Magnetization of superconducting Hg3NbF6 and Hg3TaF6

The magnetization of the bulk superconducting transitions of Hg₃NbF₆ and Hg₃TaF₆ was measured in the temperature range from 0.1 to 1 K and in magnetic fields from 0 to 50 G. Measurements were performed on both samples in the zero-field-cooled (ZFC) and field-cooled (FC) modes. The magnetization was also measured as the sample warmed through its superconducting transition in the same field in which it had been cooled; this was referred to as the return-warming (RW) mode. A large difference in magnitude between the ZFC and FC modes for both samples indicated the presence of multiply connected regions (or type-II superconductivity). Evidence for both samples of the trapping of flux due to inhomogeneities and defects was provided by the hysteresis measured between the FC and RW modes. The observation of excess magnetization above the critical temperature in magnetic fields below 30 G was attributed to a proximity effect between the observed free mercury and the Hg₃NbF₆ and Hg₃TaF₆ metals. For the average of the data from the three modes, the Hg₃NbF₆ sample exhibited a critical field at absolute zero, H_c(0), of 150 G and a critical temperature, T_c, of 0.38 K. The Hg₃TaF₆ had a lower H_c(0) of 60 G, with a T_c of 0.44 K.