  Simple elastic models of spin-crossover compounds are known empirically to
exhibit classical critical behavior. We demonstrate how the long-ranged
interactions responsible for this behavior arise naturally upon integrating out
mechanical fluctuations of such a model. A mean field theory applied to the
resulting effective Hamiltonian quantitatively accounts for both thermodynamics
and kinetics observed in computer simulations, including a barrier to
magnetization reversal that grows extensively with system size. For
nanocrystals, which break translational symmetry, a straightforward extension
of mean field theory yields similarly accurate results.
