Yan Qi-wei

The copper isotope effect in Ba2Y Cu3O7

Abstract The effect of isotopic substitution on the superconducting Tc in the 90 K superconductors Ba2Y Cu3O7 by replacing ordinary copper Cu with 63Cu and 65Cu has been studied. The transition temperatures are found to be the same within experimental error of ± 0.2K. This indicates that the copper isotope effect is absent in this material, as in the cases of oxygen.

Crystal structure and magnetic properties of La1.2Sr1.8-xCaxMn2O7

The effect of divalent cation substitution on the structure and magnetic properties of La1.2Sr1.8-xCaxMn2O7 (x = 0-0.900) is investigated in this paper. Partly replacing divalent cation Sr2+ by Ca2+ ions results in the weakening and then disappearance of long-range ferromagnetic ordering and the formation of spin canting and low-temperature spin-glass. Based on structural analysis by Rietveld profile fitting, we suggest that this variation of magnetic property be related to a Jahn-Teller-type lattice distortion of MnO6 octahedra due to the introduction of the smaller sized Ca2+ ions.

The structure and magnetic properties of La2-2xSr1Ca2xMn2O7 (x = 0.25-1.00)

In this paper, the effect of divalent cation substitution on the structure and magnetic properties in La2-2xSr1Ca2xMn2O7 have been investigated systematically using bulk samples with a wide doping concentration range 0.25 ≤ x ≤ 1.00. Replacing trivalent La ions by divalent Ca ions results in the weakening and then disappearance of the long-range ferromagnetic (FM) ordering, the formation of spin canting, antiferromagnetic (AFM) ordering and low-temperature spin-glass. These results show that increasing the hole-doping concentration significantly suppresses the FM state. We suggest that this variation of magnetic properties is related to the competition of the FM and AFM interactions resulting from the change of Mn3+/Mn4+ ratio and Jahn-Teller-type lattice distortion of MnO6 octahedra due to the introduction of Ca2+ ions.

EFFECT OF MANGANESE SUBSTITUTION IN R 2 AlFe 16-x Mn x (R=Y,Ho,Nd AND Pr)COMPOUNDS ON STRUCTURAL AND MAGNETIC PROPERTIES

The structural and magnetic properties of R2AlFe16-xMnx (0 ≤ x ≤ 8 for Y, Ho, 0 ≤ x ≤ 10 for Pr, 0 ≤ x ≤ 16 for Nd) compounds have been investigated by means of x-ray diffraction and magnetization measurements. The R2AlFe16-xMnx compounds have a hexagonal Th2Ni17-type structure for R = Y and Ho, and a rhombohedral Th2Zn17-type structure for Nd and Pr. With increasing x, the unit-cell volumes have a small increase initially, followed by a greater linear increase. The Curie temperature and the saturation magnetization of these compounds show marvelous drop with increasing x. Compensation point was observed in the temperature dependence of the magnetization for Ho2AlFe12Mn4 compound.

Magnetic properties of Dy2Fe17-xCrx and Er2Fe17-xCrx (x = 0-3) compounds

The crystal structure and magnetic properties of R2Fe17-xCrx (R = Dy, Er, 0 ≤ x ≤ 3) compounds have been investigated by means of x-ray diffraction and magnetization measurements. These compounds have hexagonal Th2Ni17-type structure. The unit-cell volumes decrease with the increase of Cr concentration x. The Curie temperature Tc of the Er2Fe17-xCrx compounds increases from 320 K for x = 0 to 403 K for x = 1.0 and then decreases with further increase of x. The Curie temperature Tc of Dy2Fe17-xCrx compounds increases from 364 K for x = 0 to 435 K for x = 1.0 and then decreases with further increase of x. The saturation magnetization of these compounds shows an approximately linear decrease with the increase of x. Spin reorientation transitions occurs in Er2Fe17-xCrx (x = 2.0 and 3.0).

Formation and magnetic properties of Dy2Fe17-x Mnx C(x = 0-17) prepared by arc-melting

We report on alloys formed by replacing iron with manganese in the compound Dy2Fe17C1.0. Samples were characterized by x-ray diffraction and magnetic measurements. The 2:17-type structure can be crystallized in the whole range of manganese substitution. The Curie temperature of the series of Dy2Fe17-xMnxC1.0(x = 0-17) drops down considerably from 515 K for x = 0 to about 20 K for compounds with high manganese concentration, and the compensation point was observed in a narrow range of x ≈ 4-6. The rapid decrease of the magnetization shows that the manganese substitution is not a simple magnetic dilution to the transition-metal sublattice moment, it indicates that the moment of Mn may be antiparallel to that of Fe. The field dependence of the magnetization of Dy2Fe17-xMnxC1.0 may indicate the existence of the nonlinear magnetic structure for the samples with high Mn concentrations.

NEUTRON-POWDER-DIFFRACTION STUDY OF ErFe 11.35 Nb 0.65 AND ErFe 11.35 Nb 0.65 N y

ErFe11.35Nb0.65 and ErFe11.35Nb0.65Ny have been synthesized and neutron-powder-diffraction experiments at room temperature performed. The ErFe11.35Nb0.65Ny nitride, obtained by gas-solid reaction, retains the ThMn12-type structure of its parent compound. The Nb atoms occupy 8i sites and the nitrogen atoms are located at 2b sites. The atomic magnetic moments of the Er ions are antiparallel to those of the Fe atoms. Upon nitrogenation, the lattice cell expands mainly along the a-axis and the atomic magnetic moments of Fe are enhanced.

A neutron powder diffraction study of the structure in Nd2Fe17Nx(x = 2.5, 3.0, 5.5)

The crystallographic and magnetic structures of Nd2Fe17Nx (x=2.5, 3.0, 5.5) at room temperature were refined by Rietveld analysis of neutron powder diffraction data. We found that Nd2Fe17Nx has a Th2Zn17 type structure (S. G. Rm) and the nitrogen atoms occupy both 9e and 18g sites simultaneously and at different rates.

Neutron diffraction study of Al-Mn quasicrystal

Powder neutron diffraction measurements have been carried out on the Al-Mn quasicrystal at room temperature. Nine diffraction peaks of the quasicrystal phase were observed, and indexed on the basis of six independent reciprocal vectors pointing to the vertices of an icosahedron.