A previously unknown neptunium-transition-metal binary compound Np 2Co 17 has been synthesized and characterized by means of powder x-ray diffraction, 237Np Mössbauer spectroscopy, superconducting-quantum-interference-device magnetometry, and x-ray magnetic circular dichroism (XMCD). The compound crystallizes in a Th 2Ni 17-type hexagonal structure with room-temperature lattice parameters a=8.3107(1)= Å and c=8.1058(1).Å Magnetization curves indicate the occurrence of ferromagnetic order below T C350 K. Mössbauer spectra suggest a Np3 + oxidation state and give an ordered moment of μ Np=1.57(4) μ B and μ Np=1.63(4) μ B for the Np atoms located, respectively, at the 2b and 2d crystallographic positions of the P6 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 [μ S=-1.88(9) μ B, μ L=3.48(9) μ B]. The ratio between the expectation value of the magnetic-dipole moment and the spin magnetic moment (m md/μ 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 μ Co1.6 μ 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.
|Original language||American English|
|Journal||Physical Review B - Condensed Matter and Materials Physics|
|State||Published - 30 Jan 2012|