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.