Mössbauer and magnetization studies of some neptunium heavy-electron intermetallics

Mössbauer studies of the heavy-electron magnet Np${\mathrm{Be}}_{13}$ and new data on the compounds Np${\mathrm{Ru}}_2$, Np${\mathrm{Os}}_2$, and Np${\mathrm{Ir}}_2$ are reported. Whereas Np${\mathrm{Ru}}_2$ remains paramagnetic to lowest temperatures, the three other materials exhibit long-range magnetic order which, however, is very sensitive to defects and strain and may easily be suppressed. A correlation between isomer shift and electronic specific heat is presented.