Asymmetrical Dependence of Exchange Coupling in Synthetic Antiferromagnets FeMn/Co/Ru/Co on the Ferromagnetic Layer Thickness

The exchange field is found to change as an exponential function of 1/|t₁ — t₂| in two wedged synthetic antiferromagnets FeMn/Co(t₁ = 12.0 nm)/Ru(0.7 nm)/Co(t₂) and FeMn/Co(t₁)/Ru(0.7 nm)/Co(t₂ = 12.0 nm). It has opposite signs and different magnitudes in the two above series. Based on a simple model of magnetization, coherent rotation and magnetically non-rigid coupling, numerical calculations showed that this asymmetrical and exponential dependence is attributed to a non-collinear alignment of the magnetizations in the low field range. It is interesting to find that the minor loop has a pronounced asymmetrical shape when the two ferromagnetic layers are close in thickness, hinting the formation of a planar domain wall in the FeMn layer during the magnetization reversal of the ferromagnetic layers.