Spin and Momentum Mapping of Highly Oriented Spinterfaces

Structurally defined interfaces between magnetic substrates and molecular layers, so-called <i>spinterfaces</i>, represent a critical frontier in the design of spin-functional devices. Here, we show that long-range molecular order enables coherent Umklapp scattering of spin-polarized substrate electrons, modifying the spin-resolved electronic structure at the interface. Using spin-resolved momentum microscopy and photoemission tomography, we compare iron (FePc) and metal-free phthalocyanine (H₂Pc) monolayers assembled on an oxygen-passivated iron surface. We find that both molecular lattices give rise to distinct Umklapp replicas of the substrate valence bands. By selectively probing the momentum space, we demonstrate that spin polarization near normal emission is predominantly governed by scattering rather than direct contributions from possibly spin-polarized molecular orbitals. This work underscores the intricate relationship between structural order and spin functionality, providing valuable insights into engineering spinterfaces.