The crystal-field interactions in NpGa2

The zero-temperature value of the local Np ordered magnetic moment (2.39 mu(B)) in the ferromagnet NpGa2, its temperature-dependence and the value of T-c (55 K) have been very well reproduced within the crystalline-electric-field (CEF) approach in combination with spin-dependent interactions considering the 5f(4) configuration of the Np3+ ion. The derived fine electronic structure of the I-5(4) multiplet, with the set of CEF parameters B-2(0) = -9.4 K, B-4(0) = -45 mK, B-0(6) = +0.7 mK and B-6(6) = -14 mK, has the non-Kramers doublet Gamma(5) as the charge-formed ground state and the overall CEF splitting of 435 K. The good description provides strong evidence for the localized 5f magnetism of the neptunium ions in NpGa2 and the trivalent state of the Np ion.