[en] Synthetic method of YBa₂Cu₄O₈ (124) under a condition of ambient pressure of O₂ gas using citrate pyrolysis technique is examined in order to improve the reproducibility. A new reaction tube device is designed to ensure complete calcination reaction, and the process from the precursor to 124 phase is traced successively by X-ray diffraction analyses. From the experiments, enough contact of flowing O₂ gas with the precursor, and slow heating rate (1 deg. C/min) to the optimal reaction temperature 780 deg. C are suggested to be essential. Long reaction process (for 70 h or more) is necessary for higher purity. A transient mixture state of Y₂Cu₂O₅, BaCO₃ and CuO phases is found to grow up in early stage of the calcination process. This mixture state leads to the formation of final 124 phase with good reproducibility

[en] YBa₂Cu₄O₈ (124) is synthesized at ambient pressure condition using citrate pyrolysis precursor method, and the weakly sintered 124 ceramics are studied by both SEM observation and ac magnetization measurement. Such ceramics can be regarded as a system of weak-coupled super fine grains, so that we may study experimentally the intergrain superconductive phase transition considering the effects of mesoscopic sintered structure. For this purpose, several specimens at different treatment time stage in the sintering process are examined. As the sintering is advanced, very small grains are fused into larger ones and the adjacent large grains tend to be fused at their interface. From magnetic measurements, the ceramics are equally confirmed to be weak-coupled grain systems which show the intergrain ordering at each T_c2 determined consistently by sintering temperature. Utilizing the series of ceramics, we will be able to study the intergrain ordering further systematically

[en] Local structures around Y and Zr solutes in bcc-Fe are studied for the melt-quenched ribbons by X-ray absorption fine structure (XAFS). The melt-quenched ribbons of Fe₉₉Y₁ and Fe₉₉Zr₁ are confirmed as a super-saturated solid solution by X-ray diffraction and DSC. XAFS results indicate that the local structures around Y and Zr for the melt-quenched Fe₉₉Y₁ and Fe₉₉Zr₁ are similar to those for the Fe-rich intermetallic compounds; Fe₁₇Y₂ for Fe₉₉Y₁ and Fe₂Zr for Fe₉₉Zr₁. Prior to the apparent precipitation of the intermetallic compounds by annealing, the precursor of intermetallic compounds, about 1 nm in a diameter, are formed in the melt-quenched Fe₉₉Y₁ and Fe₉₉Zr₁. The large size mismatch of the atom between solutes (Y and Zr) and host (Fe) is responsible for it. This result shows a sharp contrast to that for the case of Cu and Ga in bcc-Fe, in which Cu and Ga occupy a substitutional site of bcc-Fe

No abstract available

[en] The ordering process of a dilute quasi two-dimensional (2d) system is investigated. A free energy consideration leads us the conclusion that the successive features of ordering in random systems may be realized in such a case in the form of bifurcation of one single phase transition for the pure system into two successive ones by dilution, which results in a regional long range order with 2d local order character and another perfect 3d long range order over the whole system, respectively. Such a hierarchy of ordering explains well the experimental results for a quasi 2d dilute series Mnsub(1-x)Znsub(x)(HCOO)₂x2H₂O and the deuterated salts. (orig.)

[en] Weakly sintered ceramic samples of YBa₂Cu₄O₈ (124) are prepared as systems of Josephson-coupled sub-micron grains, with varying sintering time. The obtained ceramic systems are studied by observing fundamental and harmonic magnetic responses. Each specimen shows characteristic temperature dependence of the responses caused by intergrain ordering. The ordering temperature estimated by singular peak of the nonlinear response changes consistently with the sintering time, indicating that the coupling strength is systematically changed. Furthermore, dual anomalies of the nonlinear response are found. The results suggest that two different origins of nonlinear response inhere in the intergrain ordering

[en] X-ray absorption fine structure (XAFS) measurements were done for the Zr K-edge of Nd₁₆Fe_75.5Zr_0.5 B₈ magnets in the disproportionated, partially and fully recombined states of HDDR process. XAFS results reveal that Zr preferentially occupies Fe(j₂) and Fe(j₁) sites in the Nd₂Fe₁₄B structure for the disproportionated, partially and fully recombined states. The partially recombined sample shows larger magnetic anisotropy but a smaller coercivity than the fully recombined one. Fourier analyses of the XAFS results show the rearrangement of Zr atoms as the HDDR process proceeds; some Zr at Fe(j₁) sites move to Fe(j₂) sites in the partially recombined state but they turn back to Fe(j₁) sites in the fully recombined state. The interchange of preferential site occupancy of Zr between Fe(j₁) and Fe(j₂) sites with proceeding recombination may be responsible for the degree of magnetic anisotropy

[en] The effects of the thermal history and shear stress in the casting process using the single-roll melt-spinning method have been studied by X-ray diffraction. The strong textured crystallines were found in the Fe/sub 91/Zr/sub 9/, Ni/sub 90/Zr/sub 10/ and Ni/sub 20/Zr/sub 80/ melt-spun ribbons. The textured layer is located near the free-surface side of the ribbons for the Fe/sub 91/Zr/sub 9/ and Ni/sub 20/Zr/sub 80/, but penetrates the ribbon to opposite side for the Ni/sub 90/Zr/sub 10/ alloy. The structural differences in an atomic scale have been found to exist between a free- and roll-surface of the melt-spun Fe/sub 89/Zr/sub 11/ metallic glass

[en] Studies on the structure and the crystallographic site of Mn in LaCo_13-xMn_x compounds were carried out by using X-ray diffraction and X-ray absorption fine structure (XAFS) of the Mn K-edge. These compounds with x≤3.0 adopt a NaZn₁₃-type structure consisting of icosahedral clusters. The lattice constant increases with the Mn concentration. The calculated XAFS curves of the center and the corner sites in the icosahedral clusters for the Mn K-edge are obtained by using the program FEFF. The fitting result for the corner site agrees much better with the observed XAFS spectrum than that for the center site. Therefore, the Mn site is determined to be the corner Co site in the icosahedral clusters of all the compounds. In comparison with the crystallographic parameters of LaCo₁₃, the icosahedral clusters composed of Mn atoms expand and the crystallographic Mn site is slightly more close to the La atom

No abstract available