Direct observation of frozen moments in the NiFe/FeMn exchange bias system

Abstract

We detect the presence of frozen magnetic moments in an exchange biased NiFe ferromagnet at the NiFe/FeMn ferromagnet/antiferromagnet interface by magnetic circular dichroism in x-ray absorption and resonant reflectivity experiments. Frozen moments are detected by means of the element-specific hysteresis loops. A weak dichroic absorption with unidirectional anisotropy can be linked to frozen magnetic moments in the ferromagnet. A more pronounced exchange bias for increasing the thickness of the FeMn layer correlates with an increase in orbital moment for interface Ni atoms carrying a frozen moment. These atoms compose about a single monolayer, but only a fraction of the atoms contributes by means of a strongly enhanced orbital moment to the macroscopic exchange bias phenomenon. The microscopic spin–orbit energy associated with these few interface frozen moment atoms appears to be sufficient to account for the macroscopic exchange bias energy.