Superconducting Pb(_1-x)Eu(_x)Mo(_6)S(_8) measured between 300 mK and T(_c) in high magnetic fields, using a new field-gradient torque magnetometer

Abstract

A field-gradient torque magnetometer has been designed and constructed which measures the magnetic moment of isotropic superconductors using a Quantum Design torque chip. The magnetometer fits onto the base of an Oxford Instruments Heliox probe; thus the temperature range is 300 mK to 20 K, in magnetic fields up to 15 T. The superconductors measured on the torque magnetometer in vacuum were NbTi wire, PbMo(_6)S(_8), and Pb(_0.75)Eu(_0.25)Mo(_6)S(_8) as a function of magnetic field, magnetic field- gradient and temperature. The utility of the torque magnetometer was demonstrated by determining the critical current density (J(_c)) and the upper critical field (B(_c2)(0)) of the samples. These superconducting properties were compared with the vibrating sample magnetometer (VSM) values, for the same samples. The change in the superconducting parameters and properties of Pb(_1-x)Eu(_x)Mo(_6)S(_8), for x = 0.0 & 0.25 were investigated. The samples were fabricated using two different heat treatment methods, which finished with hot isostatic pressing (HIP'ing) at 2000 bar and 800 C for 8 hours. To characterise the superconductors, ac resistivity, ac susceptibility, specific heat and magnetic measurements were carried out, as a function of magnetic field and temperature. From the magnetisation data, the GL parameter (k) and the Sommerfeld constant (y) were determined for the magnetic superconductor x = 0.25. For both samples, k was determined in the temperature range 6 K to T(_c), from the magnetisation data. For x = 0.0 & 0.25, the critical temperature (T(_c)), the upper critical field (B(_c2(0)), the critical current density (J(_c)) and the irreversibility fields (B(_irr)) were determined from the experimental data.