Positive Exchange Bias in FeF2-Fe Bilayers

We have discovered a positive unidirectional exchange anisotropy in antiferromagnetic (Fe${\mathrm{F}}_2$) and ferromagnetic (Fe) bilayers cooled through the antiferromagnetic critical temperature $T_N$ in large magnetic fields. For low positive cooling fields, the ferromagnet's magnetization ( $M-H$) loop center shifts to negative fields, as is normally observed in other systems. In contrast, large cooling fields can cause the shift to be positive. This can be explained if the Fe${\mathrm{F}}_2$ surface spins couple to the external magnetic cooling field above $T_N$ and the Fe${\mathrm{F}}_2$-Fe interaction is antiferromagnetic.