Topological spin textures in 2D altermagnetic chromium chalcogenides: Interplay between magnetic frustration and Dzyaloshinskii-Moriya interaction

Abstract

Altermagnetism has recently emerged as a distinct magnetic paradigm, yet the exploration of topological spin textures within two-dimensional (2D) altermagnets remains limited. Using first-principles calculations and atomistic spin simulations, we investigate monolayer $\mathrm{Cr_2X_2}$ and Janus monolayer $\mathrm{Cr_2XY}$ altermagnets ($\mathrm{X, Y}=\mathrm{O, S, Se, Te}$, and $\mathrm{X \neq Y}$). Both systems exhibit strong intrinsic exchange frustration arising from competition between nearest-neighbor and further-neighbor exchange interactions. In Janus $\mathrm{Cr_2XY}$, breaking the out-of-plane mirror symmetry additionally activates a substantial in-plane Dzyaloshinskii-Moriya interaction (DMI), mediated by the strong spin-orbit coupling of heavy chalcogen ligands. The interplay between exchange frustration and DMI gives rise to distinct frustration- and DMI-dominated regimes, while magnetic anisotropy determines the preferred spin orientation, leading to a diverse range of non-collinear states. Systematic phase-space scans and geodesic nudged elastic band (GNEB) calculations reveal two distinct mechanisms governing this interplay: a synergistic effect, in which frustration lowers the critical DMI for the onset of non-collinear states, and a competitive effect, in which frustration favors high-topological-charge states while DMI favors low-charge states. Our results establish a general framework for understanding the interplay between frustration and DMI and guiding the design of topological spin textures in two-dimensional altermagnets.