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Sanford KernAssociate Professor 
Neutron ScatteringCrystal Fields in Rare Earth and Actinide Materials The rare earth and actinide oxides and halides exhibit a variety of magnetic responses at low temperatures; among them are reduced magnetic moments in the ordered state. The reasons for this behavior are complex and are often tied to electron-phonon interactions within the first several crystal field levels. The primary means that we are using for studying these materials are magnetic susceptibility and neutron scattering techniques. Most of our attention has been given to the high symmetry compounds, such as the dioxides and the simple perovskites, both of which present the f-electron system to a cubic, or near-cubic environment. Some, such as PrO2 and UO2, are antiferromagnets, while NpO2 and BaPrO3 undergo transitions that indicate highly reduced magnetic moments at low temperatures. More recent experiments have been performed on the low symmetry actinide tetrafluorides, where complex crystal field effects on the magnetic and thermal properties are being explored.Many of the experiments are carried out at the Intense Pulsed Neutron Source at Argonne National Laboratory. Neutron diffraction is used to characterize the nuclear and magnetic structure, and inelastic measurements are used to study the crystal field structure. Excitations to 250 meV have been seen. Quasi-elastic techniques are employed to determine the low-lying excitations that split the ground states when they are subjected to interactions with the lattice; these excitations are thought to be connected to the interactions that drive low temperature cooperative transitions. |