1. Ultrafast Dynamic Exchange Coupling in Ferromagnet/Oxide/Semiconductor Heterostructures
Yu-Sheng Ou, PhD
Ezekiel Johnston-Halperin’s group
Department of Physics
The Ohio State University

2. Introduction to Spintronics
Concatenation meaning: spin electronics
Conventional electronics: reply on charge property of electrons for application
Spintronics: reply on either solely on the spin or spin plus charge for application

3. Introduction to Spintronics
Concatenation meaning: spin electronics
Conventional electronics: reply on charge property of electrons for application
Spintronics: reply on either solely on the spin or spin plus charge for application
Discovery of giant magnetoresistance (GMR) effect
Fert et al. and Grunberg et al. (Nobel prize 2007)
FM/non-magnetic metal/FM
Low (magneto)resistance, “0” state
High (magneto)resistance, “1” state I FM NM

4. Introduction to Spintronics
Concatenation meaning: spin electronics
Conventional electronics: reply on charge property of electrons for application
Spintronics: reply on either solely on the spin or spin plus charge for application
Discovery of giant magnetoresistance (GMR) effect
Fert et al. and Grunberg et al. (Nobel prize 2007)
Applications: read head of HDD and magnetoresistive random access memory (MRAM, TMR effect)
FM/non-magnetic metal/FM
Low (magneto)resistance, “0” state
High (magneto)resistance, “1” state I FM NM
Static magnetization
Read head of hard disk drive
MRAM

5. Emergent direction: dynamic Spin Pumping
FMR-driven spin pumping (Py/Pt bilayer) Js M
Beff FM
MDC
MAC NM
Saitoh et al., APL 88, (2006)
Thermally-driven spin dynamics (spin Seebeck effect, Py/Pt bilayer)
K. Uchida et al., Nature 455, 778 (2008)

6. Emergent direction: dynamic Spin PumpingJs M
Beff FM
MDC
MAC NM
Pu et al., PRL 115, (2015)
AC component of magnetization dynamics is crucial to spin pumping efficiency
Dynamic spin pumping attracts lots of attentions for its potential to realize spin battery
Magnetization dynamics can be driven either coherently by FMR or by thermal current
Injected spin current is often measured by measuring the DC voltage induced by the inverse spin Hall effect

7. Dynamic Exchange Coupling in Fe/GaAs Heterostructures
Ultrafast pump-probe spectroscopy
Time domain study
High Bandwidth (up to THz)
Can be applied to a variety of
materials: semiconductors, FM,AFM Fe
GaAs
𝑩 𝑎𝑝𝑝
θKR(Δt)= θ0 exp(-Δt/T2*)cos(ΩL Δt)
CP pump
θ KR
LP probe Δt
T2*: spin lifetime
Ω=gµBBtot /h (Larmor frequency); Btot=Bapp+Blocal

8. Dynamic Exchange Coupling in Fe/GaAs Heterostructures
Ultrafast pump-probe spectroscopy
Time domain study
High Bandwidth (up to THz)
Can be applied to a variety of
materials: semiconductors, FM,AFM
4 separate spin systems:
“Pure” GaAs spins ( 𝑺 𝑟𝑒𝑙 )
Exchange coupled GaAs spins ( 𝑺 𝑭𝑷𝑷 )
Fe spins ( 𝑺 𝐹𝑒 )
Nuclear spins (Ga, As)(I)
𝑩 𝑡𝑜𝑡 = 𝑩 𝑎𝑝𝑝 + 𝑩 𝑛 𝑡𝑜𝑡 Fe
𝑺 𝐹𝑒
GaAs
FPP
𝑩 𝑛 𝑡𝑜𝑡
𝐴𝑰⋅ 𝑺 𝑭𝑷𝑷
𝑩 𝑎𝑝𝑝
𝐴𝑰⋅ 𝑺 𝑟𝑒𝑙 I
𝑺 𝑟𝑒𝑙
𝑺 𝑭𝑷𝑷
𝑺 0
θKR(Δt)= θ0 exp(-Δt/T2*)cos(ΩL Δt) Δt
T2*: spin lifetime
Ω=gµBBtot /h (Larmor frequency); Btot=Bapp+Blocal
θ KR
LP probe
CP pump

9. Time-Resolved Kerr Rotation Spectroscopy
Ultrafast pump-probe spectroscopy
Time domain study
High Bandwidth (up to THz)
Can be applied to a variety of
materials: semiconductors, FM,AFM
4 separate spin systems:
“Pure” GaAs spins ( 𝑺 𝑟𝑒𝑙 )
Exchange coupled GaAs spins ( 𝑺 𝑭𝑷𝑷 )
Fe spins ( 𝑺 𝐹𝑒 )
Nuclear spins (Ga, As)(I)
𝑩 𝑡𝑜𝑡 = 𝑩 𝑎𝑝𝑝 + 𝑩 𝑛 𝑡𝑜𝑡 Fe
𝑺 𝐹𝑒
GaAs
FPP
𝑩 𝑛 𝑡𝑜𝑡
𝐴𝑰⋅ 𝑺 𝑭𝑷𝑷
𝑩 𝑎𝑝𝑝
𝐴𝑰⋅ 𝑺 𝑟𝑒𝑙 I
𝑺 𝑟𝑒𝑙
𝑺 𝑭𝑷𝑷
𝑺 0
θKR(Δt)= θ0 exp(-Δt/T2*)cos(ΩL Δt)
LP probe Δt
θ KR
CP pump (~817nm)
T2*: spin lifetime
Ω=gµBBtot /h (Larmor frequency); Btot=Bapp+Blocal

10. Ferromagnetic Proximity Polarization
Ultrafast pump-probe spectroscopy
Time domain study
High Bandwidth (up to THz)
Can be applied to a variety of
materials: semiconductors, FM,AFM
4 separate spin systems:
“Pure” GaAs spins ( 𝑺 𝑟𝑒𝑙 )
Exchange coupled GaAs spins ( 𝑺 𝑭𝑷𝑷 )
Fe spins ( 𝑺 𝐹𝑒 )
Nuclear spins (Ga, As)(I)
𝑩 𝑡𝑜𝑡 = 𝑩 𝑎𝑝𝑝 + 𝑩 𝑛 𝑡𝑜𝑡 Fe
𝑺 𝐹𝑒
GaAs
FPP
𝑩 𝑛 𝑡𝑜𝑡
𝐴𝑰⋅ 𝑺 𝑭𝑷𝑷
𝑩 𝑎𝑝𝑝
𝐴𝑰⋅ 𝑺 𝑟𝑒𝑙 I S0 M
𝑺 𝐹𝑒
CP pump
𝑩 𝑎𝑝𝑝
𝑺 𝑟𝑒𝑙
𝑺 𝑭𝑷𝑷
𝑺 0
θKR(Δt)= θ0 exp(-Δt/T2*)cos(ΩL Δt)
LP probe Δt
θ KR
CP pump
T2*: spin lifetime
Ω=gµBBtot /h (Larmor frequency); Btot=Bapp+Blocal

11. Hyperfine Coupling in GaAs
Ultrafast pump-probe spectroscopy
Time domain study
High Bandwidth (up to THz)
Can be applied to a variety of
materials: semiconductors, FM,AFM
4 separate spin systems:
“Pure” GaAs spins ( 𝑺 𝑟𝑒𝑙 )
Exchange coupled GaAs spins ( 𝑺 𝑭𝑷𝑷 )
Fe spins ( 𝑺 𝐹𝑒 )
Nuclear spins (Ga, As)(I)
𝑩 𝑡𝑜𝑡 = 𝑩 𝑎𝑝𝑝 + 𝑩 𝑛 𝑡𝑜𝑡 Fe
𝑺 𝐹𝑒
GaAs
FPP
𝑩 𝑎𝑝𝑝
𝐴𝑰⋅ 𝑺 𝑟𝑒𝑙
𝑩 𝑛 𝑡𝑜𝑡
𝐴𝑰⋅ 𝑺 𝑭𝑷𝑷 I
Dynamic nuclear polarization (DNP)
𝑺 𝑟𝑒𝑙
𝑺 𝑭𝑷𝑷
𝐴𝑰⋅𝑺
Nuclear spin bath
electron spin bath
𝑩 𝒏 𝒕𝒐𝒕 ∝− 𝑩 𝒂𝒑𝒑 𝑩 𝒂𝒑𝒑 ∙ 𝑺 𝑭𝑷𝑷 − 𝑺 𝒓𝒆𝒍 𝐵 𝑎𝑝𝑝 2
𝐴𝑰⋅𝑺
Nuclear spin bath
electron spin bath
𝑺 0
θKR(Δt)= θ0 exp(-Δt/T2*)cos(ΩL Δt)
LP probe Δt
θ KR
CP pump
T2*: spin lifetime
Ω=gµBBtot /h (Larmor frequency); Btot=Bapp+Blocal

12. Dynamic Exchange Coupling in Fe/GaAs Heterostructures
Ultrafast pump-probe spectroscopy
Time domain study
High Bandwidth (up to THz)
Can be applied to a variety of
materials: semiconductors, FM,AFM
4 separate spin systems:
“Pure” GaAs spins ( 𝑺 𝑟𝑒𝑙 )
Exchange coupled GaAs spins ( 𝑺 𝑭𝑷𝑷 )
Fe spins ( 𝑺 𝐹𝑒 )
Nuclear spins (Ga, As)(I)
𝑩 𝑡𝑜𝑡 = 𝑩 𝑎𝑝𝑝 + 𝑩 𝑛 𝑡𝑜𝑡 Fe
𝑺 𝐹𝑒
GaAs
FPP
𝑩 𝑛 𝑡𝑜𝑡
𝐴𝑰⋅ 𝑺 𝑭𝑷𝑷
𝑩 𝑎𝑝𝑝
𝐴𝑰⋅ 𝑺 𝑟𝑒𝑙 I
𝑺 𝑟𝑒𝑙
𝑺 𝑭𝑷𝑷
𝑺 0
θKR(Δt)= θ0 exp(-Δt/T2*)cos(ΩL Δt) Δt
T2*: spin lifetime
Ω=gµBBtot /h (Larmor frequency); Btot=Bapp+Blocal
θ KR
LP probe
CP pump

13. Exchange-driven spin relaxation Impurity-band hyperfine coupling
Research Overview
Exchange-driven spin relaxation
Tuning the exchange coupling
Impurity-band hyperfine coupling
Ou et al., in preparation
Ou et al., Proc. SPIE 9551, Spintronics VIII, 95510E (2015).
Ou et al., PRL 116, (2016)

14. Sample processing@OSU
Sample Development
(2nd
Sample
GaAs substrate
400nm n-InGaP
120nm n-GaAs
(1st
As capping
GaAs substrate
400nm n-InGaP
120nm n-GaAs Fe
MgO
GaAs substrate
400nm n-InGaP
120nm n-GaAs Fe
MgO
400nm n-InGaP
120nm n-GaAs
100-μm sapphire
InGaP and n-GaAs are grown by Metal-organic chemical vapor deposition (MOCVD)
InGaP is lattice matched to GaAs
Carrier concentration (7e16/cm3) of n-GaAs is chosen to maximize spin lifetime.
Remove native oxide of n-GaAs and add As capping layer in an As-filled molecule beam epitaxy (MBE) chamber.
Remove As capping and anneal GaAs surface for 2x4 reconstruction for MgO and ferromagnet deposition inside MBE chamber.
Selective wet etching removes the GaAs substrate (etchant: citric acid, ammonia hydroxide, hydrogen peroxide)
InGaP acts as a stop-etching layer (selectivity: 105)

15. Ferromagnetic Imprinting of Nuclear Spins
Nuclear spin polarization tracks the magnetization
ωL = gµBBtot /h
Bn= Btot- Bapp
Btot (Bn) in Fe/MgO/GaAs tracking the magnetization is indicative of ferromagnetic imprinting of nuclear spin polarization

16. Field Dependence of nuclear field: competition between FPP and Zeeman effect
component
dominated
FPP component
FPP
dominated
𝑩 𝑡𝑜𝑡 = 𝑩 𝑎𝑝𝑝 + 𝑩 𝑛 𝑡𝑜𝑡 Fe
𝑺 𝐹𝑒
GaAs
𝐴𝑰⋅ 𝑺 𝑟𝑒𝑙
𝑩 𝑛 𝑡𝑜𝑡
𝐴𝑰⋅ 𝑺 𝑭𝑷𝑷
𝑩 𝑎𝑝𝑝 I
𝑺 𝑟𝑒𝑙
𝑺 𝑭𝑷𝑷
𝑺 0
𝑩 𝒏 𝒕𝒐𝒕 ∝− 𝑩 𝒂𝒑𝒑 𝑩 𝒂𝒑𝒑 ∙ 𝑺 𝑭𝑷𝑷 −𝑺 𝒓𝒆𝒍 𝐵 𝑎𝑝𝑝 2

17. FPP Effect on Spin Relaxation
Field dependence of T2* & nuclear field strength
Field dependence of T2* & nuclear field
Field dependence of T2*
Inhomogeneous nuclear field model
Low temperature/high field y
Spatial distribution of Si donors
GaAs x
Si donor
Polarized e- wave function x
potential
Strong dependence of spin relaxation time on hyperfine field strength
Inhomogeneous nuclear field due to random donor distribution leads to coherent spin relaxation
Strong dependence of spin relaxation time on hyperfine field strength

18. FPP Effect on Spin Relaxation & Inhomogeneous nuclear field theory
Field dependence of T2* & nuclear field strength
Inhomogeneous nuclear field model
Low temperature/high field
GaAs
GaAs x x
Energy
Si donor
Polarized e- wave function
Polarized nuclei cloud Bn
Strong dependence of spin relaxation time on hyperfine field strength
Inhomogeneous nuclear field due to random donor distribution leads to coherent spin relaxation

19. Test the validity of inhomogeneous nuclear field model: temperature
Low temperature/ high field
high temperature/ high field y
GaAs y x
GaAs x
GaAs
Energy
Energy
KBT x x
Si donor
Polarized e- wave function
Polarized nuclei cloud Bn
Turn off hyperfine interaction by increasing temperature

20. Test the validity of inhomogeneous nuclear field theory : temperature
Temperature dependence of nuclear field strength
Temperature dependence of spin relaxation time
Bapp= 0.18 kG
hyperfine ?
The peak at 40 K suggests a transition of spin relaxation mechanism from hyperfine-dominated to DP-dominated
40 K is close to the thermal depopulation temperature of Si donors

21. Temperature dependence of T2* for GaAs
Fe/MgO/GaAs
GaAs
hyperfine
D’yakonov-Perel’
Kikkawa et al., Phys. Rev. Lett. 80, 4313 (1998)

22. Test the validity of inhomogeneous nuclear field model: applied field
Large applied field
small applied field
Bapp
Bapp
Nuclear spin
Nuclear field
Turn off hyperfine interaction by nuclear dipole-dipole field

23. Test the validity of inhomogeneous nuclear field model: applied field
Bapp= 0.10 kG
Temperature dependence of nuclear field strength
Temperature dependence of spin relaxation time
Bapp= 0.18 kG
Hyperfine-dominated spin relaxation is turned off at low field due to the nuclear depolarization by nuclear dipole field

24. Summary
Hyperfine interaction is dominant spin relaxation mechanism in Fe/MgO/GaAs at low temperature
Resolving the long-lasting dispute about the spin dissipation mechanism of GaAs at low temperature and finite magnetic fields

25. Acknowledgement Sample synthesis Kawakami’s group (OSU & UCR)
Patrick Odenthal (Utah)
Theory and modeling
Flatté group (U of Iowa)
Dr. Nicholas Harmon
Measurement
Johnston-Halperin’s group (OSU)
Yi-Hsin Chiu (PSU)
Matt Sheffield
Michael Chilcote
Michel Swartz