Kazuyoshi Ito

Magnetic Susceptibility, Thermal Expansion and Electrical Resistivity of MnSe

Measurements of magnetic susceptibility, thermal expansion and electrical resistivity were carried out at temperatures ranging from 77 K to room temperature on a polycrystalline sample of the antiferromagnetic compound MnSe with rock salt structure. The curves obtained show a large thermal hysteresis and a sharp transition at the Neel temperature. The change in the susceptibility at TN (197 K) as the sample is cooled is -2.2×10-5 emu/g. The value of exchange striction is estimated to be 2.0. The change in the thermal expansion at TN is -5.6×10-4. The exchange striction and the first-order transition due to it cause the anomalous magnetic, crystalline and electrical behaviors at TN in MnSe.

Effect of hydrogen on the formation of interstitial loops in hydrogen-irradiated aluminum

Abstract The role of hydrogen atoms in the formation of interstitial type dislocation loops in aluminum irradiated with 15 keV hydrogen ions was investigated by electron microscopy. It was found that the loop density satisfied and Arrhenius relation with the pre-irradiation temperature, yielding an apparent activation energy of about 0.3 eV. Experimental results are interpreted in terms of the mechanism that small hydrogen-vacancy complexes formed by the pre-irradiation of hydrogen ions act as nucleation sites of dislocation loops in the subsequent electron irradiation. A basically identical mechanism seems to be acting in the irradiation with hydrogen ions at room temperature.

In-situ observation of the migration and growth of helium bubbles in aluminum

Abstract Brownian motion of helium bubbles, their disappearance, and coalescence in high purity aluminum were demonstrated in real time by electron microscopy and the recording of these whole processes on VTR. It was experimentally established for the first time that the mean square of the migration distance of the bubble during the time duration, t , is proportional to t at least for t ≥ 1 min. The growth of the bubble was caused by the coalescence between bubbles during Brownian motion and also by annealing at higher temperature unless the coalescence takes place. A dependence of the diffusivity of the bubble on its diameter and a rapid formation of new bubble by the coalescence of two bubbles seem to be supported through the surface diffusion of aluminum on the bubble surface.

Antiphase Domain Structure in Gd75Sc25 Single-Crystal Alloys

Magnetization, low field ac susceptibility, electrical resistivity, ultrasonic attenuation and specific heat have been measured for Gd75Sc25 single crystals. Low field ac susceptibility curves for the c-axis sample exhibit a sharp peak at the highest ordering temperature Tc (= 187 K) and a drop between Tc and T2 (= 126 K), which suggest the existence of an antiphase domain ferrimagnetic structure. A similar conclusion has been reached from the results of electrical resistivity, elastic property and specific heat measurements.

Conversion of hydrogen bubbles to disk-shaped defects in hydrogen irradiated aluminum

Abstract Electron microscopy was carried out on hydrogen bubbles in high purity aluminum. It was found for the first time that small hydrogen bubbles formed by irradiation with 15 keV hydrogen ions at 423 K converted to disk-shaped defects by annealing at higher temperature, depending on the initial concentration of bubbles. The many beam lattice image of the defects suggested that they mostly lay over multiple layers of (111) planes, containing characteristic distortions of the lattice on (111) planes. The formation of the disk-shaped defects is interpreted in terms of small hydrogen bubble coalescence during a random migration and collapse on (111) planes, losing their internal pressure, which may be allowed by the formation of aluminum hydride at high temperature.

Synchrotron Radiation Topographic Camera for Continuous Observation

An X-ray topographic camera in the high-speed X-ray topographic station (BL-15B) at the Photon Factory, High Energy Accelerator Research Organization was improved for continuous observation with nuclear plates. Using this camera, several topographs were taken during heating process of nearly-perfect aluminum single crystals. As a result, an obscure topograph, in which not only defect images but also fringe are unclear, was taken in spite of very short exposure time (2.0 s) just prior to the appearance of interstitial-type dislocation loops grown as vacancy sources.

Interaction between implanted hydrogen or nitrogen atoms and oxygen-related defects in germanium

Abstract Experimental results by DLTS measurement are presented which demonstrate the interaction between implanted hydrogen or nitrogen atoms and the oxygen-related defects introduced by electron irradiation. The electron trap at Ec -0.26 eV associated with the germanium A-center was found to be formed by 1.0 MeV electron irradiation. Another level at Ec -0.21 eV was observed on annealing at 120°C, accompanied with the decay of the Ec -0.26 eV level. As for the sample implanted with 27 keV hydrogen ions following electron irradiation, the trap concentration is four times as large as that for electron irradiation alone. After 25 keV nitrogen implantation following electron irradiation, the trap at Ec -0.21 eV was not observed. It is most probable that the germanium A-centers capture hydrogen atoms, increasing electrically active centers, and that the reduction in the oxygen-related defect growth is due to the prevention of defect migration with nitrogen atoms.

Deep level defects produced by electron irradiation and their annealings in undoped germanium

Abstract The defect levels produced in undoped p-type germanium by irradiation with 1.5 MeV or 2.0 MeV electrons and the annealing behaviour of the levels were studied by the DLTS technique. The positions of the trap levels, their concentrations and the cross sections for hole trapping of these traps were determined and the variation of the introduction rates of the traps with the energy of irradiation is reported. The level located at E v +0.22 eV should be ascribed to a divacancy-oxygen complex and the E v +0.31 eV levels to a defect-containing oxygen with regard to the effects of electrically inactive impurities. From the results of annealing under reverse or forward biases, the defect associated with the E v +0.31 eV level is considered to be annealed by an ionization- enhanced mechanism.

Interaction of Point Defects with Hydrogen in Germanium

The defect levels produced in undoped germanium crystals by implantation with 50 keV hydrogen ions of irradiation with 1.5 MeV electrons and the annealing behaviour of the materials were studied by DLTS technique. The positions or the trap levels and the cross sections of hole trapping of these traps were determined. The level located at Ev+0.42 eV produced by electron irradiation should be ascribed to a divacancy. In the samples implanted with hydrogen ions, the trap levels at Ev+0.38 eV and Ev+0.42 eV annealed at the same temperature. The two traps are attributable to the defect levels associated with a divacancy-hydrogen complex.

Indistinct Defect Images in Topographs of Nearly Perfect Aluminum Crystals Just Prior to Appearance of Dislocation Loops

Vacancy generation and annihilation mechanisms by the growth of two types of dislocation loops, the interstitial and the vacancy type, in nearly perfect aluminum single crystals at fairly high temperatures were investigated by synchrotron radiation topography. Topographs were continuously taken with white beam X-ray at an elevated temperature. Just prior to the appearance of the loops, an indistinct image topograph, in which not only defect images but also fringes were unclear, was taken for both types of loops in spite of a short exposure time (<3 s). This phenomenon was interpreted as being due to the scattering of the diffracted X-ray beam by very small clusters of interstitial atoms or vacancies which are invisible on the X-ray topograph. Moreover, some of them grow into interstitial or vacancy type dislocation loops as revealed by X-ray topography.