Atomic-scale visualization of strain-tailored noncollinear spin textures in an antiferromagnetic ultrathin film

Crystalline strain is typically considered as an effective approach to engineer low-dimensional antiferromagnets. However, a direct visualization of strained-tailored noncollinear spin textures in antiferromagnetic atomic layers has so far not been achieved. Here, we uncover a strain-induced transition from a three-dimensional noncollinear spin state in pseudomorphic Mn bilayer to a cycloidal spin spiral with a canted rotation plane in reconstructed Mn bilayer on the Ag(111) surface. These spin states are spatially imaged on the atomic scale by spin-polarized scanning tunneling microscopy revealing the correlation of atomic and magnetic structures. As demonstrated via first-principles electronic structure theory, the three-dimensional noncollinear spin state arises from the superposition of spin spiral and antiferromagnetic order due to higher-order exchange interactions. In reconstructed Mn bilayer, by contrast, the antiferromagnetic order is hindered by interlayer exchange coupling resulting in a pure spin spiral state. Our work highlights the complex interplay of atomic structure, intra- and interlayer exchange, as well as higher-order exchange interactions at antiferromagnetically coupled interfaces.