Field-induced states and thermodynamics of the frustrated Heisenberg antiferromagnet on a square lattice

We investigate the ground-state and finite-temperature properties of the J 1 – J 2 Heisenberg antiferromagnet on the square lattice in the presence of an external magnetic field. We focus on the highly frustrated regime around J 2 ≈ J 1 /2. The h – T phase diagram is investigated with particular emphasis on the finite-temperature transition into the “up-up-up-down” state that is stabilized by thermal and quantum fluctuations and manifests itself as a plateau at one half of the saturation magnetization in the quantum case. We also discuss the enhanced magnetocaloric effect associated to the ground-state degeneracy that arises at the saturation field for J 2 = J 1 /2. For reference, we first study the classical case by classical Monte Carlo simulations. Then, we turn to the extreme quantum limit of spin-1/2 where we perform zero- and finite-temperature Lanczos calculations.