  Non-monotonic dependence of anomalous Hall resistivity on temperature and
magnetization, including a sign change, was observed in Fe/Gd bilayers. To
understand the intriguing observations, we fabricated the Fe/Gd bilayers and
single layers of Fe and Gd simultaneously. The temperature and field
dependences of longitudinal resistivity, Hall resistivity and magnetization in
these films have also been carefully measured. The analysis of these data
reveals that these intriguing features are due to the opposite signs of Hall
resistivity/or spin polarization and different Curie temperatures of Fe and Gd
single-layer films.
