Magnetic Properties of Mixed Spins on Hexagonal Nanosystems: Molecular Field-Type Calculation and Monte Carlo Simulation

In order to study the magnetic properties of a ferromagnetic mixed spin, on the nanolayered structures described by spin-1/2 Ising model and spin-1 Blume-Capel model, analytical and numerical results were carried out through the mean field theory and computational simulation. The mean field approximation improved with the Gibbs–Bogoliubov inequality was used in association with Monte Carlo simulation to interpret the magnetic behavior of mixed spin lattices with hexagonal cylindrical symmetry. The Gibbs free energy, magnetization and critical frontiers were obtained. Besides the stable branches of the order parameters, there were obtained the metastable and unstable parts of these curves and also found the phase transitions of the metastable and unstable branches of the order parameters. The classifications of the stable, metastable and unstable states were made by comparing the free energy values of these states. The presence of a metastable transition, identified by the Ehrenfest classification as a second-order phase transition, involving an unstable state with order parameter nonzero and the highest entropy value, was adopted as the justification for an uncommon increase in the magnetization with the temperature. Beyond it, a weak ferromagnetic order in a midland chain of Ising spins in the cylindrical nanostructure was interpreted by curves of the order parameter and magnetic susceptibility.