In spite of the fact that competing multi-spin interactions or the combination of inversion asymmetry and spin-orbit effects may lead to complex spin structures, the magnetism of nanomagnets has so far mainly been explained on the basis of the Heisenberg exchange interaction and the magnetic anisotropy. Now, spin-polarized scanning tunneling microscopy (SP-STM) allows the investigation of single compensated nanostructures with atomic resolution and reveals a wealth of surprising phenomena.
Recently, we revisited the two-dimensional atomic-scale antiferromagnetic (AFM) structure observed within a single layer of Mn atoms on tungsten (110) which has been studied earlier by spin-polarized STM [1]. We recall that earlier results were originally interpreted in terms of an antiferromagnetic configuration consisting of a checkerboard arrangement of Mn atoms of antiparallel magnetization [1]. In agreement with earlier results, our new SP-STM data reveal periodic stripes running along the [001] direction with an inter-stripe distance matching the surface lattice constant along the [110] direction. In addition, however, we observe that the magnetic amplitude is not constant but modulated on a larger lateral scale of about every 6 nm. Measurements in external fields which allow the separation of in-plane and out-of-plane magnetic components reveal that this modulation is caused by a spin spiral. First-principles calculations identify the spiral as cycloidal with specific chirality and prove that it is caused by the Dzyaloshinskii-Moriya (DM) interaction arising from structural inversion asymmetry inherent to all surfaces and interfaces.
[1] S. Heinze et al., Science 288 (2000) 1805 |