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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)
2SPINTEC – URA 2512
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)
3SPINTEC – URA 2512
SPINTEC - location
SPINTEC
MINATEC
LETI
4SPINTEC – URA 2512
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
5SPINTEC – URA 2512
Why MRAM ?
• Non-Volatility of FLASH
• Density competitive with DRAM
• Speed competitive with SRAM
6SPINTEC – URA 2512
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
7SPINTEC – URA 2512
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
8SPINTEC – URA 2512
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
9SPINTEC – URA 2512
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
10SPINTEC – URA 2512
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 ?
11SPINTEC – URA 2512
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
12SPINTEC – URA 2512
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
13SPINTEC – URA 2512
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
14SPINTEC – URA 2512
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
15SPINTEC – URA 2512
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)
16SPINTEC – URA 2512
= 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
17SPINTEC – URA 2512
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
18SPINTEC – URA 2512
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
19SPINTEC – URA 2512
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
Thermally Assisted MRAM (TA-MRAM) – exchange bias as temperature probe
20SPINTEC – URA 2512
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
Thermally Assisted MRAM (TA-MRAM) – exchange bias as temperature probe
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
21SPINTEC – URA 2512
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
Thermally Assisted MRAM (TA-MRAM) – exchange bias as temperature probe
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
22SPINTEC – URA 2512
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.