Dr. Nicolae Atodiresei (Molecular Electronics & Spintronics Subgroup in the Peter Grünberg Institut and Institute for Advanced Simulation, Jülich, Germany.) Topic: Molecular Spintronics at Hybrid Organic-Ferromagnetic Interfaces: New Insights from Ab Initio Studies Prof. Chia Liang Cheng (Department of Physics, National Dong Hwa University ) Contact Person: Time ： Nov. 03 (THU.) 3:00-4:00 P.M. Place ： B327, Science Building, National Dong Hwa University

Abstract The use of organic molecules opens a flexible route to design spintronic devices for the generation, transport and detection of spin currents, which may be used to store, manipulate and read spin information at the nanoscale. Moreover, the design of nanoscale spintronic elements in multifunctional devices relies on a clear theoretical understanding of the physics at the electrode-molecule interfaces and in particular, the functionality of specific molecules in a given organic-metal surface environment. The density functional theory provides a framework where a realistic understanding of these systems with predictive power can be expected. However, only very recently functionals are available that describe the exchange correlation of the organic molecule-metal interface reliably including the van der Waals interaction. We show that this has a great influence in particular on flat absorbing molecules. By means of ab initio calculations we demonstrate that it is possible to manipulate the magnetization direction in organic magnetic molecules by changing their oxidation state. We demonstrate this novel effect on the Eu2(C8H8)3 molecule, in which the hybridization of the outer π-ring states with the Eu 4f-states causes a redistribution of the orbitals around the Fermi level leading to a strong ferromagnetism due to a hole-mediated exchange mechanism. Furthermore, using density functional theory simulations we have investigated the role of heteroaromatic systems in the chemical and van der Waals interaction for flat adsorption of π- conjugated molecules on metallic surfaces. Our study shows that the alignment of the molecular orbitals at the adsorbate-substrate interface depends on the number of heteroatoms and as a direct consequence, the molecule-surface van der Waals interaction involves not only the π-like orbitals, which are perpendicular to the molecular plane but also σ-like orbitals delocalized in the molecular plane. I will also present conceptual studies performed to understand how to tailor the magnetic properties at a hybrid organic-ferromagnetic interface by adsorbing organic molecules containing π-electrons onto a magnetic substrate. For such hybrid systems, the magnetic properties like molecular magnetic moments and their spatial orientation can be specifically tuned by an appropriate choice of the chemical substituents. Our first-principles calculations demonstrate that, by employing an appropriate chemical functionalization of organic molecules adsorbed on a ferromagnetic surface, a fine tuning of the spin-unbalanced electronic structure can be achieved. For example, by using molecular substituents with different electronegativities attached to π-conjugated systems adsorbed on a ferromagnetic surface, the electrons with a specific spin [i.e. up (↑) and down (↓)] can selectively be injected at the molecular site from the same ferromagnetic substrate. Even more important, we show that there is direct correspondence between the substituent’s electronegativity and the size of the induced molecular magnetic moment. As regarding the stability of the magnetization direction of the hybrid organic-ferromagnetic system, we demonstrate that the adsorbed hydrogenated molecules destabilize more the out-of-plane magnetization of the ferromagnetic surface as compared to molecules containing more electronegative atoms as Cl and F which could also enhance it. Ultimately, this allows us to precisely engineer the magnetic properties of the hybrid organic-ferromagnetic interfaces which can be further exploited to design more efficient spintronic devices based on organic molecules. References: [1] N. Atodiresei, P. H. Dederichs, Y. Mokrousov, L. Bergqvist, G. Bihlmayer, S. Blugel, Phys. Rev. Lett. 100, (2008). [2] N. Atodiresei, V. Caciuc, P. Lazic, S. Blugel, Phys. Rev. Lett. 102, (2009). [3] N. Atodiresei, J. Brede, P. Lazic, V. Caciuc, G. Hoffmann, R. Wiesendanger, S. Blugel, Phys. Rev. Lett. 105, (2010). [4] J. Brede, N. Atodiresei, S. Kuck, P. Lazic, V. Caciuc, Y. Morikawa, G. Hoffmann, S. Blugel, R. Wiesendanger, Phys. Rev. Lett. 105, (2010). [5] C. Busse, P. Lazic, R. Djemour, J. Coraux, T. Gerber, N. Atodiresei, V. Caciuc, R. Brako, A. T. N'Diaye, S. Blugel, J. Zegenhagen, T. Michely, Phys. Rev. Lett. 107, (2011). Large π-conjugated organic molecules like cobalt-phthalocyanine come closer to ferromagnetic surfaces due to attractive van der Waals interaction. Therefore, the π-electrons of the molecule hybridize strong with the delectrons of the magnetic metal and, as a consequence, a complex energy dependent magnetic structure is formed. Therefore, near the Fermi level, at the molecular site an inversion of the spin-polarization with respect to the ferromagnetic surface occurs. Our studies demonstrate that electrons of different spin [i.e. up (↑) and down (↓)] can selectively be injected from the same ferromagnetic surface by locally controlling the inversion of the spin-polarization, an effect which can be used to increase the efficiency of future molecular spintronic devices. Prof. Chia Liang Cheng (Department of Physics, National Dong Hwa University ) Contact Person: