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Surface Effects Leading to Unusual Size Dependence of the Thermal Hysteresis Behavior in Spin-Crossover Nanoparticles

We analyze the size effect on spin-crossover transition nanoparticles in a 2D Ising-like model subject to a specific ligand-field at the surface. By anisotropic sampling method applied to the finite 2D square Ising lattices with various sizes, we determined the density of macro states by scanning the spin configurations. This information, which is independent on the system parameters, is used to exactly calculate the thermal behavior of spin-crossover nanoparticles whose ligand-field of the atoms at the surface is lower than those of the bulk. We found that decreasing the size of the nanoparticles leads to a global increase of the effective interaction, which has the consequence to enhance the width of the thermal hysteresis. This unusual behavior opens a new avenue in controlling the bistability characteristics at small scale, one of the important conditions of applicability of these materials at the nanometric scale.