MAGNETISM AND HYPERFINE INTERACTIONS IN (Np1-xUx)6Fe

Overview & Synopsis

Magnetic susceptibility and Mössbauer studies of the 59.5 keV transition of 237Np and the 14.4 keV transition of 57Fe in (Np1-xUx)6Fe were performed. The susceptibility and the Mössbauer experimental results prove that the actinides and the iron do not carry a localized magnetic moment. The samples do not order magnetically down to 1.9 K. The 57Fe spectrum consists of a quadrupole doublet, eqQ = 16(2) Mc/s, and isomer shift - 0.29(6) mm/s, relative to Fe metal. The 237Np Mössbauer spectra, for all temperatures and x values (T < 77 K, x = 0, 0.5, 0.8, 0.9) are composed of two pure non-axial quadrupole subspectra of relative intensities 2:1. In Np6Fe, eq1Q = 4450(50) Mc/s, η = 0.46(2) and eq2Q = 900(100) Mc/s, η = 0.9(1). These results agree very well with the known crystal structure (tetragonal 41/mcm) which contains Np ions in two low symmetry sites. The isomer shift of the two subspectra (-16(1) mm/s and -21(2) mm/s relative to NpAl2) corresponds to pentavalent neptunium. Np5+ are non Kramers ions and since they are located in very low symmetry crystalline fields, their being nonmagnetic is not surprising. On the other hand, the phenomenon that iron in an inter-metallic compound is nonmagnetic is not that common.