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

TL;DR

This study explores the phase diagram, thermodynamics, and magnetic states of the frustrated $J_1$-$J_2$ Heisenberg antiferromagnet on a square lattice, revealing a field-induced plateau and enhanced magnetocaloric effects near the highly frustrated point.

Contribution

It combines classical Monte Carlo and quantum Lanczos methods to analyze the finite-temperature behavior and field-induced states in the frustrated square lattice antiferromagnet.

Findings

Identification of the 'up-up-up-down' state and magnetization plateau at half saturation.

Observation of an enhanced magnetocaloric effect near the saturation field.

Analysis of the phase diagram around the highly frustrated regime.

Abstract

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 $J_2 \approx J_1/2$. 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.