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

We present a calculation of spin waves in coupled multilayered structu res that is based on exact evaluation of both exchange and dipolar fie lds. We compare our results with earlier continuum treatments. The gou nd-state spin configuration in antiferromagnetically coupled multilaye rs can differ significantly from the uniformly canted ground state usu ally assumed. This nonuniform ground state is found to alter radically the character of the spin-wave modes and sometimes lead to a strong l ocalization of the wave to the outermost magnetic films of the multila yer. In addition, we examine the validity of effective-medium theory-a continuum theory-and find that it does not completely describe spin-w ave excitations in finite antiferromagnetically coupled multilayers. F inally, the spin-wave frequencies are found to be nonreciprocal with r espect to propagation direction for most directions, i.e., omega(q) no t-equal omega(-q) where q is the propagation wave vector. This nonreci procal behavior is explained from basic symmetry considerations. Again , the nonreciprocity is not properly described within effective-medium theory.