Switchable Magnonic Crystals Based on Spin Crossover/CrSBr Heterostructures

ABSTRACT The progress of magnonics ultimately depends on material platforms that offer precise control of spin wave propagation. Here, we put forward a chemical approach to create locally tunable magnonic crystals by integrating switchable spin‐crossover (SCO) molecules with 2D van der Waals magnets. Specifically, we investigate from first principles a hybrid molecular/2D heterostructure formed by Fe(HB(3,5‐(CH 3 ) 2 Pz) 3 ) 2 molecules (Fe‐pz) deposited on a single layer of semiconducting CrSBr. We show that Fe‐pz molecules are stable on CrSBr while preserving its SCO bistability, particularly in densely packed molecular arrays. By patterning Fe‐pz into periodic stripes separated by pristine CrSBr regions, the interface becomes a magnonic crystal that filters spin waves at selected frequencies. Crucially, light‐driven excited spin‐state trapping (LIESST) enables LS→HS switching below T C of CrSBr, inducing up to ∼1.3% local strain, which in turn reshapes the magnonic band structure in a dynamic and reversible manner. These results establish Fe‐pz@CrSBr as a switchable platform for on‐chip, programmable magnonic devices and pave the way to chemically design artificial light‐controlled reconfigurable magnonic crystals.