Buhrman_KITP_Spintronics.pdf

7/28/2019 Buhrman_KITP_Spintronics.pdf

1/38

Spin Torque and Magnetic Tunnel

JunctionsEd Myers, Frank Albert, Ilya Krivorotov, Sergey Kiselev, Nathan

Emley, Patrick Braganca, Greg Fuchs, Andrei Garcia,

Ozhan Ozatay, Eric Ryan, J ack Sankey, J ohn Read, Phillip Mather, Dan Ralph J ordan Katine and Daniele Mauri (HGST)


2/38

Outline

Spin torque switching in spin valves

Switching speedsAsymmetry of switching currents (spin torque and spin accumulation)Reducing switching current levels

Non-uniform spin torque systemsSwitching by concentrated spin current injectionVortex spin torque oscillator

Spin torque in magnetic tunnel junctionProbing spin torque as function of tunnel junction bias


3/38

Realizing Spin Transfer Effects

Nanopillar GMRSPIN VALVE

Py (2 nm)

Py (12 nm)Cu (6 nm)

Cu

Cu

free layer

fixed layer

Conventional ferromagnet spin transfer devices require lateral dimensions 250 nm to avoid significant self-field effects from required current levels

Low impedance ~ 0.01 -m2GMR (R/R) ~ 10 -20%

High impedance ~ 1 - 100 -m2GMR (R/R) ~ < 50-90+%(varies with barrier thickness)

Critical current densities quite similar in good spin valves and MTJ sHigh polarization of MTJ s may give a ~ 2x advantage

Nanopillar MAGNETIC TUNNEL JUNCTION

Py (2 nm)

Py (12 nm) AlOx (~0.7 nm

Cu

Cu

free layer

fixed layer

Practical issues for spin-torque switching : speed, switching currents, impedance


4/38

5.1

5.3

5.5

0 600 1200

Magnetic Field [G]

d V / d I [ O h m

]

5.1

5.3

5.5

-1 -0.5 0 0.5 1

Current [mA]

d V / d I [ O h m

]

T = 4.2 KNanopillar Spin-Valve

Py (2 nm)

Py (12 nm)Cu (6 nm)

Cu

Cu

free layer

fixed layer

Spin Transfer Driven Magnetic Reversal

~120 nm

~40 nm


5/38

ChallengesIn standard nanopillar devices, initial direction of spin torque is determined by arandom thermal fluctuation from equilibrium.This leads to a random phase of the

precessional dynamics.

Time-resolved measurementsrequire devices with a non-zeroangle between the free and thefixed layers.

( ) sin~2

=

I mm I m st

fixed layer

M

free layer

m

st

free layer

m

fixed layer

M

st

1)sin(


6/38

Sampling Oscilloscope

Step Generator

dc

+25 dB

Measurements of Spin-Transfer Dynamics

Py (4 nm)

Py (4 nm)Cu (8 nm)

Cu

Cu

free layer

fixed layerIrMn (8 nm)

~ 130 nm

~ 60 nmHEB

HEB = exchange bias field

I. N. Krivorotov et al.Science 307 , 228 (2005).

Exchange biasing of the fixed Py layer at 45 to the easy axis results in a non-zeroinitial angle between magnetic moments of the fixed and free layers. Thisestablishes a well-defined phase for precessional dynamics of the magnet.


7/38

5.9

5.95

6

6.05

6.1

-400 -200 0 200 400 600

Filed (G)

d V / d I ( O h m

)

Happlied

Mfixed

Mfree

0

- data

- Stoner-Wolfarth fit

Equilibrium Configuration of Magnetization

0 ~ 35

( )( )2/cos1

2/cos12

2

0

+

+= R R R

= 0.5; H eb = 1.5 kG

Sample 2


8/38

High Speed Spin Torque Switching

switching time 1 =

0 ~ initial angle betweenmagnetizations

-set by thermalfluctuations ormagnetic pinning

Ic0 is T= 0 critical current

co

0

II

2ln

1 J .Sun, Phys Rev B. 62 , 570 (2000)

Faster reversal requires larger Iswitch

Spin polarized current mustdeliver sufficient spin angularmomentum to nanomagnet toreverse magnetic moment.

Hence ( I I c0)x = constant


9/38

How fast is spin-transfer-driven switching?

SamplingOscilloscope

Step Generator

dc

+25 dB

Switching t ime < 1 ns at high pulse amplitude

Measure time dependent response of nanopillar resistance to step pulse.

I. N. Krivorotov et al.Science 307 , 228 (2005).


10/38

I co+ = e M s Vol [H + H an + 2 M s ] / hg(0) 2 e M s2 Vol / h g(0)

I co- = e M s Vol [H - H an - 2 M s ] / hg( ) 2 e M s2 Vol / h g( )

J co+

2

e M s2

t / h g(0); J co+

2

e M s2

t / h g( )t = nanomagnet thickness, =Gilbert damping parameter, M s = magnetization

H an = shape anisotropy field

Critical Current for Spin Torque Switching

H an

4 Ms

out of plane

demagnetizationfield top view

To reduce J co - reduce t, M s and/or but must maintain nanomagnet stability

This requires U K = M s H an Vol /2 > 50 k BT - ten year bit stability


11/38

IcMs2 (Vol)U0 HanMs(Vol)

Han ~ Ms(t/t0)

U0

MRAM requirement:Bit lifetime ~ 10 years U0 = 1 eV at RTWith heating to 100 C U0 = 1.3 eV

~120 nm

~40 nm

Minimize Ms and sample volumeUse shape anisotropy to maximize H k

thick and elongated

Decreasing Switching Currents

4.5 nm Py : U 0,P-AP =0.85 eV, I c0+ = .42 mAU 0,AP-P =0.73 eV, I c0- = .39 mA

Ic0 = zero-temp critical current. Need I co < 100 ANeed to decrease damping and improve micromagnetics


12/38

Spin torque switching currents of low M s free layers

Pulse-response measurementsPulse Generator

dc

+25 dB

Apply current pulse to device.

Determine if pulse has switcheddevice.

Increase pulse duration untilprobability of switching goes tounity.

Increase current pulse amplitudeand repeat.

0.0 0.5 1.0 1.5 2.0 2.5

0.0

0.2

0.4

0.6

0.8

1.0

S w

i t c h i n g

P r o

b a b i l i t y

Pulse Amplitude (mA)

100 ns30 ns10 ns3 ns1 ns


13/38

Comparison with Single Domain LLG Simulations

0.0

0.2

0.4

0.6

0.8

1.0

S w

i t c h i n g

P r o

b a b i l i t y

0.5 1.0 1.5 2.0 2.5

simulationsdata


1 ns3 ns100 ns

0.0 0.2 0.4 0.6 0.8 1.0

0.2

0.4

0.6

0.8

1.0

1.2

1.4

p-apap-p

I

5 0 %

( m A )

-1 (ns -1 )

Fitting to LLG simulation yields empirical spin-torque function and damping

N.B. Similar AP-P and P-AP switching currents in these devicesBraganca et al. APL 05


14/38

Spin Transfer Torque Function

effect of device geometry on g( ) spin accumulation affects?

0 /2

0.050.1

0.150.2

0.250.3

g

g ( ) Slonczewski 1996

g ( ) Cornell (exp.)

I c, P - AP ~ g ( 0 ) ; I c, AP - A ~ g ( )

( ) ( )mImmmHmm eff +

=

)sin()(

2

m

g edt

d dt

d B

g ( ) Xiao, et al.

See also: Manschot et al ., APL.2004

Barnas et al. PRB 2005

)cos(1)sin(

)(

B A

g +=


15/38

>> sf > sf

Effect of Electrode Structure on Spin Torque

Net electron flow

0.050.1

0.150.2

0.250.3

gold cap g


16/38

F e

M n

>> sf > sf

Effect of Electrode Structure on Spin Torque

Net electron flow

0.050.1

0.150.2

0.250.3

gold cap

Fe-Mn cap

g


17/38

Pulsed Current Experiments

Pt Capped Devices

1 2 3 4 50.0

0.2

0.40.6

0.8

1.0

1 ns pulse data3 ns pulse data10 ns pulse data100 ns pulse datasimulations

S w

i t c h i n g

P r o b a b

i l i t y


A =0.18 =0.037

Standard Configuration

1 2 3 4 5

1 ns pulse data3 ns pulse data10 ns pulse data100 ns pulse datasimulations


A =0.52 =0.047

Inverted Configuration

AP-P switching

Spin pumping enhancement ininverted samples Better spinsinking in extended Cu lead

LLG fit deviation from data at largecurrents microwave oscillations

)cos(1)sin(

)(

B A

g +

=Torque angular dependence

A Torque amplitude from spin currentand spin accumulation

LLG simulations

+=

11

B AP P switch

P AP switch

I

I

=,

,

e-


18/38

0.08-0.130.11-0.230.32-0.330.02-0.19B

0.0470.033-0.0370.033-0.0370.025-0.030

0.45-0.520.18-0.210.12-0.160.25-0.30A

Pt inv.Pt capFe-MncapAu cap

Pt normal Pt inverted

A=0.18B=0.23

A=0.52B=0.13


19/38

30nm hole

150x250nm pillar

Pt30nm

Cu

Py20nm

Cu 8nm

Py 5nm

Cu

Spin-Transfer-Switching by Spatially Non-Uniform Currents

Al 2O 3 3nm

SiO 2 SiO 215-30nm aperture sizes

150nm

A 3nm Al 2O 3 insulating barrier with anano-orifice is inserted into a Cu/Pyspin-valve nanopillar Goal:

Result:

150x250 nm pillar


20/38

Hc~37.5Oe R~253m

150m

J ~ 1.2x10 7 A/cm2AP-PIc- = 4 mA

P-APIc+ = 7.8 mA

100 x 200 nm 2uniform current

Jpillar ~ 4x10 5 A/cm2 Jhole~1.6x10 7 A/cm2

AP-PIc- = 50

P-APIc+ = 180 A

150 x 250 nm 2with 30 nm

aperture

T=4.2K

R = 3

R = 12

The nano-aperture devicerequires much less current toinduce switching than ananopillar with uni formcurrent flow.

Current-induced switchingmay not result in fu ll reversalof the nanomagnet

11.65

11.7

11.75

11.8

11.85

11.9

11.95

-0.5 -0.4 -0.3 -0.2 -0.1 0 0.1 0.2 0.3 0.4 0.5

I(mA)

d V / d I (

)

11.6511.7

11.7511.8

11.8511.9

11.9512

-600 -400 -200 0H(Oe)

R ( )

150m

T=4.2K

Spin-Transfer-Switching by Spatially Non-Uniform Currents


21/38

- 1 10 - 7- 5 10 - 8

0

5 10 - 81 10 - 7

- 5 10- 8

0

5 10 - 8

0

2 10 64 10 6

6 10 6

0- 7

- 5 10 - 80

5 10 - 8

-

3D OOMMF Simulations

The effect of spin torque was modeled using LLG equation with the Slonczweskiterm for each cell. The simulations were performed taking into account theOersted field created by electron flow through a wire.

OOMMF is apublic

software

developed byM.J .Donahueand D.G.

Porter fromNIST


22/38

t=1.13ns t=1.6ns

t=2.3ns

t=3.3ns

t=2.06ns

t=2.5ns

t=3.96ns t=5.9ns

0.5 mA


23/38

Spin Transfer with Magnetic Tunnel Junctions

0.1 1 10 100 100010000

Bad TMR,

Pinholes

Good TMR, too highresistance to do spintransfer.

RA ( m2)

O k f o r

s p

i n t r a n s f e r

Pt 30 nm

Cu 5 nm

CoFeB 2 nmAlO x 7-8

CoFeB 8 nm

Cu 80 nm

Ta 10 nm 147 nm

56 nm

147 nm

56 nm

Challenge: Tunnel barriers with

high TMR that can withstand thecurrents necessary for switching,particularly for fast switching


24/38

Early Demonstrations with AlOx

Minor LoopH = 387 Oe

T= 77K

There is a small TMR measured with DCresistance at switching currents.

Wear-out of barriers a concern due to highcritical currents/voltages

T = 77 K

Switching currents

Huai et. al., APL 84 , 3118 (2004)

Fuchs et. al., APL 85 , 1205 (2004)

20 CoFeB

80 CoFeB

6.5 Al + Oxygen

CoFeB=Co88.2Fe9.8B2


25/38

Anti-alignedfixed layers

Alignedfixed layers

Spins from each fixed layer are in the samedirection more spin torque

Spins from each fixed layer are in oppositedirections almost no spin torque

5 nm CoFe6 nm Cu

4 nm Py~0.8 nm AlO x8 nm CoFe20 nm Ta

Increasing spin torque in MTJs with three

magnetic layers


26/38

P

APAP/P

AP/P

APAP

PP T=77K Anti-aligned fixed layers Aligned fixed layers

Ic,o+ = 0.290.01 mA

Ic,o- = -0.280.01 mAJ c,o /t = (2.90.4) x10 6 A/(cm2-nm), reduced by40% compared to a Py free layer with onefixed layer: 5x10 6 A/(cm2-nm)

(shape and size

not optimized)

G. D. Fuchs et al., Appl. Phys. Lett. 86 , 152509 (2005).

Ohmic heating reduces H c,minimal spin torque

Strong spin torque

Spin Transfer Switching in 3-layer MTJs

Note the similarity of I cs


27/38

Questions regarding spin torque in MTJs

Why does TMR decrease withincreasing bias?

How does bias affect spin-transfer torque?

What is the nature of spinpolarized transport in MgObased MTJs at f inite bias?

Models that describe TMR( V) must also beconsistent with spin torque, Nst /I(I) and I(V)

-0.3V 0 0.3V


28/38

How to measure torque vs. current

A thermally stable free layer can only provide a measure of

the spin-torque at theswitching bias

A thermally unstable free layer can provide a measure of spin-torque continuously as afunction of bias by applying H and

I so as to have opposing effects


29/38

Sample structure

Lacour et al, APL 85 , 4681, (2004)

Bottom pinned SAF nearly cancelsthe dipole field and has a verylarge exchange field (~2 kOe)

Devices are patterned with a 2:1aspect ratio

Have a range of thermal activationbarriers

CoFe = Co 86Fe14Py = Ni91.5Fe8.5

CoFe 1 nm/Py 1.8 nmMgO 0.8 nmCoFe 1.9 nmRu 0.7 nmCoFe 2.2 nm

PtMn 15.4 nm

100 nm

Katine and Mauri - HGST


30/38

Experimental approach

=

ococ

dip

B

ao AP P I

I I H

H H

T k E

Exp,

2

,/

)(11

mLifetime in thermal

activation regime

(I)=Scaling factor to parameterize N st /I variation with I - Spin Transfer Efficiency

E. B. Myers, et al , PRL 89 , 196801 (2002).Z. Li and S. Zhang, PRB 68 , 024404 (2003).I. N. Krivorotov, et al , PRL 93 166603 (2004).

Positions of equal meanlifetimes if the efficiencyis constant with bias

Positions of equal meanlifetimes if efficiency decreaseswith increasing bias

Increasing Current ( I)

M a g n e

t i c

F i e l d ( H )

0


31/38

H(I) data - Linear Response

TMR decreases by over 40%

Hd


32/38

H(I) data - Linear Response

TMR decreases by over 40%

Break in data crystalline anisotropy effect


33/38

Spin Transfer EfficiencyData are consistent with less than a 10% decrease in spin torqueefficiency out to the switching bias point (~ 0.3 V)


34/38

Tunnel Conductance Through MgOs-like

pd-likeNo s-like channels!

s-like decays inthe electrode No s-like channels!

W. H. Butler, X. G. Zhang, T. C. Schulthess, PRB 63 , 054416 (2001).

J . Mathon and A. Umerski, PRB 63 , 220403 (2001).


35/38

MgO DOS Data

-6 -5 -4 -3 -2 -1 0 1 2 3 4 5 6

Fe / 2nm MgO[eb]Fe / 2nm MgO[rf]CoFeB / 2nm MgO[rf] 375C 1 hr

D O S ( A

. U . )

Negative Tip Bias (V)

Negative Tip Bias (V)

D O S ( A

. U . )

Fe / 20 MgO[eb]Fe / 20 MgO[rf]CoFeB / 20 MgO[rf] 375 oC 1hr

5.5 eV

2 eV

STM tunneling spectroscopy evidence for O vacancy defects in MgO barrier layers


36/38

Tunnel Conductance through MgO

Simmons model fit:=1.35 0.05=0.82 0.02* pm

*apm

Magnetic state dependent effective mass (decay length):

Elastic scattering by barrier defectsreduces the TMR

P

AP

(I)~const implies that:

conductance for each spin channel varies withbias at a rate proportional to the zero biasDOS.

electron scattering rate from defects is notstrongly spin dependent!

W. H. Butler, X. G. Zhang, T. C. Schulthess, PRB 63 , 054416 (2001). J . Mathon and A. Umerski, PRB 63 , 220403 (2001).


37/38

Symmetry of Critical Currents

])(1[2

)()( 2

CosV P

V P g

+=

Polarization term

Asymmetry term ispresent to convert

Slonczewskis criticalvoltage (V c ) into acritical current (J c ).

A better approximation:

++

==

CosV TMR

V TMR

V P g

)(2)(

12

)0()(

P 2

calculated from TMR(V)

Polarization term is aconstant function of V,

consistent with our study

Diao et al. , APL 87, 232502, (2005)


38/38

Conclusions ST in MTJ s

Spin-transfer torque per unit current is independent of bias within10% up to 0.35 V (good news for spin-torque driven MRAM)

Measurement brings new information to help understand therelationship between bias and spin-polarized tunnelingResults are inconsistent with:

Free-electron, split-band tunneling modelsMagnon emission models that reduce polarization factors

Results are consistent with calculations due to Butler et al and Mathonet al for transport through ultra-thin MgO tunnel barriers allowing for

defects in non-ideal tunnel barriers.

Fuchs et al ., cond-mat/0510786

Date post:	03-Apr-2018
Category:	Documents
Upload:	gokaran-shukla
View:	214 times
Download:	0 times

Buhrman_KITP_Spintronics.pdf

Documents