Asymmetric self-organization accompanying a thermoinduced spin transition with symmetry breaking: Microscopic modeling

A microscopic elastic model allowing a symmetry breaking upon a spin transition is developed based on competing interactions between the nearest and next-nearest neighbors. The model yields a structurally degenerated high-spin state with a diamond-shaped cell and a nondegenerated low-spin state with a square-shaped cell. We investigated the effect of the symmetry change of the unit cell on the thermoinduced spin transition by monitoring the ratio of the elastic energies involved in the nearest and next-nearest neighbors. The simulations are performed on a deformable two-dimensional lattice made of spins S=±1 (representing the high-spin and low-spin molecules) coupled with springs. The numerical resolution of the model is based on the Monte Carlo metropolis approach, running on spins and positions variables. The simulations of the thermoinduced spin transition disclose asymmetric thermal hysteresis loops with quite different domain distributions on the heating and cooling branches. The analysis of the magnetic and structural properties pointed out that the spin transition with symmetry breaking is dependent on the sign of the thermal gradient. We demonstrated that the nucleation and growth process of spin domains might contrast with the structural self-organization of the lattice according to the energetic contribution of the symmetry breaking. Indeed, the examination of the spatial organization aspects revealed that the structural and elastic anisotropy in the lattice hinders the long-range character of the intermolecular interactions. The lattice configurations show a labyrinthlike structure during a transition from high-symmetry to low-symmetry phases and a multidomain structure upon a transition in the other way around regardless of the spin state in both phases. Furthermore, we found that thermal fluctuations have a crucial role depending on the thermal gradient and on the direction of evolution of the symmetry (reducing or increasing). We demonstrated that stepped and even incomplete spin transitions could be obtained by adjusting the elastic contribution responsible for the symmetry breaking.