First Principles Heisenberg Models of 2D magnetic materials: The Importance of Quantum Corrections to the Exchange Coupling

TL;DR

This paper emphasizes the importance of quantum corrections in Heisenberg models for 2D magnetic materials, showing that quantum effects can significantly alter exchange parameters and critical temperature predictions.

Contribution

It introduces a quantum eigenstate-based approach to determine exchange parameters, improving upon classical approximations for 2D magnetic systems.

Findings

Exchange parameters are reduced by about 10% when quantum effects are included.

Quantum corrections can lower predicted critical temperatures by up to 7%.

Density functional theory aligns with quantum eigenstates of the Heisenberg model.

Abstract

Magnetic materials are typically described in terms of the Heisenberg model, which provides an accurate account of thermodynamic properties when combined with first principles calculations. This approach is usually based on an energy mapping between density functional theory and a classical Heisenberg model. However, for two-dimensional systems the eigenenergies of the Heisenberg model may differ significantly from the classical approximation, which leads to modified expressions for exchange parameters. Here we demonstrate that density functional theory yields local magnetic moments that are in accordance with strongly correlated anti-ferromagnetic eigenstates of the Heisenberg Hamiltonian. This implies that density functional theory provides a description of these states that conforms with the quantum mechanical eigenstates of the model. We then provide expressions for exchange parameters based on a proper eigenstate mapping to the Heisenberg model and find that they are typically reduced by 10 % compared to a classical analysis. Finally, we calculate the corrections to critical temperature for magnetic ordering for a previously predicted set of two-dimensional ferromagnetic insulators and find that the inclusion of quantum effects may reduce the predictions of critical temperatures by up to 7 %.