Deterministic creation and deletion of a single magnetic skyrmion observed by direct time-resolved X-ray microscopy

Abstract

Spintronic devices based on magnetic skyrmions are a promising candidate for nextgeneration memory applications due to their nanometre-size, topologically-protected stability and efficient current-driven dynamics. Since the recent discovery of roomtemperature magnetic skyrmions, there have been reports of current-driven skyrmion displacement on magnetic tracks and demonstrations of current pulse-driven skyrmion generation. However, the controlled annihilation of a single skyrmion at room temperature has remained elusive. Here we demonstrate the deterministic writing and deleting of single isolated skyrmions at room temperature in ferrimagnetic GdFeCo films with a device-compatible stripline geometry. The process is driven by the application of current pulses, which induce spin-orbit torques, and is directly observed using a time resolved nanoscale X-ray imaging technique. We provide a current-pulse profile for the efficient and deterministic writing and deleting process. Using micromagnetic simulations, we also reveal the microscopic mechanism of the topological fluctuations that occur during this process.Nanoscale memory devices that employ magnetic materials and offer non-volatility and efficient electrical controllability have the potential to outperform and eventually replace conventional silicon-based memory technologies such as DRAM or NAND-Flash 1-3 . Among the many different types of spintronic memory devices available, those based on magnetic skyrmions -topologically non-trivial spin nanostructures -are of particular interest. 4-10 Key advantages of skyrmion-based memory device include their stability 9 , high-density arrangement 4,5 and low-power operation using electrical excitations 6-8 . Recently the room-temperature stabilization of magnetic skyrmions and their current pulse-induced displacement on nanotracks has been reported in magnetic heterostructures such as Ta/CoFeB/MgO 11,12 , Pt/Co/Ta 13 , Pt/CoFeB/MgO 13-15 , Pt/Co/Ir 16 and Pt/GdFeCo/MgO 17 where the strong interfacial Dzyaloshinskii-Moriya interaction (DMI)leads to the stabilization of the skyrmion structures 18,19 . While such current-controlled skyrmion motion is applicable to an actual device scheme, the deterministic writing and deleting of a single isolated magnetic skyrmion at room temperature is required for fully functional skyrmionic devices 10,20 .Theoretical studies have suggested that skyrmions can be created by applying localized vertical spin-polarized current to a confined geometry 21 or well-defined notches 22 .Moreover, there are suggestions that a single skyrmion can be formed by the conversion of a pair of domain walls 23,24 . However, the controlled creation of a single skyrmion, and the subsequent annihilation of the same skyrmion, has only been experimentally achieved at low temperatures with spin-polarized scanning tunneling microscopy (SP-STM) 8,25 . This approach requires the in situ deposition of material at cryogenic conditions, which may be hard to incorporate into actual commercial devices. Recently, several s...

Skyrmion-based artificial synapses for neuromorphic computing

“…This asymmetric ferrimagnetic multilayer stack is known to have reasonably large DMI, |D| = 0.98 mJ m -2 , where ~200 nm-size skyrmions can be stabilized at room temperature 12 . Moreover, electrical writing/deleting from natural sites 11 and efficient skyrmion motion with a reduced skyrmion Hall effect 12 have been individually observed in this ferrimagnetic stack in our previous studies, which are combined together to realize fully functional skyrmion devices in the present work. Figure 1a first shows the experimental set-up used for the simultaneous resistance measurement and domain imaging.…”

“…2b inset. The applied unipolar pulses during both potentiation and depression were designed and optimized to generate/delete the controlled number of skyrmions using current-induced spin-orbit torques (SOTs) as investigated in our previous study 11 , assisted by current-induced joule heating that contributes to the reduction of skyrmion switching thresholds 28 . In Fig.…”

“…We have calculated the average resistivity decrease induced by each skyrmion addition over all acquired states during potentiation, |Δρxy,sk- During depression, we simply reversed the polarity of gradually increasing pulses with the same amplitudes and durations (as indicated in Fig. 2b) at a slightly increased background magnetic field Bz = 144 mT, so that reversed pulses could excite skyrmion texture, eventually leading to skyrmion annihilation medicated by topological defects without creating additional skyrmions 11 . Note that strong inter-skyrmion repulsive forces at highly skyrmionpopulated states may also contribute to the annihilation upon SOT application, as local impurities in real materials may block their synchronous motion.…”

Antiferromagnetism Emerging in a Ferromagnet with Gain

“…Compared to their FM counterparts, AFM skyrmions [47] have some other advantages, such as the elevated mobility [48][49][50][51] and the unusual thermal properties [48,52], among others [53][54][55][56][57]. One recent breakthrough toward this direction is the experimental realization of ferrimagnetic skyrmions in GdFeCo films with inhibited skyrmion Hall effect [58,59]. Because of its intrinsic difficulties in materials and detections, the AFM skyrmion is yet to be observed in experiments.…”

Creating zero-field skyrmions in exchange-biased multilayers through X-ray illumination

“…To date, the electrical creations of a single skyrmion have been experimentally demonstrated 5,11 , e.g., spin-polarized currents or electric fields can favor the flip of spins and induce the creation of a single skyrmion. By leveraging a non-uniform device geometry or a naturally formed defect, a current pulse can also generate a single skyrmion 7,[12][13][14] . In addition to electrical means, optical methods can also offer an efficient way for manipulating magnetizations, with the advantages of ultrafast dynamics and flexibility in selecting the writing position 15 .…”