Antiferromagnetic coupling of Cr-porphyrin to a bare Co substrate

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Antiferromagnetic coupling of Cr-porphyrin to a bare Co substrate

Jan Girovsky, Kartick Tarafder, Christian Wäckerlin, Jan Nowakowski, Dorota Siewert, Tatjana Hählen, Aneliia Wäckerlin, Armin Kleibert, Nirmalya Ballav, Thomas A. Jung, and Peter M. Oppeneer

Phys. Rev. B 90, 220404(R) – Published 10 December 2014

Abstract

We report the discovery of an antiferromagnetic coupling of the magnetic moment of chromium(II) tetraphenyl-porphyrin (CrTPP) molecules to the magnetization of the clean ferromagnetic Co(001) substrate. We assign this unusual molecule-substrate exchange coupling to the less than half-filled chromium $3 d$ orbitals interacting with Co valence band electrons via porphyrin-ligand molecular orbitals. X-ray magnetic circular dichroism, x-ray photoelectron spectroscopy, and scanning tunneling microscopy are combined with $DFT + U$ calculations and provide evidence for a surprising type of antiferromagnetic $90^{\circ}$ indirect magnetic exchange coupling.

Received 19 March 2014
Revised 10 November 2014

DOI:https://doi.org/10.1103/PhysRevB.90.220404

Authors & Affiliations

Jan Girovsky^1,*, Kartick Tarafder^2,3, Christian Wäckerlin¹, Jan Nowakowski¹, Dorota Siewert⁴, Tatjana Hählen¹, Aneliia Wäckerlin⁴, Armin Kleibert⁵, Nirmalya Ballav⁶, Thomas A. Jung¹, and Peter M. Oppeneer^2,†

¹Laboratory for Micro- and Nanotechnology, Paul Scherrer Institute, CH-5232 Villigen PSI, Switzerland
²Department of Physics and Astronomy, Uppsala University, Box 516, S-751 20 Uppsala, Sweden
³Department of Physics, BITS-Pilani Hyderabad Campus, Shameerpet, Andhra Pradesh 500078, India
⁴Department of Physics, University of Basel, CH-4056 Basel, Switzerland
⁵Swiss Light Source, Paul Scherrer Institute, CH-5232 Villigen PSI, Switzerland
⁶Department of Chemistry, Indian Institute of Science Education and Research, Pune 411008, India

^*jan.girovsky@psi.ch
^†peter.oppeneer@physics.uu.se

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Issue

Vol. 90, Iss. 22 — 1 December 2014

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Images

Figure 1
(a) Sketch of the experimental setup with the CrTPP molecules lying flat on the Co(001) substrate. Prior to the measurements the sample is magnetized by an external magnetic field along the Co[110] direction. The [110] direction of the substrate, surface normal $\vec{n}$ , and propagation vector $\vec{k}$ of the circularly polarized x rays lies in one plane. The AFM coupling of the molecules with respect to the substrate is depicted by the yellow and white arrows. A scheme of CrTPP is shown in the inset. (b) XA (red and blue lines) and XMCD spectra (black and dashed lines) acquired at the Co $L_{3, 2}$ edges of the substrate, at 300 and 70 K. (c), (d) XA and XMCD spectra of the CrTPP molecules measured at the Cr $L_{3, 2}$ edges at 300 and 70 K, respectively.
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Figure 2
XPS data measured at the $2 p_{3 / 2}$ core levels of (a) Cr and (b) Cl for a single layer of CrTPP molecules on Co(001) (red dots) and a multilayer of CrTPPCl on Au(111) (blue dots). The solid lines give the results of a Gaussian-Lorentzian fit of the spectra. (c) STM data of CrTPP molecules on Co(001). Three different characteristic patterns are presented in the insets and are assigned to different conformations of the molecules (6-lobe, 5-lobe, 4-lobe, from left to right). $(U_{bias} = - 2.1 V, I_{t} = - 10 pA, T = 300$ K.)
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Figure 3
$DFT + U$ calculations of Cr-P adsorbed on a Co(001) substrate. (a) Plot of the magnetic spin-density isosurfaces of the Cr-P/Co system. Yellow (blue) colored hypersurfaces depict spin-majority (spin-minority) magnetization. (b) Spin-resolved density of states calculations of Cr $d$ orbitals reveal AFM coupling of the spin moment of the Cr ion to that of Co. The inset shows the spin-resolved density of states of the Co substrate with spin-up (blue) and spin-down (red) bands, and the spin-polarized total DOS of Cr in gray.
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Figure 4
Left: $3 d$ orbitals of the Cr ion and their electron occupations in CrTPP/Co(001) according to Hund's rules. Due to indirect coupling (via nitrogen) to the spin-up polarized Co substrate, the Cr electrons exhibit a spin-down orientation (red arrows). Right: The nitrogen $2 p_{x}$ and $2 p_{z}$ orbitals. The N $p$ electrons show an imbalance of spin-down (red arrows) and spin-up (blue arrows) electrons due to interaction with the Co substrate. The effective electron exchange through the $σ$ and $π$ bonds is indicated by the dashed arrows. Box: Schematic side view of the indirect $90^{\circ}$ exchange coupling of the spins in CrTPP/Co(001).
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