CoX2Y4: a family of two-dimensional magnets with versatile magnetic order†

Two-dimensional (2D) magnetic materials offer a promising platform for nanoscale spintronics and for exploration of novel physical phenomena. Here, we predict a diverse range of magnetic orders in cobalt-based 2D single septuple layers CoX₂Y₄, namely, CoBi₂Te₄, CoBi₂Se₂Te₂, CoBi₂Se₄, and CoSb₂Te₄. Notably, CoBi₂Te₄ presents intrinsic non-collinear antiferromagnetism (AFM), while the others display collinear AFM. The emergence of AFM in all CoX₂Y₄ materials is attributed to the antiferromagnetic 90° Co–Te(Se)–Co superexchange coupling. The origin of non-collinear/collinear orders lies in competing Heisenberg exchange interactions within the Co triangular lattice. A pivotal factor governing the non-collinear order of CoBi₂Te₄ is the vanishingly small ratio of exchange coupling between next-nearest neighbour Co and the nearest neighbour Co (J₂/J₁ ∼ 0.01). Furthermore, our investigation into strain effects on CoX₂Y₄ lattices demonstrates the tunability of the magnetic state of CoSb₂Te₄ from collinear to non-collinear. Our prediction of the unique non-collinear AFM in 2D suggests the potential for observing extraordinary magnetic phenomena in 2D, including non-collinear scattering and magnetic domain walls.