Electrical resistivity measurements have been made on a good qualified single crystalline magnetite (Fe3O4) at temperatures from 300 down to 3.0 K under pressure up to 10 GPa. A steep change in resistivity at the Verwey transition temperature has been observed at pressure below 6.5 GPa, which shows a quite distinct result reported in prior work. Moreover, the Verwey transition temperature has been found to decrease nonlinearly with increasing pressure and it disappears at around 8 GPa. Above 8 GPa magnetite exhibits metallic behavior. The residual-resistivity ratio of the metallic state is observed to be more than 350. This finding of a metallic ground state in magnetite provides insight into the understanding of the Verwey transition in magnetite.
By chemical liquid deposition in which an alkoxide with a carbon number of 4 or smaller was used as raw material, a Sr0.7Bi2.3Ta2O9+α (SBTO) thin film was fabricated for use as a ferroelectric memory device for the purpose of decreasing the temperature of crystallization and improving surface morphology. The crystallization process was also examined. Crystallization began when the film was heat treated in oxygen at 650° C. When it was heat treated at higher than 700° C, it showed ferroelectric properties, and the squareness (remanent polarization/saturation polarization) of its hysteresis loop was improved at 800° C. A film heat treated at temperature 650° C was a cluster of fine particles, and a film heat treated at 800° C was a cluster of large particles. A film heat treated at 700° C was a mixture of fine particles and large particles. Therefore, it is concluded that the alkoxide with a carbon number of 4 or smaller as raw material enable the lowering of the heat-treatment temperature and improvement of the surface morphology of SBTO thin films.
The effects of Pt electrode and H2 sintering on the metallic Bi content of Sr0.9Bi2.1Ta2O9 (SBT) thin films for ferroelectric memories were studied using X-ray photoelectron spectroscopy (XPS). Oxidic Bi in SBT films is reduced to metallic Bi by H2 sintering. The degree of reduction depends on the structure of the SBT film. The SBT film with a fluorite structure is more difficult to reduce than that with a Bi-layered structure. Pt upper electrode formation also leads to an increase of metallic Bi content. Both H2 sintering and the Pt upper electrode work synergistically. In the combination of the pure Pt upper electrode and H2 sintering, over 80% of Bi on the SBT film surface was metallic. Pt films of 5 nm thickness on the SBT film recrystallized during the heat treatment and were broken up into particles. Pt electrode coverage was about 20% after the 2nd annealing at 800°C for 30 min in O2 atmosphere. Electrical properties were significantly affected by the presence of metallic Bi; in particular, SBT films with higher metallic Bi content showed higher leakage current density. The metallic Bi content must therefore be suppressed to obtain SBT films with low leakage current density.
The orientation of Sr0.7Bi2.3Ta2O9+α (SBT) films, which are layer-type bismuth compounds, was controlled by varying the Sr-source. In this paper, the effect of crystal orientation on film characteristics is described. The crystal orientation of the SBT ferroelectric films did not affect the surface morphology, leakage current or fatigue characteristics, but it did affect the shape of the hysteresis loop (polarization) and the window value of the C-V characteristics when the films were connected to a metal-oxide-semiconductor (MOS) diode. Although a complete c-axis orientation film with a stoichiometry of SrBi2Ta2O9 shows no spontaneous polarization in general, the highly c-axis orientated Sr0.7Bi2.3Ta2O9+α film in this study showed some spontaneous polarization. The polarization values are larger than expected by considering orientation alone. A deviation from stoichiometry resulted in an increase made in the polarization along the c-axis. Therefore, control of the crystal orientation and composition of SBT films is quite an important factor in actual applications.
We studied how to form an MgO protective layer on an ac-type plasma display panel (PDP) using a screen-printing method. In our study, we focused on high luminous efficiency and low drive voltage. To raise the luminous efficiency, it is necessary to limit the discharge current. The firing voltage (Vf), which is the voltage at which all cells start to discharge, and the sustaining voltage (V~), which is the minimum voltage required for all cells to sustain discharge, have no clear relationship to luminous efficiency. To attain both a higher luminous efficiency and a lower drive voltage, a thinner more crystalline layer is necessary. We found that we could prepare an MgO layer by screen-printing and sintering fine vapor-deposited MgO powders of different grain sizes with an MgO liquid binder. An MgO protective layer prepared by this method provided both a low drive voltage and superior luminous efficiency 2.8 times higher than that of a sputtered layer. The screen-printing method thus enables larger ac-PDPs to be produced at lower cost.
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