The Np counterpart of the superconducting ${\mathrm{PuCoGa}}_5$ compound, ${\mathrm{NpCoGa}}_5,$ has been investigated by magnetization, resistivity, specific heat, and ${}^{237}\mathrm{Np}$ Mössbauer spectroscopy measurements. Unlike the plutonium compound, ${\mathrm{NpCoGa}}_5$ does not show any hint of superconductivity down to $T=0.4\mathrm{}\mathrm{K}$ but the onset of antiferromagnetic order below $T_N=47\mathrm{}\mathrm{K}.$ The magnetization experiments evidence a metamagnetic-like transition ${(B}_c=4.5\mathrm{}\mathrm{T}$ at $T=5\mathrm{}\mathrm{K})$ towards a canted antiferromagnet. The electronic effective mass in ${\mathrm{NpCoGa}}_5$ is moderately enhanced with a Sommerfeld specific heat coefficient $\gamma =64\mathrm{}{\mathrm{mJ}\mathrm{}\mathrm{mol}}^{-1}{\mathrm{K}}^{-2},$ comparable to that of ${\mathrm{PuCoGa}}_5.$ An ordered Np moment of $0.84{\mu }_B$ and the occurrence of a ${\mathrm{Np}}^{3+}$ charge state were inferred from the Mössbauer data. Comparison with ${\mathrm{NpGa}}_3$ suggests a moderate delocalization of the $5f$ electrons in ${\mathrm{NpCoGa}}_5.$ Similarities and differences with the isostructural ${\mathrm{PuCoGa}}_5$ and $\mathrm{Ce}M{\mathrm{In}}_5$ compounds are discussed.

• Received 16 December 2003

DOI:https://doi.org/10.1103/PhysRevB.69.184411