Magnetic origin of enhanced top exchange biasing in Py/IrMn/Py multilayers

In a substrate/ferromagnetic/antiferromagnetic/ferromagnetic trilayer, the exchange coupling occurring at the bottom ferromagnetic/antiferromagnetic interface is always found to be weaker than the one of the top antiferromagnetic/ferromagnetic interface. In spite of many reported results, in this paper we show that this effect does not exclusively depend on changes of the crystallographic quality or the roughness of the antiferromagnetic layer, or on magnetic interactions between the two ferromagnetic layers. On the contrary, this effect is associated with the different domain patterns on the two sides of the antiferromagnetic layer. Indeed, the upper interface of this layer appears unbiased after multilayer growth but it acquires a larger field biasing than the lower interface during the subsequent in-field annealing. Thus, in opposition to established growth conditions, applying a magnetic field during the multilayer growth does not appear to be the best way to get the highest exchange coupling.