Antiferromagnetic interlayer coupling in Fe/c-SiFe/Fe sandwiches and multilayers

We report the first ab-initio study of the interlayer exchange coupling in Fe/c-FeSi/Fe sandwiches and Fe/c-FeSi multilayers. We perform several structural studies, which show the stability of the CsCl arrangement seen experimentally for the spacer. We find antiferromagnetic coupling between the Fe slabs for spacer thicknesses smaller than about 15 $\AA$, for both sandwiches and multilayers structures. We also study the effect of pinholes, interface roughness or structural misconfigurations of the spacer on the sign and magnitude of the exchange constant J. We finally show that the asymptotic behavior of J is determined by a flat band of the c-FeSi spacer, located at the M point in the Brillouin zone.