Paramagnetic Curie Temperatures of Gd-Doped Europium Chalcogenides

The paramagnetic Curie temperature $\Theta$ of the Eu chalcogenides rises precipitously with the addition of small concentrations of substitutional Gd donors; this effect is discussed on the basis of a mechanism suggested by Kasuya and Yanase. We find $\Theta$ by employing an appropriate variant of the usual high-temperature expansion for the susceptibility. Although the primitive expansion would not converge at the temperatures of interest, the modified expansion yields a weakly temperature-dependent but well-defined $\Theta \mathrm{}\left(T\right)\mathrm{}$. We consider both the case of an $s$-type conduction band (as postulated for EuSe by Kasuya and Yanase) and that of a $d$-type band (as suggested by Cho's calculation for EuS). The assumption of an $s$-type band is consistent with the observed $\Theta$ only if an additional nonlocal $s-f$ exchange—of plausible magnitude—is postulated, whereas the $d$-type band is directly consistent with the observed $\Theta$.