The magnetic properties and nanostructure of polycrystalline ${\mathrm{MgCNi}}_3,$ prepared from a batch with overall composition ${\mathrm{MgC}}_{1.5}{\mathrm{Ni}}_3,$ were studied by vibrating sample magnetometry and electron microscopy. Very high critical current density, e.g., $1.8\hspace{0.5em}{\mathrm{M}\mathrm{A}/\mathrm{c}\mathrm{m}}^2$ at 1 T and 4.2 K, is deduced from the magnetic hysteresis and evident subdivision of the sample into $10\hspace{0.5em}\mu \mathrm{m}$ clusters of ${\mathrm{MgCNi}}_3$ grains by excess graphite. The bulk pinning force $F_p\mathrm{}\left(H\right)\mathrm{}$ is comparable to that of other strong flux-pinning superconductors, such as NbN, Nb-Ti, and ${\mathrm{Nb}}_3\mathrm{Sn},$ all of which have higher critical temperatures. While $F_p\mathrm{}\left(H\right)\mathrm{}$ indicates the expected grain-boundary pinning mechanism just below $T_c\approx 7.2\hspace{0.5em}\mathrm{K},$ a systematic change to a core-pinning mechanism is indicated by a shift of the $F_p\mathrm{}\left(H\right)\mathrm{}$ curve peak to higher (reduced) field with decreasing temperature. The lack of temperature scaling of $F_p\mathrm{}\left(H\right)\mathrm{}$ suggests the presence of pinning sites at a nanometer scale inside the grains, which are smaller than the diameter of fluxon cores $2\xi \mathrm{}\left(T\right)\mathrm{}$ at high temperature and become effective when the coherence length $\xi \mathrm{}\left(T\right)\mathrm{}$ 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 $\xi \mathrm{}\left(0\right)\mathrm{}\approx 5\hspace{0.5em}\mathrm{nm}$ 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 ${\mathrm{MgCNi}}_3$ is unique. These results also indicate that strong flux pinning might be combined with a technologically useful upper critical field if variants of ${\mathrm{MgCNi}}_3$ with higher $T_c$ can be found.

• Received 8 March 2002

DOI:https://doi.org/10.1103/PhysRevB.65.214518