We present the result of an extended experimental characterization of the hexagonal intermetallic Haucke compound NpNi5. By combining macroscopic and shell-specific techniques, we determine the 5f-shell occupation number nf close to 4 for the Np ions, together with orbital and spin components of the ordered moment in the ferromagnetic phase below TC = 16 K (μS = -1.88~μB and μL = 3.91~μB). The apparent coexistence of ordered and disordered phases observed in the M\"{o}ssbauer spectra is explained in terms of slow relaxation between the components of a quasi-triplet ground state. The ratio between the expectation value of the magnetic dipole operator and the spin magnetic moment (3⟨Tz⟩/⟨Sz⟩ = +1.43) is positive and large, suggesting a localized character of the 5f electrons. The angular part of the spin-orbit coupling (⟨ℓ⋅s⟩ = -5.55) is close to the value of -6.25 calculated for trivalent Np ions in intermediate coupling approximation. The results are discussed against the prediction of first-principle electronic structure calculations based on the spin-polarized local spin density approximation plus Hubbard interaction, and of a mean field model taking into account crystal field and exchange interactions.
@article{arxiv.1407.6825,
title = {Low temperature magnetic properties of NpNi$_5$},
author = {A. Hen and E. Colineau and R. Eloirdi and J. -C. Griveau and N. Magnani and F. Wilhelm and A. Rogalev and J. -P. Sanchez and A. B. Shick and I. Halevy and I. Orion and R. Caciuffo},
journal= {arXiv preprint arXiv:1407.6825},
year = {2014}
}
Comments
Accepted manuscript, to be published in Phys. Rev. B http://prb.aps.org