  We observe a strong thermally-controlled magnon-mediated interlayer coupling
of two ferromagnetic layers via an antiferromagnetic spacer in spin-valve type
trilayers. The effect manifests itself as a field-induced coherent switching of
the two ferromagnets, which can be controlled by varying temperature and the
spacer thickness. We explain the observed behavior as due to a strong
hybridization of the ferro- and antiferro-magnetic magnon modes in the trilayer
at temperatures just below the N\'eel temperature of the antiferromagnetic
spacer.
