spin-torque nano-oscillators trends and challenging¼magelm2013/talks/finocchio...self-oscillator...

Spin-torque nano-oscillators trends and challenging

Domain Microstructure and Dynamics in Magnetic ElementsHeraklion, Crete, April 8 – 11, 2013

Giovanni FinocchioDepartment of Electronic Engineering, Industrial Chemistry and Engineering

University of Messina

N S

extH�

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

Giant magneto-resistance Introduction (1/4)

Spin torque effect

Introduction (2/4)Giant MagnetoResistance

(GMR)

Electronic Transportaffected by

Magnetic State

Fert – Gruenberg (1988)

Spin-Transfer Torque(STT)

Magnetic Stateaffected by

Electronic Transport

Slonczewski – Berger (1996)two sides of the same coin

Introduction (3/4)

eff ext exch ani M TH= + + + +H H H H H H

( )0 3

2 ( , ) ( )B

EffS s

Jd d

dt M dt d M e

µαγ χ = − × + × − × ×

M MM H M M P M M P

J. Slonczweski, J. Magn. Magn. Mat. 159, L1-L6 (1996).

Amp MC+ +H H

Landau-Lifshitz-Gilbert-Slonczewski (LLGS) equation

Equation of motion

Oersted Field

MagnetostaticCoupling

Exchange

iS�

jS�

External Field

N SextH

�

Anisotropy Magnetostatic

Contribution

+ + + +

+ +

- - - - -

- - -dH�

Thermal 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.

Outline

� Introduction



– Soliton mode

– Two free layers



�Conclusions

� Technological point of view

STO very interesting

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.

, , , STO – An overview (1/2)

� Fundamental point of view Technological point of view

– nanoscale size

– broad working temperature

– low dissipation power

– large output power

– ultralow critical current

– microwave emission atzero field

– high oscillation frequency

– narrow linewidth

– …

Fundamental point of view

� compensation of the damping losses

� very rich dynamical behaviour

� smallest self-oscillator known in nature

� excitation of uniform and non-uniform modes

� …

, , , STO – An overview (2/2)

In-plane Out-of-plane

Classification

In-plane polarizer

Out-of-plane free layer

In-plane Out-of-plane

Two in-plane free layers

Two out-of-plane polarizers

Outline

� Introduction



– Soliton mode

– Two free layers



�Conclusions

, , , STO – Effect of the in-planefield

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 � Sub-critical vs Super-critical Hopf bifurcation

Outline

� Introduction



– Soliton mode

– Two free layers



�Conclusions

, , , STO – Solitonmode

� 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.

G. Finocchio, O. Ozatay, et al. Phys. Rev. B 78, 174408 (2008)

Details of device fabrication and measurement technique

O. Ozatay, et al. Appl. Phys. Lett.88, 202502 (2005).

V. L. Pokrovskii, G. V. Uimin. Sov. Phys. JETP Lett. 41, 128-132 (1985).

S. Komineas. Phys. Rev. Lett. 99, 117202 (2007).


� Excitation of a Vortex-antivortex pair!

G. Finocchio, O. Ozatay, et al. Phys. Rev. B 78, 174408 (2008)

V. L. Pokrovskii, G. V. Uimin. Sov. Phys. JETP Lett. 41, 128-132 (1985).

S. Komineas. Phys. Rev. Lett. 99, 117202 (2007).


� 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



– Soliton mode

– Two free layers

– STOwith interface perpendicular anisotropy


�Conclusions

STO - Two free layers

� Zero field, high frequency microwave emission!


( ) ( , )

( ) ( , )

eff Gt

eff GP

d d

d dd d

d d

ατ τ

ατ τ

−

−

= − × + × − = − × + × −

t tt t t t b

b bb b b t b

m mm h m T m m

m mm h m T m m

2

( , )( ) ( )( , )

e

ptBg j

dM

ε εµγ ε ε

× × − × = − × × − ×

t t b t b t pb t

m m m m m m mT m m

m m m m m m m

eff ext exch ani M Oe th MC− − − − − − −= + + + + + +t t t t t t th h h h h h h h

eff ext exch ani M Oe th MC− − − − − − −= + + + + + +b b b b b b bh h h h h h h h

�Spin Torque Effect

�Magnetostatic Contribution

G. Finocchio,et al,Phys. Rev. B 76,174408 (2007).

( , ) ( , )ε ε=p f f pm m m m

COUPLING

20

( , ) e ( , )( ) ( )s pbdMγ ε ε

= − × × − ×

b tb b t b t b p

T m mm m m m m m m

Torque from perpendicular

Polarizers

STO - Two free layers( , )sinε θt bm m

20

21

Bth

s

K TD

V M t

αµ γα

= =′∆ ∆+

H ξ ξ

Thermal fluctuationsGaussian process

�Zero mean�Unit variance

( ) ( )13 3/ 2( , ) 4 1 3 4ε η η

− = − + + + • t b b tm m m m

( , ) 0.4ε =p fm m

( , ) 0.2ε =p fm m

( )zthythxthth HHH ,,, ,,=H

( ) 0=tH kth ,

( ) ( ) ( )ttDtHtH kllthkth ′−=′ δδ,,

W. F. Brown, Jr., Phys. Rev. 130, 1677, 1963.

STO - Two free layers� Phase locking between the dynamics in the two free layer! � Frequency doubling in the GMR signal!

� Excitation of a strong non uniform mode !

� 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

Outline

� Introduction



– Soliton mode

– Two free layers



�Conclusions

, , ,

Electrostatic interaction MgO/CoFeB� surface perpendicular anisotropy

STO - Interface perpendicular anisotropy

The easy axis depends on

thethickness of the CoFeB

A B C

, , ,

To reduce the out-of-plane demagnetizing field


while maintaining the orientation of both the two magnetizations in the film plane

STO properties – demonstrated separately – frequency tunability (field and current)

– nanoscale size

– broad working temperature

– low dissipation power


, , ,

Spin-torque compensates the damping losses.




– microwave emission atzero field


– very interesting non-autonomous behavior


– …

IDEAL� all-in-one STO

, , ,

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. JiangScientific Reports 3, 1426, (2013).

Out-of-plane free layer – in-plane polarizer


, , ,

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. JiangScientific Reports 3, 1426, (2013).

STO properties– nanoscale size

– low dissipation power <60 µW

– large output power > 60nW


– large output power > 60nW

– ultralow critical current <1×10 6 A/cm2

– microwave emission at zero field

Outline

� Introduction



– Soliton mode

– Two free layers



�Conclusions

• Pt/Co/AlO

– Out-of-plane easy axis

– Switchingdrivenby anin-planecurrent

Model (3)

– Switchingdrivenby anin-planecurrent

– 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 experimentalstudy)

Self-oscillator based on Spin-Hall-Effect

Slonczewski-like torqueSlonczewski-like torque


2 20

2 20 0

1

(1 ) (1 )

(1 ) (1 )

S

J J

S S

d

M dt

d d

M M

αγ α α

αα γ α γ

= − × − × ×+ +

− × × + ×+ +

EFF EFFm

m h m m h

m m σ m σ

σ

Spin-current direction in Pt B H

JS

jd

eM d

µ α=

Slonczewski-like torque


• Ta/CoFeB/MgO

– In-plane easy axis

– Switching driven by an in-plane currentLiu et al, PRL 109, 186602 (2012)

�Dynamics (field applied along 30°with respect to the easy axis)

� Improvement of the output power!


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.

Outline

� Introduction



– Soliton mode

– Two free layers



�Conclusions

Remain STO challenges � microwave emission atzero field


synchronization of array of STOs

optimization of the geometrical and physical parameters


voltageinduced magneto-crystalline anisotropy

, , , Conclusions

voltageinduced magneto-crystalline anisotropy


two free layer devices


Synchronization

Injection locking

� microwave emission spin-orbit torque

� Spin-Hall

� Rashba

� Dzyaloshinskii-Moriya

Thank You for Your attention!Thank You for Your attention!

SHE effect

• Spin dependent scattering (Mott)

spin-torque nano-oscillators trends and challenging¼magelm2013/talks/finocchio...self-oscillator...

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