Optical properties of rare earth thin films

Abstract

Measurements have been made of the complex refractive index of thin films of several of the rare earth elements in the photon energy range from 0,33 - 3.0eV, using a conventional ellipsometric method. The observations were carried out at various temperatures between 4.2 and 300ºK corresponding to the different magnetically ordered phases of the elements neodyraium, gadolinium, terbium, dysprosium, holmium, erbium and ytterbium. In the elements which show helical ordering, no evidence was found for absorption at the gaps corresponding to the superzone boundaries. Instead the results suggest that magnetic ordering of any type is seen in a general way through direct transitions between the exchange split levels of the band structure. The exchange energies estimated this way are in reasonable agreement with theoretical values. Suggested locations are given for the regions in k space corresponding to these "magnetic transitions". The Drude contribution to the optical conductivity has been obtained and the variation of σ(w)(_w=o) for the different elements is in good agreement with static conductivity measurements although the magnitude of the optical constant is in general larger by about 15%.This is presumably due to the effects of the joint anisotropy of the electron relaxation times and velocities at the Fermi surface.