Mobile N\'eel skyrmions at room temperature: status and future

Magnetic skyrmions are topologically protected spin textures that exhibit many fascinating features. As compared to the well-studied cryogenic Bloch skyrmions in bulk materials, we focus on the room-temperature N\'eel skyrmions in thin-film systems with an interfacial broken inversion symmetry in this article. Specifically, we show the stabilization, the creation, and the implementation of N\'eel skyrmions that are enabled by the electrical current-induced spin-orbit torques. Towards the nanoscale N\'eel skyrmions, we further discuss the challenges from both material optimization and imaging characterization perspectives.

creation, and the implementation of Néel skyrmions that are enabled by the electrical current--induced spin--orbit torques. Towards the nanoscale Néel skyrmions, we further discuss the challenges from both material optimization and imaging characterization perspectives.

Introduction
After their original observation in MnSi single crystals 1--3 , magnetic (Bloch) skyrmions have stimulated tremendous research efforts in the field of spintronics 4--9 . On the one hand, thanks to the well--defined spin topology in real space, magnetic skyrmions enable many intriguing quantum--mechanical phenomena to be observed including: emergent electromagnetic dynamics 10 , effective magnetic monopole 11 , topological/skyrmion Hall effects 7,9,12 . On the other hand, due to their topological properties, magnetic skyrmions can behave as meta--stable quasi--particles and thus have been envisioned as information carriers for ultra--low power non--volatile spintronics 4,5,13 .
Although the investigation of magnetic skyrmions was pioneered using bulk materials with chiral exchange interactions due to crystal symmetries lacking inversion, there are so far only very few materials that enable the stabilization of Bloch skyrmions above room temperature 14 . In contrast, recent progress on interfacial nanomagnetism has significantly extended the paradigm of magnetic skyrmions to readily accessible materials systemheavy metal/ultra--thin ferromagnet/insulator (HM/FM/I) hetero--structures with perpendicular magnetic anisotropy 7, 13, 15--22 . The interfacial symmetry breaking in HM/F/I hetero--structures introduces a chiral interfacial Dzyaloshinskii-Moriya interactions (DMI) between the neighbouring atomic spins ! and ! that can be written as where the DMI vector !"# lies in the film plane acting as an equivalent (spatial) in-plane field and stabilizes Néel (hedgehog) skyrmions/domain walls with a fixed chirality 18,21,[23][24][25][26] . In these interfacial symmetry-breaking systems, the stabilization, generation, and manipulation of hedgehog skyrmions by electric currents have been demonstrated at room temperature 21,27,28 .
Beyond the realization of room--temperature magnetic skyrmions, this novel material system provides many unique advantages. From a material synthesis perspective: these thin films and their multilayers can be readily produced by using a magnetron sputtering technique onto SiO2 substrates. From an application perspective, as a result of the strong spin-orbit interaction of the involved heavy metals (typically Ta, Pt and W) 13,29,30 , the electrical current induced spin--orbit torques from the spin Hall effects provide very energetically efficient avenues for electrically generating, manipulating, and more importantly, implementing magnetic skyrmions at room temperature.

Experiment and Discussion
The feasibility of generating mobile magnetic skyrmions at room temperature on demand was demonstrated by using patterned heterostructures 19 . A trilayer of Ta(5nm)/Co20Fe60B20(CoFeB)(1.1nm)/TaOx(3nm) grown at room temperature by a magnetron sputtering technique onto a SiO2 substrate. A polar magneto--optical Kerr effect (MOKE) microscope in a differential mode was utilized for imaging experiments at room Furthermore, the displacement is identically reproduced for subsequent current pulses as long as the skyrmion is not in the vicinity of the additional voltage contacts. This indicates that for these higher current densities random pinning is inconsequential. However, when the skyrmion is close to the additional voltage contacts of the wire, the displacement is somewhat reduced. This could either be due to current spreading into the voltage contacts, which effectively reduce the current density, or due to dipolar interactions of the skyrmions with the magnetic material forming the voltage contacts.

Perspectives
While tremendous progresses have been made on the emerging field of magnetic skyrmions in thin film heterostructures, there are many challenges that still need to be addressed. In order to make magnetic skyrmions truly attractive for ultra--high density data storage, and to probe at room temperature the emergent new physics due to their topology, such as topological or skyrmion Hall effects, the size of the magnetic skyrmions has to be significantly reduced 21,32,33 . The size of magnetic skyrmions shown here is large (≈ 1 µm in diameter) due to its relatively weak DMI (≈ 0.5±0.1 mJ/m 2 ) at the Ta/CoFeB interface, as compared to other material systems such as Pt/Co (≈ 1.3±0.1 mJ/m 2 ). It should also be noted that the insulator layer in our Ta/CoFeB/TaOx heterostructures contributes to the perpendicular magnetic anisotropy and interfacial asymmetry, but its role for interfacial Properly designing the DMI in these heterostructures may be more complicated 34 , since the magnitude of the DMI might depend strongly on the details of local crystallographical microstructure.
Smaller skyrmions also provide additional challenges for their characterization. Upon approaching few tens of nanometer Néel skyrmions at room temperature, one has to access novel spin sensitive imaging techniques to reveal the associated chirality. Lorentz transmission electron microscopy (L--TEM) has been a powerful tool to study the Bloch-type skyrmions in B20 compounds 3 . However, the phase shift for a given Néel skyrmion/domain wall is zero in the Lorentz mode, which results in the absence of magnetic contrast. On the other hand, it is known that the spin--polarized low--energy electron microscopy (SPLEEM) 15,16,35 and spin--polarized scanning tunnelling microscopy (SP--STM) 33 are capable of quantifying all 3--dimensional components of an arbitrary spin texture. However, the sample preparation for these techniques is non--trivial and often these techniques do not work for the ex--situ patterned devices. A more suitable approach for ex--situ patterned devices may be a nitrogen vacancy (NV) center diamond microscope, is expected to be suitable for quantifying the 3 dimensional arbitrary spin textures with a sub--nm resolution 36,37 . This technique can thus in principle be used to map out the spin topology, the dynamics, and the deformation of nanometer skyrmions, driven by both spin-orbit torques and magnetic fields, but so far this has not yet been adopted to the investigation of Néel skyrmions.

Conclusions
In summary, by harvesting the interfacial Dzyaloshinskii--Moriya interactions and the current induced spin--orbit torques in the heavy metal/ultra--thin ferromagnet/insulator heterostructures, Néel skyrmion might be more advantageous as compared to Bloch skyrmion in bulk crystals, including the efficient creation, manipulation, and implementation. It could enable not only the realization of functional skyrmionic devices, but also the observation of intriguing topological transport phenomena possibly at room temperature.
Work carried out at the Argonne National Laboratory, including patterned device fabrication, magneto--optic imaging and data analysis, was supported by the U.S.