Finite-size effects and “surface spin-flop” phenomena in antiferromagnetically coupled multilayers

Antiferromagnetic superlattices with low uniaxial anisotropy have been investigated within a phenomenological approach based on a one-dimensional discretized model for the magnetic energy of laterally homogeneous states, which is due to Mills. A transformation of this energy into a system of interacting antiferromagnetic dimers allows to investigate in detail the evolution of the magnetic states in the applied field and elucidates the nature of reorientation transitions. We show that the low anisotropy low field phase diagram is ruled by a first-order transition into an inhomogeneous spin-flop phase that keeps mirror symmetry for ideal multilayer stacks. The continuum limit for Mills models allows to formulate this mechanism in simple terms by the formation and internal transformation of an antiferromagnetic domain wall in the center of the finite superlattices. Within the continuum model, generalizations of Mills model and connections with other antiferromagnetic systems in confined geometries are discussed.