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 an
d 25 Angstrom. It exhibits a striking temperature dependence which has
a positive temperature coefficient and is fully reversible in the tem
perature 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.