300kV electrons (see Fig. 1), and an external magnetic field is applied in the horizontal direction. When an electron beam passes through the vortices, their magnetic fields deflect the electron beam. Therefore, when observed in the lower plane, a vortex can be viewed as a pair of black and white spots.

Individual flux lines in superconducting specimens have been recently observed using a 300kV holography electron microscope equipped with a cold field emission gun in the transmission mode by means of the out-of-focus method. In this way it has been possible to observe the dynamic behaviour of fluxons depending upon the variations of the temperature and/or the applied magnetic field or current. The main disadvantage of this method is that correlation with structural defects is troublesome owing to the large defocus distance (of the order of 1 cm for conventional superconductors) necessary for the detection of flux lines.This drawback is not present in the Foucault technique, where the contrast is generated in the focused image by masking part of the transmitted beam by means of an aperture. Therefore, some preliminary calculations have been made recently using one-dimensional models, which allow for a quick and easy simulation of the factors affecting the experimental conditions .