Molecular order reshapes spin at the interface

Harnessing the spin of electrons is a central goal of spintronics, a technology that promises faster and more energy-efficient electronic devices. A particularly attractive route is offered by so-called spinterfaces: interfaces where organic molecular layers are placed on magnetic surfaces, combining the flexibility of molecular design with the spin functionality of ferromagnets. Yet, despite intense research, understanding where the observed spin polarization at these interfaces really comes from has remained a major challenge.

In this study, we show that structural order alone can profoundly shape spin-polarized signals at spinterfaces, even when the molecules themselves do not carry a magnetic moment. Our results reveal that long-range molecular order can diffract spin-polarized electrons emitted from a magnetic substrate, redistributing them in momentum space and creating spin-polarized replicas that may easily be mistaken for genuine molecular spin states.

We investigated monolayers of iron phthalocyanine (FePc) and metal-free phthalocyanine (H₂Pc) assembled on an oxygen-passivated iron surface. This surface provides a chemically stable and magnetically active template that allows the molecules to self-assemble into highly ordered two-dimensional lattices (Fig. 1a,b). Such long-range order turns out to be the key ingredient in shaping the electronic and spin-resolved properties of the interface.

Using spin- and momentum-resolved photoemission spectroscopy, we directly mapped how electrons are emitted from the interface, resolving both their momentum and their spin. We observed that the periodic molecular lattice acts as a diffraction grating for photoelectrons originating from the iron substrate (Fig. 1c). Through a process known as Umklapp scattering, these electrons are elastically scattered, giving rise to replicated electronic bands that inherit the spin polarization of the underlying ferromagnet (Fig. 1d).

Figure 1: (a) Top-view ball-and-stick representation of the FePc adsorption geometry on the Fe(001)-p(1 × 1)O surface; (b) Low-energy electron diffraction (LEED) pattern recorded at 20 eV kinetic energy. (c) Two-dimensional momentum maps acquired at 150 meV binding energy for the FePc/Fe(001)-p(1 × 1)O interface. (d) Spin-resolved two-dimensional momentum maps acquired at the same energy. (e) Spin-resolved band maps measured along the X̅−Γ̅−M̅ direction of the substrate’s first Brillouin. Corresponding majority- and minority-spin-resolved spectra are shown in the right column. Different momentum integration intervals, corresponding to different acceptance angles, are plotted. Spin polarization curves are displayed in the top part of each panel. Adapted from Nano Lett. 25,17138 (2025).

Crucially, we find the same behavior for both FePc and H₂Pc layers. Even in the absence of a magnetic center in the molecule, the scattered electrons remain spin-polarized, demonstrating that the detected spin signal near the Fermi level is governed by scattering effects rather than by spin-polarized molecular orbitals. This finding resolves a long-standing ambiguity in the interpretation of spin-resolved measurements at organic–magnetic interfaces.

A detailed analysis shows that this effect is particularly strong close to normal emission, the region most commonly probed in many spin-resolved experiments. Here, overlapping replicas of substrate bands with opposite spin can strongly modify — or even suppress — the apparent spin polarization (Fig. 1e). As a result, structural order at the interface can mask the true electronic origin of the measured signal.

Spectroscopic photoemission experiments were carried out at the NanoESCA beamline at Elettra, which hosts a spin-resolved momentum microscope. This unique instrument allowed us to simultaneously map the energy, momentum and spin of photoemitted electrons over large regions of reciprocal space, providing direct access to the spin-resolved electronic structure of complex interfaces.

By clearly disentangling molecular electronic contributions from diffraction-induced effects, we provide a new framework for interpreting spin polarization at spinterfaces. Beyond its fundamental relevance, our study offers concrete guidelines for the design of molecular spintronic systems, showing that controlling structural order is just as important as tailoring chemical or magnetic interactions when engineering functional spin interfaces.

I. Cojocariu^1,2,3, D. Baranowski^3,4, V. Feyer^3,5, M. Jugovac^1,2 and C. M. Schneider^3,5,6

¹ Elettra - Sincrotrone Trieste S.C.p.A., Trieste, Italy
² Physics Department, University of Trieste, Trieste, Italy
³ PGI 6 - Forschungszentrum Jülich GmbH, Jülich, Germany
⁴ Physical and Computational Sciences Directorate and Institute for Integrated Catalysis, Pacific Northwest National Laboratory, Richland, Washington, USA
⁵ Faculty of Physics and Center for Nanointegration Duisburg-Essen (CENIDE), University of Duisburg-Essen, Duisburg, Germany
⁶ Department of Physics and Astronomy, UC Davis, Davis, California, United States

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