Impurity-induced magnetic units embedded in ferromagnetic surfaces

Abstract

Induced magnetic properties at isolated nonmagnetic atoms (Cd) attached to ferromagnetic Ni surfaces are found to be independent of the local symmetry and dependent on the number of nearest Ni neighbors. This dominance of coordination-number dependence as opposed to local symmetry could be studied on differently oriented surfaces at constant coordination numbers using perturbed angular correlation spectroscopy. Mediated by the localized sd-electron hybridization, the impurities form zero-dimensional magnetic units with their nearest Ni neighbors. We discuss this finding as basic research toward future applications with self-organizing patterned structures.

Surface and interface investigations using radioactive atoms

“…Although the EFG and therefore the local geometry are very different from NN = 4 for Ni(001), see Ref. [6], we obtain the same magnetic hyperfine fields within error bars for both geometries. The s-electron polarisation at the Cd impurity, transferred via the sd-electron hybridisation from the neighbouring Ni atoms, is independent of the local symmetry even at low coordination numbers.…”

“…[5], that at 45 K all adatoms had left their adatom position and had moved to steps. For the present experiment [6], we have chosen the temperature of 39 K, with the result that most of the probe atoms moved to the steps, see Fig. 5.…”

“…In the last 10 years experiments carried out at ASPIC (Apparatus for Surface Physics and Interfaces at CERN) have followed this "bottom up" approach: starting with single atoms and based on a broad experimental study of isolated impurities on ferromagnetic surfaces and interfaces [1][2][3][4][5][6]. By careful choice of surface orientation, temperature, and the MBE (molecular beam epitaxy) growth of additional layers, radioactive probes have been positioned in selected sites, e.g., in/at terraces, in/at atomic steps, at kinks, and at interfaces.…”

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 DFT calculations also suggest that other elements should have a dependence which is different from that of Cd, and that deviations will be found on varying the magnetic substrate. 16,18 Comparing Ni͑001͒ and Ni͑111͒ surfaces for NN= 7,13 and NN= 4, 12,32 there is a strong indication that B hf does not depend on the local symmetry but essentially on the number of nearest neighbors. A vice versa argumentation can be performed for the terrace sites: while substitutional terrace sites and adatoms show C 4v / C 3v symmetry on Ni͑001͒, 12 and Ni͑111͒, respectively, their magnetic hyperfine fields differ significantly from 7 to 16 T. These findings, as well as the measured EFG and B hf on Ni͑111͒, are in good agreement with theoretical calculations.…”

“…II͒. A recent experiment by Prandolini et al 32 showed that on Ni͑53,47,50͒, also free step sites with NN= 4 ͑which we call distant free step site, Table I͒ can be occupied at a lower temperature ͑39 K͒.…”