Magnetic Properties of Al-Doped Na_(0.7)Co_(1-x)Al_xO_2

Single phase polycrystalline samples Na0.7Co1-xAlxO2 （x = 0, 0.05, 0.10, 0.15, 0.20, 0.25, 030） were prepared by solid state reaction. The magnetic properties from 5 K to 300 K have been studied by dc and ac magnetic susceptibility measurements. Samples with lower doping quantity （x = 0, 0.05, 0.10） showed paramagnetic behaviors, but those with higher doping quantity （x=0.20, 0.25, 0.30）showed spin-glass behaviors with a freezing temperature （Tf） of about 13 K.

Analysis of Vertical Oscillations of a Permanent Magnet Freely Levitated above an YBCO Bulk in an AC External Magnetic Field

The effect of the moving speed of permanent magnet （PM） on levitation force between PM and high temperature superconducting （HTS） bulk is analyzed and described in the PM-HTS levitation system. The PM vibration characteristic in the PM-HTS system is investigated. The PM may collide with the HTS in vibration if the amplitude and frequency of driving force satisfy the relationship Pmin=Afn. When the load of the system is below a threshold, the minimal collision amplitude of the driving force increases with the load increasing, however, it sharply drops to zero when the load exceeds the threshold. With the increase of the initial height of the PM, the threshold load increases, but the minimal driving force which causes a collision between PM and HTS decreases.

Hg Substitution Effect on Superconductivity and Crystal Structure of MgB_2

Polycrystalline Mg1-xHgxB2 samples with x=0, 1%, 2.5%, 5%, 7.5%, and 10% have been synthesized by solid-state reaction. Different from the substitution effect of Al, C, Li, etc. on crystal structure of MgB2, Hg substitution for Mg results in an increase of the lattice constant in both a and c directions. The super- conductivity of MgB2 is also suppressed by Hg substitution. The observed suppression of super- conductivity by Hg substitution is discussed in terms of the interband impurity scattering effect in two-band superconductors.

Effects of Doping Mn on Nd_(1.85)Ce_(0.15)CuO_4 System

Nd1.85Ce0.15Cu1-xMnxO4 samples with doping level up to 20% have been synthesized by solid-state reaction method. The influence of Mn on their normal-state transport, crystal structure, superconductivity and magnetic properties has been investigated. For the samples with x〉0.03, magnetization under zero-field cooling indicates that the magnetic state changes from ferromagnetic to paramagnetic at T≈100 K, which can be explained with the interaction between Mn4＋and Mn3＋. The electrical resistivity p of samples increases with Mn doping. For the samples with doping level lower than 0.20, p initially increases with the decrease of temperature, i.e., dp/dt〈0, and then shows superconductivity transition at ≈20 K. The results suggest the coexistence of superconductivity and ferromagnetic ordering in Mn doped Nd1.85Ce0.15CuO4.

Superconductivity in Hg-Substituted BaPb_(0.75)Bi_(0.25)O_3

A series of Hg-doped BaPb0.75Bi0.25O3 with a nominal composition of BaPb0.75-xHgxBi0.25O3 （x=0 to 0.40 with 0.05 intervals） have been synthesized by solid-state reaction. These compounds exhibit a cubic perovskite-related structure with the lattice parameter being expanded by Hg doping. Superconducting transition temperature Tc and superconducting volume fraction are suppressed by Hg doping in the low doping level region （0≤ x ≤0.25）. However, further increasing Hg content makes the superconductivity recovered at x〉0.3. The superconductivity suppression in Hg-doped BaPb0.75Bi0.25O3 can be explained by the decrease of electron carrier concentration as well as the bandnarrowing-induced electron localization.