CoX 2 Y 4 : 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 CoX 2 Y 4 , namely, CoBi 2 Te 4 , CoBi 2 Se 2 Te 2 , CoBi 2 Se 4 , and CoSb 2 Te 4 . Notably, CoBi 2 Te 4 presents intrinsic non-collinear antiferromagnetism (AFM), while the others display collinear AFM. The emergence of AFM in all CoX 2 Y 4 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 CoBi 2 Te 4 is the vanishingly small ratio of exchange coupling between next-nearest neighbour Co and the nearest neighbour Co ( J 2 / J 1 ∼ 0.01). Furthermore, our investigation into strain effects on CoX 2 Y 4 lattices demonstrates the tunability of the magnetic state of CoSb 2 Te 4 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.