  We report on a comprehensive study of the magnetic coupling between soft
magnetic Fe layers and hard magnetic Dysprosium (Dy) layers at low temperatures
(4.2 - 120K). For our experiments we prepared thin films of Fe and Dy and
multilayers of Fe/Dy by ultra-high vacuum sputtering. The magnetic properties
of each material were determined with a superconducting quantum interference
device. Furthermore, we performed magnetoresistance measurements with similarly
grown, microstructured devices, where the anisotropic magnetoresistance (AMR)
effect was used to identify the magnetization state of the samples. By
analyzing and comparing the corresponding data of Fe and Dy, we show that the
presence of a Dy layer on top of the Fe layer significantly influences its
magnetic properties and makes it magnetically harder. We perform a systematic
evaluation of this effect and its dependence on temperature and on the
thickness of the soft magnetic layer. All experimental results can consistently
be explained with exchange coupling at the interface between the Fe and the Dy
layer. Our experiments also yield a negative sign of the AMR effect of thin Dy
films, and an increase of the Dy films' Curie temperature, which is due to
growth conditions.
