Applicability of Paradigms for High Transition Temperature Superconductivity to Magnetic Relaxation and Irreversibility in Superconducting Niobium.

A commercially built SQUID magnetometer was used to make detailed measurements of the temperature and magnetic field dependence of the magnetization and the magnetic relaxation of niobium films. An irreversibility line was identified using the intersection of the zero-field-cooled and field-cooled magnetization curves. Below the irreversibility line the remanent magnetization was found to decay logarithmically in time. The data were quantitatively compared to predictions from flux-creep, vortex-glass, and vortex-lattice melting models that have been offered to explain similar magnetization data from the high-transition-temperature materials. The predictions from the flux-creep picture were found to be in good agreement with the magnetic relaxation data below the irreversibility line, but they could not consistently account for the irreversibility line itself. The vortex glass model was also found to be inconsistent with the data: no evidence for a vortex-glass state--or for a second -order vortex-glass-to-vortex-fluid phase transition--was observed. The vortex-lattice melting model was, on the other hand, in good agreement with the irreversibility line data, thus suggesting that the onset of magnetically reversible behavior in niobium is due to a vortex-lattice -to-vortex-fluid melting transition. The Lindemann number that was obtained is smaller than the values that have been quoted for similar measurements on the high-transition temperature materials, but this was found to be consistent with theoretical predictions.