Magneto-optic spectra of EuS-Gd and EuS-Tb films

Thin films were prepared by coevaporating EuS and Gd metal by abeam onto cooled substrates as previously described for EuS-Tb films. The mole percent Gd was varied from 0 to 80. The films have the f.c.c., NaCI structure, the same as EuS, at least up to 50% Od es determined by x-ray diffraction. The Iattice constant decreases more rapidly than Vegard's law when plotted against the nominal Gd mole fraction indicating possibly a high concentration of sulfur site vacancies. We suggest the general formula for these materials: Eu
_1-x
Gd
_x
S
_1-y
□
_y
where y can be less than x if some of the Eu is displaced from the EuS lattice by Gd and rejected to the grain boundaries as Eu metal. The latter process could also contribute to the more rapid decrease in lattice constant than expected based on the nominal Gd mole fraction. The Gd substituted films show increased T
_c
, H
_c
and squareness with increasing Gd concentration es previously observed in the EuS-Tb system. However, in the EuS-Gd system the coercivity increases monotonically to about 1 kOe at 50% Gd whereas in the EuS-Tb system the coercivity is over 4 kOe at 30% Tb. The lower coercivity in the EuSGd system is expected because Gd is nearly an e-state ion depending on how much d character it retains. The resistivity generally decreases with Gd concentration. The 50% Gd has a ρ of about 800 μΩcm and ρH/ ρ = 0.3 %. The effect of Gd on the resistivity suggests that it is contributing electrons to the conduction band as it does in the well known EuS-GdS solid solutions. In our films, however, there is no sulfur to take up some of the electrons donated by the Gd so more electrons are available.