Raman Scattering in Heavy Rare Earth Metals.

Inelastic light scattering spectra were measured in MBE grown single crystal films of Dysprosium, Erbium and Yttrium. In order to measure the weak Raman signals of metals a single grating monochromator was designed and constructed and added to an existing double grating monochromator to improve the stray light rejection. Polarized Raman Scattering was used to probe vibrational and electronic excitations in Dy, Er and Y and magnetic excitations in Dy and Er. The vibrational light scattering measurements have revealed an anomalous temperature dependence of the zone-center transverse optical phonon in Dy and Er. Previous measurements of the thermal expansion of bulk Dy and Er indicate that this temperature dependence cannot be explained by lattice anharmonicity and the related volume change. The hardening of this mode may be due in part to the coupling of the TO phonon to the ordered magnetic moments. The linewidth of this mode is observed to increase at low temperature in Dy and Er. This may be an unresolved splitting of this mode due to a symmetry-breaking orthorhombic distortion of the lattice in the ferromagnetic phase. The ferromagnetic acoustic spin wave gap in Dy was observed. To our knowledge this is the first observation of a metallic spin wave by Raman scattering. The light scattering measurement of the spin wave gap energy agrees with and corroborates earlier infrared resonance measurements in bulk Dy. The observation of the optical spin wave in ferromagnetic Dy is forbidden by Raman selection rules. The optical spin wave in spiral phase Dy was not observed. The light scattering observation of spin waves in Erbium is forbidden for backscattering in crystals grown along the c-axis.