- 1. 1 Spin Torque for DummiesSpin Torque for Dummies Matt Pufall, Bill Rippard Shehzu Kaka, Steve Russek, Tom Silva National Institute of Standards and Technology, Boulder, CO J. A. Katine Hitachi Global Storage Technologies San Jose, CA Mark Ablowitz, Mark Hoefer, Boaz Ilan University of Colorado Applied Math Department Boulder, CO
- 2. 2 OutlineOutline 1) Spin dynamics 2) Magnetotransport 3) Spin momentum transfer 4) Spin torque nano-oscillators
- 3. 3 But first… a puzzle!But first… a puzzle! We start with a polarized beam of spin 1/2 Ag ions… … we pass the beam through a magnetic field gradient… … and the beam “diffracts” into two beams, polarized along the axis of the magnetic field. Q: What happened to the angular momentum in the original polarization direction??? (For more details, see Feynman’s Lectures on Physics, Vol. III, page 5-1) The “Stern-Gerlach” experiment … but true!!! Very strange … H
- 4. 4 Part 1: Spin dynamicsPart 1: Spin dynamics
- 5. 5 Magnets are Gyroscopes!Magnets are Gyroscopes! 2 B L e e m µ γ = − = − h e- r v z 2 B s µ γ = − h For spin angular momentum, extra factor of 2 required. “The gyromagnetic ratio” L r p= × r r r angular momentum for atomic orbit: L IAµ = rr magnetic moment for atomic orbit: ( )2 ˆ 2 ev r z r π π = − ÷ ˆ 2 evr z= − ˆrmvz= Classical model for an atom: 2 e evr rm v = − z L zL µ γ B
- 6. 6 Larmor EquationLarmor Equation H M 0T M Hµ= × r r r T dL T dt = r r L µ γ ≡ ( )0 dM T dt M H γ γµ = = × r r r r (definition of torque) (gyromagnetic ratio) Magnetic field exerts torque on magnetization. Joseph Larmor Magnets make me dizzy!
- 7. 7 ( )0 d s T M M H M αµ = = × × r r r r damping torque 0 precession torquepT M Hµ = = × r r r Landau & Lifshitz (1935): ( ) ( ) 0 0 p d s dM T T T dt M H M M H M γ γ γ µ α γ µ = − = − + = − × − × × r r r r r r r r r α = dimensionless Landau-Lifshitz damping parameter Gyromagnetic precession with energy loss: TheGyromagnetic precession with energy loss: The Landau-Lifshitz equationLandau-Lifshitz equation L. Landau and E. Lifshitz, Physik. Z. Sowjetunion 8, 153-169 (1935). H Lev Landau Damping happens!! Lev Landau
- 8. 8 Part 2: Charge transport in magnetic heterostructuresPart 2: Charge transport in magnetic heterostructures (Magneto-transport)(Magneto-transport)
- 9. 9 Ferromagnetism in Conductors: Band StructureFerromagnetism in Conductors: Band Structure D(E) E “s-p band” EF spin-up spin-down D(E) E “d band” EF “majority” states “minority” states “Density of States” “exchange splitting” s-band states (l = 0) have higher mobility than d-band states (l = 2) in conductors due to the much lower effective mass of the s-band. Minority band electrons scatter more readily from the s-p band to the d-band due to the availability of hole states in the minority d-band.
- 10. 10 Spin-dependent conductivity in ferromagnetic metalsSpin-dependent conductivity in ferromagnetic metals “majority band” “minority band” M JJ Conductivities in ferromagnetic conductors are different for majority and minority spins. In an “ideal” ferromagnetic conductor, the conductivity for minority spins is zero. Analogy: Like water flowing through pipes. Large conductivity = big pipe, etc…
- 11. 11 Concept: interfacial spin-dependent scatteringConcept: interfacial spin-dependent scattering M “Majority” spins are preferentially transmitted. ”Minority” spins are preferentially reflected. Normal metal Ferromagnet
- 12. 12 Concept: ferromagnets as spin polarizersConcept: ferromagnets as spin polarizers M “Majority” spins are preferentially transmitted. Normal metalFerromagnet Ferromagnetic conductors are relatively permeable for majority spins. Conversely, they are impermeable for minority spins.
- 13. 13 Concept: spin accumulationConcept: spin accumulation ∆M z I ( ) sM z M M= + ∆ Non-equilibrium spin polarization “accumulates” near interfaces of ferromagnetic and non-magnetic conductors. “spin diffusion length”
- 14. 14 Part 3: Spin momentum transferPart 3: Spin momentum transfer
- 15. 15 Non-collinear spin transmissionNon-collinear spin transmission M θ What if the spin is neither in the majority band nor the minority band??? ???? ???? Is the spin reflected or is it transmitted? Quantum mechanical probabilities: ( )( ) ( )( ) 2 2 1 Pr 1 cos 2 1 Pr 1 cos 2 A B θ θ ↑ = = + ↓ = = − = +A B θ Quantum mechanics of spin: cos 2 sin 2 A B θ θ = ÷ = ÷ An arbitrary spin state is a coherent superposition of “up” and “down” spins.
- 16. 16 Spin Momentum Transfer: Small Current LimitSpin Momentum Transfer: Small Current Limit θ M + = -δT + Tdamp θ MPolarizer: At low electron flux, damping torque compensates spin torque: Magnetization is stable. e- θ M Mf ; 1Tδ θ θµ r =+ = δT Electrons:
- 17. 17 Spin Momentum Transfer: Large Current LimitSpin Momentum Transfer: Large Current Limit δT is driven by spin accumulation in the Cu spacer. Spin accumulation is proportional to current flowing through the structure. + = θ M + = δT -δT + Tdamp θ + δθ M Electrons: Polarizer: Spin torque exceeds damping torque: Polarizer reacts with changing M. Torque proportional to angle θ: Unstable!Damping torque
- 18. 18 Transverse torque via spin reorientation/reflectionTransverse torque via spin reorientation/reflection Consider only reflection events... For the electron: ( ) ( ) ( ) ˆ ˆsin 2 ˆ ˆ ˆ 2 ˆ ˆ ˆ 2 transverse transverse inc inc f s s s y s y m m p m m m θ ∆ ′∆ = − ′= − = − × × = × × r r r r h h h θ θ transverses∆ r incs rrefs r AND Consider only change in angular momentum transverse to magnetization axis. (Equivalent to assuming magnitude of M does not change.) y′ x′ where 2 ˆ incp s= r h θ ˆp ˆm−
- 19. 19 Newton’s Second LawNewton’s Second Law ( )ˆ ˆ ˆ 2 trans fs m m m∆ = × × hr If… …then… ( ) ( )ˆ ˆ ˆ 2 fM V m m mγ∆ = × × r h …per electron. For a flowing stream of electrons: ( ) ( )( )2 ˆ ˆ ˆ 2 ˆ 2 f f s m m m dM I dt V e I M M m eM V γ γ × × = ÷ ÷ = × × ÷ hr r rh Total moment of “free” magnetic layer Rate of electron impingement on “free” layer
- 20. 20 The Slonczewski Torque TermThe Slonczewski Torque Term 2 ˆ( ) 2 Sloncewski f s J T M M m e M ε δ = × × r r rh J. Slonczewski, Journal of Magnetism and Magnetic Materials, vol. 159, page L1 (1996) J M Mf δ efficiency* ~ 0.2 - 0.3 *Accounts for all those messy details: Polarization of ferromagnet, band structure mismatch at interface, spin decoherence, etc…
- 21. 21 The “FAQ Page”The “FAQ Page” Q: What if the spin is transmitted through the “free” layer rather than reflected? A: Doesn’t matter. Quantum mechanically, there is an amplitude for both transmission and reflection, but only for spin along the axis of magnetization. The transverse component of spin for the incident electrons is “lost” once the electron wavefunction is split into the transmitted and reflected components. Conservation of angular momentum dictates that the transverse component is transferred to the magnetic layer. This is a purely quantum mechanical phenomenon: There is no classical analog! Q: Don’t the reflected spins affect the spin accumulation in the spacer layer? A: Yes, they do. There are several theories that take this “back-action” on the spin accumulation into account. See J. C. Slonczewski, J. Magn. Magn. Mater. 247, 324 (2002); A. A. Kovalev, et al., Phys. Rev. B 66, 224424 (2002); J. Xiao, et al., Phys. Rev. B 70, 172405 (2004); A. Fert, et al., J Magn. Magn. Mater. 69, 184406 (2004). ( ) 0 1 0 1cos cos q q B B B B ε θ θ θ + − = + + − Tsmt ε = constant ε (θ )
- 22. 22 Back to the puzzle…Back to the puzzle… A: The quantum equivalent of a card trick. If a card “vanishes” magically from one deck, it must reappear somewhere else. No mechanism for the transfer of angular momentum need be invoked! The “Stern-Gerlach” experiment, revisited. Once the linear momentum for the two spin components is split, the transverse angular momentum is “released” to do work on the magnet system. H “Up” spin current “Down” spin current 0xS = 1 2 xS = + x y z
- 23. 23 ( )Larmor damp smt dM T T T dt γ= + + r r r r 0 eff dM M H dt γµ= × r r r Larmor term: precession My Mz Mx M Heff Tsmt ≅ -Tdamp J ~ 107 A/cm2 Slonczewski 1996 My Mz Mx Mi Mf Heff Damping term: aligns M with H 0 2 ( )eff s M M H M αγµ − × × r r r Precession Heff Spin Torque Damping M Spin Torque Term 2 ˆ( ) 2 B f s Jg M M m e M µ ε δ + × × r r Spin torque can counteract damping mf Magnetodynamics: Three TorquesMagnetodynamics: Three Torques
- 24. 24 How Can We See This? Torque ∝ to current density: must have high current densities to produce large torques 1 mm 10 µm I = 0.1 MA≈ I = 10 A≈ 100 nm I = 1 mA≈ We will use nanopillar and nanocontact structures Typical wire Size of a human hair 500 atoms across≈ X Required Idc Possible X
- 25. 25 Part 4: Spin torque nano-oscillators
- 26. 26 • Step DC current • Measure DC R, microwave power output 5 6 7 8 9 24.8 25.0 25.2 25.4 dV/dI(Ω) Current (mA) 9.6 9.7 9.8 9.9 10.0 0.0 0.1 0.2 0.3 0.4 9 mA 8.5 mA8 mA 7.5 mA 7 mA 6.5 mA 6 mA 5.5 mA Power(pW) Frequency (GHz) Devices are nanoscale current-controlled microwave oscillators Au Cu “Fixed” layer “Free” layer ~40 nm 0.7 T, θ = 10o θT = 300K Nanocontact DynamicsNanocontact Dynamics
- 27. 27 SummarySummary Magnetization dynamics tutorial: Magnets are gyroscopes. Magnetotransport tutorial: Magnets are spin filters. Spin momentum transfer: Back action of spin polarized carriers on magnet. Spin torque nano-oscillator: Spin torque compensates damping. An excellent review article!! M. D. Stiles and J. Miltat, “Spin Transfer Torque and Dynamics,” Topics in Applied Physics 101, 225-308 (2006).

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