Detailed flux penetration measurements and the harmonic magnetic response have been measured from 4.2 K up to the critical temperature in dc fields up to 10 T for a hot isostatically pressed Chevrel-phase superconductor (Pb,Gd)Mo6S8. Good agreement is found between experimental results up to the tenth harmonic and calculations derived from a critical-state model which demonstrates that on the macroscopic scale the supercurrents flow throughout the bulk of the material. The field and temperature dependence of the critical current density shows a universal scaling behavior in agreement with Kramer’s pinning law, which suggests that a single grain-boundary mechanism determines the critical current density. Complementary compositional microscopy, x-ray, resistivity, and susceptibility data are presented. When gadolinium is not incorporated into the crystal structure it acts primarily as an oxygen getter in the form Gd2S3, which enhances the density of paths of PbMo6S8 with low oxygen content and high superconducting critical parameters. When the divalent gadolinium replaces the trivalent lead to form (Pb1−xGdx)Mo6S8, it reduces the density of hole states at the Fermi energy which lowers the critical parameters, the critical current density, and the irreversibility field.
A probe has been designed and built for measuring the magnetization critical current density Jc(B, T) and magnetic field profiles inside superconducting samples as a function of temperature in large DC magnetic fields. Magnetic field profiles provide a unique opportunity to study the spatial variation of Jc within a superconducting sample. In these measurements, a small AC field is superimposed upon a much larger DC field. This is the first flux penetration probe capable of making measurements as a function of temperature, in the range 2-30 K (accurate to 100 mK), which combines high AC fields of up to 100 mT (at 19.7 Hz) with very large DC fields of up to 17 T. The probe can be used to study short sections of superconducting cables, wires, tapes, thin films or bulk samples. Its capabilities are demonstrated by presenting variable-temperature data obtained from a commercial multifilamentary NbTi wire in magnetic fields up to 10 T.
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