  We show, theoretically, that the phase of the interlayer exchange coupling
(IEC) undergoes a topological change of approximately $2\pi$ as the chemical
potential of the ferromagnetic (FM) lead moves across a hybridization gap (HG).
The effect is largely independent of the detailed parameters of the system, in
particular the width of the gap. The implication is that for a narrow gap, a
small perturbation in the chemical potential of the lead can give a sign
reversal of the exchange coupling. This offers the possibility of controlling
magnetization switching in spintronic devices such as MRAM, with little power
consumption. Furthermore we believe that this effect has already been
indirectly observed, in existing measurements of the IEC as a function of
temperature and of doping of the leads.
