Due to their exceptional topological and dynamical properties magnetic skyrmions—localized stable spin structures—show great promise for spintronic applications. To become technologically competitive, isolated skyrmions with diameters below 10 nm stable at zero magnetic field and at room temperature are desired. Despite finding skyrmions in a wide spectrum of materials, the quest for a material with these envisioned properties is ongoing. Here we report zero field isolated skyrmions at T = 4 K with diameters below 5 nm observed in the virgin ferromagnetic state coexisting with 1 nm thin domain walls in Rh/Co atomic bilayers on Ir(111). These spin structures are investigated by spin-polarized scanning tunneling microscopy and can also be detected using non-spin-polarized tips via the noncollinear magnetoresistance. We demonstrate that sub-10 nm skyrmions are stabilized in these ferromagnetic Co films at zero field due to strong frustration of exchange interaction, together with Dzyaloshinskii–Moriya interaction and large magnetocrystalline anisotropy.

In this work we report a scanning tunneling microscopy investigation of lithographically defined superconducting nanosquares. The obtained spectroscopic maps reveal the spatial evolution of both the superconducting condensate and the screening currents as a function of the applied magnetic field. The symmetry of the nanostructure is imposed on the condensate and it controls the distribution of the vortices inside the nanosquare. Our local study allows exploring the impact of small structural defects, omnipresent in these kind of structures, on both the supercurrent and vortex distribution. As a result, direct experimental evidence of vortex pinning and current crowding at the nanoscale has been obtained. DOI: 10.1103/PhysRevB.93.054514 Confinement effects play an important role in different physical phenomena especially in quantum systems like Bose-Einstein condensates, superconductors, and superfluids. For example, the ability to structure superconducting devices at length scales comparable to the characteristic sizes (penetration dept λ and coherence length ξ ) of the condensate revealed a vast world of possibilities to explore quantum phenomena (e.g., creation of artificial atoms [1], induction of quantum phase slip lines [2], confinement effects [3], to name a few). In the latter example, theoretical modeling of these systems solving the Ginzburg-Landau (G-L) [4,5] or Bogoliubov-deGennes (B-dG) [6] equations for single and/or multiband mesoscopic superconductors has been done. All these simulations unveil the importance of confinement in mesoscopic superconducting systems. For example, the symmetry of the nanostructure will compete with the vortex-vortex interaction resulting in different vortex configurations compared to the triangular Abrikosov lattice, found in bulk superconductors [3][4][5][6].These intriguing effects were experimentally investigated, using low temperature transport measurements, by probing the influence of nanostructuring on the superconductor/normal phase boundary [3]. Although these measurements proved the importance of size and shape they do not give sufficient local information about the spatial distribution of the superconducting condensate. Moreover, a different approach is needed to explore the condensate in the nondissipative (zero voltage) state. In order to tackle these issues a second set of experiments probes the magnetic field profiles, generated by the superconducting currents, by using magnetic field sensitive probes (e.g., Hall probe [7,8], scanning Hall probe microscopy [9], Bitter decoration [10], and scanning SQUID microscopy [11]). These techniques indeed visualized the symmetry-induced vortex configurations in superconducting nanostructures within the low confinement regime (i.e., nanostructure size ∼λ ξ ). The observed configurations are the results of the imposed boundary conditions and the repulsive magnetic interactions between vortices. In the strong confinement regime (i.e., nanostructure size ∼ξ λ), the distribution of the superconducting condensate is gov...

No abstract

We use spin-polarized scanning tunneling microscopy and density functional theory (DFT) to study domain walls (DWs) and the Dzyaloshinskii-Moriya interaction (DMI) in epitaxial films of Co/Ir(111) and Pt/Co/Ir(111). Our measurements reveal DWs with fixed rotational sense for one monolayer of Co on Ir, with a wall width around 2.7 nm. With Pt islands on top, we observe that the DWs occur mostly in the uncovered Co/Ir areas, suggesting that the wall energy density is higher in Pt/Co/Ir(111). From DFT we find an interfacial DMI that stabilizes Néel-type DWs with clockwise rotational sense. The calculated DW widths are in good agreement with the experimental observations. The calculated total DMI nearly doubles from Co/Ir(111) to Pt/Co/Ir(111); however, in the latter case the DMI is almost entirely due to the Pt with only a minor Ir contribution. Therefore a simple additive effect, in which both interfaces contribute significantly to the total DMI, is not observed for one atomic Co layer sandwiched between Ir and Pt.

We present a comparison of the characteristics of the magnetic domain walls in an atomic monolayer of Co on Pt(111) and a Ni/Fe atomic bilayer on Ir(111), based on spin-polarized scanning tunneling microscopy measurements. In both cases, the films exhibit a roughly triangular dislocation line pattern created by epitaxial strain relief, as well as out-of-plane ferromagnetic order. Domains with opposite magnetization are separated by domain walls with a unique rotational sense, demonstrating the important role of the Dzyaloshinskii-Moriya interaction induced by the Co/Pt and the Fe/Ir interfaces. The domain walls in Co/Pt(111) are straight and usually found in geometrical constrictions of the film, where they can minimize their length. In contrast, the domain walls in Ni/Fe/Ir(111) follow complicated paths, which can be correlated to the structural triangular pattern. The comparison between the two systems shows that the structural patterns, despite their similarity, have a different impact on the domain walls. In the Co/Pt(111) case, the magnetic state is not influenced by the dislocation line network, in contrast to the Ni/Fe/Ir(111) system in which the formation of the walls is favored at specific positions of the structural pattern.