The materials of choice for constructing spin-devices with organic materials are π-conjugated polymers. The appeal of organic compounds for spintronics is primarily due to their extremely long spin lifetimes owing to weak spin relaxation effects.

However, the practical realization of organic spin devices depends on the feasibility in manipulating the spin orientation in organic media. Interestingly, the answer to this challenge may lie in a tantalizing class of molecules generally known as spin crossover compounds, whose spin state can be changed from low spin to high spin by an external perturbation. This external stimulus can be provided by temperature, light, pressure or a static electric field.

Temperature-driven spin crossover transition

In J. Phys.: Condens. Matter 23 316001, the possibility of a temperature-driven spin crossover transition in π-conjugated polymer chains was explored theoretically with a minimal model incorporating a repulsive Hubbard interaction and electron–phonon coupling. The model was solved for finite molecules using a combination of energy minimization techniques and Monte Carlo simulations.

The work demonstrates that in a certain region of parameter space of the model, there is a spin crossover, where the system transits from its low spin state to a high spin state with temperature. The authors have demonstrated that such a transition is entropy driven and that both the spin and vibration contributions to the entropy are relevant. This is in close analogy to standard spin crossover phenomena in divalent magnetic molecules.