Elastic Frustration In 1d Spin-Crossover Chains: Evidence Of Multi-Step Transitions And Self-Organizations Of The Spin States

We consider a 1D elastic spin-crossover (SCO) chain in which each site may be in the low-spin or in the high-spin (HS) state. The sites interact elastically through a harmonic coupling, and the local equilibrium distances depend on the spin states of the interacting sites. The Hamiltonian of the system is solved by the Monte Carlo method running on the spin states and the atomic displacements. By considering the existence of an elastic frustration between the equilibrium distances of the nearest-neighbors and the next-nearest-neighbors, we succeeded to highlight a number of original behaviors of the thermal dependence of the high-spin fraction, like multistep transitions, incomplete spin transitions, emergence of self-organized structures, and re-entrant spin transitions, by adjusting only one control parameter. The obtained results allow understanding several experimental data of 1D spin-crossover materials which seem to be model systems for elastic frustration.