SPIN-WAVE STUDY OF THE MAGNETIC EXCITATIONS IN SANDWICH STRUCTURES COUPLED BY BILINEAR AND BIQUADRATIC INTERLAYER EXCHANGE

Using a microscopic and quantum approach, we study the ground-state co nfiguration and the spin-wave excitations of a model of two ferromagne tic monolayers, interacting via both bilinear and biquadratic exchange , and subject to an easy-plane anisotropy, a quartic in-plane anisotro py and an external field. In the framework of a Green's-function metho d, we obtain analytical results for the spin-wave dispersion curves. P eculiar features are found in the field dependence of the acoustic and optical frequency gaps, in correspondence to the critical fields for which the canting angle between the magnetizations of the two coupled layers presents discontinuities. In particular, a minimum for the opti cal gap at the saturation field is predicted, which could be experimen tally observed by Brillouin light scattering in real systems.