Spin dynamics in Bi 2 Se 3 /ferromagnet heterostructures

Transcription

1 Spin dynamics in Bi 2 Se 3 /ferromagnet heterostructures Hyunsoo Yang Electrical and Computer Engineering, National University of Singapore eleyang@nus.edu.sg

2 Outline Spin-orbit torque (SOT) engineering Heusler alloy Oxygen manipulation of SOT SOT in Co/Pd and Co/Ni multilayers AHE & SOT in LaAlO 3 /SrTiO 3 oxide heterostructures SOT in topological insulators (Bi 2 Se 3 /ferromagnet) 2

3 Charge electronics Spin electronics Information transfer = electron transfer Spin waves MTJs Information processing = processing electron flow Spin transistor Charge transfer and processing energy loss is huge All spin electroinics 3

4 Spin-orbit torques (SOT) Miron et al. Nature 476, 189 (2011) Liu et al. PRL 109, (2012) Heavy metal/ferromagnetic material/oxide layer. Current induced magnetization switching is observed (longitudinal field needed). Magnetization states depend on both current and field directions. Possible mechanisms: Rashba effect & spin Hall effect (SHE). 5

5 Perpendicularly magnetized trilayer structures Oxide Ferromagnet (FM) Heavy Metal (HM) M Strong Rashba field arises from asymmetric interfaces Spin Hall effect arises from HM SOT In-plane currents can switch the magnetization STT FM2 MgO FM1 7

6 Spin Hall vs. interfacial Rashba Reverse switching polarity by oxygen engineering SH 0 SH 0 -Sign of spin Hall angle changes across a transition thickness of SiO 2 (t = 1.5 nm) -Cannot be understood by spin Hall physics suggest the role of interface Nat. Nanotech. 10, 333 (2015) 8

7 Spin-orbit torque switching currents STT I 2 SOT Free FM spacer Pinned FM I 1 I I c c 2e MV S H P e M t A H SFM HM SH -No damping term great flexibility for choosing FM, high speed -No spin polarization term no need to use MgO -Can use thick MgO eliminate MgO breakdown issue -Large spin Hall angle ( SH ) or effective field (H eff ) is the key eff eff STT SOT K.J.Lee, APL (2013) H eff = ħ SH j e /(2 e M S t F ) 19

8 Large spin Hall angles from various materials FePt/Au spin Hall angle ( SH ) = 0.1 (Takanashi group) CuBi SH = (Otani group) Phys. Rev. Lett. 109, (2012) Nat. Mater. 7, 125 (2008) Co/Pd multilayer SH = 4 (NUS) β-w SH = 0.3 (Cornell) Appl. Phys. Lett. 101, (2012) Phys. Rev. Lett. 111, (2013) 20

9 Spin orbit torque from Heusler alloy Pt/Co 2 FeAl 0.5 Si 0.5 (0.8 nm)/mgo Perpendicular anisotropy CFAS Both H L and H T exist with a large SH Appl. Phys. Lett. 107, (2015) 22

10 Single magnetic layer vs. magnetic multilayer systems cap PMA Pt MgO Substrate A better choice for structural engineering Perpendicular magnetic anisotropy (PMA) originates from Pt/FM interface. Low damping (~0.02) Spin polarization (~50%) Low thermal stability (not enough volume) Co/Pd or Co/Ni interfaces contribute to PMA. Lower damping (~0.01) Higher spin polarization (~80%) Good thermal stability [ =K u V/(k B T)] 30

11 Spin orbit torques in Co/Pd multilayers Ta (4nm) Co (0.2nm) Pd (0.7nm) Co (0.2nm) Pd (0.7nm) 22 times Ru (20nm) Ta (4nm) Phys. Rev. Lett. 111, (2013) 31

12 Structural asymmetry can be added up TEM data Ta (4nm) Co (0.2nm) Pd (0.7nm) Co (0.2nm) Maesaka, IEEE Trans. Magn. 38, 2676 (2002) Pd (0.7nm) -Two successive Co/Pd and Pd/Co interfaces are structurally dissimilar. -Lattice mismatch (9%) between Pd and Co 33

13 LAO/STO 2DEG formation LaAlO 3 SrTiO 3 Nat. Comm. 4:1838 LaAlO 3 grown on TiO 2 terminated SrTiO 3 (100) SrTiO 3 (Insulator 3.2 ev) LaAlO 3 (Insulator 5.6 ev) LAO (2 nm) 2 DEG STO 2 DEG formed inside the STO side 34

14 Magnetism in LaAlO 3 /SrTiO 3 heterostructures In-plane angular measurements 2 DEG LAO (2 nm) LIA-1 LIA-2 STO I Data Fit Idc + Iac AMR ( ) AMR measurements at H = 9 T, T = 4 K R XX = a 0 +a 1 cos 2 ( + ) + a 2 cos 4 ( + ) Angle (deg) a 0, a 1, a 2 are constants Appl. Phys. Lett. 105, (2014) 36

15 Asymmetric spin-orbit fields H EFF H R β α H A AMR ( ) I dc = +250 A AMR ( ) I dc 2220 I dc = -250 A Angle (deg) 1635 H H H 2H H cos EFF A R A R R b b H cos b H cos 2 4 XX 0 1 EFF 2 EFF H R (+I) = 1.26 T H R (-I) = T H R, H A H EFF are Rashba, applied and effective fields b 0, b 1, b 2 are constants Appl. Phys. Lett. 105, (2014) 37

16 Current induced spin-orbit fields in 2DEG 3 H R (T) H = +8 T H = -8 T I ac = 20 A Assuming thickness of 2DEG t 2DEG = 7 nm Nat. Mater. 7, 621 (2008) current density = A/m µa 32.9 Tesla/10 6 A/cm I dc ( A) The highest current induced torque reported in metallic system is only 0.5 T. 3 *2 so PRL 111, (2013) eh / m H so = 1.48 T, α R = 12 mev Å, spin splitting = 3 mev (~30 T) R cf. Co/Pd multilayer α R =360 mev Å Appl. Phys. Lett. 105, (2014) 38

17 Magnetism imaging in LAO/STO by Squid Squid data 10 u.c. LaAlO 3 TiO 2 -terminated 001 STO substrates n-type similar to our samples Nat. Phys. 7, 771 (2011) Magnetic dipoles from LAO/STO Squid magnetometry is sensitive to the stray field.

18 Magnetism imaging in LAO/STO structures by MFM MFM images Nat. Comm. 5, 5019 (2014). 12 unit cell LAO films on TiO 2 -terminated (001) STO substrates Ferromagnetism arises at lower gate voltages when the 2DEG is depleted.

19 Microscopic mechanism of the anomalous Hall effect 1. Intrinsic Berry Phase Electrons have an anomalous velocity perpendicular to the electric field related to their Berry s phase curvature 2. Extrinsic Side jump deflection by the opposite electric fields experienced upon approaching and leaving an impurity. Skew Asymmetric scattering due to the effective spin-orbit coupling of the electron or the impurity. 47

20 Topological insulators (TIs) Physics 3, 62 (2010) ARPES spectra showing a linear band structure of the surface states on a 3D TI arxiv: Nobel Symposium 2010, Shoucheng Zhang Spin polarized surface currents Linear dispersion 66

21 Spin pumping Enhancement of Gilbert damping A schematic describing spin pumping J. Appl. Phys. 108, (2010) Spin pumping is a process in which a precessing magnetization induces spin currents into an adjacent magnetic layer Tserkovnyak et al., Phys. Rev. Lett. 88, (2002) 67

22 Experimental setup Magnetization oscillation provides high density spin currents into TI and a transverse voltage is detected in TI spin detector. V - Signal generator to excite magnetization dynamics in NiFe through a coplanar waveguide - Voltmeter to measure spin pumping induced ISHE - Vector network analyzer for FMR measurements Phys. Rev. B 90, (2014) 69

23 Characterization of Bi 2 Se 3 Resistivity of 20 QL Bi 2 Se 3 Carrier concentration of 20 QL Bi 2 Se 3 ( cm) QL 1 nm n 2D (10 13 /cm 2 ) T(K) T (K) Show a typical Bi 2 Se 3 feature of saturation below 30 K. Appl. Phys. Lett. 103, (2013) 70

24 FMR measurements FMR signal (norm.) Bi 2 Se 3 /Py Py f - f 0 (GHz) Linewidth (Oe) Py linewidth linear fit for Py Bi 2 Se 3 /Py linewidth linear fit for Bi 2 Se 3 /Py H H f Frequency (GHz) Increase in linewidth is indicative of spin pumping. g Md g 4 r Py B gr = m -2 Phys. Rev. B 90, (2014) 72

25 ISHE measurements V R ISHE V ISHE ( V) B (koe) θ sh = 0.01 λ sf = 6.2 nm 3 GHz 4 GHz 5 GHz 6 GHz fitting e gr hrf M M 4 sf d BiSe shwdbise tanh M 4 d BiSe 2 sf (c) V ISHE /R ( V/ ) Data Fitting Bi 2 Se 3 thickness (nm) V ISHE ~ R J s SH R is resistance of the film J s is induced spin current θ SH is spin Hall angle Phys. Rev. B 90, (2014) 73

26 Weak anti-localization in Bi 2 Se Data HLN fitting 2 e 1 G B ln B 2 2 h 4eL B 2 4eL B 2 G (e 2 /h) Magnetic field (T) c q 2 G c H e 1 3e 1 q 24 h Bso Be 48 h 4 3 Bso B B so 4el 2 so 2 B e 4el 2 e Taking l e = 10 nm, spin orbit length l so was found to be 6.9 nm l so ~ λ sf suggest that spin-orbit coupling is dominant source of spin scattering Phys. Rev. B 90, (2014) 75

27 Temperature dependence sh (%) sf (nm) Temperature (K) Both spin Hall angle and spin diffusion length increase at low temperature θ sh = and λ sf = 9.5 nm at 15 K Phys. Rev. B 90, (2014) 76

28 Bulk spin relaxation time in Bi 2 Se 3 Time-resolved magneto optical Kerr effect 20 + Kerr rotation (a.u.) (a.u.) Delay (ps) Delay (ps) - Signal sensitive to bulk due to large penetration depth of light - Oscillation frequency is 2.13 THz from coherent vibrations of the A 1g longitudinal optical phonons of Bi 2 Se 3 - Exponentially decay with a characteristic time of 1.3 ps Jean Besbas et al. (under review) 79

29 No spin momentum locking (a) (b) 12 sh (%) Surface ( sh1 ) Bulk ( sh2 ) Temperature (K) sf (nm) Temperature (K) - Assumed spin Hall angle at opposite surfaces was taken to be of opposite signs. - Spin Hall angle does not show any clear distinction between the surface and bulk value - Momentum locking signature is not detected. Phys. Rev. B 90, (2014) 80

30 Comparison with other reports Nature 511, 449 (2014) Nat. Mater. 13, 699 (2014) Spin torque ferromagnetic resonance measurements θ SH = Magnetization switching by current induced spin orbit torque θ SH = In these experiments, a charge current flows through the TI material, unlike ours. 81

31 Properties of Bi 2 Se 3 and Bi 2 Se 3 /Co 40 Fe 40 B 20 R xx ( ) Bi 2 Se 3 20 QL T(K) M/area ( emu/cm 2 ) 400 CFB 5 nm CFB 4 nm CFB 3 nm 200 CFB 2 nm CFB 1.5 nm M/area 400 MDL = 1.36 nm CFB (nm) H (Oe) 20 QL Bi 2 Se 3 films on Al 2 O 3 (0001) by MBE. A typical feature of resistivity saturation below 30 K for Bi 2 Se 3. The Co 40 Fe 40 B 20 (CFB) dead layer ~1.36 nm. Wang et al., PRL 114, (2015) 83

32 ST-FMR measurement of Bi 2 Se 3 /CoFeB K 50 K 200 K 20 K 100 K f = 8 GHz 4 V ( V) H (Oe) ST-FMR measurements with a lock-in amplifier at H = 35. ST-FMR signal (V mix ) can be fitted by a sum of symmetric and antisymmetric Lorentzian functions: V V F ( H ) V F ( H ) mix s sym ext a asym ext V s : in-plane torque on CFB V a : total out-of-plane torque PRL 114, (2015) 84

33 Two analysis methods 1 st method: from V s /V a If only Oersted field induced out-of-plane torque ( Oe ) contributes to V a ( V / V )( e M td/ )[1+(4 M / H )] s a 0 s eff ext 12 / Thickness of Bi 2 Se 3 2 nd method: from only V s and only V a separately V ( I cos /4)( dr/ d ) (1/ ) F ( H ) s rf H H sym ext s J s/ E Mt s / E / s a 12 / rf H H Oe 0 eff ext asym ext V ( I cos /4)( dr/ d )( ){[1 ( M / H )] / } F ( H ) s Js E s / Mt/ E s / Liu et al.,phys. Rev. Lett. 106, (2011) Mellnik et al., Nature 511, 449 (2014) Wang et al., Phys. Rev. Lett. 114, (2015) 85

34 In-plane spin-orbit torque ratio in Bi 2 Se 3 /CoFeB R xx ( ) Bi 2 Se 3 20 QL T(K) By V s Only D 1 D 2 D 3 By V s /V a D 1 D 2 D T (K) ( ǁ ) increases steeply and nonlinearly to ~0.42at low temperature and could be almost 10 times larger than that at 300 K. The polarization direction of is consistent with spin-momentum-locked TSS. by 1 st and 2 nd methods shows a significant difference below ~ 50 K, other out-of-plane torque may contribution besides Oe. Wang et al., PRL 114, (2015) 87

35 In-plane spin-orbit torque (ratio) in Bi 2 Se Pt Pt (Oe) T (K) Wang et al., PRL 114, (2015) Ta Wang et al., APL 105, (2014) CuIr Spin Hall mechanism from Bi 2 Se 3 bulk is not the main mechanism for the nonlinear increase of ( ǁ )inbi 2 Se 3. Hao et al., PRB 91, (2015) Niimi et al., PRL 106, (2011) The direction of spin polarization is consistent with TSS of TIs. 88

36 Out-of-plane spin-orbit torque ratio in Bi 2 Se 3 /CoFeB (Oe) D 1 D 2 D D 1 D 2 D T (K) T (K) ( ) also increases at low temperature similar to ( ǁ ). Rashba-split state in 2DEG of Bi 2 Se 3 is not the main mechanism for. Hexagonal warping in the TSS of Bi 2 Se 3 can account for ( ). Wang et al., PRL 114, (2015) 90

37 Is Rashba effect responsible for out-of-plane spin-orbit torque? Metal & 2DEG in semiconductor: H / (z ˆ ) T R k Nat. Mater (2010) Ta GaAs/AlGaAs InSb/InAlSb Sci. Rep. 4, 4491 (2014) PRB 77, (2008) JPCM (2011) Previous Rashba reports showed a smaller effect at low temperatures. But, we observed larger effects ( ) at low temperatures. Rashba effect might not be the main mechanism for and. 91

38 Out-of-plane torque in Bi 2 Se 3 Bi 2 Se 3 Wang et al., PRL 107, (2011) Nomura et al., PRB 89, (2014) -Recent reports showed there is substantial out-of-plane spin polarization due to Hexagonal warping. -Hexagonal warping in the TSS of Bi 2 Se 3 can account for ( ). 92

39 Estimation of from topological surface states (TSS) I BiSe /I CFB n 2D (10 13 /cm 2 ) Bi 2 Se T (K) k F-TSS ~ Å -1 k F-2DEG ~ Å -1 n = 2 n + 2 n + n d 2D TSS 2DEG bulk n TSS ~ cm -2 n 2DEG ~ cm -2 n bulk ~ cm -3 (~ cm -2 ) k F-bulk ~ Å T (K) k F-bulk < k F-2DEG < k F-TSS and n 2DEG < 2 n TSS By estimating I TSS :I 2DEG :I bulk, from only TSS at low temperature is ~2.1± 0.39 (with bulk contribution) ~ 1.62 ± 0.18 (without bulk contribution) If we assume TSS thickness ~ 1 nm, the 2D spin orbit torque efficiency SOT ~ nm. IREE ~ nm in Ag/Bi interface [Nat. Commun. 4, 2944 (2013)] Wang et al., PRL 114, (2015) 95

40 Exotic spin Hall angles from topological insulators spin Hall angle ( SH ) = 2~3.5 ST-FMR (Cornell) SH = 2 (low temp) ST-FMR (NUS) SH = (low temp) spin-orbit switching (UCLA) Nature 511, 449 (2014) SH = 0.01 Spin-pumping (Tohoku) PRL 114, (2015) SH = 0.01 Spin-pumping (NUS) Nat. Mater. 13, 699 (2014) SH = Spin-pumping (Minnesota) PRL 113, (2014) PRB 90, (2014) Nano Lett 15, 7126 (2015) 97

41 LAO/STO (NUS) BiSbTe (UCLA) 3000 T 480~1460 T Co/Pd (NUS) 4 Ta Bi 2 Se 3 (Cornell, NUS) 5 koe Spin orbit torque (koe per 10 8 A/cm 2 ) Switching current density (MA/cm 2 ) W Pd Pt Ta W Pt =2 SH BiSbTe (UCLA) 8.9E4 A/cm 2 θ SH 113

42 Open questions Why is the spin Hall angle so different from spin pumping, ST- FMR, and optical imaging measurements? Spin pumping, photovoltage bulk dominant ST-FMR surface dominant Are spin currents from TI big enough to switch 3d ferromagnets? Is there any compensation of spin orbit torques from the surface states and Rashba 2DEG? Can we realize a room temperature spin orbit torque TI devices? 116

43 Coexistence of surface states and Rashba bands 117

44 Dr. Yi Wang Praveen Deorani Dr. Xuepeng Qiu Dr. K. Narayanapillai Li Ming Loong Jiawei Yu T. Venkatesan (NUS) Seah Oh (Rutgers) Aurelien Manchon (KAUST) K-J. Lee (Korea Univ.) 118