Paramagnetic Curie Temperatures of Gd-Doped Europium Chalcogenides

The paramagnetic Curie temperature Θ 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 Θ 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 Θ(T). 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 Θ only if an additional nonlocal s-f exchange---of plausible magnitude---is postulated, whereas the d-type band is directly consistent with the observed Θ.