  Due to their exceptional topological and dynamical properties magnetic
skyrmions - localized stable spin structures on the nanometre scale - show
great promise for future spintronic applications. To become technologically
competitive, isolated skyrmions with diameters below 10 nm that are stable at
zero magnetic field and room temperature are desired. Despite finding skyrmions
in a wide spectrum of materials, the quest for a material with these envisioned
properties is still ongoing. Here we report zero field isolated skyrmions with
diameters smaller than 5 nm coexisting with 1 nm thin domain walls in Rh/Co
atomic bilayers on the Ir(111) surface. These spin structures are characterized
by spin-polarized scanning tunnelling microscopy and can also be detected using
non-spin-polarized tips due to a large non-collinear magnetoresistance. We
demonstrate that sub-10 nm skyrmions are stabilised in these ferromagnetic Co
films at zero field due to strong frustration of exchange interaction, together
with Dzyaloshinskii-Moriya interaction and a large magnetocrystalline
anisotropy.
