title: Magnetic properties of NpNiGa(5).
abstract: We have investigated a polycrystalline sample of NpNiGa(5) by magnetization, specific heat and (237)Np Mössbauer spectroscopy. We confirm that a ferromagnetic order sets in at T(C)≈30 K and that NpNiGa(5) undergoes a second transition at T(N)≈18 K. Mössbauer data indicate that the transition at T(N) is accompanied by a steep increase of the Np magnetic moment, which saturates to a value of 0.94 µ(B), in good agreement with the recently published neutron diffraction results. The Sommerfeld specific heat coefficient γ = 114 mJ mol(-1) K(-2) is the highest among the NpTGa(5) compounds. It compares to the ones observed in the PuTGa(5) superconductors. The trend of the isomer shift along the NpTGa(5) (T = Fe, Co, Ni, Rh) series points to a tendency towards localization of the 5f electrons when increasing the number of d electrons of the transition metal T. Neither fully localized nor fully itinerant models are able to reproduce the whole experimental data. Realistic models should include the dual nature of the 5f electrons, i.e. consider the difference in the degree of itinerancy among 5f electrons in the NpTGa(5) compounds.
authors.affiliation: European Commission, Joint Research Centre, Institute for Transuranium Elements, Postfach 2340, D-76125 Karlsruhe, Germany.
authors.forename: E
authors.initials: E
authors.lastname: Colineau
authors.forename: J P
authors.initials: JP
authors.lastname: Sanchez
authors.forename: F
authors.initials: F
authors.lastname: Wastin
authors.forename: P
authors.initials: P
authors.lastname: Boulet
authors.forename: J
authors.initials: J
authors.lastname: Rebizant