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 Neel temperature of 60 K. The state has large magnetic moment (& SIM;4 mu(B) per Mn) and sizable anisotropy (& SIM;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 (& SIM;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. Published under an exclusive license by AIP Publishing.