Investigations on surface and interface magnetism using local probes

The magnetic behaviour of isolated nonmagnetic atoms Cd on ferromagnetic single crystal surfaces of Ni is presented. Using a nuclear method, the perturbed angular correlation spectroscopy PAC , atomic resolution is achieved making it possible to distinguish between probe atoms at various sites of a surface, i.e., in terraces, in at atomic steps, at kinks, or as adatoms. These different atomic surface sites have differing Ni coordination numbers, i.e., the number of Ni nearest neighbours . A correlation as a continuous nonlinear dependence with increasing magnetic hyperfine fields Bhf versus decreasing coordination number was found. Furthermore, experimental evidence is presented, that the induced s moments on the Cd isolated impurity is independent on the details of the surface symmetry but essentially determined only by the coordination number. Replacing step by step Ni atoms by Pd atoms with induced magnetic moments a similar behavour is observed. Monolayers of Pd on Ni form large unit cells which offer selected sites to the impurity. The possibility to create ordered structures of isolated impurities on a ferromagnetic surface, in particular of rare earth atoms, by taking advantage of selected sites in unit cells, will be discusse

Magnetic anisotropy of Ni modified by extreme lattice expansion

The induced magnetism of isolated Cd adatoms, soft landed onto ultrathin Ni layers grown pseudo morphologically on Pd 001 a Pd 3.8907 angstrom , was studied using the perturbed angular correlation PAC technique. The magnitude of the induced magnetic response magnetic hyperfine field of 5.2 T was found to be ca. 30 smaller than on bulk Ni a Ni 3.524 angstrom surfaces. This result is compared to ab initio calculations. Additionally, the magnetic anisotropy of the induced response was found to be modified as compared to bulk Ni surfaces and the induced hyperfine field was observed to emerge from the plane at an angle of 50 5 degrees to the surface normal. This canted magnetic anisotropy is attributed to the large lattice expansion of the ultrathin Ni film on Pd 00

Surface and interface magnetism using radioactive probes

Magnetic properties of impurities at ferromagnetic surfaces and interfaces have been investigated performing Perturbed Angular Corelation PAC measurements in the ultra high vacuum chamber ASPIC using different PAC probes. We present the measurements of magnetic hyperfine fields at Cd 111 probe atoms i in Pd covered by Ni, ii on Pd decorated Ni surfaces, iii and on pure Ni surfaces at a variety of local structures like terraces, steps, kinks. The results yield deep insight into the interplay of structural surface roughness and magnetic roughness on the atomic scale. Correlating the experimental B hf values with the number of their nearest Ni and Pd neighbours, the coordination number, nonlinear dependences were found. These findings are compared with recent theoretical studies which are prompted by the experiment

Magnetic anisotropy of Ni modified by extreme lattice expansion

The induced magnetism of isolated (Cd) adatoms, soft-landed onto ultrathin Ni layers grown pseudo-morphologically on Pd(001) ($a_{Pd} = 3.8907$ Å), was studied using the perturbed angular correlation (PAC) technique. The magnitude of the induced magnetic response (magnetic hyperfine field of |5.2| T) was found to be ca. 30% smaller than on bulk Ni ($a_{Ni} = 3.524$ Å) surfaces. This result is compared to ab initio calculations. Additionally, the magnetic anisotropy of the induced response was found to be modified as compared to bulk Ni surfaces and the induced hyperfine field was observed to emerge from the plane at an angle of 50(5)$^{\circ}$ to the surface normal. This canted magnetic anisotropy is attributed to the large lattice expansion of the ultrathin Ni film on Pd(001)

Investigations of “soft-landed” Cd surface atoms via nuclear methods: hyperfine-field sign determination

Using refined preparation techniques, cadmium guest atoms have been positioned at different sites on the surfaces of nickel crystals. The magnetic hyperfine fields and the electric field gradients at the Cd nuclei were measured by time-dependent perturbed angular correlation (TDPAC) spectroscopy of the emitted gamma radiations. By measuring the combined interactions, electric field gradients and magnetic hyperfine fields can be unambiguously attributed to each surface site. The signs of the magnetic hyperfine fields are determined by applying an external magnetic field and choosing the appropriate γ-ray detector configuration. The measured fields correlate with the number of neighbouring host atoms. Band structure calculations confirm this finding and predict magnetic fields for various sp elements from the band structure of the s-like conduction electrons. The quadrupolar interactions are manifestations of the balance in the occupation of the guest p-sublevels. These results provide new information on the structure and formation of electronic configurations of sp elements in different local environments and will contribute to understanding electronic effects on surfaces. Copyright EDP Sciences/Società Italiana di Fisica/Springer-Verlag 200773.20.-r Electron states at surfaces and interfaces, 75.70.Rf Surface magnetism, 76.80.+y Mossbauer effect; other gamma-ray spectroscopy,