Negative remanence in synthetic antiferromagnetically coupled FeCo/Ru/FeCo films

Inverted hysteresis loops in a sputtered FeCo/Ru/FeCo bilayer have been investigated. A Stoner-Wohlfarth energy minimization calculation reproduced the negative remanence assuming a small misalignment of the easy axes of the ferromagnets. During the initial stage of the growth of the top layer, the external field fails to overcome the strong antiferromagnetic exchange coupling between the layers. The magnetizations, as a result, are no longer collinear and the easy axis of the top layer will vary with its thickness. The negative remanence disappeared following an annealing of the sample in a strong field confirming the theory.