  We argue that the interface coupling, responsible for the positive exchange
bias (HE) observed in ferromagnetic/compensated antiferromagnetic (FM/AF)
bilayers, favors an antiferromagnetic alignment. At low cooling field this
coupling polarizes the AF spins close to the interface, which spin
configuration persists after the sample is cooled below the Neel temperature.
This pins the FM spins as in Bean's model and gives rise to a negative HE. When
the cooling field increases, it eventually dominates and polarizes the AF spins
in an opposite direction to the low field one. This results in a positive HE.
The size of HE and the crossover cooling field are estimated. We explain why HE
is mostly positive for an AF single crystal, and discuss the role of interface
roughness on the magnitude of HE, and the quantum aspect of the interface
coupling.
