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

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

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

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 https://commons.wikimedia.org/w/index.php?curid=11293145 MRAM http://www.everspin.com/

Emergent direction: dynamic Spin Pumping FMR-driven spin pumping (Py/Pt bilayer) Js M Beff FM MDC MAC NM Saitoh et al., APL 88, 182509 (2006) Thermally-driven spin dynamics (spin Seebeck effect, Py/Pt bilayer) K. Uchida et al., Nature 455, 778 (2008)

Emergent direction: dynamic Spin Pumping Js M Beff FM MDC MAC NM Pu et al., PRL 115, 246602 (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

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

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

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

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

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

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

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, 107201 (2016)

Sample processing@OSU Sample Development (2nd MBE)@UCR Sample processing@OSU GaAs substrate 400nm n-InGaP 120nm n-GaAs (MOCVD)@OSU (1st MBE)@OSU 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)

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

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

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

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

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

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

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

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

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

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

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