ABSTRACTWe report the formation of the CsCI FeSi phase in Fe/FeSi multilayers with constant Fe thickness and varying FeSi layer thickness. The crystallographic and magnetic properties of this metastable FeSi phase are determined in thick (62 nm) epitaxial silicide layers with ion channeling, X-ray diffraction and Mossbauer spectroscopy measurements which clearly show that the CsCl phase becomes increasingly magnetic upon cooling down to 4.2 K. These results indicate that the transition from antiferromagnetic to ferromagnetic coupling observed at low temperatures in such multilayers is controlled by a magnetic phase transition in the FeSi spacer layer.