Onset of long-range order in superlattices: Mean-field theory

We discuss the onset of long-range order in a model superlattice that consists of two materials, each of which undergoes a phase transition, in bulk form. We suppose that each transition has Ising character, and describe the onset of long-range order, along with the response to a uniform external field, by use of Landau-Ginzburg theory. The issue explored is the circumstances under which the structure displays a single maximum (singularity) in the susceptibility, and those in which two maxima (one maximum, one singularity) appear, near the bulk transition temperature of each constitutent. The analysis is motivated by experimental studies of a ${\mathrm{CoF}}_2$/${\mathrm{FeF}}_2$ superlattice reported by Jaccarino and co-workers.