Magnetic properties of EuTe-PbTe superlattices

Abstract

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 N\'eel temperature ${T}_{N}$=8.5\ifmmode\pm\else\textpm\fi{}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.