SPIN-WAVES IN ANTIFERROMAGNETIC THIN-FILMS AND MULTILAYERS - SURFACE AND INTERFACE EXCHANGE AND ENTIRE-CELL EFFECTIVE-MEDIUM THEORY

High-quality, insulating antiferromagnetic thin and multilayers offer possibilities for new studies of exchange mechanisms at surface and ac ross interfaces between dissimilar magnetic materials. Using a microsc opic theory, we study long-wavelength spin waves to answer two questio ns: (1) Can we determine accurate values for surface and interface exc hange from spin-wave frequencies and (2) can we obtain an effective-me dium description which properly reproduces all the spin-wave excitatio ns of the antiferromagnetic structure. We find that the frequencies of surface and interface modes are particularly sensitive to interface e xchange values, even when the spin-wave frequencies for the materials are far apart and little coupling is expected. We also present a metho d for calculation the dynamic magnetic response of a superlattice stru cture that avoids a serious approximation made in conventional effecti ve-medium theory. Our effective-medium formulation gives results in ex cellent agreement with microscopic calculations and will be useful for characterizing thin antiferromagnetic films as well as large multilay ers. Susceptibilities are derived which can be directly applied to cal culations for infrared reflectivity experiments.