Novel two-dimensional magnets with an in-plane auxetic effect

The auxetic effect in two-dimensional (2D) materials can not only enhance their mechanical properties but also brings additional tunability of their physical properties. Here, we employ density-functional-theory calculations to report on a class of auxetic 2D magnets, namely, the squarely packed transition metal dichlorides MCl2 (M = Ti, V, Mn, Fe, Co, Ni). These magnets are dynamically stable and exhibit an intrinsic in-plane auxetic effect. Meanwhile, the transition metal disulfides MS2 (M = V, Cr, Mn) with the same crystal structure exhibit a positive Poisson's ratio. This indicates that the auxetic effect in MCl2 is not merely dominated by the crystal structure. We attribute the occurrence of such auxetic behavior to the weak bond stiffness governed by electronic coupling between nearest-neighboring atoms. We find that magnetic ordering of 2D magnets with an auxetic effect is robust under external strain due to the protection of super-exchange interaction coming from the auxetic effect. Super-exchange interaction is sensitive to the symmetry of the crystal structure while the auxetic effect can mitigate the variation of such symmetry. The abundant magnetic properties in combination with the auxetic effect exhibit potential for novel nanodevice applications.