HYPERFINE FIELD OF 237NP IN NPO2

Nuclear hyperfine spectra, obtained by means of the Mössbauer effect of the 59.6 keV transition in 237Np, have been investigated in NpO2 between 4.2° and 30°K. At low temperatures a broadening of the resonance line is observed which is interpreted to be a small, unresolved magnetic splitting. From the size of the magnetic hyperfine field, the magnetic moment on the Np atom is estimated to be of the order of 0.01 μB. These results confirm the presence of antiferromagnetic ordering in accordance with previous magnetic susceptibility and heat capacity data, and in addition show that the magnetic moment in the ordered state is unusually low, as has been indicated by previous neutron diffraction measurements. © 1968.