POSITIVE EXCHANGE BIAS IN FEF2-FE BILAYERS

We have discovered a positive unidirectional exchange anisotropy in an tiferromagnetic (FeF2) and ferromagnetic (Fe) bilayers cooled through the antiferromagnetic critical temperature T-N in large magnetic field s. 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 t o be positive. This can be explained if the FeF2 surface spins couple to the external magnetic cooling field above T-N and the FeF2-Fe inter action is antiferromagnetic.