Microscopic theory of spin-crossover phase transition and magnetoelectric coupling*

The molecular magnet Mn(taa) is one of the earliest studied spin-crossover system where the spin can transit from a low-spin state to a high-spin state around 45 K. Very recently, the magnetoelectric coupling was observed in Mn(taa) and a microscopic theory is needed. Here, we investigated the atomistic origin of the phase transition and magnetoelectric coupling in spin-crossover system Mn(taa) via first-principles calculations and Monte Carlo simulations. By constructing a molecular-scale Hamiltonian, we discovered that the Jahn-Teller-induced inter-molecular strain interaction is the key factor in both the spin-crossover phase transition and magnetoelectric behaviors. Our work leads to a first theory that goes beyond the mean field approximation.