NpCoGe, the neptunium analogue of the ferromagnetic superconductor UCoGe, has been investigated by dc-magnetization, ac-susceptibility, specific heat, electrical resistivity, Hall effect, 237Np Moessbauer spectroscopy and LSDA calculations. NpCoGe exhibits an antiferromagnetic ground state with a Neel temperature TN = 13 K and an average ordered magnetic moment <mNp> = 0.80 mB. The magnetic phase diagram has been determined and shows that the antiferromagnetic structure is destroyed by the application of a magnetic field (around 3 T). The value of the isomer shift suggests a Np3+ charge state (configuration 5f4). A high Sommerfeld coefficient value for NpCoGe (170 mJ mol-1K-2) is inferred from specific heat. LSDA calculations indicate strong magnetic anisotropy and easy magnetization along the c-axis. Moessbauer data and calculated exchange interactions support the possible occurrence of an elliptical spin spiral structure in NpCoGe. The comparison with NpRhGe and uranium analogues 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 a magnetic quantum critical point.
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