Skyrmion Dynamics in Thin Films of Chiral Magnets

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1 Skyrmion Dynamics in Thin Films of Chiral Magnets Yoshi Tokura Department of Applied Physics, University of Tokyo RIKEN Advanced Science Institute Skyrmions and topological transport phenomena Skyrmions in multiferoics toward E-control Forced and spontaneous dynamics of skyrmions

2 Collaborators Univ. of Tokyo (Japan) Y. Kanazawa, N. Nagaosa,, M. Mochizuki Y. Onose, T. Arima, S. Ishiwata, A. Tsukazaki, M.Ichikawa, M. Kawasaki, S. Seki,Y. Shiomi, K. Shibata, F. Kagawa, Y. Okamura RIKEN (Japan) X. Z. Yu, Y. D. Okuyama, Y. Tokunaga, Y. Taguchi NIMS (Japan) Y. Matsui, K. Kimoto, W. Z. Zhang Tohoku Univ. (Japan) K. Ohoyama JAEA (Japan) K. Kakurai, S. Wakimoto Sung Kyun Kwan Univ. (Korea) J. H. Han, J. H. Park MPI (Germany) D. S. Inosov, J. H. Kim, B. Keimer PSI (Switzerland) J. White, N. Egetenmeyer, J. Gavilano Groningen Univ. (Holland) M. Mostovoy Fudan Univ. (Peoples R China) Yufan Li, Xiofeng Jin

3 Skyrmion Crystal Superposition of three Helix without phase shift Skyrmion Q Skyrmion crystal 1 Q2 Q3 3-flod-Q S. Muhlbauer et al. Science 323, 915 (29).

4 What is magnetic skyrmion? 5 ~ 1 nm Topologically-stable spin vortex with particle-like nature Lateral component of M ofsome bubbles skyrmion number S=-1 S= a pair of Bloch lines

5 Toward real space observation of Skyrmion structure H JSi S j Dij S S ) ( i j Ferro + DM Fe.5 Co.5 Si Helical spin structure Long period~aj/d~1nm-3nm Lorentz microscope electrons M. Uchida, Y. Onose, Y. Matsui, Y. Tokura, Science (26)

6 FeGe: from helical to skyrmion crystal at 26K X.Z. Yu et al. Nat. Mater.(211) H= H=.1T

7 7 Near room-temperature formation of SkX in FeGe t = 15 nm t = 35 nm t = 6 nm t = 75 nm -Near room-temperature formation of SkX -Stability of SkX depend largely on the thickness. X. Z. Yu, N. Kanazawa, Y. Onose, K. Kimoto, W.Z. Zhang, S. Ishiwata, Y. Matsui, and Y. Tokura, Nature Mater (21

8 Real-space fictitous magnetic field in a skyrmion spin texture scalar spin chirality Solid angle W = 4p In srong coupling case One skyrmion One magnetic flux f Emergent magnetic field A: skyrmion size A. Neubauer et al, PRL (29) High skyrmion density Large topological Hall Effect

9 Ultrathin epitaxial thin films of MnSi 1nm-thick 2nm-thick MnSi Si substrate

10 Skyrmion phase mapping by topological Hall resistivity Yufan Li, Kanazawa,Kagawa 1nm-thick 5nm-thick Conventional anomalous + normal Hall effects See also the late paper on FeGe thin film; S. X. Huang and C. L. Chien Phys. Rev. Lett. 18, (212)

11 T N (K) T N (K) Magnetic phase daigrams of B2 TMSi, TMGe B2 structure Cubic but noncentrosymmetric (Chiral) 3 H JS S D S S ) i Ferro j + ij DM ( i j Helical spin structure Long period~aj/d ~1nm-3nm 1 MnGe FeGe CoGe Band filling MnSi FeSi CoSi Bandwidth

12 Small angle neutron scattering on MnGe (polyxtal) B (1T) then B= in collaboration with Keimer group λ=3-6nm q s//<1> Evidence for multiple-q structure even at B=

13 yx ( W cm) yx ( W cm) Topological Hall effect in MnGe 2 K 16 K 1 K 5 K 3 K 3 K 2 K 1 K 5 K H (T) yx ( W cm) H > H C Induced ferromagnetic state Conventional anomalous Hall 3 Keffect Solid lines: estimate of Components of THE T yx ( W cm).1 2 K yx ( W cm) 16 K 1 K 5 K K K 2 K 7 K 5 K 3 K 5 1 H (T) 15 Nearly temperature independent 13

14 H (T) topological Hall effects via Skyrmion lattice AHE H constant A yx ( W cm) Mt T (K) 1 2 T (K) M/t magnetization magnetoresistivity A xy (W -1 cm -1 ).1 T yx ( W cm) T (K) T yx ( W cm) THE t independent H MnGe t t T xy (W -1 cm -1 ) SkX subsistent to ground state

15 Possible 2D (meron) or 3D (hedgehog) Skyrmion Xtal at B= B. Binz and A. Vishwanath, Physica B 43, 1336 (28). skyrmion-antiskyrmion

16 Current drive of skyrmions and emergent EM field Domain wall motion by spin transfer torque Conduction electron Localized moment Domain wall Racetrack memory Topological Hall effect Emergent magnetic field h -Emergent electric field Skyrmion motion by spin transfer torque V appears as reduction of topological Hall effect e -counteraction of topological Hall effect (THE) skyrmion Hall effect

17 Current driven skyrmion flow in FeGe film

18 Jc<4A/cm 2!!for skyrmion (not skrew) >1 5 orders of magnitude smaller than Jc for DW

19 No pinning effect on skyrmion motion LLG Iwasaki-Mochizuki-Nagaosa (212) screw skyrmion no (minimal) pinning effect on SkX 19

20 imulation of current-driven skrymions under pinning sites 2

21 Ferro(ferri)magnetic chiral lattice hosts skrymions? c.f. JWG Bos,CV Colin, and TTM Plastra, PRB (21).

22 Lorentz TEM observation of thin flake of Cu 2 OSeO 3 Seki et al. Science (212)

23 Skyrmion crystal phase: bulk vs. thin film bulk film (5nm) Seki et al. PRB (RC), 85, 2246(212). See also Adams et al. PRL, 18, (212).

24 H//x,y on skyrmions H ω //xy Onose et al. PRL (212) Mochizuki PRL (212)

25 H//z on skyrmions Onose et al. PRL (212) H ω //c Mochizuki PRL (212)

26 Cu 2 OSeO 3 : P and ρ distributions in skyrmion polarization P E-field drive charge ρ quadrupole dipole Seki et al. PRB (212)

27 Electrodynamics of Skyrmions ~ toward Skyrmionics ~ H (T) Stabilization Skrymions and Skyrion Xtal in form of thin films Topological Hall effect as probe for SkX emergent EM fields magnetization magnetoresistivity T yx ( W cm) MnGe zero-field skyrmion crystal 1 2 T (K) -.2 Skyrmion transport and dynamics toward skyrmionics current-drive of Skyrmions (~1A/cm 2 ) multiferroic skyrmions, E-drive/optical-control of Skyrmion spontaneous ratchet motion in thermal equilibrium