High pressure Mössbauer studies of magnetic Np intermetallics

A high pressure (50 kbar) Mössbauer 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 NpOs ₂ and NpAl ₂ 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 NpCo ₂Si ₂, while the hyperfine field remains constant. This suggests that its magnetic properties must arise from different sources.