Core pinning by intragranular nanoprecipitates in polycrystalline MgCNi₃

The magnetic properties and nanostructure of polycrystalline MgCNi₃, prepared from a batch with overall composition MgC_1.5Ni₃, were studied by vibrating sample magnetometry and electron microscopy. Very high critical current density, e.g., 1.8 MA/cm² at 1 T and 4.2 K, is deduced from the magnetic hysteresis and evident subdivision of the sample into 10 μm clusters of MgCNi₃ grains by excess graphite. The bulk pinning force F_p(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 F_p(H) indicates the expected grain-boundary pinning mechanism just below T_c≈7.2 K, a systematic change to a core-pinning mechanism is indicated by a shift of the F_p(H) curve peak to higher (reduced) field with decreasing temperature. The lack of temperature scaling of F_p(H) suggests the presence of pinning sites at a nanometer scale inside the grains, which are smaller than the diameter of fluxon cores 2ξ(T) at high temperature and become effective when the coherence length ξ(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 ξ(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 T_c can be found.