High pressure Moessbauer studies of magnetic Np intermetallics

A high pressure (50 kbar) Moessbauer spectrometer for the 60 keV resonance in 237Np for temperatures between 1.4 and 100 K is described. It was used to study the magnetic properties of some neptunium intermetallics under pressure. For the cubic Laves phase compounds NpOs2 and NpAl2 a drastic decrease of the ordering temperature, the hyperfine field, and the isomer shift under increasing pressure was observed. It shows that their magnetic properties are primarily determined by the Np-Np separation which controls the width and hybridization of the 5f band. In contrast, an increase of ordering temperature coupled with a decrease of isomer shift with pressure was found in tetragonal NpCo2Si2, while the hyperfine field remains constant. This suggests that its magnetic properties must arise from different sources, most likely connected with the transition metal component. (orig.)