Abstract
NpNiSi2 represents the first transuranium-based system in the isostructural family withrare earths and uranium homologues that crystallizes into the orthorhombicCmcm structure. It exhibits strong anisotropic ferromagnetic ordering belowTC = 51.5 K with the easy magnetization axis along the shortest(c) axis. The ordered magnetic moment inferred from237Np Mössbauer spectroscopyamounts to μNp = 1.4 μB. Theisomer shift value of 6.8 mm s−1 versus NpAl2 suggests that the neptunium ions are in theNp3+ charge state withelectronic configuration 5f4 (J = 4). From the values of the quadrupolar interaction parameters, thedirection of the main principal axis of the electric field gradient(VZZ) is deduced to bealong the b axis. TheSommerfeld coefficient γ = 133 mJ mol−1 K−2 obtained from the low temperature behaviour of specific heat qualifiesNpNiSi2 as a moderately heavy fermion. Finally, the logarithmic decrease of thehigh temperature resistivity and the reduced (compared to the freeNp3+ ion) magnetic moments (both ordered and effective) point to the occurrence of a Kondo effect.NpNiSi2 hence appears as a good candidate for being described with the underscreened Kondolattice model of Perkins et al (2007 Phys. Rev. B 76 125101) which accounts for thecoexistence of the Kondo effect and ferromagnetic order.
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