ON THE BIQUADRATIC INTERLAYER COUPLING IN LAYERED MAGNETIC-STRUCTURES

New mechanisms contributing to a biquadratic term in the phenomenological expression for the interlayer coupling energy are proposed and analysed theoretically. One of them takes into account exchange coupling not only between the two magnetic atomic planes which are adjacent to the interlayer, but also coupling between further atomic planes. The second mechanism is based on the fact that the wavefunctions corresponding to electronic states responsible for the coupling depend on the relative orientation of the film magnetizations. Both mechanisms give nonvanishing terms for ideally flat interfaces, contrary to the Slonczewski's (Phys. Rev. Lett. 67 (1991) 3172) mechanism which works only in the presence of interface roughness (steps).