Atomically tweaking spin-crossover cooperativity to augment molecular memory density.

Spin-crossover complexes are promising candidates for molecular bits owing to their magnetic bistability. However, it's a formidable challenge to turn densely packed spin-crossover molecules into high-density bits due to cooperative switching dynamics inherent in spin-crossover aggregates. Herein, coordination-field engineering is demonstrated by scanning tunneling microscopy/spectroscopy and density functional theory as an efficient strategy to modulate the cooperative dynamics of surface-confined spin-crossover chains. Within these chains, Ni centers exhibit collective spin transitions upon local stimuli. This spin-crossover cooperativity can be disrupted by coordination-field modification via hetero-metal/ligand doping, which converts partial switchable coordination centers into non-switchable nodes. These nodes divide the single-bit-like spin-crossover chain into multiple independently-switchable segments and thereby augment the bit density. One-by-one erasable write-in of these spin-crossover bits is achieved by scanning tunneling microscope tip manipulation. These results highlight how to precisely tweak the cooperative dynamics of the spin-crossover aggregates to advance their potential applications for high-density memory.