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]-<i>D</i>_2<i>h</i>and [M₄]-<i>D</i>_4<i>h</i>crystal fields govern the M-3<i>d</i>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<i>d-d/p</i>hoppings lead to the tunable altermagnetic splitting in Ti₂S₂O/Cr₂Te₂O, while the parity of V-3<i>d_yz</i>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.