Itinerant antiferromagnetism in the nearly-heavy-fermion compoundNpSn3

${\mathrm{NpSn}}_3$, with a paramagnetic electronic specific-heat coefficient γ, of 242 mJ/mol ${\mathrm{K}}^2$, undergoes an antiferromagnetic phase transition at 9.5 K with a resultant drop in γ to 88 mJ/mol ${\mathrm{K}}^2$. This behavior of γ (reduction by 64%) is typical of the ‘‘heavier’’ fermion antiferromagnets ${\mathrm{U}}_2$ ${\mathrm{Zn}}_{17}$, ${\mathrm{UCd}}_{11}$, and ${\mathrm{CePb}}_3$. Self-consistent linear-augmented-plane-wave calculations within the local-density approximation have been performed for both phases to study this effect. The calculations include both spin-orbit and magnetic coupling on the same level throughout the self-consistency process. An electronic specific-heat coefficient of 102 mJ/mol ${\mathrm{K}}^2$ is found for the paramagnetic phase—the largest value ever reported from a band-structure calculation. This large value implies that large enhancements observed in other heavy-fermion materials are not present in ${\mathrm{NpSn}}_3$. A specific-heat coefficient of 39.7 mJ/mol ${\mathrm{K}}^2$ was found for the antiferromagnetic phase, yielding a calculated 61% reduction in γ as a consequence of undergoing the phase transition. A total moment of 0.21${\mu }_B$ was obtained (-0.18 spin, 0.39 orbital) indicating the itinerant nature of the magnetism. An analysis of the dramatic reduction in electronic specific heat through the transition, as well as the temperature dependence of C/T in the paramagnetic phase, is presented along with a discussion of the implications for the other heavy-fermion antiferromagnets.