Phase transitions in magnetic superlattices

We investigate the magnetic-field- and temperature-dependent equilibrium structure of magnetic superlattices formed from two ferromagnetic materials which couple antiferromagnetically at the interfaces. Both a macroscopic, Landau-Ginzburg, and a microscopic approach are used. Due to competing exchange and Zeeman interactions, a variety of phases exist in the superlattice. There are aligned phases where all the spins are either parallel or antiparallel to the applied field, and there is a twisted phase where the spins in each layer lie at a different angle with respect to the applied field. We show that small changes in the layering structure can lead to dramatic changes in the phase diagram.