Unidirectional Behavior Of Uncompensated Fe Orbital Moments In Exchange-Biased Co/Femn/Cu(001)

Exchange-bias and uncompensated magnetic moments in epitaxially grown fcc Co/FeMn/Cu (001) are studied with element-specific x-ray magnetic circular dichroism. The magnitude of the uncompensated Fe moments of the antiferromagnet FeMn and the sign of their magnetic coupling to the ferromagnet Co are indicated by the circular dichroism signal. In the biased state the orbital-to-spin-moment ratio of the uncompensated Fe moments is found to be enhanced in remanence after magnetizing into the field cooling direction but zero after magnetizing into the opposite direction. This unidirectional behavior is mainly caused by a change in the orbital moment component along the easy magnetization axis probed in the experiment. It clearly reflects the macroscopic unidirectional loop shift being characteristic for the exchange-bias effect. This result suggests that spin-orbit coupling between rotatable spin and pinned orbital moments belongs to the magnetic interactions which cause the exchange-bias effect in the present system.