Doping EuS films with Tb or Gd increases the Curie point because of strong exchange through the free carriers and increases the coercivity because of strong crystal fields associated with vacant sulfur sites. The two main features of the reflectivity spectra of the doped samples are a strong free carrier contribution below 1.5 eV and a region of low reflectance between 1.5 and 4 eV. The Kerr peak in EuS at 2.4 eV is greatly enhanced in some of the doped compositions, probably as a consequence of low values of the optical functions n( omega ) and k( omega ) in this low-reflectance region.
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