3D Ising model studies of mixed-state domain formation in spin crossover molecular systems

Compact molecular domains of largely high spin state were recently observed in photoemission electron microscopy images of thin films of the spin crossover [Fe(H₂B(pz₂)₂)(bipy)] deposited on a semiconducting NiCo₂O₄(111) substrate.  Two-dimensional Monte Carlo simulations of the Ising-like spin crossover Hamiltonian suggest that the compact domain shape arises from extensive intermolecular interactions. The cooperativity in this spin crossover system occurs on a length scale well beyond nearest-neighbor interactions.  The 2D simulation fails to predict the minority-dominated mixed-state of the domains.  The 3D Monte Carlo Ising Model simulation resembles the observed NiCo₂O₄ substrate domain structure by including substrate effects through the introduction of layers with a fixed spin state distribution into a thermalized system. By annealing the system and studying the spin-state fraction, domain formation could be followed in the simulation.  This approach allows insight into the fundamental physics governing domain nucleation and allows better identification and quantification of specific mechanisms in a spin crossover system.