Title: Magnetic Properties of NpNiSi2
Citation: JOURNAL OF PHYSICS-CONDENSED MATTER vol. 20 p. 075207-1 075207-6
Publication Year: 2008
JRC N°: JRC45875
ISSN: 0953-8984
URI: http://dx.doi.org/10.1088/0953-8984/20/7/075207
DOI: 10.1088/0953-8984/20/7/075207
Type: Articles in periodicals and books
Abstract: NpNiSi2 represents the first transuranium-based system in the isostructural family with rare earths and uranium homologues that crystallizes into the orthorhombic Cmcm structure. It exhibits strong anisotropic ferromagnetic ordering below TC = 51.5 K with the easy magnetization axis along the shortest (c) axis. The ordered magnetic moment inferred from 237Np Mössbauer spectroscopy amounts to µNp = 1.4 µB. The isomer shift value of 6.8 mm s-1 versus NpAl2 suggests that the neptunium ions are in the Np3+ charge state with electronic configuration 5f4 (J = 4). From the values of the quadrupolar interaction parameters, the direction of the main principal axis of the electric field gradient (VZZ) is deduced to be along the b axis. The Sommerfeld coefficient ¿ = 133 mJ mol-1 K-2 obtained from the low temperature behaviour of specific heat qualifies NpNiSi2 as a moderately heavy fermion. Finally, the logarithmic decrease of the high temperature resistivity and the reduced (compared to the free Np3+ 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 Kondo lattice model of Perkins et al (2007 Phys. Rev. B 76 125101) which accounts for the coexistence of the Kondo effect and ferromagnetic order.
JRC Directorate:Nuclear Safety and Security

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