A General Theory of Chiral Splitting of Magnons in Two-Dimensional Magnets

Abstract

Magnons in antiferromagnets exhibit two chiral modes, providing an intrinsic degree of freedom for magnon-based computing architectures and spintronic devices. Electrical control of chiral splitting is crucial for applications, but remains challenging. Here, we propose the concept of extrinsic chiral splitting, involving alternating and ferrimagnet-like types, which can be induced and controlled by an electric field. A symmetry framework based on 464 collinear spin layer groups is established to classify chiral splitting characteristics and electric field responses in two-dimensional magnets. We further elucidate how the spin layer group determines the type of alternating chiral splitting and the dominant lowest-order magnetic exchange interaction. We demonstrate electric-field control over the magnitude and sign of the chiral splitting, enabling control of the spin Seebeck and Nernst effects related to thermal spin transport. This work provides a general theory for electric field manipulation of magnon chirality, paving the way for low-power magnonic logic devices.

Figures (4)

• Figure 1: Characteristics of chiral splitting of magnon bands and symmetry classification of electric-field responses of 2D collinear magnets. (a) Schematic magnon band structures for degenerate AFM (D-AFM), AM, Extrinsic AM (E-AM) and FiM (E-FiM). The two magnon chiralities are distinguished by different colors. (b) Classification of the collinear spin layer groups (SLGs). The SLGs of AFM are classified into two categories: D-AFM and AM, and further subdivided into four categories based on the response of the magnon bands under an out-of-plane electric field. The symmetry operations $A$ connecting two magnetic sublattices are divided into four sets I-IV based on whether they cause chiral degeneracy and whether they can be broken by an out-of-plane electric field. The check mark and cross on the right side of the sets indicate whether elements of the sets are allowed or prohibited in the corresponding type, respectively.
• Figure 2: The lowest-order magnetic exchange interaction required for altermagnetic chiral splitting. (a) Schematic of degeneracy and non-degeneracy of intra-sublattice magnetic exchange interactions. Degenerate magnetic exchange interactions are represented by double arrows of the same color. The lowest order magnetic exchange paths determined by SLG operators and the corresponding characteristics of chiral splitting, involving (b) $d$-wave, (c) $g$-wave, and (d) $i$-wave. The blue and red areas represent the opposite signs for chiral splitting.
• Figure 3: Material candidates with different lowest-order magnetic exchange interactions and chiral splitting types. (a) Crystal structures and symmetry operations connecting two magnetic sublattices, (b) calculated intra-sublattice magnetic exchange interactions, and (c) 2D isosurfaces of chiral splitting and magnon band dispersions for Fe$_2$WS$_4$ monolayer ($d$-wave). The values of the lowest-order magnetic exchange interactions are circled in a red box, with direction marked in red. (d)-(f) and (g)-(i) show the corresponding results for MnS$_2$ ($g$-wave) and Mn$_2$P$_2$S$_3$Se$_3$ ($i$-wave), respectively.
• Figure 4: Electric-field control of chiral splitting and spin transport. (a) Crystal structure and SLG of FeSe monolayer. (b) Magnon band structures under different out-of-plane electric field values. (c) Dependence of the anisotropy of the intra-sublattice nearest-neighbor exchange interaction ($\Delta J$) and the maximum chiral splitting ($\Delta\omega$) on the electric field. (d) Calculated coefficients of SSE ($\sigma_{\parallel}$) and SNE ($\sigma_{\perp}$) as functions of temperature and electric field. (e) Schematic illustration of electric field control over thermal spin transport. Changing the sign of the electric field can reverse the spin current caused by SSE and SNE.