Surface and Interface Studies with Radioactive Ions

Investigations on the atomic scale of magnetic surfaces and magnetic multilayers were performed by Perturbed Angular Correlation (PAC) spectroscopy. The unique combination of the Booster ISOLDE facility equipped with a UHV beamline and the UHV chamber ASPIC (Apparatus for Surface Physics and Interfaces at CERN) is ideally suited for such microscopic studies. Main advantages are the choice of problem-oriented radioactive probes and the purity of mass-separated beams. The following results were obtained: $\,$i) Magnetic hyperfine fields (B$_{hf}$) of Se on Fe, Co, Ni surfaces were determined. The results prompted a theoretical study on the B$_{hf}$ values of the 4sp-elements in adatom position on Ni and Fe, confirming our results and predicting unexpected behaviour for the other elements. $\,$ii) Exemplarily we have determined B$_{hf}$ values of $^{111}$Cd at many different adsorption sites on Ni surfaces. We found a strong dependence on the coordination number of the probes. With decreasing coordination number, B$_{hf}$ incrrases strongly, from -7T in Ni bulk to +16T at Cd in adatom position on Ni(111). $\,$iii) In experiments with $^{111}$Cd in ultra thin Pd on Ni surfaces static magnetic hyperfine fields were measured which indicate an induced magnetic order in Pd. $\,$iv) Fluctuating magnetic interactions in Pd were observed, when thick Pd is in contact with Ni.