Oscillatory exchange coupling: RKKY or quantum-well mechanism?

The exchange coupling in multilayers of iron and chromium is calculated in the local spin density approximation, as a function of the Fe magnetic moment. We find a short-period and a long-period oscillation, which we attribute to a harmonic of the short period. For small moments, the interlayer coupling is found to be bilinear in the Fe moments, as assumed in RKKY theory. For moments appropriate to real iron the coupling tends to saturate, as assumed in quantum-well models, demonstrating that the quantum-well model more correctly describes the real iron-chromium system, though both models give correct periods of the oscillation.