Magnetic response of isolated cd impurities on ferromagnetic Ni surfaces and Ni-Pd interfaces : Access to ordered structures of polarized impurities with ultrahigh density

The induced $s$ electron polarization at isolated Cd atoms on ferromagnetic Ni surfaces and the $\mathrm{Pd}∕\mathrm{Ni}$ interface was studied by the measurement of magnetic hyperfine fields $({B}_{hf})$ applying perturbed angular correlation spectroscopy. For Cd on Ni, we found a surprisingly strong dependence of the ${B}_{hf}$ values on the local geometry in terms of the number of nearest neighbors (NNs) leading to the conclusion that impurities with their nearest neighbors form magnetic clusterlike units embedded in the two-dimensional ferromagnetic surface. At the $\mathrm{Pd}∕\mathrm{Ni}$ interface, the ${B}_{hf}$ values depend also on the stoichiometry (mixed NNs) due to the induced magnetic polarization of the Pd neighbors. Within the incommensurable unit cells of one monolayer of Pd on Ni, the impurities occupy selected sites. This observation might serve as a basis for the preparation of ordered structures with ultrahigh density of isolated impurities on patterned ferromagnetic substrates. The experiments prompted theoretical studies, which explain the coordination number dependence by the change of the density of polarized $s$ states at the Fermi level from site to site.