Spins in Molecular Monolayers

Applications like giant magnetoresistance (GMR) read-heads and magnetic random-access memory (MRAM) rely on so-called spin transport electronics or ‘spintronics’, where the electron spin rather than the charge is employed to carry information. The possibility to use organic compounds as spintronics materials opens new opportunities, as they allow precise modification of there properties by chemical substitution. To advance towards versatile magnetic materials with decreasing domain and layer dimensions, the understanding of local magnetic coupling in molecular monolayers is of utmost importance.
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Our pioneering work showed that monolayers of organic paramagnetic molecules can couple both ferromagnetically [1, 2] and antiferromagnetically [3] to the underlying ferromagnetic surfaces depending on their electronic structure. If sublimed onto an oxygen-reconstructed ferromagnetic substrate, the molecules are coupled antiferromagnetically with respect to the substrate [4]. Furthermore, we find that the oxygen-reconstruction allows for the self-assembly of the ad-molecules.

The magnetic moments in the ad-molecules and in the ferromagnetic substrates are determined by the element specific X-ray magnetic circular dichoism (XMCD) technique. The XMCD experiments are performed at the Swiss Light Source (SLS) in collaboration with the Microscopy and Magnetism Group on samples which are prepared and characterized in our Surface Science Lab. Thus, we employ a combination of local (Scanning Tunneling Microscopy) with spatially averaging (X-ray photoelectron spectroscopy, X-ray absorption, XMCD) techniques. The experiments are complemented by density functional theory+U (DFT+U) calculations performed by Peter Oppeneer and co-workers (University of Uppsala).

On the basis of possibility to induce a magnetic moment into molecular complexes, we explore the possibilities to control the single molecular spins by an external chemical stimulus i.e. from "spin on" to "spin off" [5].

We find that, if applied to those on-surface complexes, coordination of the external stimulus can also i) tune the spin ("spin high" to "spin low"), ii) change the sign of the exchange coupling ("spin FM" to spin "AFM") and iii) adjust the strength of the exchange coupling ("coupling strong" to "coupling weak"). In consequence to the "trans effect", we the observed magnetochemical effects provide evidence for the existence of a "surface spin trans effect" [6].



Publications

[1] Induced magnetic ordering in a molecular monolayer
A. Scheybal, T. Ramsvik, R. Bertschinger, M. Putero, F. Nolting, et al.
Chem. Phys. Lett. 411, 214 (2005)

[2] Indirect magnetic coupling of manganese porphyrin to a ferromagnetic cobalt substrate
D. Chylarecka, T. K. Kim, K. Tarafder, K. Müller, K. Gödel, et al.
J. Phys. Chem. C 115, 1295 (2011)

[3] Antiferromagnetic coupling of Cr-porphyrin to a bare Co substrate
J. Girovsky, K. Tarafder, C. Waeckerlin, J. Nowakowski, D. Siewert et al.
Phys. Rev. B 90, 220404 (2014)

[4] Self-assembly and superexchange coupling of magnetic molecules on oxygen-reconstructed ferromagnetic thin film
D. Chylarecka, C. Wäckerlin, T. K. Kim, K. Müller, F. Nolting, et al.
J. Phys. Chem. Lett. 1, 1408 (2010)

[5] Controlling spins in adsorbed molecules by a chemical switch
C. Wäckerlin, D. Chylarecka, A. Kleibert, K. Müller, C. Iacovita, et al.
Nature Comm. 1, 61 (2010)

[6] On-surface coordination chemistry of planar molecular spin systems: novel magnetochemical effects induced by axial ligands
C. Wäckerlin, K. Tarafder, D. Siewert, J. Girovsky, T. Hählen, et al.
Chem. Sci. doi:10.1039/c2sc20828h (2012)