Strain-Controlled Magnetic Phase Transitions through Anisotropic Exchange Interactions: A Combined DFT and Monte Carlo Study

TL;DR

This study demonstrates how epitaxial strain can induce magnetic phase transitions in materials by altering exchange interactions, using combined DFT and Monte Carlo methods to reveal strain as a key control parameter.

Contribution

The paper introduces a combined DFT and Monte Carlo approach to show how strain influences magnetic phases through anisotropic exchange interactions in correlated materials.

Findings

Strain causes anisotropic magnetic interactions in BiFeO3.

Structural distortion drives magnetic phase transitions.

Strain stabilizes different magnetic ground states in a Hubbard model.

Abstract

Epitaxial strain provides a powerful, non-chemical route to tune the properties of functional materials by manipulating the coupling between spin, charge, and lattice degrees of freedom. Using density functional theory (DFT) calculations and $\rm BiFeO_3$ as a model system, we first demonstrate how epitaxial strain exactly leads to anisotropic magnetic interactions where the exchange coupling along the $c$-axis differs from that in the $ab$-plane. We show that subtle structural modifications, specifically the distortion from a cubic to a tetragonal lattice, drive a magnetic phase transition from a G-type to a C-type antiferromagnetic (AF) phase. The anisotropy in magnetic interactions, which becomes prominent in the lower symmetry tetragonal phase, provides a direct link between the structural distortion and the potential change in magnetic ordering. For a more comprehensive study, we next investigate the role of strain in driving magnetic phase transitions within a half-filled one-band Hubbard model in three dimensions. In this framework, strain is introduced through anisotropic hopping processes between nearest- and next-nearest-neighbor sites, inspired by the DFT calculations. Using a semiclassical Monte Carlo (s-MC) approach, we construct ground state phase diagrams in the nonperturbative regime, which show how uniaxial strain stabilizes distinct magnetic ground states: Compressive strain drives a transition from a G-type to a C-type AF insulator, whereas tensile strain suppresses the C-type AF order, favoring an A-type AF phase. Overall, our combined DFT and s-MC calculations highlight that strain is a powerful tuning parameter for controlling competing magnetic phases by governing exchange coupling mechanisms in correlated systems, offering valuable insights for the design of strain-controlled materials.