EXCITATION SPECTRUM OF AN ANTIFERROMAGNETIC FILM WITH A NONUNIFORM GROUND-STATE

Using a microscopic approach, we study the elementary excitations for a uniaxial antiferromagnetic him with an external magnetic field H app lied along the easy axis. For H greater than a critical value, H-S, th e system has a non-uniform ground state which is obtained, very rapidl y and with high accuracy, in terms of a two-dimensional area preservin g map, where the surfaces are introduced as appropriate boundary condi tions. The consequent spectrum is calculated in the whole Brillouin zo ne and the behaviour of the localized surface modes is carefully analy sed. It is possible to observe that the lowest-energy modes have subst antially a bulk character for low values of k(parallel to) (the waveve ctor in the plane parallel to the surfaces), whilst the localized surf ace modes have the lowest energy for high values of k(parallel to). Th e relevance of our results for the superlattices composed by ferromagn etic films antiferromagnetically coupled across non-magnetic layers, w hich present the giant magnetoresistance phenomenon, is discussed, too .