Mini Volume Collapse as Evidence for a 3-Body Magnetic Polaron in $Sm_{1-x}Eu_xS$

Samarium sulfide (SmS) is a non-magnetic narrow-gap (0.06 eV) semiconductor which undergoes a transition to a metallic intermediate valence state at 6.5 kbar. Europium sulfide (EuS) is a ferromagnetic semiconductor with a Curie temperature of 16K and a gap of 1.6 eV. Here we present a study of the lattice constant, magnetic susceptibility, and resistivity of the substitution series $Sm_{1-x}Eu_xS$ for 0 < x < 1. We observe a smooth interpolation of magnetic and transport behavior across the series, consistent with a virtual crystal scenario and Vegard's law. Surprisingly, however, the lattice constant deviates below Vegard's law in a manner that suggests parametric control of the Sm-Sm distance by the Eu moment in the manner of a magnetic polaron.