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Room temperature electrically detection of single nanoscale skyrmions, a topological magnetic quasi-particle

Davide Maccariello, William Legrand, Nicolas Reyren, Karin Garcia, Karim Bouzehouane, Sophie Collin, Vincent Cros & Albert Fert "Electrical detection of single magnetic skyrmions in metallic multilayers at room temperature"
Nature Nanotechnology (2018)

Magnetic skyrmions are nanoscale non collinear spin configurations having topological features which give them a finite robustness against external perturbations. These skyrmions allow the unique interplay between topology, chiral magnetism and spin dependent transport properties at the nanoscale to be investigated. Indeed, the topological nature of skyrmions should underpin unique phenomena, such as the Topological Hall Effect (THE), which involves the appearance of an emergent electromagnetic field that affects the transport properties of conduction electrons, as demonstrated in bulk crystals with break of the inversion symmetry.

From a technological perspective, the unique properties associated to their topological nature can be exploited in a new generation of technological applications, ranging from logic or memory devices to nano-scale neuromorphic devices. One of the main challenges for implementing magnetic skyrmions in standardized and industrialized devices was concerning the possibility to detect them without using complex laboratory imaging techniques. To tackle this issue, researchers of the joint laboratory CNRS-Thales have been devoting their efforts in demonstrating that it is possible to electrically detect all the way down to the single magnetic skyrmion in a writing/erasing process at room temperature. This research has been published in Nature Nanotechnology.

Electrical detection and imaging of nanoscale skyrmions. a, Scheme of the experimental set-up for electrical measurements integrated in a Magnetic Force Microscopy (MFM). Blue (red) indicates an attractive (repulsive) magnetic force on the MFM cantilever and hence reveals the magnetization in the sample. The sample shape is its actual topography measured by atomic force microscopy. b, Scanning electron microscopy image of a standard device. c, The profile of the of the STXM absorption difference μp − μm acquired at the Co L3-edge (the average of the profiles is taken along the core of three skyrmions) is plotted together with the simulated profile of a skyrmion of diameter 80 nm convoluted with a Gaussian beam with a full-width at half-maximum of 100 nm (using Δ = 10 nm). The inset images show the map of the STXM absorption difference μp − μm (blue frame) and the simulated skyrmion at the same scale (red frame).

These swirling spin textures are stabilized by chiral interactions that can be observed in a given class of materials (known as B20 materials) in which their crystals have broken inversion symmetry or in artificially engineered magnetic thin multilayers where the symmetry is broken at interfaces. Until recently, the electrical properties of skyrmions were reported mostly for the case of the first class of materials in which was generally possible to stabilize skyrmions in form of lattices. In this case, the electrical signal of skyrmions were mainly distinguished by an extra transversal “Hall” voltage contribution due to novel electrodynamics properties linked to the topological charge of skyrmions. However, all this was mainly possible at low temperatures and the isolation of single skyrmions was not achieved yet. In the present study, the researchers used magnetic multilayers made of the stacks of ultrathin ferromagnetic layers between non-magnetic thin films, mainly heavy metals, in which it is possible to stabilize skyrmions at room temperature. By adopting to concomitant usage of the electrical transport measurements and the magnetic force microscopy (MFM), they succeeded to relate the variation of the Hall resistance with the appearance or the erasing of magnetic skyrmions with size of about 80nm. Remarkably, they especially report also about a large electrical signal due to nucleation of one single skyrmion achieved thanks to electrical pulses in nanostructured tracks, with transversal size approaching that of skyrmion. Contrary to the former case, in multilayered systems the electrical contribution mainly arises from the anomalous Hall effect, commonly observed in ferromagnets, with a negligible contribution from the topological Hall effect, more specifically related to the topological nature of skyrmions.

Variation in Hall due to a single skyrmion. The RT Hall resistivity shows a large variation when one single skyrmion is nucleated in the “Hall” cross area (dashed yellow square) in a track of width 400 nm. The red and green dots correspond to the Hall resistivity of the uniform ferromagnetic and single skyrmion states, respectively, shown in the MFM images. After the current injection, applied between iteration numbers 5 and 6), the Hall resistivity shows a change of 〖Δρ〗_xy^SK = 25±4 nΩcm associated with the single-skyrmion formation.

These observations advance the technological possibilities for skyrmion-based devices and they could represent a new avenue for further fundamental studies of their very rich physics.