  For epitaxial trilayers of the magnetic rare-earth metals Gd and Tb, exchange
coupled through a non-magnetic Y spacer layer, element-specific hysteresis
loops were recorded by the x-ray magneto-optical Kerr effect at the rare-earth
$M_5$ thresholds. This allowed us to quantitatively determine the strength of
interlayer exchange coupling (IEC). In addition to the expected oscillatory
behavior as a function of spacer-layer thickness $d_Y$, a temperature-induced
sign reversal of IEC was observed for constant $d_Y$, arising from
magnetization-dependent electron reflectivities at the magnetic interfaces.
