Strain-tunable Dzyaloshinskii--Moriya interaction and skyrmions in two-dimensional Janus Cr${}_2$X${}_3$Y${}_3$ (X, Y = Cl, Br, I, X $\ne$ Y) trihalide monolayers

Abstract

Recently, great effort has been devoted to the search for two-dimensional (2D) ferromagnetic materials with inherent strong Dzyaloshinskii--Moriya interaction (DMI). Here, through a first-principles approach, we systematically investigate the effect of biaxial strain on the DMI, the Heisenberg exchange interaction, and the magnetic anisotropy energy (MAE) of Janus Cr${}_2$X${}_3$Y${}_3$ (X, Y = Cl, Br, I, X $\ne$ Y) monolayers. Both DMI and MAE can be significantly enhanced by tensile strain, while a reversal of the chirality of DMI in Cr${}_2$Cl${}_3$Br${}_3$ and a switch of MAE from off-plane to in-plane in Cr${}_2$I${}_3$Cl${}_3$ are induced by a compressive strain of $2\%$. Microscopically, DMI and MAE are associated mainly with the large spin--orbit coupling of the heavy nonmagnetic halogen atoms rather than that of the magnetic Cr atoms. In particular, the peculiar magnetic transition of Cr${}_2$I${}_3$Cl${}_3$ is explained by competition between direct exchange and superexchange interactions. Micromagnetic simulations show that a small external magnetic field of 65~mT stabilizes a skyrmion with a diameter of 9.8~nm in the Cr${}_2$I${}_3$Cl${}_3$ monolayer. Our results will provide guidance for further research on DMI and skyrmions in 2D Janus materials, as well as a basis for the potential applications in spintronic devices.

8 pages, 5 figures

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