Excitation spectrum of an antiferromagnetic film with a non-uniform ground state

Using a microscopic approach, we study the elementary excitations for a uniaxial antiferromagnetic film with an external magnetic field H applied along the easy axis. For H greater than a critical value, H S , the system has a non-uniform ground state which is obtained, very rapidly and with high accuracy, in terms of a two-dimensional area preserving map, where the surfaces are introduced as appropriate boundary conditions. The consequent spectrum is calculated in the whole Brillouin zone and the behaviour of the localized surface modes is carefully analysed. It is possible to observe that the lowest-energy modes have substantially a bulk character for low values of k/sub /// (the wavevector in the plane parallel to the surfaces), whilst the localized surface modes have the lowest energy for high values of k/sub ///. The relevance of our results for the superlattices composed by ferromagnetic films antiferromagnetically coupled across non-magnetic layers, which present the giant magnetoresistance phenomenon, is discussed, too.