Spin-torque nano-oscillators trends and challenging

Domain Microstructure and Dynamics in Magnetic Elements Heraklion, Crete, April 8 11, 2013 Spin-torque nano-oscillators trends and challenging N H ext S Giovanni Finocchio Department of Electronic Engineering, Industrial Chemistry and Engineering University of Messina

Outline Introduction Spin-Transfer-torque Oscillators (STO) An overview Effect of the in-plane field angle Soliton mode Two free layers Interface perpendicular anisotropy Self-oscillator based on Spin-Hall-Effect Conclusions

Introduction (1/4) Giant magneto-resistance Spin torque effect

Giant MagnetoResistance (GMR) Introduction (2/4) Spin-Transfer Torque (STT) Electronic Transport affected by Magnetic State Magnetic State affected by Electronic Transport Fert Gruenberg (1988) two sides of the same coin Slonczewski Berger (1996)

Equation of motion Introduction (3/4) Landau-Lifshitz-Gilbert-Slonczewski (LLGS) equation dm α dm 2µ J = γ ( M H Eff ) + M χ( M, P) M ( M P) dt M dt d M e B 0 3 S s J. Slonczweski, J. Magn. Magn. Mat. 159, L1-L6 (1996). Heff = Hext + Hexch + Hani + HM + HTH + H + H Amp MC N External Field H ext S Exchange S i S j Anisotropy Magnetostatic Contribution + + + + + + - - - - - - - - H d Magnetostatic Thermal Field Coupling Oersted Field

Introduction (4/4) Landau-Lifshitz-Gilbert-Slonczewski (LLGS) equation Semi-implicit algorithm based on the Adams-Bashforth algorithm as a predictor, and a second order Adams-Moulton procedure as a corrector in the Landau Lifshitz Gilbert-Slonczewski equation.

STO An overview (1/2), After the first experimental observation of persistent magnetization dynamics reported by Tsoi and co-workers in point contact geometries with an out-of-plane bias field, there was a huge numbers of experimental and theoretical works. Fundamental point of view compensation of the damping losses very rich dynamical behaviour STO very interesting smallest self-oscillator known in nature excitation of uniform and non-uniform modes Technological point of view nanoscale size broad working temperature low dissipation power large output power ultralow critical current microwave emission at zero field high oscillation frequency narrow linewidth

STO An overview (2/2), Classification In-plane Out-of-plane In-plane polarizer Out-of-plane free layer Two in-plane free layers Two out-of-plane polarizers

STO Effect of the in-plane field, In-plane The linewidth is strongly dependent on the in plane field angles. Non-uniform mode near easy axis uniform mode near hard in-plane axis Sub-critical vs Super-critical Hopf bifurcation

STO Soliton mode, Self-oscillation driven by injection of a non uniform current! Excitation of a sub-ghz mode at zero field. Is a non-uniform mode? solving the Poisson equation. V. L. Pokrovskii, G. V. Uimin. Sov. Phys. JETP Lett. 41, 128-132 (1985). S. Komineas. Phys. Rev. Lett. 99, 117202 (2007). Details of device fabrication and measurement technique O. Ozatay, et al. Appl. Phys. Lett. G. Finocchio, O. Ozatay, et al. 88, 202502 (2005). Phys. Rev. B 78, 174408 (2008)

STO Soliton mode, Excitation of a Vortex-antivortex pair! V. L. Pokrovskii, G. V. Uimin. Sov. Phys. JETP Lett. 41, 128-132 (1985). S. Komineas. Phys. Rev. Lett. 99, 117202 (2007). G. Finocchio, O. Ozatay, et al. Phys. Rev. B 78, 174408 (2008)

STO Soliton mode, Excitation of a different soliton modes! A stable vortex quadripole! G. Finocchio, S. Komineas, et al in preparation. V. Puliafito, G. Finocchio, S. Komineas, et al, Poster Tuesday.

Outline Introduction Spin-Transfer-torque Oscillators (STO) An overview Effect of the in-plane field angle Soliton mode Two free layers STO with interface perpendicular anisotropy Self-oscillator based on Spin-Hall-Effect Conclusions

STO - Two free layers Zero field, high frequency microwave emission!

STO - Two free layers dmt dmt = ( mt ht eff ) + αgtmt T( mt, mb ) dτ dτ dmb dmb = ( mb hb eff ) + αgpmb T( mt, mb ) dτ dτ T ( m, m ) b t g µ (, )( ) pt ( ) B j mt ε mt mb mt mb ε mt mp = 2 e γ 0dM s ε (, )( ) ε pb ( ) mb mb mt mb mt mb mp ε ( m, m ) = ε ( m, m ) p f f p Spin Torque Effect Magnetostatic Contribution COUPLING Torque from perpendicular Polarizers G. Finocchio, et al, Phys. Rev. B 76, 174408 (2007). h = h + h + h + h + h + h + h eff t ext exch t ani t M t Oe t th t MC t h = h + h + h + h + h + h + h eff b ext exch b ani b M b Oe b th b MC b

STO - Two free layers ε ( m, m )sinθ t b ( ) ( ) Thermal fluctuations Gaussian process Zero mean Unit variance H B th = ξ α K T 2 1 2 + α µ γ V M t = ξ 3 3/ 2 ε ( m, ) = 4 + 1+ η 3 + 4η t mb m b mt Hth, k ( t) Hth, l ( t ) = Dδ kl δ ( t t ) ε ( mp, mf ) = 0.4 W. F. Brown, Jr., Phys. Rev. 130, 1677, 1963. ε ( m, m ) = 0.2 p f 1 H th = 0 (, H H ) H th, x th, y, th, z s H th, k ( t) = 0 D

STO - Two free layers Excitation of a strong non uniform mode! Phase locking between the dynamics in the two free layer! Frequency doubling in the GMR signal!

STO - Two free layers

STO - Two free layers Phase locking between the dynamics in the two free layer! Systematic study Elliptical cross section (170nm x 130nm) Circular cross section (140nm) Symmetric polarizer (CoNi) Asymmetric polarizer (CoNi - CoPt) Injection locking at zero field! 23

STO - Interface perpendicular anisotropy, Electrostatic interaction MgO/CoFeB surface perpendicular anisotropy The easy axis depends on the thickness of the CoFeB A B C

STO - Interface perpendicular anisotropy, To reduce the out-of-plane demagnetizing field while maintaining the orientation of both the two magnetizations in the film plane

STO - Interface perpendicular anisotropy STO properties demonstrated separately frequency tunability (field and current) nanoscale size broad working temperature low dissipation power large output power ultralow critical current microwave emission at zero field high oscillation frequency very interesting non-autonomous behavior narrow linewidth, Spin-torque compensates the damping losses. IDEAL all-in-one STO

STO - Interface perpendicular anisotropy Ultralow current density and bias-field-free spin-transfer nano-oscillator Z. M. Zeng, G. Finocchio, B. S. Zhang, J. A. Katine, I. Krivorotov, Y. Huai, J. Langer, B. Azzerboni, P. Khalili Amiri, K. L. Wang, and H. W. Jiang Scientific Reports 3, 1426, (2013)., Out-of-plane free layer in-plane polarizer

Model (3) Pt/Co/AlO Out-of-plane easy axis Switching driven by an in-plane current Main source: Rashba Effect Pt/Co/AlO Out-of-plane easy axis Switching driven by an in-plane current Main source: Spin-Hall Effect (systematic experimental study)

Self-oscillator based on Spin-Hall-Effect Slonczewski-like torque

Self-oscillator based on Spin-Hall-Effect dm 1 α = m h 2 EFF m m h 2 γ M dt (1 + α ) (1 + α ) 0 S d αd m m σ + m σ (1 + α ) γ (1 + α ) γ M J J 2 2 0M S 0 S EFF d J = jµ α B S H em d σ Spin-current direction in Pt

Self-oscillator based on Spin-Hall-Effect Slonczewski-like torque Ta/CoFeB/MgO In-plane easy axis Switching driven by an in-plane current Liu et al, PRL 109, 186602 (2012)

Self-oscillator based on Spin-Hall-Effect Improvement of the output power! Dynamics (field applied along 30 with respect to the easy axis) Excitation of a uniform mode Slightly dependence of the oscillation frequency on current Liu et al, PRL 109, 186602 (2012) R. H. Liu, W. L. Lim, and S. Urazhdin, arxiv:1210.2758.

Remain STO challenges microwave emission at zero field large output power synchronization of array of STOs optimization of the geometrical and physical parameters ultralow critical current Conclusions, voltage induced magneto-crystalline anisotropy high oscillation frequency two free layer devices narrow linewidth Synchronization Injection locking microwave emission spin-orbit torque Spin-Hall Rashba Dzyaloshinskii-Moriya

Thank You for Your attention!

SHE effect Spin dependent scattering (Mott)