Tuning Noncollinear Spin Structure and Anisotropy in Ferromagnetic Nitride MXenes.

Recent experimental success in the realization of two-dimensional magnetism has invigorated the search for low-dimensional material systems with tunable magnetic anisotropy that exhibit intrinsic long-range ferromagnetic order. Here we report that modifying the surface termination and transition metal in monolayer M2NT x nitride MXenes gives rise to a rich diversity of noncollinear spin structures and finely tunable magnetocrystalline anisotropy. Based on first-principles simulations, we predict that manipulating the strength of the spin-orbit interaction and electron localization via the chemical degrees of freedom can induce sufficient anisotropy to counteract thermal fluctuations that suppress long-range magnetic order. We find that Ti2NO2 and Mn2NF2 MXenes have continuous O(3) and O(2) spin symmetries, respectively, that may be broken by an applied field, while Cr2NO2 and Mn2NO2 are intrinsic Ising ferromagnets with out-of-plane easy axes and magnetic anisotropy energies up to 63 μeV/atom. These systems also exhibit both gapped and gapless Dirac points near the Fermi level. Our study suggests that nitride MXenes offer a promising avenue for achieving both practical spintronic devices and investigating fundamental spin processes in two-dimensional materials.