Facing the ever-growing demand for data storage will most probably require a new paradigm. Magnetic skyrmions are anticipated to solve this issue as they are arguably the smallest spin textures in magnetic thin films in nature. We designed cobalt-based multilayered thin films where the cobalt layer is sandwiched between two heavy metals providing additive interfacial Dzyaloshinskii-Moriya interactions, which reach about 2 mJ/m 2 in the case of the Ir|Co|Pt multilayers. Using a magnetization-sensitive scanning x-ray transmission microscopy technique, we imaged magnetic bubble-like domains in these multilayers. The study of their behavior in magnetic field allows us to conclude that they are actually magnetic skyrmions stabilized by the Dzyaloshinskii-Moriya interaction. This discovery of stable skyrmions at room temperature in a technologically relevant material opens the way for device applications in a near future.A major societal challenge is related to the continually increasing quantity of information to process and store. The hard disk drives, in which information is encoded magnetically, allow nowadays the storage of zettabytes (10 21 ) of information, but this technology should soon reach its limits. An up-and-coming avenue has been opened by the discovery of magnetic skyrmions [1], i.e. spin windings that can be localized within a diameter of a few nanometers and can move like particles [2]. These magnetic solitons, remarkably robust against defects due to the topology of their magnetic texture [3], are promising for being the ultimate magnetic bits to carry and store information. The topology of the skyrmions also appears to further underlie other important features such as their current-induced motion induced by small dc currents that is crucial for real applications but also the existence of a specific component in Hall Effect [4][5][6] that can be used advantageously for an electrical read-out of the information carried by nano-scale skyrmions. We proposed recently that these skyrmions could be used in future storage devices and information processing [2].The existence of skyrmion spin configuration has been predicted theoretically about thirty years ago [1] but it was only recently that skyrmion lattices have been observed in crystals with noncentrosymmetric lattices, e.g. B20 crystallographic structure in MnSi [7][8][9] FeCoSi [10] or FeGe [5] crystals. In 2011, skyrmions have also been identified in single ultrathin ferromagnetic films with out-of-plane magnetization (Fe and FePd) deposited on a heavy metal substrate such as Ir(1 1 1) [11,12]. Thin magnetic films appear to be more compatible with technological developments, though the observation of skyrmions in thin films has been limited up to now to low temperature and also needs, in some cases, the presence of a large applied magnetic field [12]. The study of these new magnetic phases associated with chiral interactions has generated a

Magnetic skyrmions are nanoscale windings of the spin configuration that hold great promise for technology due to their topology-related properties and extremely reduced sizes. After the recent observation at room temperature of sub-100 nm skyrmions stabilized by interfacial chiral interaction in magnetic multilayers, several pending questions remain to be solved, notably about the means to nucleate individual compact skyrmions or the exact nature of their motion. In this study, a method leading to the formation of magnetic skyrmions in a micrometer-sized track using homogeneous current injection is evidenced. Spin-transfer-induced motion of these small electrical-current-generated skyrmions is then demonstrated and the role of the out-of-plane magnetic field in the stabilization of the moving skyrmions is also analyzed. The results of these experimental observations of spin torque induced motion are compared to micromagnetic simulations reproducing a granular type, nonuniform magnetic multilayer in order to address the particularly important role of the magnetic inhomogeneities on the current-induced motion of sub-100 nm skyrmions for which the material grains size is comparable to the skyrmion diameter.

No abstract

Magnetic skyrmions are arguably the smallest stable magnetic configuration in films, and therefore could be the ultimate magnetic storage bit [1,2] . They have also triggered a wide interest due to the new fundamental phenomena related to their topology . Numerical simulations have shown that the interfacial Dzyaloshinskii-Moriya interaction (DMI) can stabilize such skyrmions in nanoscale disks or tracks for a rather large range of DMI amplitudes for which the skyrmion can either be the ground state or metastable relative to the uniform state [4,5,6] . Here, we demonstrate experimentally the presence of skyrmions in metallic multilayers structures engineered to exhibit a strong DMI interaction . In this work, beyond the study of skyrmions in thin films [3], we focus our investigation on sputtered multilayers with vertical magnetization consisting of stacks of trilayers composed of 0 .6-nm-thick Co layers sandwiched between 5d transition metal layers, namely Pt, Ir and W . Asymmetric sandwiches were designed in order to introduce additive DMI from the top and bottom interfaces of Co [2,5,6] while also obtaining a considerable perpendicular magnetic anisotropy . We will present our results on two types of metallic multilayers grown at room temperature: |Pt10|-Co0 .6|Pt1|{Co0 .6|Pt1}x10|Pt3 and |Pt10|Co0 .6|Pt1|{Ir1|Co0 .6|Pt1}x10|Pt3 (thickness in nm) . The magnetic anisotropy is determined using standard magnetometry, while the DMI amplitude is estimated by two original methods . Based on detailed mapping of the magnetization obtained using scanning transmission X-ray microscopy (STXM) combined with the XMCD effect, STXM allows the magnetic imaging of patterned structures in a non-invasive way with nanoscale resolution (<50 nm) [7] . We acquired such images at different perpendicular magnetic fields in both symmetric Pt|Co|Pt and asymmetric Pt|Co|Ir multilayers [8] . From the analysis of the magnetic domain configurations, either at zero field after demagnetization (not shown) or following the evolution of the size of bubbles with perpendicular magnetic field (Figure 1), we evaluate consistently a DMI amplitude D as large as 2 mJ/ m 2 in Pt|Co|Ir . The process for estimating the skyrmion radius is displayed in Figure 2 . A direct consequence of having such a large DMI is that the bubble-like domains that we have identified are indeed isolated magnetic skyrmions . In micromagnetic simulations, trivial bubbles (winding number equal to zero) are not stable and vanish in a few nanoseconds or less, while skyrmions (winding number equal to one) are stable . The good agreement of the size dependence as well as the stability of the bubble domains is strong evidence that these domains are, in fact, skyrmions . The value of D is further confirmed by studying the typical width of the worm domains, which are observed at zero magnetic field, indicating consistency in the analysis .In conclusion, we demonstrate the presence, at room temperature, of skyrmions stabilized by interfacial DMI in metallic multilayers, opening the...

We study numerically the dynamics of a magnetic bubble in a disc-shaped magnetic element which is probed by a pulse of a magnetic field gradient. Magnetic bubbles are nontrivial magnetic configurations which are characterized by a topological (skyrmion) number N and they have been observed in mesoscopic magnetic elements with strong perpendicular anisotropy. For weak fields we find a skew deflection of the axially symmetric N = 1 bubble and a subsequent periodic motion around the center of the dot. This gyrotropic motion of the magnetic bubble is shown here for the first time. Stronger fields induce switching of the N = 1 bubble to a bubble which contains a pair of Bloch lines and has N = 0. The N = 0 bubble can be switched back to a N = 1 bubble by applying again an external field gradient. Detailed features of the unusual bubble dynamics are described by employing the skyrmion number and the moments of the associated topological density.

No abstract

No abstract

Magnetic sensing and logic devices based on the motion of magnetic domain walls rely on the precise and deterministic control of the position and the velocity of individual magnetic domain walls in curved nanowires. Varying domain wall velocities have been predicted to result from intrinsic effects such as oscillating domain wall spin structure transformations and extrinsic pinning due to imperfections. Here we use direct dynamic imaging of the nanoscale spin structure that allows us for the first time to directly check these predictions. We find a new regime of oscillating domain wall motion even below the Walker breakdown correlated with periodic spin structure changes. We show that the extrinsic pinning from imperfections in the nanowire only affects slow domain walls and we identify the magnetostatic energy, which scales with the domain wall velocity, as the energy reservoir for the domain wall to overcome the local pinning potential landscape.