Strain-tunable Dzyaloshinskii-Moriya interaction and skyrmions in two-dimensional Janus Cr2X3Y3 (X, y = Cl, Br, I, X≠Y) trihalide monolayers

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 Cr2X3Y3 (X,Y = Cl, Br, I, X≠Y) monolayers. Both DMI and MAE can be significantly enhanced by tensile strain, whereas a reversal of the chirality of DMI in Cr2Cl3Br3 and a switch of MAE from off plane to in plane in Cr2I3Cl3 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 Cr2I3Cl3 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 Cr2I3Cl3 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.