Spin-wave spectrum of a superlattice with antiferromagnetic interfacial coupling

We investigate the spin-wave spectrum of a superlattice composed of two alternating ferromagnetic films with antiferromagnetic coupling at the interfaces. Depending on the value of temperature and of an externally applied magnetic field, this system can exist in any of four distinct phases. The temperature- and field-dependent behaviors of the spin-wave modes in three of these phases are investigated. Propagation both perpendicular and parallel to the layers is considered. The frequency of one mode is found to increase with increasing temperature, in contradiction to the usual expectation; this behavior is explained in terms of a ferrimagnet analogy.