Xiaonong Xu

The spin-entropy enhancement induced by Ce doping in Ca3Co4O9+δ

We present measurements of the thermopower as a function of temperature under different magnetic fields, for Ce-doped Ca3Co4O9+δ. A strong magnetic-field suppression of the thermopower indicates a large spin-entropy contribution. The magnetothermopower is enhanced in all doped samples, which provides an experimental evidence for the enhanced spin entropy by Ce doping. Our magnetic results allow us to determine the decrease in Co4+ concentration which results in the spin-entropy enhancement. We adopt a suitable theoretical model to explain our experimental observations. This investigation gives strong evidence that the enhanced thermopower mainly originates from enhancement of the spin entropy.

Enhancement of the spin entropy in NaxCo2O4 by Ni doping

The effects of Ni doping on the spin entropy in NaxCo2O4 (x=1.2) has been carefully studied. A strong magnetic-field suppression of the thermopower indicates the emergence of large spin-entropy effect. The magnetothermopower increases with increasing Ni doping level, suggesting that Ni doping improves the spin entropy of NaxCo2O4. Based on analysis of x-ray absorption spectra, we propose a spin-entropy competition mechanism to explain well the spin-entropy enhancement.

Synthesis and X-ray powder diffraction analysis of the high-Tc superconductor HgBa2Ca2Cu3O8+δ

Abstract The third member (Hg-1223) of the recently-discovered homologous series HgBa 2 Ca n −1 Cu n O 2 n +2+δ , which having a T c of 118 K (assynthesized sample) and 133 K (oxygen-annealed sample), was successfully prepared by solid state reaction and short time annealing technique using the mixtures of metal oxides HgO, BaO, CaO and CuO with starting composition HgBa 2 Ca 2 Cu 3 O y . We found that the combination of very short annealing time with optimum temperatures 715–725°C gives rise to a very effective sintering of the constituents and avoids an excessive loss of Hg. This also means that the reaction should be followed by X-ray powder diffraction analysis of the material at each step. Sample so obtained displays a sharp superconducting transition determined both magnetically and resistively. The X-ray diffraction patterns of the 118 K phase and the 133 K phase both indicate a nearly single phase, corresponding to the tetragonal structure of space group of P 4/ mmm and with lattice parameters a = 0.385 nm and c = 1.585 nm. Energy dispersive X-ray spectroscopic analysis (EDX) data of several flat grains of the Hg-1223 sample shows that the cation stoichiometry is an agreement with the proposed chemical formulae.

OPTICAL AND MAGNETO-OPTICAL PROPERTIES OF CO(001)/GE/CO SANDWICHES

A series of bcc‐Co(001)/Ge/Co trilayers was grown on a GaAs(001) substrate by the molecular beam epitaxy technique. The optical and magneto‐optical properties of the samples were studied. An oscillatory variation of the magneto‐optical Kerr effect (MOKE) with the thickness of the Ge spacers was observed. We conclude that the effective optical constants as well as the real and imaginary off‐diagonal element of the dielectric tensor should be considered in explanation of the MOKE activities since both have a large influence on the MOKE oscillations. Moreover, the coercivity and magnetization also oscillate with the Ge layer thickness.

13C NMR STUDY ON ONE DIMENSIONAL FULLERENE POLYMER

We have prepared three C60 polymers under high-temperature and high-pressure conditions, and confirmed them to be orthorhombic (1D), tetragonal and rhombohedral (2D) polymer by XRD method, respectively. The 13C MAS NMR spectra of both orthorhombic and rhombohedral polymer have been measured and two resonance peaks observed. One resonance at 73 ppm resulted from the sp3 carbons with an intermolecular bonding, the other at 145 ppm is from the inequivalent sp2 carbons on the C60 molecule. For 1D polymer, the fine structure of the main peak at 145 ppm has been analyzed. By simulation of the 13C MAS NMR peak shape of the inequivalent carbons, we propose that there exist nine inequivalent carbons on a C60 molecule in the orthorhombic (1D) polymer.

Analytic solution of field distribution and demagnetization function of ideal hollow cylindrical field source

The Halbach type hollow cylindrical permanent magnet array (HCPMA) is a volume compact and energy conserved field source, which have attracted intense interests in many practical applications. Here, using the complex variable integration method based on the Biot-Savart Law (including current distributions inside the body and on the surfaces of magnet), we derive analytical field solutions to an ideal multipole HCPMA in entire space including the interior of magnet. The analytic field expression inside the array material is used to construct an analytic demagnetization function, with which we can explain the origin of demagnetization phenomena in HCPMA by taking into account an ideal magnetic hysteresis loop with finite coercivity. These analytical field expressions and demagnetization functions provide deeper insight into the nature of such permanent magnet array systems and offer guidance in designing optimized array system.