Coupling of pinned magnetic moments in an antiferromagnet to a ferromagnet and its role for exchange bias

The interaction between uncompensated pinned magnetic moments within an antiferromagnetic (AFM) layer and an adjacent ferromagnetic (FM) layer responsible for the existence of exchange bias is explored in epitaxially grown trilayers of the form FM2/AFM/FM1 on Cu3Au(0 0 1) where FM1 is ~12 atomic monolayers (ML) Ni, FM2 is 21-25 ML Ni, and AFM is 27 ML or 50 ML Ni~25Mn~75. Field cooling for parallel or antiparallel alignment of the out-of-plane magnetizations of the two FM layers does not make a difference for the temperature-dependent coercivity (HC), magnitude of exchange bias field (Heb), AFM ordering temperature (TAFM), and blocking temperature for exchange bias (Tb). We explain this by a model in which the uncompensated pinned magnetic moments distributed within the volume of the AFM layer interact with both of the FM layers, albeit with different strength. Parallel and antiparallel coupling between the magnetization of the pinned moments and the FM layers equally exists. This leads to the experimentally observed independence of HC, Heb, as well as of TAFM and Tb on the magnetization direction of the FM layers during field cooling. These results provide new and detailed insight into revealing the subtle and complex nature of the exchange bias effect.