A multiferroic polycrystalline BiFeO3 film has been fabricated by a chemical solution deposition followed by the post deposition annealing at 823 K in air. The nanostructure of the BiFeO3 film was characterized by transmission electron microscopy (TEM). The nano-beam electron diffraction and the fast Fourier transform pattern image from the high resolution TEM image were compared with the electron diffraction patterns of the multislice simulation, and it was revealed that the BiFeO3 has R3c rhombohedral structure. Formation of any additional phase or phases was not found in the sample. The BiFeO3 film shows the small saturation magnetization of 5.2emu/cm(3) without spontaneous magnetization at room temperature, which behavior is typical for the weak ferromagnetic materials. The ferroelectric hysteresis loop of the BiFeO3 film was measured at low temperature in order to reduce the leakage current. The remanent polarization and the electric coercive field at 90 K were 52 mu C/cm(2) and 0.51 MV/cm at an applied electric field of 1.4 MV/cm, respectively. The structure-magnetic properties relationship is also discussed.

Ni-Zr metallic glasses have been recognized to be unstable in comparison with Cu-Zr metallic glasses. An analysis of Voronoi polyhedra in the RMC simulations based on the diffraction data could characterize the atomic configurations around Ni and Cu atoms. The polyhedra around Ni atoms are dominated by trigonal prisni-like, Archimedian antiprism-like, and similar polyhedra. In contrast. icosahedron-like polyhedra are preferred for Cu. The Ni-Zr glasses have been reported to stabilize by adding Al. Therefore, in this work, the analysis of Voronoi polyhedra around Ni, Zr and Al atoms for Ni25Zr60Al15 ternary metallic glass was carried out in order to clarify the difference between the atomic structures for the binary and ternary metallic glasses. Trigonal prism-like, Archimedian antiprism-like and similar polyhedra, which are dominated in the Ni-Zr metallic glasses, decreased in number by adding Al to the Ni-Zr system. On the contrary, the number of icosaliedron-like polyhedra was found to increase. The results apparently indicate that the addition of Al into Ni-Zr binary system promote the formation of icosahedron-like polyhedra in the structure. Therefore, from these results. we can easily recognize that icosahedron-like polyhedra play an important role to stabilize the structure of metallic glasses.

Local atomic structures in Zr66.7Ni33.3 and Zr 66.7Cu33.3 metallic glasses were examined by using nanobeam electron diffraction (NBED), energy-filtered selected area electron diffraction (SAED) and high-resolution electron microscopy (HREM). Locally ordered regions of atomic medium range order (MRO) were observed in both of the specimens by NBED, although it was difficult to recognize the regions using HREM. Statistical analyses for NBED patterns revealed such a difference in the extended MRO regions between the specimens that the MRO structure in Zr 66.7Ni33.3 is more complex with a large dispersion of interplanar spacings than those in Zr66.7Cu33.3. To understand nearest-neighbor atomic coordination, we performed electron intensity analyses using energy-filtered SAED patterns and constructed structure models including about 5000 atoms with the help of reverse Monte Carlo simulation. The nearest-neighbor atomic environments around Ni atoms in Zr66.7Ni 33.3 are also different from those around Cu atoms in Zr 66.7Cu33.3, consistent with the NBED study. The local structural difference between the two glasses was discussed in relation to their glass-forming abilities. ©2007 The Japan Institute of Metals.

In amorphous alloys, crystalline atomic clusters as small as 1-2nm are frequently observed as local lattice fringe images by high-resolution electron microscopy (HREM). These clusters can be understood as local structures of amorphous alloys corresponding to "medium-range-order (MRO)". The MRO structure can be observed only under suitable defocusing conditions of the objective lens in HREM. A clear imaging of the MRO structure is difficult in conventional TEMs, mainly due to the delocalization of the image, caused mainly by the spherical aberration of the objective lens and eventually by the chosen defocus. In the present study, we have examined MRO in a Pd-based bulk metallic glass (Pd40Ni40P20) using a high-resolution TEM (acceleration voltage 200 kV) fitted with a spherical aberration constant corrector (C, corrector) for aberration correction. We found that when C, was close to zero and defocus values were near the Gaussian focus, MRO regions with an FCC-Pd structure could be clearly observed with a low image disturbance. Under these conditions, the phase-contrast transfer function was understood to act as an ideal filter function, which distinctly selects specific lattice periods of the FCC-Pd clusters. The obtained atomic images of the glass structure including the FCC-Pd clusters are in good agreement with those expected from image simulation according to our amorphous structure model. In this study, we have demonstrated that the C-s-corrected HREM is a powerful tool to directly image locally ordered structures in metallic glasses. (c) 2006 Elsevier B.V. All rights reserved.

次世代のリチウムイオン電池の負極材として期待されているアモルファスシリコンに関して、その充放電機構の理解へ向け、アモルファスの局所構造、特にこれまで不明な点が多かった中距離秩序構造の解明を試みた。本研究では、回折実験を再現するアモルファスシリコンの構造モデルを分子動力学法に作製し、幾何学的手法であるボロノイ多面体解析を用いて特に第2近接配位に関する解析を行った。その結果、中距離秩序構造を特徴づけるボロノイ指数が見いだされ、それらと関連結晶相の構造の比較も行った。今後、シリコンの電池利用において、充放電時における構造変化の理解のために、本研究で提案した解析は有用な手法であると思われる。