Heat Induced Antiferromagnetic Coupling in Multilayers with Ge Spacers

We report on heat induced antiferromagnetic exchange coupling in a new system: ferromagnetic Fe films separated by a spacer of amorphous Ge. Antiferromagnetic coupling occurs at spacer thicknesses between 20 and 25 {}. It exhibits a striking temperature dependence which has a positive temperature coefficient and is fully reversible in the temperature range between 40 and 230 K. Our findings about the importance of the interfaces support the interpretation that resonant tunneling through localized states in the gap of the spacer mediate the magnetic exchange.