Strong changes in the magnetic properties of ultrathin Co/Cu(001) films due to submonolayer quantities of a nonmagnetic overlayer

We have studied the evolution of the magnetic properties of the Co/Cu(001) system as a Cu overlayer is deposited, using the magneto-optical Kerr effect (MOKE) in situ. We observe striking, nonmonotonic variations in the coercive field ${\mathit{H}}_{\mathit{c}}$, the M-H loop amplitude ${\mathit{M}}_{\max }$, and the ratio of remanent to saturation magnetization S upon the deposition of submonolayer quantities of Cu. We propose that the observed effects arise due to overlayer-induced changes in the electronic structure. As a consequence both the magnetic anisotropy of the films and the magneto-optical response vary strongly. We show that the entire Co film is affected, in that a single Cu atom at the surface can affect the behavior of more than 40 Co atoms throughout the thickness of the film, illustrating the importance of the segregation of substrate atoms during the growth of such films. Distinct behavior occurs according to the range of overlayer thickness, implying that separate physical mechanisms dominate for different coverages. At submonolayer thicknesses we propose that a step-induced uniaxial magnetic anisotropy term is very strongly modified by the presence of a partial Cu overlayer. Concurrent changes in the magneto-optical signal are attributed to the electronic structure of the partial Cu/Co interface. For overlayer thicknesses in the monolayer range, electronic effects associated with the completed interface occur, in particular a reduction of the Curie temperature ${\mathit{T}}_{\mathit{C}}$ and a possible suppression of the magnetic moment of the Co atoms.