Molecular field and Monte Carlo study for the mixed-spin axial next-nearest-neighbor Ising model

The mixed-spin axial next-nearest-neighbor Ising model, in which spin S=1 and spin μ=1 2 ferromagnetic layers are stacked along the c axis alternately, is studied by using the molecular field approximation (MFA) and Monte Carlo (MC) simulation. The finite-temperature magnetic phase diagram obtained by the MFA shows a partially disordered phase (PDP) and the crossover behavior between two PDPs of periodicity 4, one including paramagnetic S spins and the other including paramagnetic μ spins. The appearance of the PDP and the crossover behavior predicted by the MFA are confirmed by MC simulation. The crossover behavior of the PDP is a coupled effect between the mixed spin and the frustration of the exchange interactions.