Antiferromagnetic ground state in NpCoGe

Abstract

NpCoGe, the neptunium analog of the ferromagnetic superconductor UCoGe, has been investigated by dc magnetization, ac susceptibility, specific heat, electrical resistivity, Hall effect, Np M\"ossbauer spectroscopy, and local spin-density approximation (LSDA) calculations. NpCoGe exhibits an antiferromagnetic ground state with a N\'eel temperature ${T}_{N}\ensuremath{\approx}13$ K and an average ordered magnetic moment $\ensuremath{\langle}{\ensuremath{\mu}}_{\mathrm{Np}}\ensuremath{\rangle}=0.80\phantom{\rule{0.16em}{0ex}}{\ensuremath{\mu}}_{B}$. The magnetic phase diagram has been determined and shows that the antiferromagnetic structure is destroyed by the application of a magnetic field (\ensuremath{\approx}3 T). The value of the isomer shift suggests a Np${}^{3+}$ charge state (configuration $5{f}^{4}$). A high Sommerfeld coefficient value for NpCoGe (170 mJ mol${}^{\ensuremath{-}1}$ K${}^{\ensuremath{-}2}$) is inferred from specific heat. LSDA calculations indicate strong magnetic anisotropy and easy magnetization along the $c$ axis. M\"ossbauer 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 a magnetic quantum critical point.