In ultrathin magnetic films deposited on heavy elements substrates, the interface-induced Dzyaloshinkii-Moriya interaction allows to stabilize non-collinear spin structures like magnetic skyrmions. These topologically protected objects are interesting for future data storage technologies like racetrack memories.
We present spin-polarized scanning tunneling microscopy investigations on a three-atomic-layer thick Fe film deposited on Ir(111). In this system, the large epitaxial strain is relieved by the formation of a dense dislocation lines network. This particular structure of the film induces a symmetry breaking with dramatic consequences on the magnetic state. In zero field, spin spirals propagate along the dislocations lines and their period depends on the spacing between these lines, i.e. on the strain relief, which is locally varying . We attribute this effect on the spirals to modifications of the exchange coupling.
Single skyrmions appear in external magnetic field. We demonstrate that they can reliably be written and deleted by an STM tip . The strong-bias polarity dependence and the linear behavior of the threshold voltage for switching with the tip-sample distance shows that electric field plays the dominant role in the switching mechanism. This switching between the topologically distinct magnetic states by electric fields may be beneficial in future spintronic devices employing skyrmions as information carriers.
We acknowledge financial support by the European Union via the Horizon 2020 research and innovation program under Grant No.665095.
 Finco et al, PRB 94, 214402 (2016)
 Hsu et al, Nat. Nanotech. (2016)
An interesting class of interface-driven non-collinear spin structures, i.e., chiral domain walls, cycloidal spin spirals and Néel-type skyrmions, have been observed in ultrathin transition metal films grown on heavy-element substrates making use of spin-polarized scanning tunneling microscopy (SP-STM) . Due to a lack of structural inversion symmetry at interfaces, they exhibit a unique rotational sense as a consequence of interfacial Dzyaloshinskii-Moriya (DM) interactions. In this talk, I will present our results based on the investigations of such chiral spin textures under the influence of strain relief and the effect of local electric fields. While a nanoskyrmion lattice was revealed for Fe monolayers (ML) grown on Ir(111), a cycloidal spin spiral ground state has been resolved on Fe double-layers (DL) by employing SP-STM with vectorial magnetic field. As a result of a large lattice mismatch between the epitaxially grown Fe-DL film and the underlying Ir(111) substrate, local uniaxial strain relief occurs, leading to dislocation line patterns. Interestingly, the wavevector of spin spirals is strictly guided along the dislocation lines, while the spin spiral's wavefront exhibits a zigzag deformation . By further increasing the Fe coverage to triple-layers (TL), the zigzag spin spiral remains the magnetic ground state, but with an enhanced periodicity as compared to that of Fe-DL. A magnetic phase transition from the spin spiral to a skyrmionic state, and finally to a saturated ferromagnetic state occurs for Fe-TL by applying an external magnetic field. STM-induced writing and deleting of individual skyrmions is demonstrated with a pronounced bias-polarity dependence, suggesting the decisive role of the local electric field between STM tip and Fe film for the switching mechanism .
 K. von Bergmann, A. Kubetzka, O. Pietzsch, and R. Wiesendanger, J. Phys.: Condens. Matter 26, 394002 (2014)
 P.-J. Hsu, A. Finco, L. Schmidt, A. Kubetzka, K. von Bergmann, and R. Wiesendanger, Phys. Rev. Lett. 116, 017201 (2016)
 P.-J. Hsu, A. Kubetzka, A. Finco, N. Romming, K. von Bergmann, and R. Wiesendanger, arXiv:16001.02935 (2016)