Switching off the magnetic exchange coupling by quantum resonances

We clarify the role of quantum-well states in magnetic trilayer systems of the majority carrier in the ferromagnetic configuration and all carriers in the antiferromagnetic configuration. In addition to numerical and analytic calculations, heuristic pictures are provided to explain the effects of a capping layer and side-layer modulation in recent experiments. This immediately offers individual answers to two unexplained subtle findings in experiments and band-structure calculations. Furthermore, it allows a more flexible tuning of or even a turning off of the interlayer exchange coupling.