Europium cyclooctatetraene nanowire carpets: a low-dimensional, organometallic ferromagnet

*H. Wende (1), F. Huttmann (2), N. Rothenbach (1), S. Kraus (2), K. Ollefs (1), L.M. Arruda (3), M. Bernien (3), D. Thonig (4), A. Delin (4,5,6), J. Fransson (4), K. Kummer (7), N.B. Brookes (7), O. Eriksson (4,8), W. Kuch (3), T. Michely (2)
(1) Faculty of Physics and CENIDE, University of Duisburg-Essen, Germany, (2) II. Physikalisches Institut, Universität zu Köln, Germany, (3) Institut für Experimentalphysik, Freie Universität Berlin, Germany, (4) Department of Physics and Astronomy, Materials Theory, Uppsala University, Sweden, (5) Department of Applied Physics, KTH, Stockholm, Sweden, (6) SeRC (Swedish e-Science Research Center), KTH, Sweden, (7) ESRF, Grenoble, France, (8) School of Science and Technology, Örebro University, Sweden

Sandwich molecular wires are a particular 1D class of organometallic structures. They consist of a periodic sequence of 4f rare-earth metal cations, predominantly ionically bound and eight-fold coordinated to planar aromatic anions, based on the cyclooctatetraene (C8H8 (Cot)) molecule as a ligand. Because of organometallic hybridization between the metal atomic states and the extended π orbitals of the Cot, the metal ions in the wire were proposed to couple magnetically [1]. Here we investigate the magnetic and electronic properties of europium cyclooctatetraene (EuCot) nanowires by means of low-temperature X-ray magnetic circular dichroism (XMCD) and scanning tunneling microscopy (STM) and spectroscopy (STS) [2]. The EuCot nanowires are prepared in situ on a graphene surface. STS measurements identify EuCot as an insulator with a minority band gap of 2.3 eV. By means of Eu M5,4 edge XMCD, orbital and spin magnetic moments of (−0.1±0.3) μB and (+7.0±0.6) μB, respectively, were determined. Field-dependent measurements of the XMCD signal at the Eu M5 edge show hysteresis for grazing X-ray incidence at 5 K, thus confirming EuCot as a ferromagnetic material. Our density functional theory calculations reproduce the experimentally observed minority band gap. Modeling the experimental results theoretically, we find that the effective interatomic exchange interaction between Eu atoms is on the order of millielectronvolts, that magnetocrystalline anisotropy energy is roughly half as big, and that dipolar energy is approximately ten times lower. We are confident that the finding of ferromagnetic ordering in an experimentally well-accessible, surface-supported, organometallic system will provide new inspiration for the field of molecular spintronics.

This work is supported by Deutsche Forschungsgemeinschaft (DFG, German Research Foundation) through projects WE 2623/17-1 and MI 581/23-1 and DFG Projektnummer 278162697 - SFB 1242, eSSENCE, the Swedish Research Council, the KAW foundation (projects 2013.0020 and 2012.0031), and the Foundation for Strategic Research. Further financial support through the Institutional Strategy of the University of Cologne within the German Excellence Initiative, BMBF (no. 05K13KEA “VEKMAG”) and CAPES (no. 9469/13-3) is acknowledged.

[1] N. Atodiresei et al., Phys. Rev. Lett. 2008, 100, 117207.

[2] F. Huttmann et al., J. Phys. Chem. Lett. 2019, 10, 911.