THEORY OF INTRINSIC AND THERMALLY-INDUCED INTERLAYER MAGNETIC COUPLING BETWEEN FERROMAGNETIC-FILMS SEPARATED BY AN INSULATING LAYER

A formalism developed previously to study the interlayer exchange coupling between ferromagnetic layers separated by a nonmagnetic metal spacer is applied to the case of an insulating spacer. It allows a unified treatment of both cases (metal and nonmetal spacer), provided one introduces the concept of a complex Fermi surface. In contrast to the metal-spacer case, where the exchange coupling decreases with increasing temperature, the coupling across an insulating spacer is found to increase with temperature. This finding is in agreement with recent experimental observations.