Transition to the perpendicular easy axis of magnetization in Ni ultrathin films found by x-ray magnetic circular dichroism

Using soft-x-ray magnetic circular dichroism we have observed a change in the easy axis of magnetization in Ni ultrathin films on Cu(001) from parallel to the surface for 5--9 monolayers (ML) to perpendicular for 10--75 ML. This is an unusual effect which is counter to the predictions of magnetic surface anisotropy arguments. We conclude that the effect is due to a shape anisotropy, introduced either by a change in the Ni lattice structure or by three-dimensional morphological changes in the film.