Monte Carlo study of the two-step spin transition in [Fe X Zn 1∓X (2-pic) 3 ]Cl 2 · EtOH

The Monte Carlo method has been applied to reproduce the two-step spin transition in [Fe x Zn 1−x (2-pic) 3 ]Cl 2 · EtOH (2-pic = 2-picolylamine). The partition function of the Hamiltonian has been factorized in two parts describing the inner degrees of freedom of the spin transition molecules and the interaction between them. The factorization is equivalent to an effective Hamiltonian which can be mapped on an Ising system in an applied magnetic field. The interaction consists of an infinite range and a short range interaction part. Qualitatively the two-step transition under pressure and the Zn diluted system have been well reproduced with infinite range and the next nearest neighbour interaction of ferromagnetic type and the nearest neighbour interaction of antiferromagnetic type in a simple cubic lattice.