The fabrication of chevrel phase superconductors and the origin of the irreversibility field

Abstract

This thesis describes the fabrication and characterisation of high quality Chevrel phase superconductors and investigates the origin of the irreversibility field when measuring the magnetic moment of a superconductor using a Vibrating Sample Magnetometer. A stoichiometry investigation into hot isostatically pressed (HIP'ed) lead molybdenum sulphide (PMS) across the compositional range Pb(_1+y)Mo(_6)S(_8+x) where x=-0.5, 0, 0.5 and 1.0 and y=0.0 and 0.1 was completed. T(_c), J(_c), B(_IRR) and B(_cC2) were measured for each sample and were similar to those reported for high quality bulk HIP'ed PMS. A sample of tin molybdenum sulphide (SMS) was fabricated and the critical current density determined from dc magnetisation measurements. The reduced pinning force exhibited the Peak Effect and could be scaled in both the high and low field regimes using a Kramer dependence. Three europium doped SMS samples of compositions Eu(_x)Sn(_1+x)Mo(_6)S(_8) with x = 0, 0.35 and 0.5 were characterised. The x = 0.35 sample exhibited the highest dB(_C2)/dT and B(_C2)(0) of any tin based Chevrel phase material reported. The europium doped samples simultaneously exhibited paramagnetism and superconductivity. The effect of the field inhomogeneity of the magnet used in a VSM system on the measurement of the magnetic moment of a superconductor was calculated. It was found that when the effective ac field caused by the movement of the sample is comparable to the self field of the sample, the voltage measured at the driver frequency does not represent the magnetic moment or the critical current density of the sample. The calculations show that the irreversibility field (B,(_IRR)) occurs when the effective ac field penetrates the entire volume of the sample and is not when the critical current density falls to zero. Harmonics of the driver frequency were also calculated and it was shown that measurement of these harmonics allow the determination of the field at which the critical current density is zero. Harmonic measurements were performed and showed excellent agreement with the calculations.