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

Abstract 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 M 2 X 2 O (M = Ti–Ni; X = S–Te; excluding Co 2 Te 2 O). It has been unveiled that [X 4 O 2 ]- D 2 h and [M 4 ]- D 4 h crystal fields govern the M-3 d orbital splittings in M 2 X 2 O. The splittings further lead to the antiferromagnetic (AFM) orderings in Ti 2 S 2 O/Fe 2 S 2 O/Fe 2 Se 2 O/M 2 X 2 O (M = V, Cr, Mn and Ni; X = S–Se) as well as the ferromagnetic orderings in Ti 2 Se 2 O/Ti 2 Te 2 O/Fe 2 Te 2 O/Co 2 S 2 O/Co 2 Se 2 O through kinetic and superexchange mechanisms. Notably, all the AFM M 2 X 2 O are 2D altermagnets, and Ti 2 Se 2 O/Ti 2 Te 2 O/Co 2 S 2 O/Co 2 Se 2 O are 2D half-metals. In particular, the anisotropic d-d/p hoppings lead to the tunable altermagnetic splitting in Ti 2 S 2 O/Cr 2 Te 2 O, while the parity of V-3 d yz orbital contributes to the symmetry-protected altermagnetic splitting within V 2 X 2 O. These altermagnetic and half-metallic monolayer M 2 X 2 O provide promising candidates applied in low-dimensional spintronic devices. In addition, the potential 2D altermagnetic Weyl semimetal of Fe 2 S 2 O/Fe 2 Se 2 O, nodal-loop half-metal of Ti 2 Se 2 O and half-semi metal of Ti 2 Te 2 O facilitate to uncover novel low-dimensional topological physics. These theoretical results would expand the platform in particular for 2D altermagnets and nontrivial systems.