Critical current behaviour in reduced magnesium titanate spinel showing zero resistance

The current-voltage characteristics of a reduced magnesium titanate spinel showing the 50 K electrical transition of Cogle et al. [J. Mater. Chem. 1 (1991) 289] have been investigated. Above ca. 90 K, Ohm's law is obeyed and the resistance is independent of the current applied; below this value the resistance is a function of the current. For lowest values of temperature (<50 K) and current (<15 mA), a zero electrical resistance is observed. At 20 K, the critical current was found to be 13 mA. which corresponds to a critical current density of approximately 0.05 A cm-2. These data, in conjunction with the absence of any large diamagnetic transition at 50 K in samples investigated to date, are consistent with filamentary superconductivity in the samples.