Analytical solution of 1D Ising-like systems modified by weak long range interaction - Application to spin crossover compounds

It is well-known that 1D systems with only near est neighbour interaction e xhibit no phase transition. It is shown that the presence of a small long r ange interaction treated by the mean field approximation in addition to str ong nearest neighbour interaction gives rise to hysteresis curves of large width. This situation is believed to exist in spin crossover systems where by the deformation of the spin changing molecules, an elastic coupling lead s to a long range interaction, and strong bonding between the molecules in a chain compound leads to large values fur nearest neighbour interaction co nstants. For this interaction scheme an analytical solution has been derive d and the interplay between these two types of interaction is discussed on the basis of experimental data of the chain compound [Fe(Htrz)(2)(trz)] (BF 4)(2) which exhibits a very large hysteresis of 50 K above RT at 370 K, The width and shape of the hysteresis loop depend on the balance between long and short range interaction. For short range interaction energies much larg er than the transition temperature k(B)T(t) the hysteresis width is determi ned by the long range interaction alone.