Ruriko Tsuneta

Three-Dimensional Observation of Magnetic Vortex Cores in Stacked Ferromagnetic Discs

Electron holographic vector field electron tomography visualized three-dimensional (3D) magnetic vortices in stacked ferromagnetic discs in a nanoscale pillar. A special holder with two sample rotation axes, both without missing wedges, was used to reduce artifacts in the reconstructed 3D magnetic vectors. A 1 MV holography electron microscope was used to precisely measure the magnetic phase shifts. Comparison of the observed 3D magnetic field vector distributions in the magnetic vortex cores with the results of micromagnetic simulations based on the Landau-Lifshitz-Gilbert equation showed that the proposed technique is well suited for direct 3D visualization of the spin configurations in magnetic materials and spintronics devices.

Investigation of effect of strain-compensated structure and compensation limit in strained-layer multiple quantum wells

Abstract We investigated the effect of the strain-compensated structure of InGaAs(+)/InGaAs(-) multiple quantum wells (MQWs). Photoluminescence (PL) spectroscopy and thickness fringes by a transmission electron microscopy were used to evaluated the optical and structural crystal quality. For the +0.5/ −0.5% strain-compensated cases, lattice distortion was not observed, and PL intensity increased when the number of periods increased from 5 to 15. Crystal quality was improved by using the +0.5/ −0.5% strain-compensated MQW instead of the +0.5/0% compressively strained MQW. It was possible to increase the stack thickness of the strained MQWs. However, crystal quality was degraded with increase in strain. For the +1.25/ −1.25% cases, PL intensity decreased when the number of periods increased from 5 to 15, and defects were observed in a 15-period sample. Moreover, crystal quality was degraded by using the +1.25/ −1.25% strain-compensated MQW instead of the +1.25/0% compressively strained MQW.

Comparison of Relaxation Process of Compressive and Tensile Strains in InGaAs Lattice-Mismatched Layers on InP Substrates

This paper investigates the difference in crystal quality between strained-layer multiple quantum wells with compressive (+0.5%) and tensile strains (-0.5 %). For the compressive strain, the photoluminescence intensity decreased and the length of fringe bending increased from 250 A to 500 A when the number of periods increased from 5 to 15. The amount of fringe bending increased when the InP thickness decreased, especially when the strain was compressive. We also investigated the relaxation process in an InGaAs layer as a function of the layer thickness (from 25 nm to 2 µ m). For a compressive strain (+1.1%), misfit dislocations were observed near the interface between InGaAs and InP substrate. On the other hand, for a tensile strain (-1.1 %), we observed cracks instead of misfit dislocations. Moreover, the cracks were considered to increase the X-ray full width at half maximum of both the InGaAs lattice-mismatched layer and the InP substrate.

Simulation Study of Noise Influence in 3-Dimensional Reconstruction using High-Angle Hollow-Cone Dark-Field Transmission Electron Microscope Images.

Three-dimensional reconstruction of crystal structures by filtered convolution back projection of high-angle hollow-cone dark-field transmission electron microscope images (HADF images) of a specimen is investigated. To determine the distribution of Cu particles in Si crystal from reconstructed images, the necessary conditions are estimated. Undistorted distribution of particles is obtained from a simulated reconstruction when the specimen is inclined between ±50°, the inclination axis direction error is ±2° and Poisson noise is 30 db. Under these conditions, the distribution of Cu particles in a Si crystal is experimentally reconstructed.

Atomic Species Analysis and Three-Dimensional Observation by High-Angle Hollow-Cone Dark-Field Transmission Electron Microscopy

Methods of determining atomic species and reconstructing three-dimensional images of the specimen structure were examined by high-angle hollow-cone dark-field transmission electron microscopy (HADF-TEM). The contrast of the HADF image systematically changed depending on the atomic number and the size of particles deposited on carbon films. The relationship between the contrast and the atomic number was well simulated using the theory of multiple electron scattering. The specimens, which were Cu particles precipitated in Si crystal, were observed with an inclination from -50 to 50° by steps of 2°. The images were processed using computed tomography. It became clear that the spatial distribution of the Cu particles differed depending on whether the precipitated position was a dislocation or a (111) stacking fault.

Dual-axis 360° rotation specimen holder for analysis of three-dimensional magnetic structures

A dual-axis 360° rotation specimen holder was developed for use in reconstructing the three-dimensional (3D) distribution of a magnetic field using a combination of electron holography and tomography. Pillar-shaped specimens are used to obtain accurate reconstruction without a missing angle. The holders rotation rod can be turned >360°; the pillar is set ±45° to the azimuth for both x- and y-axis rotation. Two rotation series of holograms in individual axes are recorded for vector field tomography. The two vector components of the magnetic field are reconstructed directly from the two series of holograms, and the remaining component is calculated using Maxwells equation, div B = 0. As a result, all 3D magnetic fields are reconstructed.

Vector Field Tomography by Electron Holography

Electron tomography has rapidly developed in the last decade with the progress of modern computationally controlled electron microscopy and the development of algorithms for the Radon transformation and image processing with interpolation [1]. On the other hand, electron holography has been one of the standard techniques for observing phase maps of electron waves since the commercialization of the field emission electron gun. The technological combination of tomography and holography has also led to elucidation of the distributions of the mean inner potential of materials [2]. In the case of the magnetic field, the component By parallel to the rotation axis (see Fig. 1) can be calculated from the phase shift Δy by using the conventional tomography algorithm [3], but the other two components (Bx, Bz) perpendicular to the rotation-axis cannot be calculated, because the two components are mixed together with a rotation angle θ. In the case of a magnetic field in free space, however, the phase shift Δθ projected to the optical axis is described simply [4] as Eq. 2, for which Bx and Bz can be separated.

Analytical electron microscope based on scanning transmission electron microscope with wavelength dispersive x-ray spectroscopy to realize highly sensitive elemental imaging especially for light elements

An analytical electron microscope based on the scanning transmission electron microscope with wavelength dispersive x-ray spectroscopy (STEM-WDX) to realize highly sensitive elemental imaging especially for light elements has been developed. In this study, a large-solid-angle multi-capillary x-rays lens with a focal length of 5 mm, long-time data acquisition (e.g. longer than 26 h), and a drift-free system made it possible to visualize boron-dopant images in a Si substrate at a detection limit of 0.2 atomic percent.

Three-Dimensional Magnetic Vortex Cores Visualized by Electron Holographic Vector Field Tomography

Spintronics, which uses electron spins, is expected to become a widely used device technology because it has advantages in terms of nonvolatility, data processing speed, electric power consumption, and integration densities compared with conventional semiconductor technologies. Since the functions of spintronics devices are controlled by changing the spin configuration, i.e., by changing the magnetic field vector distribution, their direct observation is important for understanding the mechanisms of spintronics devices. Vector field electron tomography (VFET) using a transmission electron microscope (TEM) is a powerful technique for visualizing three-dimensional (3D) magnetic vector distributions on the nanometer scale [1,2]. Here we report 3D magnetic vortices in stacked ferromagnetic discs in a nanoscale pillar using a 1 MV electron holography microscope and a dual-axis 360° rotation sample holder for the VFET [3].

A Scale Estimation Algorithm Using Phase-Based Correspondence Matching for Electron Microscope Images

This paper proposes a multi-stage scale estimation algorithm using phase-based correspondence matching for electron microscope images. Consider a sequence of microscope images of the same target object, where the image magnification is gradually increased so that the final image has a very large scale factor S (e.g., S=1,000) with respect to the initial image. The problem considered in this paper is to estimate the overall scale factor S of the given image sequence. The proposed scale estimation technique provides a new methodology for high-accuracy magnification calibration of electron microscopes. Experimental evaluation using Mandelbrot images as precisely scale-controlled image sequence shows that the proposed method can estimate the scale factor S=1,000 with approximately 0.1%-scale error. This paper also describes an application of the proposed algorithm to the magnification calibration of an actual STEM (Scanning Transmission Electron Microscope).