Nobuyuki Osakabe

Observation of Aharonov-Bohm Effect by Electron Holography

In this experiment, an electronand optical-holographic technique is employed with small toroidal ferromagnets each forming a magnetic-Qux closure. The holographic interferometry proves that a phase difference between two electron beams having passed through the field-free regions agrees well with the fundamental relation known as the Aharonov-Bohm effect. It is also confirmed from the same hologram that Qux leakage from the toroids does not affect the conclusion.

Fine crystal lattice fringes observed using a transmission electron microscope with 1 MeV coherent electron waves

A transmission electron microscope with a 1 MeV cold field-emission electron source has been developed for coherent and penetrating electron waves. We confirmed the coherence and overall stability of the microscope by observing Au(337) lattice fringes. These fringes have a 0.498 A spacing.

Observation of recorded magnetization pattern by electron holography

Electron holography was employed for experiments involving a high‐density magnetic recording, in which it was possible to directly observe streams of magnetic flux. The magnetic flux distribution in recorded films, and the maximum packing density in high‐coercivity evaporated cobalt film were investigated. With magnetic longitudinal recording the resultant highest density was 170 000 bits per inch. This experiment has proven that electron holography is useful for the study of magnetic recording.

Aberration corrected 1.2-MV cold field-emission transmission electron microscope with a sub-50-pm resolution

Atomic-resolution electromagnetic field observation is critical to the development of advanced materials and to the unveiling of their fundamental physics. For this purpose, a spherical-aberration corrected 1.2-MV cold field-emission transmission electron microscope has been developed. The microscope has the following superior properties: stabilized accelerating voltage, minimized electrical and mechanical fluctuation, and coherent electron emission. These properties have enabled to obtain 43-pm information transfer. On the bases of these performances, a 43-pm resolution has been obtained by correcting lens aberrations up to the third order. Observations of GaN [411] thin crystal showed a projected atomic locations with a separation of 44 pm.

Observation of microscopic distribution of magnetic fields by electron holography

A newly devised method for measuring magnetic fields localized in a submicron region is described. In this method, magnetic lines of force are observed as the contour lines for the transmitted electron phase with two neighboring lines containing the constant magnetic flux of h/e. Actual microscopic distributions of magnetic fields which were inaccessible by any other method, such as stray fields from a fine ferromagnetic particle, are measured.

Time-resolved observation of thermal oscillations by transmission electron microscopy

Time-dependent acoustic oscillations driven by stochastic thermal force have been observed by means of transmission electron microscopy. An electron-beam current passing in the vicinity of the edge of the vibrating sample, and thereby modulated, was led through an aperture at the image plane and measured with the electron counting technique as the power spectral density function, allowing the resonant frequency and the Q factor to be found. This enables estimation of the Young’s modulus and the internal friction. The method can be extended to the investigation of the elastic properties of nanoscaled samples.

Electron holography available in a non-biprism transmission electron microscope

Abstract A method for taking electron holograms with a single crystal instead of an electron biprism is described. The method is so simple that only a single crystal needs to be placed on the specimen grid in the specimen holder. Then, the crystal lattice fringes are focused on the recording photographic film to form an off-axis Fresnel hologram. The procedures for both hologram formation and reconstruction are described. Experimental results show that holograms can also be achieved with a conventional transmission electron microscope having a low-coherence electron source.

Direct Evidence of the Anisotropic Structure of Vortices Interacting with Columnar Defects in High-Temperature Superconductors through the Analysis of Lorentz Images

Two types of Fresnel contrasts of superconducting vortices in a Lorentz micrograph, corresponding to pinned and unpinned vortices, were obtained by a newly developed 1 MV field-emission transmission electron microscope on a Bi 2 Sr 2 CaCu 2 O 8+δ (Bi-2212) thin specimen containing tilted linear columnar defects introduced by heavy ion irradiation. The main features of the Fresnel contrasts could be consistently interpreted by assuming that the vortices are pinned along the tilted columnar defects and by using a layered or an anisotropic model to calculate the phase shift of the electron wave. The confirmed validity of both models strongly indicates that superconducting vortices in high-critical temperature (high- T c ) layered materials have an anisotropic structure.

Observation of Atomic Steps by Reflection Electron Holography

Reflection electron holography has been carried out successfully for the first time. Two regions in a reflection electron image of a Pt(111) surface at glancing angle incidence are overlapped by means of an electron biprism to form an off-axis electron hologram. The optically reconstructed interferogram displays the phase distribution of the diffracted electron wave which reflects surface topography. This method has been proved to have high sensitivity of the order of 0.01 nm for quantitatively measuring surface undulation by observing mono-atomic-height surface steps.

Electron Holographic Interference Micrograph of a Single Magnetic-Domain Particle

Electron interference micrographs of a single magnetic-domain particle of barium ferrite are obtained experimentally and theoretically. In the experiment, a hologram of the particle with size about 1 µ m is recorded on film and the phase distribution of the object electron wave is reconstructed digitally by the Fourier transform method. In the theoretical calculation, the phase distribution is obtained by integrating the vector potential around the particle along the electron path, assuming that the particle is spherical and is in the single-domain state. The above two phase distributions are converted to interference micrographs where magnetic flux lines are observed, and compared with each other. The theoretically calculated result agrees well with the experimental one, proving that the particle is in the single-domain state.