Fabrication and Measurement of Spin Devices Zhihong Chen School of Electrical and Computer Engineering Birck Nanotechnology Center, Discovery Park Purdue University Purdue Birck Presentation zhchen@purdue.edu 7 th NCN - - NEEDS Summer School July 24, 2014 1
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
1. Spin Injection! Choices of Contacts and Interfaces 3
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
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
Spin Injection Ferromagnetic Properties 4µm$ 6
Spin Injection Ferromagnetic Properties Coercive Field of Py Nanomagnets from Spin Valve Measurements Switching Field (mt) 25 20 15 10 5 W = 100nm W = 300nm FM Width H C! FM Thickness H C 0 0 5 10 15 20 25 30 35 40 45 Py Thickness (nm) C.- C. Lin, et al., Nano Lett., 13, 5177 (2013) 7
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
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] 3 2 200nm R A =0.4 nm 0 nm 1 0-40 -20 0 20 40 AFM V G -V Dirac [V] 9
Tunnelling Barrier Interface Control 2 Py contact w/ 0.6nm Al 2 O 3 I Ds [µa/µm] 1 0 5-40 -20 0 20 40 V G -V Dirac [V] 2- Probe graphene I,V 90nm SiO 2 Si Al 2 O 3 R c [kω] 6 5 4 3 Tunnelling Resistance ρ? [kω/?] 4 3 2 1 4- Probe graphene + V - 90nm SiO 2 Si Al 2 O 3 2 1-40 -20 0 20 40 V G -V Dirac [V] 0-40 -20 0 20 40 V G -V Dirac [V] 10
Tunneling Barrier Interface Control Well Controlled Interface for Spin Injection into Graphene 11
2. Spin Detection! Non- local Spin Valve Measurements 12
Non- local Spin Valve Device V 13
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, 085319 (2003) Non- local Spin Valve Measurement: Fully separate charge current Less sensitive to AMR, Hall effect, and resistance fluctuations 14
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
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, 80-83 (2012) 16
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, 80-83 (2012) 17
Spin Valve Measurements in Graphene Room Temperature Operation 0.5µm 0.42µm 0.2µm 0.4µm 1 2 4 3 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, 80-83 (2012) 18 µ µ
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
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, 132405 (2006) 20
3. Spin Relaxation! Choice of Spin Transport Channel 21
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, 161416 (2012) Guimaraes, M. et al., Nano Lett., 12, 3512 (2012)
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, 052409 (2003) 23
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, 80-83 (2012) 24
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] 0.5 0.4 0.3 0.2 0.1 λ S ~ 5.1 µm; P J ~ 4.0% 0 2 4 6 8 10 L [µm] 25
Optimized Graphene Spin Devices Tunability in Graphene 1) Graphene Layer Number 2) Carrier Concentration λ S [µm] 5 4 3 2 1 0 R S ρ Nλ S W Ch graphene + P 2 J e L/λ S 1 3 5 7 9 11 13 # 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 3-0.8-1 -1.2 1 layer -1.4 0 5 10 15 20 0.3 0.1-0.1 0 5 10 15 20 2.1 12 layer 1.9 1.7 R s =0.44Ω 0.5 7 layer R s =0.38Ω R s =0.04Ω 1.5 0 5 10 15 20 26
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 3 5 4 Larger spin diffusion length is found at higher carrier concentration T"="300K" I λ S [µm] 3 2 1 V + V - 0-40 -20 0 20 40 Vg [V] 27
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, 012412 (2006) Py/Cu/Py Room Temperature λ S ~ 350±50nm P J ~ 2% F. Jedema, et al., Nature, 410, 345 (2001) 28
4. Spin Manipulation! Lateral Spin Transfer Torque 29
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
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
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
Magnetic Field Assisted Spin Torque Measurement Torque B =+4mT P I + - V P R [Ω] Pulse I+ AP B R [Ω] AP Time [sec] 4 5 14 20 B [mt] P à AP Switching B =+4mT P I + V - P No Torque R [Ω] Pulse I+ Time [sec] AP B R [Ω] 4 5 14 20 B [mt] AP 35
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
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] 4 5 14 20 B [mt] AP - 4.5mA pulse 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 38
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
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
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
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
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
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
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
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