NpCoGe, the neptunium analog of the ferromagnetic superconductor UCoGe, has been investigated by dc magnetization, ac susceptibility, specific heat, electrical resistivity, Hall effect, 237Np Mossbauer spectroscopy, and local spin-density approximation (LSDA) calculations. NpCoGe exhibits an antiferromagnetic ground state with a Néel temperature TN ≈ 13 K and an average ordered magnetic moment μNp = 0.80μB. The magnetic phase diagram has been determined and shows that the antiferromagnetic structure is destroyed by the application
of a magnetic field (3 T). The value of the isomer shift suggests a Np3+ charge state (configuration 5f 4). A high Sommerfeld coefficient value for NpCoGe (170 mJ mol−1 K−2) is inferred from specific heat. LSDA calculations indicate strong magnetic anisotropy and easy magnetization along the c axis. Mossbauer data and calculated exchange interactions support the possible occurrence of an elliptical spin-spiral structure in NpCoGe. The comparison with NpRhGe and uranium analogs suggests the leading role of 5f-d hybridization, the rather delocalized character of 5f electrons in NpCoGe, and the possible proximity of NpRuGe or NpFeGe to amagnetic quantum critical point.