Crystal field effects using polarised neutron spectroscopy

Full text: The ground state of a Pr3+ ion is ninefold degenerate. In the hexagonally symmetric crystal field that the ion experiences in PrAl3, this degeneracy is partially lifted and three doublets and three singlets come about. Transitions take place between these levels. As each level has a different magnetic moment, a transitions alters the magnetic induction in the sample. The energy of the transition can therefore be measured using magnetic neutron scattering. If polarised neutrons are used, and the polarisation vector is parallel to the scattering vector, magnetic neutron scattering occurs with neutron spin flip. Nuclear scattering will be non-spin flip. This means that an instrument fitted with both polarisation and energy analysis should be useful in the measurement of crystal field transitions, as it can discriminate nuclear from magnetic scattering. One such instrument is the LONGPOL, long wavelength polarised neutron scattering instrument at Lucas Heights. In this experiment, LONGPOL was used to measure transition energies in PrAl3. The transitions were successfully seen. The ground state transition was found to be of energy 4.5 + 0.4 meV, and a second transition was seen at ∼3.5 meV. Further, nuclear and magnet elastic scattering were seen, the latter quasielastic. Analysis of these features was aided by the polarisation analysis, as it could immediately distinguish the magnetic components of the scattering from the nuclear