MICROSCOPIC CALCULATION OF SPIN-WAVES IN ANTIFERROMAGNETICALLY COUPLED MULTILAYERS - NONRECIPROCITY AND FINITE-SIZE EFFECTS

We present a calculation of spin waves in coupled multilayered structures that is based on exact evaluation of both exchange and dipolar fields. We compare our results with earlier continuum treatments. The gound-state spin configuration in antiferromagnetically coupled multilayers can differ significantly from the uniformly canted ground state usually assumed. This nonuniform ground state is found to alter radically the character of the spin-wave modes and sometimes lead to a strong localization of the wave to the outermost magnetic films of the multilayer. In addition, we examine the validity of effective-medium theory-a continuum theory-and find that it does not completely describe spin-wave excitations in finite antiferromagnetically coupled multilayers. Finally, the spin-wave frequencies are found to be nonreciprocal with respect to propagation direction for most directions, i.e., omega(q) not-equal omega(-q) where q is the propagation wave vector. This nonreciprocal behavior is explained from basic symmetry considerations. Again, the nonreciprocity is not properly described within effective-medium theory.