Magnetic properties of EuTe-PbTe superlattices

We report the magnetic susceptibility as a function of temperature (4<T<200 K) and magnetic field (1000<H<7000 Oe) of EuTe/PbTe superlattices grown by molecular-beam epitaxy. Three superlattices were used, consisting of four EuTe monolayers alternating with four PbTe monolayers (EuTe${)}_4$/(PbTe${)}_4$, (EuTe${)}_2$/(PbTe${)}_6$, and (EuTe${)}_1$/(PbTe${)}_3$ monolayers. The magnetic field was applied both in the plane of the layers and perpendicularly to it. Only the superlattice with four Eu monolayers shows an antiferromagnetic phase transition, at a nearly isotropic Néel temperature $T_N$=8.5±0.7 K comparable to that of bulk EuTe. In the paramagnetic regime, the susceptibility follows a Curie-Weiss behavior, with an anisotropic Curie-Weiss temperature FTHETA. This was explained with use of molecular-field theory by considering the influence of the strain in the superlattice layers on the Eu-Eu nearest-neighbor and next-nearest-neighbor interactions: FTHETA depends on both, while $T_N$ depends only on the latter, which is less strain dependent. The (EuTe${)}_4$/(PbTe${)}_4$ and the (EuTe${)}_2$/(PbTe${)}_6$ superlattices have roughly the same values for FTHETA, but the latter has no phase transition, presumably because the Eu atoms do not have all six of their nearest neighbors.