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Stabilization and Reversal of Skyrmion Lattice in Ta/CoFeB/MgO Multilayers Zhaogang Qin, Ying Wang, Shimeng Zhu, Chendong Jin, Jiecai Fu, Qingfang Liu, and Jiangwei Cao ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b12694 • Publication Date (Web): 02 Oct 2018 Downloaded from http://pubs.acs.org on October 3, 2018

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ACS Applied Materials & Interfaces

Stabilization and Reversal of Skyrmion Lattice in Ta/CoFeB/MgO Multilayers Zhaogang Qin‡, Ying Wang‡, Shimeng Zhu, Chendong Jin, Jiecai Fu, Qingfang Liu* and Jiangwei Cao* Key Laboratory for Magnetism and Magnetic Materials of the Ministry of Education, Lanzhou University, Lanzhou, 730000, Peoples Republic of China *Corresponding authors. E-mail addresses: [email protected]; [email protected] ‡

These authors contributed equally to this work

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ABSTRACT Recently, magnetic skyrmion attracted much attention due to its potential application in racetrack memory and other nano devices. In bulk chiral magnets with non-centrosymmetric crystal structures, skyrmion lattice phase has been extensively observed. While in film or multilayers with interfacial Dzyaloshinskii–Moriya (DM) interaction, individual skyrmion is often observed. Here, we report a short-ordered skyrmion lattice observed in [Ta(5.0 nm)/CoFeB(1.5 nm)/MgO(1.0 nm)]15 multilayer in a remnant state. The structure, stabilization and reversal of these skyrmions are discussed. Applying a slightly tilted in-plane magnetic field caused reversal of the skyrmion lattice. This reversal came from skyrmions disappeared and new skyrmions nucleated in the interstitial regions of the lattice. Also, we investigated how the skyrmion lattice depended on the CoFeB thickness. Our findings provide a pathway to stabilize and reverse skyrmions in multilayers films. KEYWORDS: skyrmion lattice, multilayers, zero field, room temperature, reversal.

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INTRODUCTION Magnetic skyrmions are attracting much interest both in fundamental science1,2 and applications3,4 especially for nonvolatile magnetic memory. This topologically non-trivial, particle-like swirling spin texture is mostly stabilized by the Dzyaloshinskii–Moriya interaction (DMI)5,6, which is associated with broken inversion symmetry and strong spin-coupling. DMI emerges in some bulk materials with non-centrosymmetric crystal structures or at the interface of thin-film stacks, which commonly stabilize Bloch-type skyrmions7-9 and Néel-type skyrmions10-15, respectively. Skyrmion (Skyrmion bubble), stabilized by both dipolar and DM interaction energies16, triggered much attraction recently due to their homochirality comparing with classical bubble. For applications, skyrmion in thin multilayer films are more appealing because they are compatible with existing semiconductor technology as well as the existence of both DM interaction and dipolar energies. Individual skyrmions have been experimentally observed in multilayer systems with sizes from several tens of nanometers10 to micrometers17,18. Skyrmion lattices, which are typically found in chiral magnets such as B20 compounds19-21 and multiferroic Cu2OSeO3,22 have also been reported in multilayer systems without DM interaction (dipolar skyrmion23) and Fe monolayer (ML) on an Ir/YSZ/Si(111) multilayer system recently24. However, it is still a big challenge to achieve a robust skyrmion lattice state that exsit at zero-field and room-temperature in multilayers film. In addition, there are reports on reversal of individual skyrmion using micromagnetic simulation25-27 and an experiment in field-driven polarity reversal of a target skyrmion28. In practice, however, we need to manipulate plenty of skyrmions. Therefore, the reversal of 3



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skyrmions in skyrmions congeries especially skyrmion lattice is more important issue in skyrmions application. And it has important guiding significance to figure out the mechanism of reversal of skyrmions in skyrmion lattice. In this letter, we demonstrate a robust short-ordered skyrmion lattice state at both zero-field and room-temperature in a [Ta(5.0 nm)/CoFeB(1.5 nm)/MgO(1.0 nm]15 multilayer. We also investigate the reversal of skyrmions in skyrmion lattice under a slightly tilted in-plane field in this system. EXPERIMENTAL SECTION Sample preparation. The multilayer films of substrate/[Ta/CoFeB/MgO]15/Ta were fabricated using magnetron sputtering with a background pressure of ~1.2×10−7 Torr; the Ta and MgO layers were deposited by radio-frequency (RF) sputtering, while the CoFeB layer was deposited by DC magnetron sputtering (with a thickness of 1.1 nm, 1.3 nm, or 1.5 nm). A 2-nm Ta cap was deposited to prevent oxidation of the stack. The deposition pressure was kept at 5 mTorr for all layers. Films were deposited on two types of substrates: on thermally oxidized Si wafers (used for Magnetic Force Microscopy (MFM) measure) and on 15-nm-thick Si3N4 membranes (used for Lorentz transmission electron microscopy (LTEM) measure). M–H hysteresis loops were measured using vibrating sample magnetometry (VSM, ADE technologies, EV9). All the measurements were done at room temperature. The micromagnetic simulation of the Landau–Lifshitz–Gilbert (LLG) equation was done to investigate the spin structure and reversal process of our skyrmion and skyrmion lattice. Magnetization imaging. Transmission electron microscopy (FEI Tecnai F30) was done at 300 kV with high spatial resolution (≤ 0.14 nm). No magnetic contrast arises in LTEM from the Néel 4


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skyrmion if the electron beam penetrates the sample perpendicularly29,30. Thus, the sample was tilted with respect to the beam direction. MFM was performed with a commercial MFP-3D AFM/MFM (Asylum Research), keeping a sharp magnetic tip at a distance h from the sample surface. The MFM tips (ASYMFHM) were ~84 nm in radius and were kept at a distance h of 50 nm, explaining why the skyrmion appeared to be large. The scanning probe system was operated at the resonance frequency of the magnetic tips, about 70 kHz. Micromagnetic simulations. The simulation was carried out based on a [CoFeB (0.9 nm)/nonmagnetic layer (4.6 nm)]15 multilayer film. The reason why the thickness of CoFeB is chosen to 0.9 nm is due to the dead magnetic layer of 0.6 nm in our CoFeB (1.5 nm) multilayer, which is proved by our former experimental result31. The total area is 1038 × 600 nm2 (x × y) with cell size of 3 nm × 3 nm × 1.5 nm3 (x × y × z). Saturation magnetization Ms, perpendicular magnetic anisotropic energy K and exchange constant A for CoFeB layer are 1200 × 103 A/m, 1.0 × 106 J/m3 and 1.6 × 10-11 J/m, respectively. 2D periodical boundary condition was applied in the plane. The state of magnetic moments was solved using the Landau-Lifshitz-Gilbert equation. The effective magnetic field includes the external, exchange, anisotropy, demagnetizing, and DMI fields. The DMI effective field is given by

r r 2D r r ∇ ⋅ m zˆ − ∇m Z H DMI = µ0 M S

[(

)

]

(1)

with D the DMI constant. In our simulation D=0.2 × 10-3 J/m2. At first, we simulated the stable state of magnetic skyrmion in our multilayers and obtained the distribution of magnetic moment in every layers and the skyrmion lattice in our multilayer with CoFeB 1.5 nm. Then A 300 mT magnetic field was tilted 85 ° from z axis to xy plane to investigate the reversal process. RESULTS AND DISCUSSION 5



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Using magnetron sputtering, we fabricated film stacks of substrate/[Ta(5.0 nm)/CoFeB(1.1–1.5 nm)/MgO(1.0 nm)]15/Ta(2.0 nm), where “15” denotes the layer repetition number, as shown in Fig. 1(a). Figure 1(b) and (c) shows the out-of-plane and in-plane hysteresis loops of the [Ta(5.0 nm)/CoFeB(x nm)/MgO(1.0 nm)]15/Ta(2.0 nm) multilayers for x=1.1, 1.3, and 1.5 nm. All three samples showed perpendicular magnetic anisotropy at room temperature (perpendicular anisotropy constant K (~0.9–1.4×106 J/m3)). The saturated magnetization Ms (~600×103 A/m) for all three samples was measured with vibrating sample magnetometry (VSM). The bow-tie shape of the out-of-plane hysteresis loops suggests a strong dipolar interaction in all three samples, which may help to stabilize the skyrmions32 as the DM interaction does. With increasing CoFeB thickness, the saturated field increased and the slope of the perpendicular hysteresis loops decreased, which indicates a strengthening dipolar interaction. The D in Ta/CoFeB/MgO stuctures can be obtained by measuring the in-plane field dependence of the shift of the anomalous Hall loops33, which gives a D of ~0.22 mJ/m2. Note that the effective DMI field |HDMI| was about 30 mT for Ta/CoFeB/MgO acquired by electrical measurement, which has been shown to stabilize Néel skyrmions by Yu et al.34,35 and Jiang et al.17,18. To investigate the domain structure, we used LTEM and MFM to observe the multilayers in the remnant state and at a non-zero magnetic field. Figure 1(d–f) show the MFM images for x=1.1 nm. First, an out-of-plane magnetic field was applied. In the remnant state, the labyrinth domain structure along the +z direction (white domain) was stable (Fig. 1(d)). As an out-of-plane field of 18 mT was applied, the density of the domains decreased. Individual skyrmions with a size of ~250 nm appeared (Fig. 1(e)). At 24 mT, only individual skyrmions remained (Fig. 1(f)). Because some of our LTEM results are similar to those reported in other multilayers13,15,29,36, we 6


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put these results and the hysteresis loops for the sample with a CoFeB thickness of x=1.1 nm in the Supplemental Material, the detailed results are described in Section S1 and S2, which indicates a Néel-skyrmion structure 29. We also investigated the thickness dependence of the micro magnetic structures of our multilayers (substrate/[Ta(5.0 nm)/CoFeB(x nm)/MgO]15/Ta(1.0 nm)) by using MFM. The remanence states for three samples are shown in Fig. 2(a), (c) and (e). A typical maze domain is appeared in these three samples. Compared to x=1.1 nm the domain shows a more regular arrangement for x=1.3 nm and x=1.5 nm. At x=1.5 nm, the stripe domain was as long as several micrometers. The ordering of these stripe domains relate to the competition of magnetic energy. As x increases, the dipolar energy increases while the DM interaction decreases with the thickness37, the total energy (including exchange energy, dipolar energy, DM energy) prefer to an ordered stripe domain structure. Note that Wang et al.38 have demonstrated experimentally the stability of skyrmions under tilted magnetic field. Here, we magnetized the samples 5 s by applying a tilted in-plane magnetic field B about 1 T with a tilted angle (within 10° to the xy plane), then removed the field. In new remnant state, the samples were investigated using MFM ( the results measured by LTEM is shown in Section S2). The observed results are shown in Fig. 2(b), (d), and (f). At x=1.1 nm, a disordered skyrmion appeared (Fig. 2(b)). At thicker CoFeB, however, hexagonal skyrmion lattices appeared. At x=1.5 nm (Fig. 2(f)), almost all skyrmions were located at one of the lattices. Because these lattices are in a short-ordered array, we call it a “multi-domain skyrmion lattice”. It is different from the dipolar skyrmion lattice formed in Fe/Gd multilayers23. In Fe/Gd multilayers, the DMI is absence. Fe or Gd layer is thin (0.34 or 0.4 nm), but the number 7



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of repeated layers is 80. The dipolar field is significant. Therefore, the formation of skyrmion lattice in Fe/Gd multilayers is attributed to the competition between the dipolar field and exchange energy. However, the stabilization of skyrmion lattice in Ta/CoFeB/MgO system is mostly attributed to the competition among exchange energy, dipolar interaction and DM interaction energy. Such skyrmions state is robust and can exist in a stable form at room-temperature and above room-temperature (see Section S3 of Supplemental Material). We noticed that some work has shown such a multi-domain skyrmion lattice state at low temperature or strong field in single-crystal39,40 and polycrystalline samples41,42. However, little is known about the occurrence of skyrmion domains in these systems. Our results below indicate that material defects seems to play an important role in the formation of multi-domain skyrmion lattice in our system. Figure 2(g) shows another enlarged image of the skyrmion lattice at x=1.5 nm, with the two domains labeled A and B. Within each domain, each skyrmion is surrounded by six neighbors (indicated by black circles), making a hexagonal lattice. The transition between A and B occurs from clockwise rotation of A, which is different from the other areas shown in Fig. 2(h) (shows the enlarged view of the skyrmions in the black rectangle in Fig. 2(f)). The angle between the upper domain and lower domain in Fig. 2(h) is about 150°, and the transition between them can also occur by rotation. However, the transition areas contain a twisted skyrmion lattice, so some of the skyrmions have seven neighbors (labeled with blue circles).The skyrmion lattice shows short-ordered features, which relate to the microstructure and initial magnetization state. In the three samples with different thicknesses, the initial magnetic states were all labyrinth domains, though their detailed microstructures were different (Figs. 2(a), (c) and (e)). In Fig. 2(a), namely at x=1.1 nm, besides the magnetic moments pointing up (+z), there 8


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are some magnetic moments in the opposite direction (black point at the end or side of each domain). These magnetic moments are distributed randomly, which indicates that some pinning sites exist. The number of pinning sites decreased with increasing CoFeB thickness. These pinning sites will influence the skyrmions nucleation43 and arrangement after the magnetization and demagnetization from the applied field. These results reversely indicate that the surface of our multilayers at x=1.1 nm and 1.3 nm are not quite perfect. At x=1.5 nm, the component of the in-plane magnetic field drags the maze domain toward its direction easily because there are fewer pinning sites, so a longer and ordered striped domain appears, while the out-of-plane component of the magnetic field cuts the stripe shorter, once the magnetic field decreases to zero, the hexagonal skyrmion lattice phase appears. In addition, micromagnetic simulation is also performed to simulate the skyrmion states shown in three samples using material magnetic parameters measured from experiments. The detailed simulation process is shown in method part. However, the long-ordered single domain skyrmion lattice were always obtained in simulations. Because the repulsive force (the repulsive force between two skyrmions is proportional to K 1 d H k A D ,44 where K1 is the modified Bessel function, d is the distance between the two skyrmions, Hk is the perpendicular anisotropy, A is the exchange constant, and D is the DMI constant) exists among skyrmions, and they are always tend to formed a perfect hexagonal lattice without pinning of material defects. To confirm the structure of skyrmions in our multilayer, micromagnetic simulation is performed to simulate the magnetic moment of every layer. The skyrmion lattice is formed under the experimental parameter of CoFeB (1.5 nm) multilayers, which is accordance with the MFM result except the long ordered skyrmion lattice in simulation. Because we didn’t consider the 9



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material defects during the simulation. For each skyrmion, in the first layer (top layer) (Fig. 2(i)), the magnetic moment distribution gives a Néel skyrmion with right-handed chirality, whose in-plane magnetization component parallel along radial direction due to the DMI. From the second layer (Fig. 2(j)), the magnetic moments begin to orientate, that leads to a non-zero, in-plane tangential magnetization component due to the dipolar interaction. At the seventh layers (Fig. 2(k)), spirals have helicity γ = π 2 and are referred to as Bloch configurations. After that, from the tenth to fifteenth layer (Figs. 2(l-n)), the chirality is reversal and Néel configuration is seen again. The above result shows that the skyrmions in multilayers shows a mixed structure that combining the Néel and Bloch structure, which is due to the competition between the DM and dipolar interaction. We have shown that our multilayer samples (x=1.1 to 1.5 nm) contain disordered and ordered skyrmion lattice at zero magnetic field. Here, we investigate the reversal of the skyrmions in lattice at x=1.5 nm. We produced a skyrmion lattice using B with a tilted angle θ of 97° (Fig. 3), the magnetizations of the core of skyrmions pointing up (+z) (as shown in Fig. 3(a)). In fact, by turnning θ to 83°, and the out-of-plane component of field is turn to +z, we obtained a skyrmion lattice with negative contrast, that is the magnetizations of the core of skyrmions pointing down (-z) (Fig. 3(b)). Thus, we can change the polarity of the skyrmions by changing the direction of the normal component of the magnetic field. To investigate how the skyrmion reversed to the new one, we first applied an out-of-plane magnetic field B⊥ to the -z polarized skyrmions. Because the skyrmion lattice was stable under 20 mT (above this field, the skyrmion lattice vanished or changed into a striped domain), we changed the B⊥in the range of +17 mT to −17 mT to observe the reversal. From +17 mT to −14 10


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mT, the skyrmion showed breathing-like behavior—it enlarged or shrank depending on the field direction—as shown in Fig. 3(c–e). Once the magnetic field was turned off, the skyrmion returned to its equilibrium size due to DMI (Fig. 3(d)). In addition, the B⊥dependence of lattice parameter b is shown in Figure 3(f) as well. The increase of b at -B⊥ mainly attribute to the increase of skyrmion diameter a. However, although a decrease when +B⊥exceeds 7 mT, b increases slightly. This is because skyrmions annihilation and random vacancies are induced in the skyrmion lattice when increasing the field. At −14 mT (Fig. 3(e)), we also found white areas (where the magnetic moments pointing up) around the skyrmion. Further increasing the B did not cause reversal of the skyrmions. Although a simple perpendicular magnetic field can make a skyrmion reversal in nanodisks28, it is not easy to reverse a skyrmion in skyrmion lattice because of the connection of skyrmions. In addition, the results of Zhao et al.45 in FeGe nanodisks also confirmed that an external magnetic field aligned perpendicular to the disk break skyrmion lattice into individual skyrmions finally. Then a tilted field B with θ= 85° was applied to the skyrmion lattice, as shown in Fig. 4(a). Figure 4(b) shows MFM images of the skyrmion lattice under a tilted field of 160 mT. Figure 4(f) shows the corresponding magnified image. Compared with the 0-mT field (Fig. 4(a) and (e)), some new skyrmion with -z polarized appeared randomly, accompanied by some slightly deformed initial skyrmions (+z polarized) . As the tilted magnetic field increased to 190 mT (Fig. 4(c)), more -z polarized skyrmions appeared. The magnified image in Fig. 4(g) shows that these skyrmions nucleated in the interstitial positions of the initial skyrmion lattice. Also, some initial skyrmions were connected to domains. When we then reduced the magnetic field to zero, new skyrmions prevailed over the initial ones and formed a new lattice (Fig. 4(d) and (h)), completing 11



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the reversal. In our experiments, we find the magnitude of the tilted field should be appropriate. If out-of-plane component is too large new skyrmion cannot be created in the interstitial positions. In contrast, if in-plane component of magnetic field is too large, the chiral domain cannot exist stably and neither can new skyrmions. ( see Section S4 of Supplemental Material). As shown in Fig. 4, the skyrmion lattice reversed because the initial skyrmions disappeared and new skyrmions nucleated in the interstitial positions. To understand the role of the tilted field, we used micromagnetic simulation to investigate the reversal of the skyrmions in lattice. The model and parameters (used the experimental results) are described in Methods. The initial ground state is set as a perfect hexagonal skyrmion lattice, as shown in Fig. 5(a). A 300 mT magnetic field was then applied along the direction that is tilted 85 ° from z axis to xy plane, which is close to the experimental value. The detailed reversal at different times (0.16, 0.3, 3, 3.5 and 8 ns) are shown in Figs. 5(b–f). At first, the skyrmions grow due to the out-of-plane component of the magnetic field (Fig. 5(b)). In each skyrmion, there exists a circular area where the magnetic moment tried to parallel off z-direction due to the in-plane component of the field. Meanwhile, outside the skyrmion, the in-plane magnetic moment area (green area) is also observed. At 0.3 ns, these skyrmions, with the distorted shape, connect to each other, producing a large area of domains. All domains arrange along the in-plane field direction and the interstitial regions were separated to small oblong regions where magnetic moments pointing down (-z) (Fig. 5(c)). After that, theses oblong regions adjust their shapes to the field (Fig. 5(d)). As the field is switched off, the oblong regions turn to skyrmion due to DMI (Fig. 5(e) and (f)). The reversal is totally completed. More details about skyrmions reversal with the increase of magnetic field is shown in Section S5 of Supplemental Material. In addition, helicityγ for every layer of the stack is 12


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shown in Fig. 5(g). The result indicates that the reversal happened among every layer not only a few of cap layers, and thus it is stable. The out-of-plane magnetization at the skyrmion core turn up to down after reversal, and the in-plane magnetization rotation turn clockwise into counterclockwise due to the sign of the DMI28. Such two possible polarized direction or two possible rotation directions is probably regarded as switchable bit elements in memory devices26. The experiments and simulation results indicate that the reversal of the skyrmions in skyrmion lattice is intrinsically consist of skyrmions disappeared and new skyrmions nucleated. The role of the in-plane component of field is to resist the DM energy and thus the skyrmions are broken and connect into domains. While out-of-plane field combined with in-plane component is to squeeze the domains in interstitial regions to form new skyrmions. CONCLUSION In conclusion, we have demonstrated a transition from disordered skyrmions state to an ordered skyrmion lattice state with increasing thickness of the CoFeB in the Ta/CoFeB/MgO multilayers. These skyrmion states are all stable at room temperature and in a remnant state. Moreover, we have investigated the reversal of skyrmions in skyrmion lattice under a slightly tilted in-plane field, revealing that the reversal came from skyrmions disappeared and new skyrmions nucleated in the interstitial regions of the lattice. SUPPORTING INFORMATION Supplemental Material for detailing the results of stabilization of skyrmions under LTEM ( Section S1 and S2), stability of skyrmion lattice above room-temperatures (Section S3), the reversal of skyrmions under tilt angles 90°and 76°(Section S4), and simulation images at stable magnetization configurations under different magnetic field (Section S5) . 13



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ACKNOWLEDGMENTS The research was supported by the National Natural Science Foundation of China (Nos. 11574121, 11674142) and the Fundamental Research Funds for the Central Universities (lzujbky-2017-179).

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Figure 1. Static Magnetic properties and Skyrmion stabilization under magnetic field (a) Schematic of (Ta/CoFeB/MgO)15/Ta multilayers. (b) Out-of-plane and (c) in-plane VSM hysteresis loops for [Ta(5.0 nm)/CoFeB(x)/MgO(1.0 nm)]15/Ta(2 nm) multilayers grown on a Si wafer. (d) MFM images of [Ta(5.0 nm)/CoFeB(1.1 nm)/MgO(1.0 nm)]15/Ta(2 nm) stack in the remnant state, (e) under an out-of-plane magnetic field of 18 mT, and (f) under an out-of-plane magnetic field of 24 mT.

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Figure 2. Transition from disordered skyrmions to skyrmion lattice and simulation results of skyrmions in every layer. (a, c, e) MFM images for the CoFeB (1.1 nm), CoFeB (1.3 nm), CoFeB (1.5 nm) multilayers at the remanent state, and room-temperature skyrmion states for the (b) CoFeB (1.1 nm), (d) CoFeB (1.3 nm), and (f) CoFeB (1.5 nm) multilayers, acquired after applying a nearly in-plane magnetic field of 1 T and then decreasing it to zero. (g) Typical skyrmion lattice with two oriented domains A and B in the CoFeB (1.5 nm) multilayers. (h) Magnified view of the skyrmion lattice marked in (f). Simulation results of skyrmion in (i) layer 1 (top layer), (j) layer 3, (k) layer 6, (l) layer 7, (m) layer 9, (n) layer 15.

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Figure 3. Reversal of the skyrmion lattice under an out-of-plane field. MFM images of short-ordered (a) Skyrmions (+z polarized) and (b) Skyrmions ( -z polarized) for the CoFeB (1.5 nm) multilayers at zero magnetic field, and out-of-plane magnetic fields B of (c) +17 mT, (d) 0 T, and (e) −14 mT. (f) The skyrmion diameter a and lattice parameter b ( the core to core distance between adjacent skyrmions) with increasing B.

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Figure 4. Reversal of the skyrmion lattice under an in-plane tilted magnetic field. Sequential MFM images of the skyrmion lattice in a tilted in-plane field of (a) 0 mT, (b) 160 mT, (c) 190 mT, and (d) 0 mT. (e, f, g, h) Enlarged views of (a), (b), (c), and (d), respectively.

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Figure 5. Micromagnetic simulation of the reversal of the skyrmion lattice. States at (a) 0, (b) 0.16, (c) 0.3 ns in the presence of a tilted magnetic field of 300 mT, (d) 3 ns, after this time, the titled field is removed (e) 3.5 ns and (f) 8 ns. (g) Spin helicity γ for every layer and magnetization configurations of skyrmions for layer 1, 7 and 15 before and after reversal happened.

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