Spintronics: Molecules Stabilizing Magnetism

Organic molecules allow producing printable electronics and solar cells with extraordinary properties.
From Karlsruhe Institute of Technology.

The magnetic moments of the three organic molecules
and the cobalt surface align very stably relative to each other.
(Photo: M. Gruber, KIT)

In spintronics, too, molecules open up the unexpected possibility of controlling the magnetism of materials and, thus, the spin of the flowing electrons. According to what is reported in Nature Materials by a German-French team of researchers, a thin layer of organic molecules can stabilize the magnetic orientation of a cobalt surface. (DOI: 10.1038/NMAT4361 – see below).

“This special interaction between organic molecules and metal surfaces could help to manufacture information storage systems in a more simple, flexible and cheaper way,” explains Dr. Wulf Wulfhekel, Karlsruher Institut für Technologie (KIT), Physikalisches Institut.

Microscopic magnets with constant orientation are used in hard disks, for example. With a view to ‘printable electronics’, organic molecules indeed could open up new simple production methods utilizing the self-organization of molecules.

In the present study, three molecular layers of the dye phtalocynine were applied to the surface of ferromagnetic cobalt. Whereas the magnetic moments of the molecules alternatingly align relative to the cobalt and relative to each other, the molecules form a so-called antiferromagnetic arrangement. The magnetic orientation of this combination of antiferromagnetic and ferromagnetic materials remains relatively stable even in the presence of external magnetic fields or cooling.

“Surprisingly, the ‘lightweight’ molecule wins this magnetic arm wrestling with the “heavyweight” ferromagnetic material and determines the respective properties,”  Wulfhekel says.

Systems of antiferromagnetic and ferromagnetic materials, among others, are used in hard disk reading heads. So far, manufacturing of antiferromagnets has been quite complex and time-consuming. Should molecules be suitable for use in the production, the antiferromagnets one day will simply come out of the printer.

The present publication (see below) is the result of a cooperation of researchers from KIT, University of Strasbourg, and Synchrotron SOLEIL. First author Manuel Gruber was member of the German-French Graduate School Hybrid Organic- Inorganic Nanostructures and Molecular Electronics, where different aspects of nanoelectronics, spintronics, and organic electronics are investigated.

Karlsruhe Institute of Technology (KIT) is a public corporation pursuing the tasks of a state university of Baden-Wuerttemberg and of a national research center of the Helmholtz Association. The KIT mission combines the three core tasks of research, higher education, and innovation. With about 9,400 employees and 24,500 students, KIT is one of the big institutions of research and higher education in natural sciences and engineering in Europe.

Reference :

Nature Materials has published an article written by Manuel Gruber, Institut de Physique et Chimie des Matériaux de Strasbourg, Université de Strasbourg, CNRS UMR 7504, 23 rue du Loess, BP 43 F-67034 Strasbourg Cedex 2, France, Physikalisches Institut, Karlsruhe Institute of Technology, Wolfgang-Gaede-Strasse 1 76131 Karlsruhe, Germany, Fatima Ibrahim, Samy Boukari, Institut de Physique et Chimie des Matériaux de Strasbourg, Université de Strasbourg, CNRS UMR 7504, 23 rue du Loess, BP 43 F-67034 Strasbourg Cedex 2, France, Hironari Isshiki, Physikalisches Institut, Karlsruhe Institute of Technology, Wolfgang-Gaede-Strasse 1 76131 Karlsruhe, Germany, Loïc Joly, Institut de Physique et Chimie des Matériaux de Strasbourg, Université de Strasbourg, CNRS UMR 7504, 23 rue du Loess, BP 43 F-67034 Strasbourg Cedex 2, France, Moritz Peter, Physikalisches Institut, Karlsruhe Institute of Technology, Wolfgang-Gaede-Strasse 1 76131 Karlsruhe, Germany , Michal Studniarek, Institut de Physique et Chimie des Matériaux de Strasbourg, Université de Strasbourg, CNRS UMR 7504, 23 rue du Loess, BP 43 F-67034 Strasbourg Cedex 2, France, Synchrotron SOLEIL, L’Orme des Merisiers, Saint-Aubin – BP 48, 91192 Gif-sur-Yvette, France, Victor Da Costa, Hashim Jabbar, Vincent Davesne, Ufuk Halisdemir, Institut de Physique et Chimie des Matériaux de Strasbourg, Université de Strasbourg, CNRS UMR 7504, 23 rue du Loess, BP 43 F-67034 Strasbourg Cedex 2, France, Physikalisches Institut, Karlsruhe Institute of Technology, Wolfgang-Gaede-Strasse 1 76131 Karlsruhe, Germany, Jinjie Chen, Physikalisches Institut, Karlsruhe Institute of Technology, Wolfgang-Gaede-Strasse 1 76131 Karlsruhe, Germany, Jacek Arabski, Institut de Physique et Chimie des Matériaux de Strasbourg, Université de Strasbourg, CNRS UMR 7504, 23 rue du Loess, BP 43 F-67034 Strasbourg Cedex 2, France, Edwige Otero, Fadi Choueikani, Kai Chen, Philippe Ohresser, Synchrotron SOLEIL, L’Orme des Merisiers, Saint-Aubin – BP 48, 91192 Gif-sur-Yvette, France, Wulf Wulfhekel, Physikalisches Institut, Karlsruhe Institute of Technology, Wolfgang-Gaede-Strasse 1 76131 Karlsruhe, Germany, Institute of Nanotechnology, Karlsruhe Institute of Technology, 76021 Karlsruhe, Germany, Fabrice Scheurer, Wolfgang Weber, Mebarek Alouani, Eric Beaurepaire, and Martin Bowen, Institut de Physique et Chimie des Matériaux de Strasbourg, Université de Strasbourg, CNRS UMR 7504, 23 rue du Loess, BP 43 F-67034 Strasbourg Cedex 2, France.

Figure 3: Adsorption geometry and magnetic properties
of the MnPc stacking on Co deduced from DFT calculations.
The red arrows represent the orientation of the spin moments.
Note that the third layer dominantly exhibits a paramagnetic behaviour.
The left inset represents the second-ML molecule on top
of the semi-transparent first-ML molecule…
Click to enlarge

Abstract: “Molecular semiconductors may exhibit antiferromagnetic correlations well below room temperature. Although inorganic antiferromagnetic layers may exchange bias single-molecule magnets, the reciprocal effect of an antiferromagnetic molecular layer magnetically pinning an inorganic ferromagnetic layer through exchange bias has so far not been observed. We report on the magnetic interplay, extending beyond the interface, between a cobalt ferromagnetic layer and a paramagnetic organic manganese phthalocyanine (MnPc) layer. These ferromagnetic/organic interfaces are called spinterfaces because spin polarization arises on them. The robust magnetism of the Co/MnPc spinterface stabilizes antiferromagnetic ordering at room temperature within subsequent MnPc monolayers away from the interface. The inferred magnetic coupling strength is much larger than that found in similar bulk, thin or ultrathin systems. In addition, at lower temperature, the antiferromagnetic MnPc layer induces an exchange bias on the Co film, which is magnetically pinned. These findings create new routes towards designing organic spintronic devices.“