Activating magnetoelectric optical properties by twisting antiferromagnetic bilayers

TL;DR

Twisting antiferromagnetic bilayers like CrI3 induces new optical and electric properties, such as pyroelectricity and Kerr effects, controllable by twist angle and chirality, opening avenues for spintronic and optoelectronic applications.

Contribution

This work reveals how twisting antiferromagnetic bilayers activates hidden properties like pyroelectricity and Kerr effects through symmetry breaking, a novel approach in 2D magnetic heterostructures.

Findings

Twisting lowers symmetry, activating pyroelectricity and Kerr effect.

Electric polarization and Kerr angle depend on twist angle and chirality.

Unconventional spin vortices with opposite chiralities in momentum space.

Abstract

Twisting in bilayers introduces structural chirality with two enantiomers, i.e., left- and right-hand bilayers, depending on the oriented twist angle. The interplay between this global chirality and additional degrees of freedom, such as magnetic ordering and the local octahedral chirality arising from the geometry of the bonds, can yield striking phenomena. In this work, we focus on collinear antiferromagnetic CrI$_3$ twisted homo-bilayers, which are characterized by a staggered octahedral chirality in each monolayer. Using symmetry analysis, density functional theory and tight-binding model calculations we show that layers twisting can lower the structural and magnetic point-group symmetries, thus activating pyroelectricity and the magneto-optical Kerr effect, which would otherwise be absent in untwisted antiferromagnetic homo-bilayers. Interestingly, both electric polarization and Kerr angle are controllable by the twist angle and their sign is reversed when switching from left- to right-twisted bilayers. We further unveil the occurrence of unconventional vortices with spin textures that alternate opposite chiralities in momentum space. These findings demonstrate that the interplay between twisting and octahedral chirality in magnetic bilayers and related van der Waals heterostructures represents an extraordinary resource for tailoring their physical properties for spintronic and optoelectronic applications.