Domain processes in the magnetization reversal of exchange- biased IrMn/CoFe bilayers

We have used transmission electron microscopy to study directly the way magnetization reversal proceeds in the ferromagnetic layer for sets of bilayers in which the ferromagnetic layer (CoFe) was of constant thickness while the thickness of the antiferromagnetic layer (IrMn) was varied. The first set studied was in the as-deposited state while the second was subjected to rapid thermal processing. For IrMn thicknesses of 20 Angstrom, no shift fields were observed, although significant coercivities (similar to70 Oe) were recorded and reversal involved rather simple domain processes. By contrast, complex small-scale domains dominated the reversal processes for samples where the IrMn thickness exceeded 60 Angstrom; here, strong exchange biasing and higher coercivities were the norm. For all thicknesses of IrMn, an unexpected variation in the dominant orientation of domain walls on the outward and return parts of the magnetization cycle tended to be observed. A possible origin of this, together with the differences between the as-deposited samples and those subjected to rapid thermal processing, are discussed