Ruzhang Ma

The low temperature Mössbauer spectra of YBa2Cu2.88Fe0.12O7-x

Abstract The occupied position of Fe in the superconducting phase of Fe-doped YBa2Cu2.8807-x was estimated by point charge model. The Mossbauer spectra were measured in the temperature range from 78 K to 273 K. Lattice softenings were found near 110 and 220 K. The observed variations of hyperfine parameters can be considered tobe related to the lattice softening near 110 K. It is suggested that the lattice softening near 110 K is related to the Peierls instability of the one-dimensional Cu(1)−0 chains.

Mössbauer study of quadrupole interaction at the different Cu sites in the Bi-system superconductor

Abstract 57 Fe Mossbauer measurements have been performed on Fe-doped samples of Bi 1.6 Pb 0.4 Sr 2 Ca 3 Cu 4- x Fe x O y ( x =0.06 and 0.08) at room temperature. The Mossbauer spectra show three quadrupole doublets that can be related to different Cu sites. Two doublets (B and C) with larger quadrupole splitting (QS) are assigned with a 57 Fe probe atom on the Cu-O square planar sites of the compound with different local charge environments. Another doublet A with smallest quadrupole splitting is associated with a Fe atom in a planar pyramid site. the doublet with smallest QS possesses relatively the smaller area indicating that the Fe atom preferentially occupies a square planar site rather than a pyramid site. To assure our assignment, we calculated the EFG (electric field gradient) on different Cu sites with the use of the point charge model and of the calculation of an effective ions valence.

Structural defects and internal stress field distribution in nanocrystalline Fe73.5Cu1Nb3Si13.5B9

Abstract The positron annihilation Doppler broadening technique and micromagnetism method have been employed to investigate structural defects in nanocrystalline Fe 73.5 Cu 1 Nb 3 Si 13.5 B 9 alloy. The results show that the precipitation of α-Fe(Si) in the amorphous matrix involves few defects at the early stage of crystallization, while at the second stage a large amount of defects associated with grain boundaries becomes dominant. The pinning centers resulting from quasi-dislocation dipoles have an intense influence on the soft magnetic properties. In view of these structural defects, the optimum soft magnetic properties of nanocrystalline Fe 73.5 Cu 1 Nb 3 Si 13.5 B 9 can be explained.

The amorphous-to-nanocrystalline transformation in Fe73.5Cu1Nb3Si13.5B9 studied by thermogravimetry analysis

Abstract The magnetic properties and isothermal crystallization process of amorphous Fe 73.5 Cu 1 Nb 3 Si 13.5 B 9 alloy have been investigated by means of the thermogravimetry analysis method (TGA) and X-ray diffraction measurements. The TGA curves for samples annealed at various temperatures indicate three distinct regions corresponding to the different magnetic characteristics of the as-quenched, the nanocrystalline and the polycrystalline structures. The results of a kinetic analysis show that Cu element plays an important role in the nucleation of ultrafine grains. The formation of Cu-rich regions reduces the activation energy for the nucleation of α-Fe(Si), so that the α-Fe(Si) phase can nucleate and precipitate preferentially in the Cu-rich regions. A comparison with isothermal DSC analysis shows that more information about nucleation at the early stage of crystallization can be obtained by the TGA method.

Interfacial structure of nanocrystalline Fe73.5Cu1Nb3Si13.5B9 studied by positron annihilation

Nanocrystalline Fe73.5Cu1Nb3Si13.5B9 alloy prepared by the crystallization of amorphous alloy has been studied by using the positron annihilation technique. Positron parameters, i.e., lifetime τ1, τ2, and line shape parameter S are obtained as a function of the annealing temperature. The results show that there exist two types of defects at the interfaces of nanocrystalline Fe73.5Cu1Nb3Si13.5B9 alloy: vacancy‐like and vacancy‐like group microvoids characterized by the lifetime τ1 and τ2. The former is in overwhelming majority. The changes of the structural defects corresponding to different stages, structural relaxation, and crystallization are discussed.

Study of lattice softening on Bi-system superconductor by means of 119Sn Mössbauer spectroscopy

Abstract Anomalous behavior of the Mossbauer factor for tin impurity atoms in Bi-system Superconductors is found by Mossbauer spectroscopy. This indicates the occurrence of lattice softening above Tc.

X-RAY-DIFFRACTION STUDY OF SOFTENING IN LATTICE VIBRATION IN THE HIGH-TC PHASE OF BI-BASED SUPERCONDUCTOR

Abstract X-ray diffraction study was conducted over the range from 77K to room temperature about high-Tc superconducting phase Bi-system. Temperature dependent intensity of (002) peak and lattice parameter show evidence of lattice vibrations softening, which occurs at about 130K and 210K.

The interfacial structure of nanocrystalline Fe73.5Cu1Mo13Si13.5B9 studied by Mössbauer spectroscopy

Abstract Nanocrystalline Fe73.5Cu1Mo3Si13.5B9 prepared by crystallization of the amorphous alloy was investigated by using Mossbauer spectroscopy and X-ray diffractometry. The present study focuses on the interfacial composition and the important role of the interfacial component in the development of the nanocrystalline structure. On the basis of a newly developed fitting program, the amorphous phase is denoted by a low field component and a high field one. Upon crystallization, the former, which corresponds to the boundary regions adjacent to the grains become more significant. The different stages associated with the crystallization are discussed.

Compositional evolution and magnetic properties of nanocrystalline Fe81.5Cu0.5Mo0.5P12C3Si2.5

The amorphous alloy Fe81.5P12C3Cu0.5Mo0.5Si2.5 has been prepared and the crystallized alloy exhibits an ultrafine structure with a grain size of about 20 nm and excellent soft magnetic properties. The coercivity and the core loss as low as 6.7 A/m, 0.26 W/kg, respectively, and the maximum permeability as high as 10.2×104 at an optimal annealing temperature of about 360 °C were obtained. By means of x‐ray diffraction, transmission electron microscope, and Mossbauer spectroscopy measurements, microstructures of the alloy were investigated as a function of the annealing temperature. The primary crystallization produces ultrafine grains of α‐Fe(Si) solid solution with a grain size of about 20 nm precipitated in the residual amorphous matrix. The Si in the α phase was estimated near to be completely disorder ranged and the Si concentration was determined to be about 2%–5%. The Fe3P phase appears in the residual amorphous phase upon annealing at 420 °C.

POSITRON-ANNIHILATION STUDY ON THE TL-1223 SUPERCONDUCTOR

We have measured the Doppler-broadened radiation spectra at the various temperatures from 77K to room temperature for the sample of T1-system 1223 phase. The temperature dependence of the line-shape parameters (S, W, S/W) show the anomalous changes in the temperature range 10-20K above Tc, which indicates that electron momentum distribution alters anomalously due to the localization of the electrons. the result is discussed.