A previously unknown neptunium-transition-metal binary compound Np${}_2$Co${}_{17}$ has been synthesized and characterized by means of powder x-ray diffraction, ${}^{237}$Np Mössbauer spectroscopy, superconducting-quantum-interference-device magnetometry, and x-ray magnetic circular dichroism (XMCD). The compound crystallizes in a Th${}_2$Ni${}_{17}$-type hexagonal structure with room-temperature lattice parameters $a=8.3107\left(1\right)$ $\AA$ and $c=8.1058\left(1\right)$ Å. Magnetization curves indicate the occurrence of ferromagnetic order below $T_C>350$ K. Mössbauer spectra suggest a Np${}^{3+}$ oxidation state and give an ordered moment of ${\mu }_{\mathrm{Np}}=1.57\left(4\right)$ ${\mu }_B$ and ${\mu }_{\mathrm{Np}}=1.63\left(4\right)$ ${\mu }_B$ for the Np atoms located, respectively, at the $2b$ and $2d$ crystallographic positions of the $P{6}_3/mmc$ space group. Combining these values with a sum-rule analysis of the XMCD spectra measured at the neptunium $M_{4,5}$ absorption edges, one obtains the spin and orbital contributions to the site-averaged Np moment [${\mu }_S=-1.88\left(9\right)$ ${\mu }_B$, ${\mu }_L=3.48\left(9\right)$ ${\mu }_B$]. The ratio between the expectation value of the magnetic-dipole moment and the spin magnetic moment ($m_{\mathrm{md}}/{\mu }_S=+1.36$) is positive as predicted for localized $5f$ electrons and lies between the values calculated in intermediate-coupling (IC) and $jj$ approximations. The expectation value of the angular part of the spin-orbit-interaction operator is in excellent agreement with the IC estimate. The ordered moment averaged over the four inequivalent Co sites, as obtained from the saturation value of the magnetization, is ${\mu }_{\mathrm{Co}}\simeq 1.6$ ${\mu }_B$. The experimental results are discussed against the predictions of first-principles electronic-structure calculations based on the spin-polarized local-spin-density approximation plus the Hubbard interaction.

• Received 30 November 2011

DOI:https://doi.org/10.1103/PhysRevB.85.014434