Austauschwechselwirkung und Spinschalten einzelner Atome
Experimentelle Physik der kondensierten Materie
Zusammenfassung der Projektergebnisse
The exchange interaction is of fundamental importance in the theory of magnetism. Typically, its strength – expressed within the Heisenberg model in terms of exchange constants – is determined indirectly in experiments by measuring the effect of inelastic spin excitations for example by neutrons or electrons. It is also possible to probe the exchange interaction directly in the setup of an atomic force microscope (AFM) equipped with a magnetic tip. This technique, which has been first demonstrated in 2007, is coined magnetic exchange force microscopy. It has been used to obtain the exchange interaction between a magnetic tip and magnetic surfaces. However, it has so far not been possible to directly probe the exchange interaction with single adatoms or molecules. On the other hand, contact measurements of single adatoms with the tip of a scanning tunneling microscope (STM) have been performed by various groups. Very recently, it has been demonstrated that one can combine an STM and an AFM using magnetic tips in order to simultaneously measure the spin-polarized (SP) tunneling current and the exchange (EX) interaction – a technique which is known as SPEX imaging and spectroscopy. In the current project we aimed to understand and guide SPEX experiments using density functional theory (DFT) calculations. In the past, DFT has become an indispensable tool to explain scanning tunneling or atomic force microscopy experiments. Here, we have applied DFT to study the exchange interaction between a magnetic tip and magnetic Co and Mn adatoms on an ultrathin Mn film which exhibits nearly antiferromagnetic order on an atomic scale and a spin spiral state on a nanometer length scale. Our DFT calculations show that very large exchange energies and forces can be obtained for single magnetic adatoms. The large exchange interactions obtained here should allow the switching of spins at the single atom level. Our analysis reveals that competing direct and indirect exchange mechanisms can lead to ferro- and antiferromagnetic coupling depending on the tip-adatom separation and 3d transition-metal adatom. We also predict significant exchange interactions with 5d transition-metal adatoms such as Ir which do not possess an intrinsic moment but only exhibit small induced magnetic moments. We anticipate that future experiments using SPEX performed by our experimental partners from the Radboud University, Nijmegen, will confirm the predicted exchange energy and force curves. We have further explored the effect of magnetic interactions on the diffusion of single atoms on an antiferromagnetic surface. In collaboration with experimental partners from the University of Hamburg we demonstrated quasi onedimensional diffusion of Co and Rh adatoms on an antiferromagnetic surface due to magnetic interactions – an effect not shown previously.
Projektbezogene Publikationen (Auswahl)
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Controlled Formation of Porous 2D Lattices from C3‐symmetric Ph6−Me‐Tribenzotriquinacene‐OAc3. Chemistry – A European Journal, 29(10).
Toksabay, Sinem; Leisegang, Markus; Christ, Andreas; Härtl, Patrick; Krebs, Johannes; Marder, Todd B.; Haldar, Soumyajyoti; Heinze, Stefan; Bode, Matthias & Krueger, Anke
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Critical behavior and exchange splitting in the two-dimensional antiferromagnet Mn on Re(0001). Physical Review B, 107(14).
Elmers, H. J.; Haldar, Soumyajyoti; Medjanik, K.; Babenkov, S.; Fedchenko, O.; Vasilyev, D.; Heinze, S. & Schönhense, G.
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Kicking Co and Rh atoms on a row-wise antiferromagnet
F. Zahner, S. Haldar, R. Wiesendanger, S. Heinze, K. von Bergmann & A. Kubetzka
