1 SPINTEC – URA 2512 Thermally Assisted MRAM How does it work ? Cristian PAPUSOI, Bernard DIENY...

1SPINTEC – URA 2512

Thermally Assisted MRAM

How does it work ?

Cristian PAPUSOI, Bernard DIENYSPINTEC, Grenoble, France

Acknowledgments: O.Redon, A.Astier, (LETI), J.P.Nozières (CROCUS Tech.), S.Cardoso, P.Freitas (INESC-MN Portugal)


Introduction

1993 – 1998

1987 – 1992

EDUCATIONPh. D., Faculty of Physics, “Al.I.Cuza” University, Iasi, Romania

Diploma in Physics, Faculty of Physics, “Al.I.Cuza” University, Iasi, Romania

2006 – 2007

2004 – 2006

2003 – 2004

2000 – 2003

1992 – 2000

2006

ACADEMIC AND PROFESSIONAL EXPERIENCEPostdoctoral Research Fellow, Grenoble, FranceInvestigation of RAM devices with thermal assisted switching

Postdoctoral Research Fellow, Center for Materials for Information Technology (MINT) Center, University of Alabama, Tuscaloosa, USAFabrication and characterization of CPP (Current Perpendicular to the Plane) spin valves

Postdoctoral Research Fellow, RWTH University,2 Physikalisches Institut A, Aachen, GermanyRole of non-magnetic defects inserted in metallic antiferromagnets on exchange bias

Postdoctoral Research Fellow, Information Storage Materials Laboratory, Toyota Technological Institute, Nagoya, Japan Thermal stability and recording performance of hard-disk media

Lecturer, Department of Electricity and Physical Electronics, Faculty of Physics, "Alexandru Ioan Cuza" University, 11 Blvd. Carol I, 700 506 Iasi, Romania

AWARDSOutstanding REU student/postdoc mentor, University of Alabama (11/8/2006)


SPINTEC - location

SPINTEC

MINATEC

LETI


Main topics of research at SPINTEC

GOAL Bridge fundamental research and advanced technology in spin

electronics

Modeling

Analytical models

Micromagnetism

Finite elements

CAD tools

Basicphenome

na

GMR

TMR

Spin transfer

Functionalmaterials

Ferro/Antiferro coupling

Materials with perpendicular magnetization

Nanoparticles

Data storage

Patterned media

GMR/TMR sensors

Recording heads

Thermally assisted recording

Spintronics

MRAM

Microwave oscillators

Magnetic logic gates

Created January 2002 as joint CEA/CNRS laboratory affiliated to MINATEC R&D center

27 permanent staff members, 16 PhD students and 7 post-docs


Why MRAM ?

• Non-Volatility of FLASH

• Density competitive with DRAM

• Speed competitive with SRAM


Why Thermally Assisted MRAM ?

Problems in conventional MRAM

Selectivity –> difficulty in writing a single junction

Scalability –> electromigration in magnetic field lines with decreasing in-plane size

Thermal stability -> reduced life-time of written information

New approaches

1. Thermally assisted MRAM (Spintec Patent + lab. demo) - good thermal stability ensured by exchange coupling of the storage

layer with an Antiferromagnet;- high selectivity;- low power consumption during writing at high temperature.

2. Current induced magnetization switching- linear decrease of power consumption with decreasing junction in-plane area

3. Possibility to integrate 1 and 2


1. TA-MRAM. Definition, structure and principle of operation.

2. Electric characterization

3. Regimes of operation. Power of the electric pulse PHP vs.

junction temperature TAF.

4. Exchange bias as a temperature probe. Electric pulse width

vs. junction temperature TAF.

5. Conclusions

Outline


Thermally Assisted MRAM (TA-MRAM) – principle

Status 0 0

OFF

Writing

ONON

Heating Cooling

OFF

HHww

IHP

Iw

Status 1 1

OFF

R

H

HHexex

PHP = 0Hw = 0

T = Room Temperature

00

R

HHw < 0

T Blocking Temperature

HHww

T = Room Temperature

R

H

HHexex

PHP = 0Hw = 0

11

PHP = 0


Thermally Assisted MRAM (TA-MRAM) – structure

TMR = 30%

Vbias = 40 mV

250

nm

PtMn (20nm)

CoFe (2.5nm)

Ta (50nm)

Ru (0.8 nm)CoFe (3nm)

NiFe (3nm)

Al (20nm)

Ta (90nm)

CoFe (1.5nm)

IrMn (5nm)

AlOX (0.5 nm)

Hard mask(thermal barrier)

Reference layer

Tunnel barrier

Storage layer

Hard mask(thermal barrier)

450 nm


MTJ

PPB

I MR Tester MR Tester HH(I>60 (I>60 A, |H|<1000 Oe)A, |H|<1000 Oe)

Pulse Pulse GeneratorGenerator

((>1.5 ns, U>1.5 ns, UPGPG<3.8 V)<3.8 V)

SwitchSwitch0

1 2

Trigger

OSC

Equivalent electric circuit of the MRAM device (by network analyser)

UAPPL UMTJ

RMTJ

≈ 800 Ω

4.2 Ω 54 pH

54 pF~

ZPG = 50 Ω

2UPG

ZOSC = 50 Ω

No amplitude attenuation (UAPPL=UMTJ) for > 1 ns pulse width

0 50 1000

10

20

30

40

50

UOSC (m

V)

Time (ms)

Thermally Assisted MRAM (TA-MRAM) – electric characterization

UMTJ = 2UPG – (ZPG+ZOSC)I

I = UOSC / ZOSC

PHP = UMTJ I

What do we measure ?


Thermally Assisted MRAM (TA-MRAM) – measurement procedure

Incr

ease

heati

ng p

uls

e p

ow

er

PH

P

Saturate HEX (negative magnetic field pulse -HSET and heating pulse of power PHP, MAX)

Quasi-Static MR cycle (HEX, HC, R)

Attempt to reverse HEX (positive magnetic field pulse HSET and heating pulse of power PHP)

NO

YES

Decr

ease

puls

e w

idth

. Set

PH

P t

o P

HP,m

in

Set pulse width to MAX. Set pulse power PHP to PHP, MIN

10-9 10-8 10-7 10-6 10-5 10-4 10-3 10-2 10-1 1000

1

2

3

4

5

6

WRT P H

P (x

10-3 W

)

(s)

HEX = 0 Oe

WRT

-500 0 500

650

700

750

800

850

SAT

EXH

EX

SAT

H

WRT

EXR

()

H (Oe)

H

How do we measure ?

PHP ()WRT

HEX<HEX ?WRT

Time

PH

P

HSE

T

Magneti

c field

puls

e

Ele

ctri

c puls

e


S

PTT

d

k2

tT

dcdc HPRT

TB

TBTBTBTB

iiii

TRHPRTAF

texp1PTT

Sk2

d

TB

TB

TBTBTB

iiii

TB

TBTR dcdc

k2

d

Thermally Assisted MRAM (TA-MRAM) – operation regimes

TA

F

PH

P

Time

Stationary regime

(TAF=TRT+PHP)

Transient regime

c - specific heat capacity J/(Kg K)k – thermal conductivity W/(K m) - density Kg/m3

d - layer thickness m

Thermal barrier 1 (cTB, TB, dTB)

Magnetic stack (ci, i, di) Thermal barrier

2 (cTB, TB, dTB)

Layer position z

Tem

pera

ture

T

TAF

1-D model of heat diffusion in the MTJ stack


Thermally Assisted MRAM (TA-MRAM) – transient regime

PUMP PROB

Time

PP

UM

P

HS

ET

Mag

neti

c field

p

uls

e

PP

RO

B

Ele

ctri

c p

uls

e

Ele

ctri

c p

uls

e

0 5 10 15 202

3

4

5

6

PHP = 2.8 mW

WRT

PHP = 4.6 mW

WRT

P HP (x

10-3 W

)

(x10-9 s)

= 10 ns

= 3 ns

WRT

0 20 40 60 80 1002

3

4

5

WRTPHP ( = 10 ns)

PHP ( = 3 ns)

WRT

P PROB(x1

0-3 W

)

(x10-9 s)

WRT

TR=2.7 ns

120 MHz maximum

writing frequency

TR=2.7 ns


Thermally Assisted MRAM (TA-MRAM) – stationary regime

20 70 120 170-200

-100

0

100

200

HC

HEXH

EX, H

C (Oe)

TAF (oC)

=105 K/W

- measured as a function of temperature

- measured as a function of PHP and converted into temperature according to

TAF = TRT + PHP

Measurement time per point is 50 s >> TR -> always in stationary regime


Thermally Assisted MRAM (TA-MRAM) – consistency of results

Al

Material kg/m3

c*

J/(K kg)k**

W/(K m)

Ta*** () 16327 144 4.3

PtMn 12479 247 4.9

CoFe 8658 446 37

Ru 12370 239 120

AlOx 3900 900 27

IrMn 10181 316 5.7

NiFe 8694 447 37

Al 2700 904 235

SiO2 2200 730 1.4*for metallic alloys calculated according to Dulong-Petit law

**for metallic alloys calculated according to Widemann-Franz law

***confirmed by electrical resistivity measurements (170 xcm) and XRD scan

TB = 15846 kg/m3

cTB = 154.1 J/(K kg)

kTB = 4.38 W/(K m)

)mJ/(K 0.306dcdc 2TBTBTB

iiii

Theory – 1D model of heat diffusion

)mJ/(K

0.315Sdcdc

2

TRTBTBTBi

iii

Experiment

m)W/(K 53.4S2dk TBTB

( = 105 K/W, TR = 2.7 ns)


= 0.84 x 105 K/W

0 1 2

50

100

150

200

T AF (

o C)

PHP (x10-3 W)

Thermally Assisted MRAM (TA-MRAM) – consistency of results

SiO2

electrodes

junction stack

TR = 2.3 ns

3-D simulations of heat diffusion in the MTJ stack(measured write power is used as input)

Electric pulse

Ta

TaSiO2

231 nm

300 oC

200 oC

100 oC

25 oC


Two temperature regimes of the MTJ evidenced:

transient temperature regime for pulse widths < 9 ns;

stationary temperature regime for longer pulse widths; in this regime, the relationship between the temperature of the storage layer TAF and the power of the electric pulse PHP is

linear: TAF = TRT + 105 (K/W) PHP.

Use of thermal barrier layers reduces the electric power density required for writing but also decreases the writing frequency

The writing power density increases with decreasing pulse width from 0.6 mW for = 1 s up to 6 mW for = 2 ns; even in the range of pulse widths 1 s - 10 ns, where the storage layer reaches the stationary temperature regime by the end of the electric pulse, the writing power shows a 500 % increase.

Thermally Assisted MRAM (TA-MRAM) – conclusions


0 45 90 135 180

(deg)

E/S

JE

JE

2KAF

tAF

< JE

2KAF

tAF

> JE

eb

cosJsintKS/E E2

AFAF

DaJJ INTE 0

FM layer = single domain

AFM layer = collection of non-interacting single domain grains

(+)(-)

D

mAFM

HES

AFINTE JH 0aDm AFAF

AFFINT JJ 0

tiniteq epeptp

1

TkSJeq BEep 211

TkSJeTkSJe BEbBEb eef 0

1

2411 AFAFEAFAFb tKJtKe (+)

(-)

Thermally Assisted MRAM (TA-MRAM) – exchange bias as temperature probe


15 20 25 30 35 40 450.0

0.2

0.4

0.6

0.8

1.0

AF grain size D (nm)T

B/T

N

-200

-100

0

100

200

300

400 = 2 ns

TB (

oC

)

R.White et al, J.Appl.Phys. 92, 4828 (2002)

15 20 25 30 35 40 450.0

0.2

0.4

0.6

0.8

1.0

AF grain size D (nm)

TB/T

N

-200

-100

0

100

200

300

400 = 100 s

tAF

= 4 nm

tAF

= 6 nm

tAF

= 8 nm

tAF

= 10 nm

tAF

= 12 nm

TB (

oC

)

Ni80Fe20/Ir20Mn80 Ferromagnetic/Antiferromagnetic bilayer

TNeel=350 oC, a0=2.7 Å, =0.33JINT = 8.11x10-15 ergKAF = 2.44x105 erg/cm3

Decreasing the pulse width may require a considerable increase of the writing temperature !!!

t

BTT

TB = 50 oC TB = 275 oC



0 50 100 150 200

-200

-100

0

100

HEX

HCH

EX, H

C (O

e)

Temperature (oC)

10 20 30 400

1

2

Prob

abilit

y (a

.u.)

AF grain size (nm)

200 nm


25 100 175 250

-1.0

-0.5

0.0

0.5

1.0

HC

HEX, H

C (a.u.)

TAF (oC)

HEX

=() 1 s() 10-3 s(▲) 10-7 s

10-7 10-6 10-5 10-4 10-3 10-2 10-1

-1.0

-0.5

0.0

0.5

1.0

HEX (a

.u.)

(s)

PHP=() 0.38 mW -> 58 oC() 0.69 mW -> 85 oC(▲) 0.92 mW -> 105 oC(▼) 1.24 mW -> 133 oC(◄) 1.47 mW -> 153 oC(►) 1.7 mW -> 173 oC() 2.24 mW -> 220 oC

TAF = TRT + 8.97 x103 PHP

KAF = 7.3 x 105 erg/cm3

KF = 1.3 x 104 erg/cm3

JINT = 8.5 x 10-15 erg


10-9 10-8 10-7 10-6 10-5 10-4 10-3 10-2 10-1 10050

100

150

200

250

300

AF te

mpe

ratu

re T

AF (

o C)

Pulse width (s)10-9 10-8 10-7 10-6 10-5 10-4 10-3 10-2 10-1 1000

1

2

3

4

5

6

W

Write

pow

er P

HP (x

10-3 W

)

Pulse width (s)

WRT


3-D simulation of heat diffusion – experimental write power PHP () is used as input

WRT

Exchange bias model - temperature required to set HEX = 0 for heating time equal to the pulse duration ; temperature pulse shape is assumed rectangular.

WRT

Exchange bias model - temperature required to set HEX = 0 for heating time equal to the pulse duration ; temperature pulse shape is calculated by 3-D simulation of heat diffusion.

WRT

HEX = 0 OeWRT


ConclusiConclusionon

Writing temperature increases with decreasing pulse width as a consequence of thermal relaxation in the Antiferromagnetic storage layer and approaches the Neel temperature in the limit -> 0.

Exemple: writing with 2 ns pulses imply heating at about 300 oC with possible negative effects on the integrity of tunnel barrier and storage layer antiferromagnet.

Solution: decrease the writing temperature by using antiferromagnets of lower Nèel temperature than IrMn (TN ≈ 350 oC) for pinning the storage layer.

1 SPINTEC – URA 2512 Thermally Assisted MRAM How does it work ? Cristian PAPUSOI, Bernard DIENY...

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