Frustrated magnets and quantum paramagnetic phases at finite temperature

TL;DR

This paper introduces a thermodynamic hierarchical mean-field approach combining exact diagonalization and variational principles to study frustrated magnets at finite temperature, revealing phase diagrams and thermal crossovers relevant for materials like vanadium oxides.

Contribution

It presents a novel framework for analyzing frustrated magnetic systems at finite temperature, capturing quantum to classical crossovers and complex magnetic phases.

Findings

Identifies a thermal crossover temperature T_0 from quantum to classical paramagnet.

Discovers magnetic phases with non-trivial spin textures stabilized by magnetic fields.

Shows the disappearance of a magnetization plateau at T ~ T_0.

Abstract

We develop a general framework, which combines exact diagonalization in small clusters with a density matrix variational principle, to study frustrated magnets at finite temperature. This thermodynamic hierarchical mean-field technique is used to determine the phase diagram and magnetization process of the three-dimensional spin-1/2 $J_1$-$J_2$ antiferromagnet on a stacked square lattice. Its non-magnetic phase exhibits a thermal crossover from a quantum to a classical paramagnet at a temperature $T=T_0$ which can be extracted from thermodynamic measurements. At low temperature an applied magnetic field stabilizes, through order-by-disorder, a variety of phases with non-trivial spin textures and a magnetization plateau at half-saturation which continuously disappears at $T\sim T_0$. Our results are relevant for frustrated vanadium oxides.