Magnetism, spintronics

Phase transitions of the mixed-spin Ising model on the square lattice

M. A. Magomedov, A. K. Murtazaev, S. Sh. Gasanov

Daghestan Institute of Physics after Amirkhanov

Abstract: The mixed-spin Ising model $S$ = (1/2, 1) on a square lattice has been investigated using a highly efficient replica exchange Monte Carlo algorithm. The system was studied with fixed exchange interaction and anisotropy parameters: $J_1$ = -1 (between spins in sublattices A and B), $J_2$ = -0.5 (between spins in sublattice $B$), and $D$ = 1.0 (anisotropy for spins in sublattice $B$). Temperature and field dependencies of the main thermodynamic characteristics (energy, specific heat, entropy, magnetization) were calculated. Ground state structures were visualized. The existence of two successive phase transitions was revealed: at $T_{C1}$ = 0.285 a transition to a partially disordered state occurs, and at $T_{C2}$ = 0.35 a transition to a paramagnetic state. A detailed analysis of the field dependencies reveals a complex, multi-step magnetization curve, indicating multiple field-induced phase transitions. We identify a series of magnetization plateaus, determine the corresponding magnetic structure for each, and calculate the critical field values for the transitions between these phases, leading to a comprehensive understanding of the system's phase diagram and its response to external perturbations.

Keywords: mixed-spin Ising model, ground state structure, phase transitions, replica exchange algorithm, Monte Carlo method.

Received: 13.05.2025
Revised: 08.10.2025
Accepted: 22.10.2025

DOI: 10.61011/FTT.2025.11.62134.116-25

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