Dresden 2009 – wissenschaftliches Programm

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MA: Fachverband Magnetismus

MA 9: Surface Magnetism II

MA 9.5: Vortrag

Montag, 23. März 2009, 16:15–16:30, HSZ 403

Origin of the spin polarization of magnetic scanning tunneling microscopy tips — •Paolo Ferriani, Cesar Lazo, and Stefan Heinze — Institut für Angewandte Physik, Universität Hamburg, Jungiusstr. 11, 20355 Hamburg

Spin-polarized scanning tunneling microscopy (SP-STM) is a very powerful tool to study magnetism at the nanoscale. The capability to resolve magnetic structures down to single atoms is often achieved by using tips coated with a magnetic 3d-metal, e.g. Fe, Cr. Although SP-STM is a well established technique, there has been a long debate about the origin of the spin-polarization in the vicinity of the Fermi level from such tips. We would intuitively expect the d-electrons to provide a large spin-polarization at the tip apex atom, with small contribution coming from s- and p-electrons. On the other hand, the local density of state (LDOS) in the vacuum region above the apex atom, which is the key quantity for the tunneling current, should be dominated by s-electrons. In order to clarify this issue, we performed density functional theory calculations using the FLAPW method. We calculated STM tips formed by 3d metal adatoms and pyramids on the (001) and (110) surfaces of Fe. By decomposing the LDOS in the vacuum into the contributions of s-, p-, and d- electrons at the apex atom, we analyzed the origin of the spin-polarization. Surprisingly, the spin-polarization in the vacuum region, is positive for Fe and dominated by s-electrons, although the LDOS at the apex atom is dominated by minority d-electrons. This result is independent of the surface symmetry and can be explained on the basis of sd-hybridization at the tip apex.