Monolayer M2X2O as potential 2D altermagnets and half-metals: a first principles study

Realizing novel two-dimensional (2D) magnetic states would accelerate the development of advanced spintronic devices and the understandings of 2D magnetic physics. In this paper, we have examined the magnetic and electronic properties of 20 dynamically stable and exfoliable M2X2O (M = Ti-Ni; X = S-Te; excluding Co2Te2O). It has been unveiled that [X4O2]-D-2h and [M-4]-D-4h crystal fields govern the M-3d orbital splittings in M2X2O. The splittings further lead to the antiferromagnetic (AFM) orderings in Ti2S2O/Fe2S2O/Fe2Se2O/M2X2O (M = V, Cr, Mn and Ni; X = S-Se) as well as the ferromagnetic orderings in Ti2Se2O/Ti2Te2O/Fe2Te2O/ Co2S2O/Co2Se2O through kinetic and superexchange mechanisms. Notably, all the AFM M2X2O are 2D altermagnets, and Ti2Se2O/Ti2Te2O/Co2S2O/Co2Se2O are 2D half-metals. In particular, the anisotropic d-d/p hoppings lead to the tunable altermagnetic splitting in Ti2S2O/Cr2Te2O, while the parity of V-3dyz orbital contributes to the symmetry-protected altermagnetic splitting within V2X2O. These altermagnetic and half-metallic monolayer M2X2O provide promising candidates applied in low-dimensional spintronic devices. In addition, the potential 2D altermagnetic Weyl semimetal of Fe2S2O/Fe2Se2O, nodal-loop half-metal of Ti2Se2O and half-semi metal of Ti2Te2O facilitate to uncover novel low-dimensional topological physics. These theoretical results would expand the platform in particular for 2D altermagnets and nontrivial systems.