Fabrication and Measurement of Spin Devices. Purdue Birck Presentation

Transcription

1 Fabrication and Measurement of Spin Devices Zhihong Chen School of Electrical and Computer Engineering Birck Nanotechnology Center, Discovery Park Purdue University Purdue Birck Presentation 7 th NCN - - NEEDS Summer School July 24,

2 Spin Transport in Lateral Structures V Spin Injection Contacts & Interfaces Spin Detection Non- local Measurements Spin Relaxation Transport Channel Spin Manipulation Spin Transfer Torque 2

3 1. Spin Injection! Choices of Contacts and Interfaces 3

4 Spin Injection Contact Choices E E E D D D D D E F E F E F Paramagnetic Contact P = 0 Ferromagnetic Contact P < 1 Half Metal Contact P = 1 Electron Spin Polarization: P = D (E F ) D (E F ) D (E F ) + D (E F ) 4

5 Spin Injection Ferromagnetic Properties 5µm(L) 5µm(W) 30nm(t) NiFe (Py) Film H 4µm(L) 800nm(W) 20nm(t) NiFe (Py) Bar H 0 AFM Topography Image MFM Phase Change Image H 5µm 0 5µm 0 Dimension Dependent Domain Structures 5µm 0 5µm 5

6 Spin Injection Ferromagnetic Properties 4µm$ 6

7 Spin Injection Ferromagnetic Properties Coercive Field of Py Nanomagnets from Spin Valve Measurements Switching Field (mt) W = 100nm W = 300nm FM Width H C! FM Thickness H C Py Thickness (nm) C.- C. Lin, et al., Nano Lett., 13, 5177 (2013) 7

8 Spin Injection Efficiency F µ (r C, P σc ) µ µ (r F, P σf ) µ Int (r N, P j ) N P j = r C P σc + r F P σ F r F + r C + r N r C = 0,r F << r N P j r F r N P σ F P σ F r F, Spin resistance in FM; P σf, Conductivity polarization in FM r N, Spin resistance in channel; Pj, Current polarization r C, Interface resistance; P σc, Conductivity polarization at Interface In the case of, r C = 0,r F r N P j = r F r N P σ F = λ F λ N σ N σ F P σ F If λ F = 10nm,λ N = 300nm, σ N σ F = 0.01,P σ F = 60% P j = 0.02% Ohmic Contact (r F = λ F σ F,r N = λ N σ N ) r C r F,r N P j P σc Resistive Contact / Spin Filter I. Zutic, et al., Reviews of Modern Physics, 76, 323 (2004) 8

9 Tunnelling Barrier Interface Control Thin Mismatched Conductivity Al thickness Optimal Thickness Thick Limited Current SiO 2 20 nm 5 4 Pd contact w/o Al 2 O 3 Py contact w/ 0.6nm Al 2 O 3 Py contact w/ 1.0nm Al 2 O 3 Al 2 O 3 /Graphene I Ds [µa/µm] nm R A =0.4 nm 0 nm AFM V G -V Dirac [V] 9

10 Tunnelling Barrier Interface Control 2 Py contact w/ 0.6nm Al 2 O 3 I Ds [µa/µm] V G -V Dirac [V] 2- Probe graphene I,V 90nm SiO 2 Si Al 2 O 3 R c [kω] Tunnelling Resistance ρ? [kω/?] Probe graphene + V - 90nm SiO 2 Si Al 2 O V G -V Dirac [V] V G -V Dirac [V] 10

11 Tunneling Barrier Interface Control Well Controlled Interface for Spin Injection into Graphene 11

12 2. Spin Detection! Non- local Spin Valve Measurements 12

13 Non- local Spin Valve Device V 13

14 Why Non- local? V" Local"Spin"Valve"" Local Local V" Non- local Non- local Non&local"Spin"Valve" Spin valve measurements of two Py/Cu/Py devices from reference: PRB, 67, (2003) Non- local Spin Valve Measurement: Fully separate charge current Less sensitive to AMR, Hall effect, and resistance fluctuations 14

15 Hanle Spin Precession Measurements Z Y B Field (z) V" X V(B z ) = ±I P j 2 e 2 NS 0 P(t)cos(ω Lt)exp( t /τ sf )dt ω L = gµ B B /! P(t) = 1 4π Dt exp( L2 / 4Dt) P j, τ sf, λ sf = Dτ sf F. Jedema, et al., Nature, 416, 713 (2002) 15

16 Graphene Non- local Spin Valve Device Injector: Py [300nm(w) 25nm(t) 2µm(l)] Detector: Py [400nm(w) 25nm(t) 2µm(l)] Interface Barrier: Oxidized 0.6nm Al B field X graphene - Au V + Py Py Au Al 2 O 3 Non- FM Contact: 30nm thick Ti/Pd/Au! Graphene Channel Width: W ch =900nm Spacing Between Py Contacts: L ch =300nm 90nm SiO 2 Si Gao, Y. et al., IEDM, (2012) 16

17 Spin Valve Measurements in Graphene Z Y X X B field (Y) graphene Au - V + Py Py 90nm SiO 2 Si Au Injector Detector Parallel 400nm 300nm Room Temperature Anti- parallel Gao, Y. et al., IEDM, (2012) 17

18 Spin Valve Measurements in Graphene Room Temperature Operation 0.5µm 0.42µm 0.2µm 0.4µm B field X graphene + 90nm SiO 2 Si V Py Py Py Py - Al 2 O 3 (a) µ µ µ (b) µ µ µ Another Type of Non- local Spin Devices: All four electrodes are ferromagnetic (c) µ µ (d) µ µ Gao, Y. et al., IEDM, (2012) 18 µ µ

19 Spin Valve Measurements in Metal (Py/Cu/Py) Py1: 500nm(w) 40nm(t) 2µm Py2: 100nm(w) 40nm(t) 14µm Spacing between Py: L ch =250nm Minor Loop F. Jedema, et al., Nature, 410, 345 (2001) 19

20 Junction Size Dependent Spin Valve Signal S j1 =1µm 200nm R~50µΩ S J1 S J2 S j2 =200nm 200nm R~120µΩ S j3 =60nm 200nm R~300µΩ S j4 =30nm 200nm R~600µΩ S J =l p w I+ I- w l p Py Cu V- S J3 S J4 ΔR R Py R Cu = λ Py / (σ PyS J ) λ Cu / (σ Cu S Cu ) 1 S J V+ T. Kimura, et al., Phys. Rev. B, 73, (2006) 20

21 3. Spin Relaxation! Choice of Spin Transport Channel 21

22 Spin Transport in Single Layer Graphene λ S =1.5~2µm Tombros, N. et al., Nature, 448, 571 (2007) λ S =4.5µm λ S =4.7µm 22 Zomer, P.J. et al., PRB, 86, (2012) Guimaraes, M. et al., Nano Lett., 12, 3512 (2012)

23 Extraction of Spin Diffusion Length and Polarization X graphen - Au V + Py P Au Al 2 O 3 90nm SiO 2 S R S = R i R P J P F 2 R F 4R N e L/λ S ( N 2 1 P + R N ) 2 J 1 P F 2 i=1 (1+ i=1 2 R i 2 R F R N 2 1 P + R N J 1 P ) 2 e 2 L/λ S F Spin Resistances: Interface Resistances: Polarization: R N = ρ Nλ S W Ch P J R i : R 1 = R 2 P F R F = ρ Fλ F R i R N S J R F R N S. Takahashi, et al., Phys. Rev. B., 67, (2003) 23

24 Extraction of Spin Diffusion Length and Polarization R S 4R N P J 2 ( R i R N ) 2 e L/λ S (1+ 2 R i R N ) 2 e 2 L/λ S Room Temperature: λ S ~ 4 µm; P J ~ 3.5% ρ Nλ S W Ch P 2 J e L/λ S R S(λ S,P J ) Same graphene materials Same device structures Gao, Y. et al., IEDM, (2012) 24

25 Extraction of Spin Diffusion Length and Polarization 0.4µm 0.5µm 0.42µm 0.2µm 0.4µm Py 3.6µm Py Py 3.6µm Py R S ρ Nλ S W Ch 90nm SiO 2 Si P 2 J e L/λ S Room Temperature: R s [Ω] λ S ~ 5.1 µm; P J ~ 4.0% L [µm] 25

26 Optimized Graphene Spin Devices Tunability in Graphene 1) Graphene Layer Number 2) Carrier Concentration λ S [µm] R S ρ Nλ S W Ch graphene + P 2 J e L/λ S # of layers V Py Py Au 90nm SiO 2 Si V G = 40V, V Dirac = - 20V Larger spin diffusion length found in multi- layer graphene Au - Al 2 O layer layer R s =0.44Ω layer R s =0.38Ω R s =0.04Ω

27 Optimized Graphene Spin Devices Tunability in Graphene 1) Graphene Layer Number 2) Carrier Concentration 7 layer graphene + Au V Py Py Au 90nm SiO 2 Si - Al 2 O Larger spin diffusion length is found at higher carrier concentration T"="300K" I λ S [µm] V + V Vg [V] 27

28 Spin Transport in Metal Co/Al 2 O 3 /Al/Al 2 O 3 /Co Room Temperature λ S ~ 350±50nm P J ~ 8±1% M. Costache, et al., Phys. Rev. B, 74, (2006) Py/Cu/Py Room Temperature λ S ~ 350±50nm P J ~ 2% F. Jedema, et al., Nature, 410, 345 (2001) 28

29 4. Spin Manipulation! Lateral Spin Transfer Torque 29

30 Spin Current Induced Magnetization Reversal in Py/Cu/Py W Cu =170nm, t Cu =65nm S Py, inj =80nm 170nm, t Py =20nm S Py, det =75nm 170nm, t Py =4nm L ch =270nm Achieved at 10K Larger spin valve signals due to: 1. Ultra- clean interface to minimize interfacial scattering 2. ΔR R Py R Cu S Cu S Py 1 P AP: Positive Current AP P: Negative Current T. Yang, et al., Nature Physics, 4, 851 (2008) 30

31 Spin Transfer Torque in Multi- layer Graphene C.- C. Lin, et al., Nano Lett., 13, 5177 (2013) Injector: 300nm (W) x 25 nm (H) Detector: 100nm (W) x 5nm (H) Spin Valve Measurement at 77k Switching field v.s. Py Thickness Detector [100nm (W) x 5nm (H)] Switching Injector [ 300nm (W) x 25 nm (H) ] Switching 31

32 Magnetic Field Assisted Spin Torque Measurement + V - Injector: 300nm (W) x 25 nm (H) Detector: 100nm (W) x 5nm (H) Spin Valve Measurement at 77k B =- 50mT + V - B =+4mT Detector [100nm (W) x 5nm (H)] Switching 33

33 Magnetic Field Assisted Spin Torque Measurement Torque B =+4mT P I + - V P R [Ω] Pulse I+ AP B R [Ω] AP Time [sec] B [mt] P à AP Switching B =+4mT P I + V - P No Torque R [Ω] Pulse I+ Time [sec] AP B R [Ω] B [mt] AP 35

34 Magnetic Field Assisted Spin Torque Measurement Torque V G = +40V, B = +4mT, T = 77k P à AP +4.5mA pulse No Torque +4mA pulse 36

35 Magnetic Field Assisted Spin Torque Measurement V G = +40V, B = +4mT, T = 77k B =+4mT P + V - P R [Ω] No Torque Pulse I- Time [sec] AP B R [Ω] B [mt] AP - 4.5mA pulse 37

36 Magnetic Field Assisted Spin Torque Measurement I + V - Injector: 300nm (W) x 25 nm (H) Detector: 100nm (W) x 5nm (H) B =+10mT Spin Valve Measurement at 77k I + V B =- 4mT Detector [100nm (W) x 5nm (H)] Switching 38

37 Magnetic Field Assisted Spin Torque Measurement I + V - Injector: 300nm (W) x 25 nm (H) Detector: 100nm (W) x 5nm (H) B =+10mT Spin Valve Measurement at 77k I + V B =- 4mT Detector [100nm (W) x 5nm (H)] Switching 39

38 Magnetic Field Assisted Spin Torque Measurement Torque V G = +40V, B = +4mT, T = 77k AP à P - 4.5mA pulse No Torque - 4mA pulse 40

39 Non- local Resistance vs. DC current at 77K Critical current 4.5mA for both Pà AP and APà P switching No spin torque observed for magnetic field < 4mT B assist =4mT B assist =-4mT V G =40V, T=77K C.- C. Lin, et al., Nano Lett., 13, 5177 (2013) 41

40 Control Experiment - - Current Induced Heating +4mA pulse Coercive Field Detector: ±3.5mT Injector: ±22mT! B ext =+2.5mT - 4mA pulse Only positive current pulse results in Pà AP switching evidence to exclude switching by current induced heating! 42

41 Control Experiment Oersted Field Spin Torque Effect O e r s t e d F i e l d Effect Switching Direction Magnetization Directions of I n j e c t o r a n d Detector Assisting Magnetic Field P AP +2.5mT +4mA P AP -2.5mT +4mA AP P -2.5mT -4mA P AP +2.5mT +4mA P AP -2.5mT -4mA AP P -2.5mT -4mA Current Pulse Coercive Field Detector: ±3.5mT Injector: ±22mT! B ext =- 2.5mT - 4mA pulse Only positive current pulse results in Pà AP switching evidence to exclude switching by Oersted field! 43

42 Asymmetric Contacts for Graphene Spin Valve L =0.4µm, W =1.0µm ρ =0.8kΩµm, λ S ~4µm, P C ~4% R c =2kΩ (with tunneling barrier) R c =1kΩ (without tunneling barrier) C.- C. Lin, et al., ACS Nano, 8, 3807 (2014) Noise Reduction: 8.4% à 4.5% 44

43 Improving Spin Transfer Torque Critical charge current for spin transfer torque at B =1mT:!! J double =45mA/µm 2! J single =33mA/µm 2 C.- C. Lin, et al., ACS Nano, 8, 3807 (2014) 45

44 Acknowledgements Students: Chia- Ching Lin, Yunfei Gao, Ashish V. Penumatcha, Vinh Quang Diep! Collaborators: Prof. Appenzeller, Prof. Da a! Funding Support: NRI INDEX Center, NCN NEEDS 46