We have studied the effect of the interface structure on the exchange bias in the ${\mathrm{FeF}}_2-\mathrm{Fe}$ system, for ${\mathrm{FeF}}_2$ bulk single crystals or thin films. The exchange bias depends very strongly on the crystalline orientation of the antiferromagnet for both films and crystals. However, the interface roughness seems to have a strong effect mainly on the film systems. These results indicate that the exchange bias depends strongly on the spin structure at the interface, especially on the angle between the ferromagnetic and antiferromagnetic spins. We have also found a strong dependence of the hysteresis loops shape on the cooling field direction with respect to the antiferromagnetic anisotropy axis, induced by a rotation of the ferromagnetic easy axis as the sample is cooled through $T_N.$ For the single crystal systems the results imply the existence of a perpendicular coupling between the antiferromagnetic and ferromagnetic spins at low temperatures.

• Received 10 April 1998

DOI:https://doi.org/10.1103/PhysRevB.59.6984