We present the result of an extended experimental characterization of the hexagonal intermetallic Haucke compound NpNi$_{5}$. By combining macroscopic and shell-specific techniques, we determine the 5$f$-shell occupation number $n_f$ close to 4 for the Np ions, together with orbital and spin components of the ordered moment in the ferromagnetic phase below T$_C$ = 16 K ($mu_{S}$ = -1.88~$mu_{B}$ and $mu_{L}$ = 3.91~$mu_{B}$). The apparent coexistence of ordered and disordered phases observed in the M{o}ssbauer spectra is explained in terms of slow relaxation between the components of a quasi-triplet ground state. The ratio between the expectation value of the magnetic dipole operator and the spin magnetic moment ($3langle T_{z}rangle/ langle S_{z}rangle$ = +1.43) is positive and large, suggesting a localized character of the 5$f$ electrons. The angular part of the spin-orbit coupling ($langlevec{ell}cdotvec{s}rangle$ = -5.55) is close to the value of -6.25 calculated for trivalent Np ions in intermediate coupling approximation. The results are discussed against the prediction of first-principle electronic structure calculations based on the spin-polarized local spin density approximation plus Hubbard interaction, and of a mean field model taking into account crystal field and exchange interactions.
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