Mixed spin Ising model in a magnetic field: a Monte Carlo study

We investigated the mixed-spin Ising model $S=(1/2, 1)$ on a square lattice using a highly efficient replica exchange Monte Carlo algorithm. The simulations were performed with fixed exchange interactions and anisotropy, conditions under which the system exhibits a frustrated, highly degenerate ground state in the absence of an external magnetic field. We calculated the temperature and field dependence of key thermodynamic parameters (energy $E$, specific heat $C$, entropy $S$, magnetization $m$), determined ground state structures with and without an external field, and explored the influence of the field on these structures and parameters. Results show that the system’s magnetization is zero without a field; however, the application of even an infinitesimal field leads to an abrupt increase to $m=0.25$. Further, the system transitions to a new state with a magnetization plateau at $m=0.5$ for $h>4.5$, reaching saturation ($m=0.75$) when $h>6.5$, at which point all spins align with the field. Ground state structures with and without a field were also determined, revealing how the field lifts the initial ground-state degeneracy.