Chiral magnetic order at surfaces driven by inversion asymmetry

Chirality is a fascinating phenomenon that can manifest itself in subtle ways, for example in biochemistry ( in the observed single-handedness of biomolecules(1)) and in particle physics ( in the charge-parity violation of electroweak interactions(2)). In condensed matter, magnetic materials can also display single-handed, or homochiral, spin structures. This may be caused by the Dzyaloshinskii - Moriya interaction, which arises from spin - orbit scattering of electrons in an inversion-asymmetric crystal field(3,4). This effect is typically irrelevant in bulk metals as their crystals are inversion symmetric. However, low-dimensional systems lack structural inversion symmetry, so that homochiral spin structures may occur(5). Here we report the observation of magnetic order of a specific chirality in a single atomic layer of manganese on a tungsten ( 110) substrate. Spin-polarized scanning tunnelling microscopy reveals that adjacent spins are not perfectly antiferromagnetic but slightly canted, resulting in a spin spiral structure with a period of about 12 nm. We show by quantitative theory that this chiral order is caused by the Dzyaloshinskii - Moriya interaction and leads to a left-rotating spin cycloid. Our findings confirm the significance of this interaction for magnets in reduced dimensions. Chirality in nanoscale magnets may play a crucial role in spintronic devices, where the spin rather than the charge of an electron is used for data transmission and manipulation. For instance, a spin-polarized current flowing through chiral magnetic structures will exert a spin-torque on the magnetic structure(6,7), causing a variety of excitations or manipulations of the magnetization(8,9) and giving rise to microwave emission, magnetization switching, or magnetic motors.