  Indirect exchange coupling in magnetic multilayers, also known as the
Ruderman-Kittel-Kasuya-Yosida (RKKY) interaction, is known to be highly
effective in controlling the interlayer alignment of the magnetization. This
coupling is typically fixed at the stage of the multilayer fabrication and does
not allow ex-situ control needed for device applications. It is highly
desirable, in addition to the orientational control, to also control the
magnitude of the intralayer magnetization, ideally switch it on/off by
switching the relevant RKKY coupling. Here we demonstrate a magnetic multilayer
material, incorporating thermally -- as well as field-controlled RKKY exchange,
focused on to a dilute ferromagnetic alloy layer and driving it though it's
Curie transition. Such on/off magnetization switching of a thin ferromagnet,
performed repeatably and fully reproducibly within a low-field sweep, results
in a giant magnetocaloric effect, with the estimated isothermal entropy change
of -10 mJ/K cm^3 under an external field of ~10 mT, which greatly exceeds the
performance of the best rare-earth based materials used in the
adiabatic-demagnetization refrigeration systems.
