Skyrmion Dynamics and Topological Transport Phenomena

Transcription

1 Skyrmion Dynamics and Topological Transport Phenomena Yoshi Tokura RIKEN Center for Emergent Matter Science (CEMS) Department of Applied Physics, University of Tokyo skyrmion, the concept originally introduced by Tony Skyrme ( ) to describe the state of nucleon: to model a particle as a topological soliton Skyrmions and topological transport phenomena Skyrmions in multiferoics toward E-control and light control

2 RIKEN (Japan) Collaborators X. Z. Yu,, S. Seki N. Nagaosa, Y. Tokunaga, D. Okuyama, Y. Taguchi, M. Kawasaki, T. Arima, N. Ogawa, M. Kubota Univ. of Tokyo (Japan) Y. Kanazawa, Y. Onose, S. Ishiwata, A. Tsukazaki, M. Ichikawa, Y. Shiomi, K. Shibata, F. Kagawa, Y. Okamura, M. Mochizuki NIMS (Japan) Y. Matsui, K. Kimoto, W. Z. Zhang 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 Xiuazhen Yu Naoya Kanzawa Shinichiro Seki

3 What is magnetic skyrmion? 5 ~ 100 nm Topologically-stable spin vortex with particle-like nature Lateral component of M of some bubbles skyrmion number S=-1 S=0 a pair of Bloch lines

4 S i e S j Spin Chirality gauge flux Φ non-coplanar spin structure Aharonov - Bohm effect e solid angle Ω :flux quantum S 1 Fictitious field 1 S 3 S 2 magnetic Φ= 2 Ω = 2 S1 S 2 S 3

5 Skyrmion gauge flux Φ S k conduction electron Mapping to a sphere S i S j Cf. Spin chirality S S S ) i ( j k =1/2 Ω Solid angle Continuum approximation Solid angle Ω = 4π Total spin Chirality 1 2 d rs ( xs 3 ys = 4πS ) = N S Skyrmion number Skyrmion carries emergent magnetic field.

6 Helical spin order in B20-type crystals 6 Crystal structure CW CCW : Transition-metal element : Group 14 element Cubic (P2 1 3) Noncentrosymmetric Magnetic structure Three well-separated energy scales Chiral lattice structure ferromagnetic interaction(s i S j ) > Dzyaloshinsky-Moriya interaction(s i S j ) > magnetic anisotopy one-handed helical spin structure (a long wavelength nm, weakly locked helix direction <111> or <100>)

7 Toward real space observation of Skyrmion structure H = i j ij ( i j ) ( JS S + D S S ) Ferro + DM Fe 0.5 Co 0.5 Si Helical spin structure Long period~aj/d~10nm-300nm Lorentz microscope electrons M. Uchida, Y. Onose, Y. Matsui, Y. Tokura, Science (2006)

8 Helical spin order in B20-type crystals 8 Crystal structure : Transition-metal element : Group 14 element Cubic (P2 1 3) Noncentrosymmetric Magnetic structure Three well-separated energy scales ferromagnetic interaction(s i S j ) > Dzyaloshinsky-Moriya interaction(s i S j ) > magnetic anisotopy one-handed helical spin structure (a long wavelength nm, weakly locked helix direction <111> or <100>)

9 Small angle neutron scattering for Skyrmion Xtal MnSi S. Mühlbauer et. al., Science (2009)

10 Real Space Observation of Skyrmion crystal Fe 0.5 Co 0.5 Si T=25K H=0 H=50mT X.Z. Yu, Y.T et al. Nature (2010).

11 H- T Phase diagram Bulk sample 20nm-thick film (Lorentz TEM) SkX phase Skx: Skyrmion Crystal 2D simulation

12 FeGe: from helical to skyrmion crystal at 260K X.Z. Yu et al. Nat. Mater.(2010) H=0 H=0.1T

13 Real-space fictitous magnetic field in a skyrmion spin texture Solid angle Ω = 4π In srong coupling case One skyrmion One magnetic flux φ 0 Emergent magnetic field one skyrmion/nm 2 B eff ~4000T A. Neubauer et al, PRL (2009) A: skyrmion size High skyrmion density Large topological Hall Effect

14 Ultrathin epitaxial thin films of MnSi 10nm-thick MnSi Si substrate

15 Skyrmion phase mapping by topological Hall resistivity Yufan Li, Kanazawa,Kagawa et al. PRL (2013) 10nm-thick 50nm-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. 108, (2012)

16 Magnetic phase diagram in B20 compounds 16 T N (K) 300 T N (K) Helicity reversal TGe TSi Mn Fe Co 0 Mn Fe Co : conventional helical/skyrmion phase : new skyrmion phase

17 Mn 1-x Fe x Ge (Control of DM interaction) Shibata et al. Nat. Nanotech. (2013) H = i j ij ( i j ) ( JS S + D S S ) J D 17

18 Magnetic phase diagram in B20 compounds 18 T N (K) 300 T N (K) Helicity reversal TGe TSi Mn Fe Co 0 Mn Fe Co : conventional helical/skyrmion phase : new skyrmion phase

19 Small angle neutron scattering on MnGe (polyxtal) B (10T) then B=0 -T N =170 K -Helical structure -modulation vector:q <100> -Helical period: λ = 3 nm 6 nm Helical period λ (nm) T (K) Q (1/nm) Huge topological Hall effect? Evidence for multiple-q <100> structure even at B=0 in collaboration with Keimer group

20 Topological Hall effect in MnGe 20 ρ yx (µω cm) ρ yx (µω cm) K 160 K 100 K 50 K 30 K 30 K 20 K 10 K 5 K µ 0 H (T) H > H C Induced ferromagnetic state Conventional anomalous Hall effect Solid lines: estimate of Components of THE ρ T yx (µω cm) K 10 K K 70 K 50 K 30 K µ 0 H (T) Nearly temperature independent

21 Real-space fictitous magnetic field in a skyrmion spin texture MnGe topological Hall effect B eff ~ T! Solid angle Ω = 4π In strong coupling case One skyrmion One magnetic flux φ 0 Emergent magnetic field one skyrmion/nm 2 A: skyrmion size B eff ~4000T High skyrmion density Large topological Hall Effect ρ H T (μωcm) No cutoff Cutoff: M < 1/6 T = 30 K µ 0 H (T) µ 0 H (T) magnetization magnetoresistivity ρ T yx (µω cm) T (K) D skyrmion X

22 Nernst effect Topological Nernst Effect Shiomi et al. PRB (2013) 22

23 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 (THE) Emergent magnetic field b V -counteraction of THE skyrmion Hall effect Emergent electric field e

24 J c <100A/cm 2 J c ~10 7 A/cm 2 for ordinary domain walls no intrinsic / minimal extrinsic pinning effect on SkX

25 Manipulation of single skyrmion in FeGe by pulse currents I (ma)

26 Lorentz TEM observation of thin flake of Cu 2 OSeO 3 space group P2 1 3 the same as B 20 (MnSi) Mott insulator with quantum spins bulk film (50nm)

27 PRB(2012)

28 Skyrmion excitations as electromagnons showing directional dichroism Y. Okamura et al. Nature Commun. (2013). 28

29 Electrodynamics of Skyrmions ~ toward Skyrmionics ~ Stabilization Skrymions in form of thin films, not only DM but also uniaxial magnets Topological Hall effect as probe/detection for SkX emergent EM fields µ 0 H (T) magnetization magnetoresistivity T (K) ρ T yx (µω cm) Skyrmion transport phenomena low-current drive of Skyrmions (<100A/cm 2 ) processing speed I*(Sk density); energy-cost per bit I optical, e-beam (spin-current) control,; E-drive (multiferroics)