Second-order topological magneto-optical effects in noncoplanar antiferromagnets

TL;DR

This paper predicts and analyzes second-order topological magneto-optical effects in noncoplanar antiferromagnets, expanding understanding of spin texture detection beyond collinear systems using first-principles calculations.

Contribution

It generalizes first-order topological magneto-optical effects to second-order effects in noncoplanar antiferromagnets, specifically in $ ext{Fe}_x ext{Mn}_{1-x}$ alloys, through theoretical and computational methods.

Findings

Identification of second-order Voigt and Schäfer-Hubert effects in noncoplanar antiferromagnets.

Comparison of effects in collinear and noncollinear spin states.

Distinct optical signatures to differentiate effects from natural birefringence.

Abstract

The second-order magneto-optical effects, represented by Voigt and Sch\"{a}fer-Hubert effects, are effective methods to detect the spin textures in antiferromagnets, whereas the previous studies are usually limited to collinear antiferromagnets. In noncollinear antiferromagnets, the spin textures characterized by spin chirality have been revealed to play a critical role in many exciting physics. In particular, the first-order topological magneto-optical effects originated from scalar spin chirality have been discovered recently. In this work, using the first-principles calculations and group theory analysis, we generalize the first-order topological magneto-optical effects to the second-order cases, that is, topological Voigt and Sch\"{a}fer-Hubert effects, by taking the noncoplanar 3Q spin state of $\gamma$-Fe$_{x}$Mn$_{1-x}$ alloy as an example. The conventional Voigt and Sch\"{a}fer-Hubert effects are comparatively studied in the collinear 1Q and 2Q spin states of $\gamma$-Fe$_{x}$Mn$_{1-x}$ alloy. In addition, the natural linear birefringence due to crystal anisotropy is discussed in the strained 1Q, 2Q, and 3Q states, and a unique fingerprint for experimentally distinguishing the second-order topological magneto-optical effects and natural linear birefringence is identified. Our work brings a topological insight into the second-order magneto-optical effects in noncoplanar antiferromagnets and also provides $\gamma$-Fe$_{x}$Mn$_{1-x}$ as an attractive material platform for future experimental exploration.