SPIN-WAVE STUDY OF THE MAGNETIC EXCITATIONS IN A LAYERED STRUCTURE WITH 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. Peculiar features are found in the field de pendence of the acoustic and optical frequency gaps in correspondence to the critical fields for which the canting angle between the magneti zations of the two coupled layers presents discontinuities. In particu lar, we suggest that a minimum in the optical gap at the saturation fi eld could be experimentally observed by Brillouin light scattering in real systems, like Fe/Cr/Fe trilayers.