Quantum-well theory of the exchange coupling in magnetic multilayers

Magnetic multilayers are novel man-made materials with great potential for applications, especially in magnetic storage technology. It is observed that the configuration of the magnetic moments of neighboring magnetic layers separated by a nonmagnetic layer can be either ferromagnetic or antiferromagnetic depending on the thickness of the intervening spacer layer. This implies an oscillatory indirect coupling between the layers mediated by itinerant electrons in the spacer layer. It is shown that the physical origin of the effect is quantum-mechanical confinement of electrons in the spacer layer by spin-dependent potentials of the magnetic layers. The confinement can be either complete (bound states) or partial (resonances) depending on the ferromagnet and spacer materials and on the layer orientation. A general method for calculating the interlayer coupling, which is based on a formal analogy with de Haas-van Alphen effect is described and illustrated for specific systems such as Co/Cu(100). It is shown that the oscillation period is directly related to the size and shape of the Fermi surface of the bulk spacer material. It is also shown that the coupling strength is determined by detailed matching of the ferromagnet and spacer bands in the direction perpendicular to the layers. Finally, band mismatch mechanism is invoked to explain the observed trends in coupling strength across the Periodic Table.