Rare earth crystal field spectroscopy by neutron magnetic scattering: From xenotime to high Tc superconductors

Optical spectroscopy is one of the traditional methods used to determine the overall splitting of the rare earth ion energy states within an fN configuration in either solution or solid state of transparent materials. This technique can provide data over a wide energy range with good resolution, and the parameters obtained for the empirical Hamiltonian reflect the `best fit' of the observed energies. Absolute intensity measurements of optical transitions and their comparison with theory, however, are difficult. Magnetic scattering of thermal neutrons arises from an interaction of the neutron magnetic moment with the convection and spin current of the scatterer. Such a weak interaction does not involve any excited intermediate states of the system and requires only a first-order perturbation treatment to calculate the scattering cross-section. However, neutron spectroscopy probes only states at energies less than approximately 1 eV. It is demonstrated that a combined treatment of neutron and optical data can provide the complementary information necessary for a proper characterization of the level splittings and wavefunctions of the rare earth ions in xenotime (RPO4, R = Tb to Yb). Next we discuss the analyses of neutron scattering data for the understanding of the rare earth energy levels and wavefunctions in various high Tc copper oxide superconductors and related compounds. The importance of a consistent refinement of the crystal field parameters across a series of isostructural rare earth compounds and a systematic comparison of the low temperature magnetic properties with model calculations are emphasized.