Untersuchung der magnetischen Austauschwechselwirkung in der Rasterkraftmikroskopie mittels ab initio Rechnungen basierend auf der Dichtefunktionaltheorie
Zusammenfassung der Projektergebnisse
Utilizing density functional theory we have investigated the magnetic exchange forces acting on magnetic tips approached towards the model system of one monolayer Fe on W(001) which exhibits an atomic-scale checkerboard antiferromagnetic ground state. Such first-principles calculations shed light on the microscopic mechanisms allowing successful magnetic exchange force microscopy (MExFM) experiments and can help to establish this novel and promising technique. Our study revealed that a single Fe atom is not an adequate tip model as the obtained magnetic exchange forces are even qualitatively different from those calculated with pyramid tips of five and fourteen atoms. Surprisingly, the magnetic exchange forces on the tip atoms in the nearest layer from the apex atom are non-negligible and can be opposite to that on the apex atom. We demonstrate that the apex atom interacts not only with the surface atom directly underneath but also with nearest-neighbor atoms in the surface. Interestingly, structural relaxations of tip and sample due to their interaction depend sensitively on the magnetic alignment of the two systems. As a result the onset of significant magnetic exchange forces is shifted towards larger tipsample separations which facilitates their detection in MExFM. Using the calculated tip-sample forces we simulated MExFM images of the Fe monolayer on W(001). Our accurate DFT based calculations implementing realistic multi-atom tips and relaxation effects revealed the details of the electron-mediated chemical and magnetic interactions. Based on our ab-initio simulations we proposed 1 ML Fe/W(001) as a model system to measure magnetic exchange forces on the atomic scale. Our study motivated experimental efforts at our institute in the group of Prof. Wiesendanger and our experimental colleagues carried out successful experiments. We were able to explain the contrast obtained in their MExFM images. Interestingly, the observed image is the result of the competition among chemical and magnetic forces between tip and sample. Moreover, the observed image with atomic resolution sensitively depends on the distance between tip and sample. Through the cooperation with the experimental group in Hamburg, we could advance the technique of magnetic exchange force microscopy and established the pioneering role of Germany in this area. In order to understand the influence of the chemical composition of the tip on the exchange forces, we have carried out calculations for Cr tips. We found that indeed larger magnetic exchange forces can be obtained with five-atom Cr tips than with five-atom Fe tips. This result confirms our initial calculations using single atom tips showing a greater delocalization of the Cr 3d-orbitals. For the fourteen-atom tips, the differences are not so striking, however, if we consider only the Cr apex atom we observe a similar effect and larger exchange forces than for the Fe apex atom. Finally, we started to tackle the question of how the interaction changes if the sample is not a magnetic film, but a single magnetic molecule. We performed calculations of a Fe tip approaching simple magnetic molecules composed of isoelectronic transitionmetal atoms (vanadium, niobium, and tantalum) adsorbed on a benzene ring. We have observed magnetic switching for these systems. This opens a very interesting area of research, as the manipulation of such structures correspond to the write and read procedure in a storage medium based on molecules. In the future, we would like to investigate the role of a realistic substrates, relaxation effects and the influence of the chemical identity of the magnetic atom on the sign, size, and distance-dependence of the exchange interaction.
Projektbezogene Publikationen (Auswahl)
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Ab initio Simulation of Exchange Force Microscopy of Fe/W(001). 10th International Conference on Non-Contact Atomic Force Microscopy September 16–20, 2007 Antalya, Turkey
C. Lazo, V. Caciuc, H. Hölscher, and S. Heinze
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Ab-initio simulation of magnetic exchange force microscopy of Fe on W(001). International Vacuum Congress, International Conference on Surface Science, and International Conference on Nano Science and Technology (IVC-17/ICSS-13 and ICN+T2007) July 2–6, 2007 Stockholm International Fairs Stockholm, Sweden
C. Lazo, V. Caciuc, H. Hölscher, and S. Heinze
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Ab initio simulation of magnetic exchange force microscopy. International Conference on Nanoscience and Technology July 20–25, 2008 Keystone, Colorado, USA
C. Lazo, V. Caciuc, H. Hölscher, and S. Heinze
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Ab-initio simulation of magnetic exchange force microscopy of Fe on W(001). 72. Annual Meeting of the DPG and DPG Spring Meeting of the Condensed Matter Division February 25–29, 2008 TU Berlin Berlin, Germany
C. Lazo, V. Caciuc, H. Hölscher, and S. Heinze
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Ab-initio simulation of magnetic exchange force microscopy of the antiferromagnetic Fe Monolayer on W(001). March Meeting of the American Physical Society March 10–14, 2008 New Orleans Convention Center New Orleans, Louisiana, USA
C. Lazo, V. Caciuc, H. Hölscher, and S. Heinze
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Ab-initio Simulation of Magnetic Exchange Force Microscopy. 11 International Conference on Non-Contact Atomic Force Microscopy (NCAFM2008) September 16–19, 2008 Universidad Autonoma de Madrid Madrid, Spain
C. Lazo, V. Caciuc, H. H¨lscher, and S. Heinze
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Ab-initio simulation of magnetic exchange force microscopy. 25th European Conference on Surface Science (ECOSS 25) July 27 to August 1, 2008 University of Liverpool Liverpool, UK
C. Lazo, V. Caciuc, H. Hölscher, and S. Heinze
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Ab-initio simulation of magnetic exchange force microscopy. Joint European Magnetic Symposia (JEMS 08) September 14–19, 2008 Trinity College Dublin, Ireland
C. Lazo, V. Caciuc, H. Hölscher, and S. Heinze
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How to reveal atomic-scale magnetic order with scanning probe microscopes. International Workshop At the Frontiers of Condensed Matter IV December 9–12, 2008 Buenos Aires, Argentina
S. Heinze
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Role of tip size, orientation, and structural relaxations in ?rst-principles studies of magnetic exchange force microscopy and spin-polarized scanning tunneling microscopy, Physical Review B 78, 214416 (2008)
C. Lazo, V. Caciuc, H. Hölscher, and S. Heinze
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Simulation of Atomic-Scale Imaging and Manipulation in Non-contact Atomic Force Microscopy. July 17, 2008 Yale University, Center for Research of Interface Structures and Phenomena (CRISP)
H. Hölscher
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How to reveal magnetism on the atomic scale using scanning probe microscopes. January 21, 2009 Seminar of the Nanoscale Science Department Max-Planck-Institute for Solid State Research Stuttgart, Germany
S. Heinze
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Probing the Magnetic Exchange Forces on Iron on the Atomic Scale, Nano Lett. 9, 200 (2009)
R. Schmidt, C. Lazo, H. Hölscher, U. H. Pi, V. Caciuc, A. Schwarz, R. Wiesendanger, and S. Heinze
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Simulation of Atomic-Scale Imaging and Manipulation in Non-contact Atomic Force Microscopy. February 16, 2009 Fritz-Haber-Institut der Max-Planck-Gesellschaft, Dep. Chemical Physics Berlin, Germany
H. Hölscher
