Theory and simulations of critical temperatures in CrI3 and other 2D materials: Easy-axis magnetic order and easy-plane Kosterlitz-Thouless transitions

TL;DR

This paper combines density functional theory with simulations to accurately predict critical temperatures in 2D magnetic materials, exploring easy-axis and easy-plane transitions and the effects of finite size on magnetic order.

Contribution

It introduces a combined theoretical and computational approach to predict critical temperatures and magnetic phases in 2D materials, accounting for finite size effects.

Findings

DFT and Monte Carlo simulations predict Curie temperatures matching experiments.

Finite size effects can induce observable magnetic order despite Mermin-Wagner theorem.

Easy-plane Kosterlitz-Thouless transitions are discussed in the context of 2D magnetism.

Abstract

The recent observations of ferromagnetic order in several two-dimensional (2D) materials have generated an enormous interest in the physical mechanisms underlying 2D magnetism. In the present prospective article we show that Density Functional Theory (DFT) combined with either classical Monte Carlo simulations or renormalized spin-wave theory can predict Curie temperatures for ferromagnetic insulators that are in quantitative agreement with experiment. The case of materials with in-plane anisotropy is then discussed and it is argued that finite size effects may lead to observable magnetic order in macroscopic samples even if long range magnetic order is forbidden by the Mermin-Wagner theorem.