Ferromagnetic and antiferromagnetic coupling of molecular systems driven by orbital symmetry

Highlight by Andrea Ferretti published in Activity Report 2018 CNR Nano

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The moiré superstructure of graphene grown on metals can drive the assembly of molecular architectures, such as metal−phthalocyanines (MPcs), allowing for the production of artificial molecular configurations. Once FePc molecules are adsorbed on the Gr/Co surface they couple antiferromagnetically with the Co layer(s) underneath, with a magnetic remanence stable up to room temperature. At variance, CuPc molecules undergo a weaker and ferromagnetic coupling. Large scale DFT simulations confirm the magnetic properties of these interfaces and highlight the role of molecular orbital symmetry as a driving force to determine the magnetic coupling.

Single atoms or molecular units with magnetic remanence at zero field have been demonstrated to enable information processing and storing at ultimate length scales. In this context, paramagnetic molecules become potential building blocks in spintronics when their magnetic moments are stabilized against thermal fluctuations, e.g. by a controlled interaction with a magnetic substrate. Though room temperature (RT) operation is still elusive in standard molecular systems, magnetic remanence at RT can open the route to engineer highly spin-polarized, nanoscale current sources. The need to fully control the organic spin interface and the tuning of ferromagnetic (FM) or antiferromagnetic (AFM) coupling to achieve a stable conductance has motivated a vast experimental interest.In this work, we propose to optimize the thermal stability and the magnetic remanence of molecular systems, preserving their electronic state, by exploiting interlayer exchange coupling within an advanced organic spin-interface architecture: arrays of metal phthalocyanine (MPc, M=Fe, Cu) arranged on Co layer(s) with a graphene spacer. First, we characterize the structural and electronic properties of the Gr/Co/Ir interface by comparing XPS data with ab initio simulations, showing how the XPS chemical shifts can be correlated to Gr/Co distance and registry [1]. Then we demonstrate how the super-exchange interaction can be mediated by the organic ligands and the graphene spacer, preserving the magnetic state of the molecule and favouring a tuneable FM or AFM coupling with Co layer(s), as deduced by X-ray magnetic circular dichroism (XMCD) measurements and confirmed by theoretical predictions. Our results unveil the extreme sensitivity of the exchange interaction to the symmetry of the orbitals responsible for the magnetic state.

According to our simulations, as reported in Fig. 2, for FePc, the spin of the central ion is oriented opposite to the one of N and C, and anti-ferromagnetically coupled with the underlying Co spin moment. In the case of CuPc, the spin imbalance is located at the central ion and on the surrounding N atoms, and coupled ferromagnetically with the Co spin moment. A detailed analysis of Lowdin charges can also be used to further discriminate the symmetry of the molecular orbitals involved in the magnetic coupling. The above picture is very robust against the use of different exchange and correlation functionals (LDA, PBE, PBE+U, discussed in the Supplemental Material), and is in excellent agreement with the experimental data, fully supporting the picture where the magnetic coupling of MPc with Gr/Co/Ir is mainly driven by the symmetry of the involved molecular orbitals.

[1] “FePc adsorption on the moire’ superstructure of graphene intercalated with a Co layer”, G. Avvisati, S. Lisi, P. Gargiani, A. Della Pia, O. De Luca, D. Pacilè, C. Cardoso, D. Varsano, D. Prezzi, A. Ferretti, M.G. Betti,  J. Phys. Chem. C  121, 1639 (2017).

Figure 1:

Top left panel: a cartoon of the corrugated Graphene as given by the egg-box model. Bottom left panels: C1s core-level shifts (CLS) computed for the two non-equivalent C atoms of Gr@Co(0001) in top-fcc registry (C atoms at fcc-hollow and on-top sites), with increasing graphene-Co distance and represented as Lorentzian functions with a width of 0.1 eV. The average of the CLS computed for GR@top-fcc at 2.05 Ang distance was used as reference, here set to zero. Aside: top view of the studied fcc-top Gr@Co(0001) geometry with grey representing C and blue Co atoms. The top Co layer is represented in lighter blue. The 1×1 unit cell is reported. Circle and triangle symbols refer to fcc and top adsorption sites, respectively. Diamond symbols correspond to the hcp site.
Right panels: (a) Core-hole shift dependence with C-Co distance computed for Gr@Co. The red line gives the average of the values computed for the two non-equivalent C sites of four different registries of commensurate Gr@Co, shown by the symbols in different colours. (b) Height distribution for corrugated Gr@Co as given by the egg-box model.  (c) XPS spectra computed as a sum of Lorentzian functions centred in the core-hole energies given in the left panel. (d): Experimental C1s XPS spectra.

Figure 2:

XMCD from Fe and Cu L2,3 absorption edges (a,d), hysteresis loops of FePc (b) and CuPc (e) on Gr/1ML Co. The change in the sign of the XMCD (a,d) and of the field-dependent  magnetization (b,e) indicate an AFM for FePc/Gr/Co and a FM for CuPc/Gr/Co. Spin-density isosurface plots for FePc and CuPc on Gr/Co/Ir (side and top view with hidden substrate), computed  at the DFT-PBE+U level, U=4 eV (c,f). The geometry optimization includes PBE-D2 van der Waals corrections. Green (red) isosurfaces correspond to the up (down) spin density.