Antiferromagnetic nodal loop and strain-controllable magnetic phase transition in monolayer MnAl

Exploring novel two-dimensional (2D) materials with intrinsic magnetism or topological band features is a focus of current research. Here, based on first-principles calculations, we study a 2D structure of MnAl, which, in the bulk form, is a well-known permanent magnet. We show that in 2D, MnAl can stabilize in a square lattice with single-atom thickness. The ground state is an antiferromagnet (AFM) with checkerboard type magnetic ordering and an estimated Néel temperature of 60 K. The state has large magnetic moment (∼4 μ B per Mn) and sizable anisotropy (∼0.27 meV/f.u.), analogous to bulk MnAl. In the electronic band structure, the state exhibits a single type-I AFM nodal loop at the Fermi level, which is protected by mirror symmetry in the absence of spin–orbit coupling. Spin–orbit coupling opens only a small gap at the loop, preserving the band inversion feature. Furthermore, we show that a small strain (∼1%) can drive a magnetic phase transition from the checkerboard AFM to a stripe-type AFM state, accompanied by a significant change in the band structure. Our result offers an intriguing platform for exploring the interplay among magnetism, topology, and phase transitions in low dimensions.