Structural and magnetic properties of ultrathin Ni/Fe bilayers grown epitaxially on Ag(001)

Ultrathin epitaxial single bilayers of Ni/Fe were grown on Ag(001) substrates by means of molecular-beam epitaxy. The growths were monitored by reflection high-energy electron diffraction, x-ray photoemission, and Auger-electron spectroscopies. The Ni grows in the pure bcc structure for the first 3–6 monolayers and then reconstructs into a modified bcc phase for larger Ni thicknesses. The magnetic properties of the bilayers were studied using ferromagnetic resonance and Brillouin light scattering. The pure bcc Ni overlayers have no effect on the magnetic properties of the ultrathin Fe layers. Reconstructed Ni overlayers create a large in-plane anisotropy with fourfold symmetry in the bilayers with the easy axis along the Fe[100] crystallographic direction. A theory of exchange-coupled layers is presented. It is shown that for strongly exchange-coupled layers the bilayer magnetic properties are given by a linear combination of the magnetic properties of the individual layers, with coefficients for each layer given by the product of the magnetic moment per atom and the number of monolayers. The fourfold anisotropy agrees well with the bilayer theory and its value can be controlled by the appropriate choice of the individual layer thicknesses. These large in-plane anisotropies with fourfold symmetry are not common in 3d transition metals and it is unique that they are a result of misfit dislocations created during the Ni lattice reconstruction.