Jungiusstr. 11, Hörsaal AP
SFB 668 - Kolloquium
Prof. Dr. K. Franke (FU Berlin):
Magnetic properties of single molecules probed by scanning tunneling spectroscopy
Magnetic properties of single molecules
and metal-organic networks on surfaces
Freie Universität Berlin
The magnetic properties of atoms and molecules on a surface are significantly affected by details in the atomic-scale surrounding. Manipulation of this surrounding provides the possibility to tune the electronic and magnetic functionality of surfaces on a nanometer scale.
Using scanning tunnelling spectroscopy (STS), we show that the lifetime of excited spin states in the paramagnetic Fe-Octaethylporphyrin-Chloride (FeOEP-Cl) is orders of magnitude longer when the molecule is adsorbed on a superconductor as compared to a normal metal substrate. We ascribe this increase in spin relaxation time to the superconducting energy gap at the Fermi level, which prohibits efficient pathways of energy quenching into the substrate . The small spin relaxation rates allow for pumping into higher spin states by large current densities.
We further manipulate the magnetic properties of individual FeOEP molecules on a gold surface by an in-situ chemical reaction of the organic ligand. A temperature-induced step-wise electrocyclic ring closure of the ethyl groups results in the final product Fe-Tetrabenzoporphyrin (FeTBP). The chemical modification is accompanied by an increased magnetic interaction with the metallic substrate as resolved by changes in the shape and width of a Kondo resonance .
Employing a combination of STM and X-ray circular magnetic dichroism (XMCD), we show that Fe atoms embedded in a bilayer of 2,4,6-Tris(4-pyridyl)-1,3,5-triazine (T4PT) on a Au(111) surface exhibit a sizable magnetic moment with an out of plane magnetic anisotropy. The field dependence of the XMCD signal reveals a ferromagnetic coupling of the Fe atoms within the highly ordered network .
 B. W. Heinrich, L. Braun, J. I. Pascual, K. J. Franke, Nature Physics 9, 765 (2013)
 B. W. Heinrich, G. Ahmadi, V. L. Müller, L. Braun, J. I. Pascual, K. J. Franke, NanoLetters 13, 4840 (2013)
 T. R. Umbach, M. Bernien, C. F. Hermanns, A. Krüger, V. Sessi, I. Fernández-Torrente, P. Stoll, J. I. Pascual, K. J. Franke, and W. Kuch, Phys. Rev. Lett. 109, 267207 (2011)