Magnetic and electronic properties of the antiferromagnetNpCoGa5

Abstract

The Np counterpart of the superconducting ${\mathrm{PuCoGa}}_{5}$ compound, ${\mathrm{NpCoGa}}_{5},$ has been investigated by magnetization, resistivity, specific heat, and ${}^{237}\mathrm{Np}$ M\ossbauer spectroscopy measurements. Unlike the plutonium compound, ${\mathrm{NpCoGa}}_{5}$ does not show any hint of superconductivity down to $T=0.4\mathrm{K}$ but the onset of antiferromagnetic order below ${T}_{N}=47\mathrm{K}.$ The magnetization experiments evidence a metamagnetic-like transition ${(B}_{c}=4.5\mathrm{T}$ at $T=5\mathrm{K})$ towards a canted antiferromagnet. The electronic effective mass in ${\mathrm{NpCoGa}}_{5}$ is moderately enhanced with a Sommerfeld specific heat coefficient $\ensuremath{\gamma}=64{\mathrm{mJ}\mathrm{}\mathrm{mol}}^{\ensuremath{-}1}{\mathrm{K}}^{\ensuremath{-}2},$ comparable to that of ${\mathrm{PuCoGa}}_{5}.$ An ordered Np moment of $0.84{\ensuremath{\mu}}_{B}$ and the occurrence of a ${\mathrm{Np}}^{3+}$ charge state were inferred from the M\ossbauer 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.