The role of vacancies in the magnetic and magneto-optic properties of Tb-doped EuO and EuS films

Rocksalt structure solid solutions were prepared by coevaporating Eu 2O3 and Tb metal. The films have the general formula Eu1-xTbxO1-yD'Alembertian signy where y, the oxygen vacancy fraction, is given by y=(3x-1)/2. We have also prepared Tb-doped EuS solid solutions where y≅x so we can compare the effects of anion vacancies in both the oxide and sulfide systems. The coercivity, Hc, increases rapidly with Tb doping at about x=0.15 in the sulfide system and x=0.45 in the oxide system. The Hc of both systems falls on a universal curve, however, when plotted vs vacancies/RE (D'Alembertian sign/RE) with the rapid increase occurring at D'Alembertian sign/RE=0.15. This value is close to 1/6 (0.166) which in octahedral coordination corresponds to one anion vacancy/RE. This suggests that the near neighbor vacancy is producing a strong crystal field at the Tb site which causes strong single ion anisotropy and thus a high coercivity. The MO spectra of the oxides show a strong dependence on x. At x=0.25, peaks are observed at 1.5 eV (-3°), 2.2 eV (5.5°), and 2.7 eV (-2°). At x=0.33, the spectra change completely. A narrow peak with a rotation θK=-20° appears at 2.5 eV, dominating the spectrum. The reflectivity, R, goes through a sharp minimum at the same energy (R<1%), possibly causing the enhancement in θK.