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978-3-8439-1251-8, Reihe Physik
Spin-Resolved and Inelastic Scanning Tunneling Spectroscopy of Magnetic Atoms and Clusters
210 Seiten, Dissertation Universität Hamburg (2013), Hardcover, A5
In this thesis, the magnetic properties of Fe adatoms and FeH complexes on Pt(111), the magnetization dynamics of Fe-nanomagnets on Cu(111), and the electronic and magnetic properties of Fe adatoms on Bi2Se3 as well as of Fe substitutional atoms in the topmost layers of Bi2Se3 are investigated by means of (spin-polarized) scanning tunneling microscopy (SP-STM) and inelastic scanning tunneling spectroscopy (ISTS).
Fe adatoms and two FeH complexes on Pt(111) are adsorbed on both fcc or hcp hollow sites. The magnetic anisotropy is found to exhibit a site dependence in strength as well as orientation. While the species on the fcc site show an out-of-plane easy-axis, easy-plane anisotropy is found for both Fe adatoms and one FeH complex on the hcp site. The excitation energies and lifetimes exhibit a distinct behavior for the species residing on the two sites. The second FeH complex on the hcp site shows Kondo screening.
Artificially constructed Fe-nanomagnets on Cu(111) consisting of 5 to 7 atoms show an out-of-plane easy-axis anisotropy. Their magnetization is probably stable at low temperatures. In SP-STM measurements, the tunneling current induces magnetization reversals between the two metastable ground states. The coupling to the substrate causes an energetic scenario where equilibrium quantum tunneling (QT) between the ground states through the anisotropy barrier is prohibited. However, non-equilibrium QT where a spin excitation preceeds QT contributes to the magnetization reversal. In addition, sequential spin excitations due to tunneling electrons from the tip and thermally-excited substrate electrons cause transitions. Since the tunneling electrons are spin-polarized, one transition direction is statistically favored. Hence, a zero-field asymmetry of the ground states is observed.
Fe adatoms on the Bi2Se3 surface are localized on the fcc and hcp hollow adsorption sites, showing strong vertical relaxation. They exhibit a magnetic easy axis within the surface plane and act as charge donors. This leads to a disturbance of the surface electronic structure by band bending and the formation of quantum well states. In an alternative surface doping scheme, Fe adatoms are incorporated into the surface of Bi2Se3 by gentle heating. The Fe atoms most likely substitute Bi and change their doping behavior to neutral or electron acceptors. In addition, they carry a considerable magnetic moment allowing for magnetic surface doping without charge doping.