Core pinning by intragranular nanoprecipitates in polycrystalline MgCNi3

The magnetic properties and nanostructure of polycrystalline MgCNi3, prepared from a batch with overall composition MgC1.5Ni3, were studied by vibrating sample magnetometry and electron microscopy. Very high critical current density, e.g., 1.8 MA/cm at 1 T and 4.2 K, is deduced from the magnetic hysteresis and evident subdivision of the sample into 10 mm clusters of MgCNi3 grains by excess graphite. The bulk pinning force Fp(H) is comparable to that of other strong flux-pinning superconductors, such as NbN, Nb-Ti, and Nb3Sn, all of which have higher critical temperatures. While Fp(H) indicates the expected grain-boundary pinning mechanism just below Tc'7.2 K, a systematic change to a core-pinning mechanism is indicated by a shift of the Fp(H) curve peak to higher ~reduced! field with decreasing temperature. The lack of temperature scaling of Fp(H) suggests the presence of pinning sites at a nanometer scale inside the grains, which are smaller than the diameter of fluxon cores 2j(T) at high temperature and become effective when the coherence length j(T) approaches the nanostructural scale with decreasing temperature. High-resolution transmission electron microscopy imaging and electron diffraction revealed a substantial volume fraction of cubic and graphite nanoprecipitates comparable to j(0)'5 nm in size, consistent with the hypothesis above. Dirty-limit behavior seen in previous studies may thus be tied to electron scattering by the precipitates. To our knowledge, no other fine-grained bulk intermetallic superconductor exhibits a similar change from grain boundary to core pinning with decreasing temperature, suggesting that the arrangement of pinning sites in MgCNi3 is unique. These results also indicate that strong flux pinning might be combined with a technologically useful upper critical field if variants of MgCNi3 with higher Tc can be found.