Magnetic Properties of Actinide Intermetallics Under High Pressure

The magnetic properties of intermetallics of the light actinides can vary from primary local moment behaviour through itinerant electron magnetism to non-magnetic behaviour. One parameter which strongly influences the magnetic character of the compound is the actinide-actinide separation which can experimentally be varied by the application of high pressure. Combining this with Mossbauer spectroscopy of the 60 keV transition in 237Np allows a detailed study of local magnetic properties of the Np ion as a function of volume. Recent measurements on three Np intermetallics will be reviewed: (1) NpCo2Si2. Neutron diffraction and Mossbauer data at ambient pressure classify this material as a highly localized magnet. Under pressure the hyperfine field remains constant while the Neel-temperature TN increases with smaller volume. The variation of TN can be explained quantitatively within the rigid spin model of Ruderman-Kittel-Arrott. (2) NpAl2 and NpOs2. These are cubic Laves phase materials, where NpAl2 has the largest lattice parameter (a = 7.785 A) and shows largely localized behaviour, while NpOs2 with a = 7.459 A is mostly itinerant. In both materials the reduction of lattice constant initiates a reduction of hyperfine field and Curie temperature, which is thought to arise from a delocalization of the magnetic 5f-electrons. In addition line broadenings are observed which show that the delocalization process is a dynamical phenomenon. It is analyzed in terms of a fluctuation between a fully localized and a fully itinerant electron state.