Switchable Magnonic Crystals Based on Spin Crossover/CrSBr Heterostructures.

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-(CH3)2Pz)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 TC 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.