Continuous field-driven phase transition from the Ising universality class of a frustrated spin-1/2 Heisenberg FM/AF square bilayer

Low-temperature magnetothermodynamics of a highly frustrated spin-1/2 Heisenberg square bilayer with the antiferromagnetic intra-dimer interaction and ferromagnetic inter-dimer interaction is governed by localized many-magnon eigenstates, which can be described by the effective ferromagnetic spin-1/2 Ising model on a square lattice. The mapping correspondence implies that the spin-1/2 Heisenberg FM/AF square bilayer displays in a zero-temperature magnetization curve discontinuous magnetization jump, which diminishes upon increasing of temperature until a continuous field-driven phase transition from the Ising universality class is reached at a certain critical temperature. The exact results and Monte Carlo simulations of the effective Ising model are confronted with full exact diagonalization data of the frustrated Heisenberg square bilayer in order to corroborate these findings.