Brillouin light-scattering experiments on exchange-coupled ultrathin bilayers of iron separated by epitaxial copper (001)

The magnetic properties of bilayers of bcc Fe(001) separated by bcc Cu(001) films of variable thickness have been investigated by means of Brillouin light scattering. For copper thicknesses less than 9 monolayers (ML) the coupling between the iron films was found to be ferromagnetic; for thicknesses between 9 and 12 ML the coupling was found to be antiferromagnetic. The data are compared with the results of a model calculation that assumes a uniform static magnetization across each film: The magnetizations of the two films, ${\mathit{M}}_{\mathit{A}}$ and ${\mathit{M}}_{\mathit{B}}$, are assumed to be coupled by an interaction energy of the form J${\mathbf{M}}_{\mathit{A}}$⋅${\mathbf{M}}_{\mathit{B}}$/(‖${\mathit{M}}_{\mathit{A}}$‖‖${\mathit{M}}_{\mathit{B}}$‖), where J is a surface energy. For small applied fields and for antiferromagnetic coupling the magnetizations in the films need not be parallel. The magnetic-field dependence of frequencies calculated using this model display features that are similar to the observations.