Synthesis and characterisation of superconducting MgCNi3

The intermetallic perovskite MgCNi3, superconducting with a Tc of 8 K, is particularly interesting from several points of view: in spite of the large amount of nickel present the compound is not ferromagnetic; nevertheless band structure calculations put in evidence a Van Hove singularity just above the Fermi level suggesting that MgCNi3 is on the border of magnetic instability and that may be driven to ferromagnetism by partial substitution of Mg with a monovalent metal as Li or Na. The material is also interesting because of the presence of two kinds of pinning centres, grain boundaries and nanometer-scale precipitates inside the grains, combination which is not found in other metallic superconductors. We have synthesized polycrystalline samples with composition MgCNi3 by a solid state route. Both the resistive and the magnetic measurements show a double superconductive transition, the first one occurring at a temperature of 10 K, higher than that reported in literature (8 K) and a second one at 6 K. High temperature resistivity shows a metallic behaviour.