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Current-driven motion of domain boundaries between skyrmion lattice and helical magnetic structure Kiyou Shibata, Toshiaki Tanigaki, Tetsuya Akashi, Hiroyuki Shinada, Ken Harada, Kodai Niitsu, Daisuke Shindo, Naoya Kanazawa, Yoshinori Tokura, and Taka-hisa Arima Nano Lett., Just Accepted Manuscript • DOI: 10.1021/acs.nanolett.7b04312 • Publication Date (Web): 18 Jan 2018 Downloaded from http://pubs.acs.org on January 18, 2018
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Nano Letters
Current-driven motion of domain boundaries between skyrmion lattice and helical magnetic structure Kiyou Shibata∗,†, Toshiaki Tanigaki‡, Tetsuya Akashi‡, Hiroyuki Shinada‡, Ken Harada†, Kodai Niitsu†, ⊥, Daisuke Shindo†,¶, Naoya Kanazawa§, Yoshinori Tokura†,§, and Taka-hisa Arima†, ǁ
†RIKEN Center for Emergent Matter Science (CEMS), Wako 351-0198, Japan, ‡ Research & Development Group, Hitachi Ltd, Hatoyama 350-0395, Japan, ¶Institute of Multidisciplinary Research for Advanced Materials, Tohoku University, Sendai 980-8577, Japan, §Department of Applied Physics, the University of Tokyo, Tokyo 113-8656, Japan, and ǁDepartment of Advanced Materials Science, the University of Tokyo, Kashiwa 277-8561, Japan * E-mail:
[email protected]; Tel: +81-48-462-1111 (ext. 6073)
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ABSTRACT To utilize magnetic skyrmions, nano-scale vortex-like magnetic structure, experimental elucidation of their dynamics against current application in various circumstances such as in confined structure and mixture of different magnetic phases is indispensable. Here, we investigate the current-induced dynamics of the coexistence state of magnetic skyrmions and helical magnetic structure in a thin plate of B20-type helimagnet FeGe in terms of in-situ realspace observation using Lorentz transmission electron microscopy (TEM). Current pulses with various heights and widths were applied, and the change of the magnetic domain distribution was analyzed using a machine-learning technique. The observed average driving direction of the twomagnetic-state domain boundary is opposite to the applied electric current, indicating ferromagnetic s-d exchange coupling in the spin-transfer torque mechanism. The evaluated driving distance tends to increase with increasing the pulse duration time, current density (>1×109 A/m2), and sample temperature, providing valuable information about hitherto unknown current-induced dynamics of the skyrmion-lattice ensemble.
KEYWORDS magnetic skyrmion; in-situ transmission electron microscopy; Lorentz transmission electron microscopy; chiral magnet; current-driven magnetic domain boundary motion; metastable state
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The magnetic skyrmion1–3, vortex-like magnetic structure stabilized in systems with broken inversion symmetry, has been actively researched for the possible application to next-generation magnetic memory devices owing to its particle nature and mobility4–8. Skyrmions and their lattice form, skyrmion lattice, have been observed in chiral-lattice helimagnets with bulk Dzyalloshinski-Moriya interaction (DMI)2,3,9–13 and thin films with interfacial DMI14–17. For Bloch-type skyrmions in chiral-lattice helimagnets, micromagnetic simulations of currentinduced skyrmion motion in constricted geometries show a low critical current density, velocity of the order of 1 m/s, and stability of skyrmions during the transport7. Experimentally, response against application of electric current density of the order of 1×106 A/m2 has been observed as rotation of neutron diffraction spots of a skyrmion lattice in MnSi in the presence of temperature gradient18 and change of Lorentz TEM image contrast of skyrmion lattice in B20-type FeGe19. As an indirect evidence, electrical detection of current-induced skyrmion velocity in MnSi as a reduction of topological Hall resistivity estimates the velocity of 0.12 mm/s at the current density of 106 A/m2
20
. More recently, the observation of current-induced skyrmion nucleation and
annihilation was reported in (Co, Zn)Mn21. However, the direction and speed of current-driven dynamics have still remained elusive in these experiments. In contrast, for Néel-type skyrmions stabilized in thin films, skyrmion dynamics has been directly observed by means of nano-scale pump-probe imaging16 and polar magneto-optical Kerr microscopy17. Direct observations of the current-induced magnetic structure motion, such as direction and speed, for not only Bloch type skyrmions but also helical magnetic structures in chiral lattice helimagnets are limited in the bulk in comparison. In this letter, we report Lorentz TEM on the dynamics of a skyrmion-helix coexistence state in B20-type FeGe thin plate9 by applying pulse electric current. Through a statistical analysis, we
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have confirmed that the direction of the domain boundary motion is against the electric current direction (parallel to the electron flow), and that the length of motion depends on the current density and pulse width. The result indicates that the s-d exchange coupling is ferromagnetic in the spin-transfer torque mechanism. We have also found that the boundary of the skyrmion lattice and helical domains tend to be more mobile at higher temperature, which indicates thermal agitation from pinning potential affects the dynamics. Single crystals of B20-type FeGe were synthesized using a chemical vapor transport method22. To achieve in-situ current application in an electron microscope, we fabricated a sample using a focused ion beam (FIB) method. Scanning electron microscopy images, schematics, and a TEM overview image of the sample are shown in Figs. 1a-c, d, and e, respectively. An Si substrate was mechanically thinned to 100 µm, and a slit was formed by FIB. An FeGe (112) block with dimensions of ∼20 µm×55 µm×5 µm was picked up from a single crystal by micro-sampling method and mounted on the Si substrate across the slit by FIB-assisted W deposition. Note that cooling below room temperature can induce uniaxial strain in the FeGe thin plate along the bridging direction due to the difference in the thermal expansion coefficients between FeGe and Si, which induces anisotropic deformation23. To reduce the strain effect, a stress relaxation structure was fabricated, as shown in Fig. 1d. Finally, the thin areas for TEM observations were fabricated. The plate consisted of five regions: two thin rectangular regions with thicknesses of ∼ 150 nm and ∼200 nm, and thick regions with a thickness of ∼500 nm. All the regions have the same width of 5 µm. Lorentz TEM observation for thin region was carried out in the area with thickness of ∼200 nm indicated in Figs. 1(c) and (e). The left panel in Fig. 1(e) is the electron diffraction pattern of (112) plane taken in the observation area. Assuming that the current
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density in each cross section is uniform, the current pulse with height of V = 1.0 V in this device approximately corresponds to current densities of J = 2.5×109 A/m2 and 1.0×109 A/m2 for 200 nm-thick and 500 nm-thick regions, respectively. Lorentz TEM observations (Fresnel mode) on magnetic structures in the thin rectangular region with thickness of ∼200 nm were carried out using a 300-kV microscope equipped with a charge-coupled device camera (HF-3000, Hitachi). The magnetic field was applied normal to the specimen plate by controlling the object lens current of the microscope3. For Lorentz TEM observation using the 300-kV microscope, a holder equipped with liquid nitrogen cooling system and with two electric terminals was used. The sample holder was connected with two resistance of 200 Ω in series and the pulse voltage was supplied by a source-measure instrument (KEITHLEY 2602B). To observe magnetic structure in the thicker plate region, Lorentz TEM was carried out using a 1.0-MV microscope (Hitachi), which provided us with higher electron transmission capability. In the thicker region, we observed the formation of helical magnetic structure after zero-field cooling from room temperature to 180 K. Meanwhile we observed no magnetic contrast after field cooling at 0.2 T from room temperature to 190 K, which implied possible formation of conical magnetic structure with propagation vector perpendicular to the thin plate (See Supplemental Information). We checked the metastability of helical magnetic structure and skyrmion lattice. After zerofield cooling to 173 K followed by increasing magnetic field and heating up to 203 K, we observed a stripe pattern by Lorentz TEM, as shown in Fig. 2a, corresponding to a proper-screw type helical magnetic structure24. Meanwhile, after field cooling at 0.2 T from room temperature to 203 K, we observed a triangular lattice of dark dot contrast, as shown in Fig. 2b,
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corresponding to a triangular lattice of skyrmions3. Figure 2c shows the magnetic phase diagram based on the observation. We prepared coexistence states of quenched metastable skyrmion lattice and helical magnetic structure by applying some current pulses to metastable skyrmion lattice phase at 173 K and 203 K with applied magnetic field of 0.2 T, and then recorded Lorentz TEM movies which contained single current pulse application during the recording. We analyzed numerous Lorentz TEM movie data and evaluated the change of the magnetic domain distribution with respect to temperature T, pulse width t, and pulse height V. From the analysis, we identified the magnetic domain distribution and evaluated the shift of domain boundaries along the applied electric current. Figures 3a and 3b are analyzed images of Lorentz TEM movie recorded at 203 K and 0.2 T before and after a single current pulse application, respectively (See Supplementary Information). The images consist of stripe and dot contrast corresponding to helices and skyrmions, respectively. We analyzed the magnetic domain distribution in the TEM images by machine-learning technique using Python module Keras25, and obtained magnetic domain-type maps. Blue and red regions correspond to domains of skyrmion lattice and helical magnetic structure, respectively. Figures 3c-f are magnified images of areas in Figs. 3a, b. Red-colored helical magnetic domains with stripe contrast and blue-colored skyrmion lattice domains with dark dot contrast confirm a good identification of the domain types. Likewise, we analyzed Lorentz TEM image pairs for various pulse height and width at 203 K and 173 K (See Supplementary Information for the typical data set at 173 K) We extracted information of domain wall motion by taking the cross correlation along x axis between the domain-type maps before and after pulses (See Supplementary Information).
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Although skyrmions are anticipated to show Hall motion4, 16, here we neglect the transverse motion of domain boundary since the matching of the corresponding local magnetic structures in before and after images is impossible in principle due to the high translational symmetry. Figures 4a and 4b show pulse-width t dependence of the displacement ∆x in the +x direction for various pulse heights V at T=173 K and 203 K, respectively. The distribution of ∆x obtained for each T, V, and t is displayed as bubble markers, the sizes of which correspond to the numbers of data points. In Fig.4a, for each pair of V and t, the largest marker locates at ∆x=0, which indicates that the domain boundary remains unchanged at most y positions. Meanwhile, in the region where both V and t are large, ∆x distributes asymmetrically, and their average locates in the ∆x
