Nano-Spintronics for Very Low Power and
High Performance Logic and Memory
Stu Wolf
University of Virginia
www.virginia.edu/nanostar
Outline
• Spintronics and Nanomagnetics
–
–
–
–
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Spin Dependent Tunneling
Magnetic Memory (MRAM)
Spin Torque Transfer (STT-RAM)
Nano-Oscillators for High Throughput Data
Nanomagnetics
• Reconfigurable Array of Magnetic Automata
Spintronics 1996 - DARPA
Spintronics: New “Degree of Freedom” for Electronics
• Develop non-volatile, radiation
hard memory chip for space,
missile and avionics
applications with the speed of
SRAM (<3ns), the density of
DRAM (up to 4 Gbit), low
power (0.1 - 0.01x), low cost
(0.1x) and is
infinitely cyclable
• Develop high sensitivity, low
cost magnetic sensors for
precision motion and rotation
control for avionics, perimeter
defense and detection of
mines
Slide and
3 buried ordnance
Integrated
Magnetic
Sensor
Spin Dependent Tunneling: MTJ
Juliere (1975)
RAP  RP 2 PP
TMR 
 1 2
RP
1  PP
1 2
N  N
with P 
N  N
IP  N N  N N
1

2

1

2

I AP  N 1 N 2  N 1 N 2
AlOx  TMR~ 70%
MgO  TMR ~800%
Everspin Products
Conventional MRAM (1T-1MTJ)
K uV
k BT
 Thermal Stability Factor
Beyond 65 nm this becomes a problem
As cell size decreases  switching current increases
Spin Torque Transfer (STT)
Absorbed Angular Momentum Torque
I
S  N t 
t
2
2e
S
I

Torque   
t 2 e
Polarizing “fixed”
layer (thick)
Active “free”
layer (thin)
Net change in
S 
per e
Spin polarized current generates torque on magnetization of free layer
As cell size decreases  switching current decreases
Katine et al, Phys. Rev. Lett. 84, (2000) 3149 .
Challenges for STT-RAM
STT-RAM architecture  1MTJ-1T for
maximum device density
Switching current supplied by CMOS
transistor
Switching current density needs to be
lowered to 5×105 A/cm2
Switching energy needs to be reduced
Key Advantages of STT-RAM at 45 nm
[Wolf, Chtchelkanova and Treger, IBM J. Res. & Dev. (2006) 50 101]
Materials engineering for STT-MRAM
Landau-Lifshitz Gilbert (LLG) equation:
Critical current density
Thermal Stability Factor
Δ = HkmV/2kBT
S. Mangin, et. al. , AAPPS Bulletin, 2008. 18: p. 41
Desired properties for free layer in STT-MTJ
a bcc structure with small mismatch with MgO for a large TMR
Reduced magnetization m
Low damping constant α
High Spin polarization P
Perpendicular magnetic anisotropy (PMA)
Materials Synthesis
 Thin film deposition
 Magnetron sputtering system (1 x 10-6
torr)
 Biased Target Ion Beam Deposition
system (BTIBD) (8 x 10-8 torr)
o Allow depositions at low voltage and
process pressure ;
o automated recipe control;
o Co-sputtering up to three targets
simultaneously;
 Post annealing treatments
 Conventional Furnace Annealing (1 x 105 torr; 1 hour)
 Rapid Thermal Annealing (1 x 10-5 torr;
10s~300s)
GMR and TMR structures by BTIBD
 Top and bottom pinned spin-valves with GMR~6%
using BTIBD.
 AlOx-barrier MTJs with TMR~20%.
 MgO-MTJs: (TMR ~ 70%)
Seeding/CoFe(6)/FeMn(10)/CoFe(1.5)/CoFeB(5)/MgO
(1.8)/CoFeB(5)/Capping
MTJ with MgO barrier
Spin valve
7
CoFe/Cu/CoFe
MR (%)
6
5
4
MR =6.1%
3
2
1
0
-300
T=305 K
-200
-100
0
100
200
H (Oe)
MR%
Materials we explored
CoFeCrB
Co2FeAl
MnAl
Lattice
Mismatch
with MgO
Ms
(emu/cc)
Sample
condition
Damping α
PMA
(erg/cc)
3.9%
44~800
(Varied with
Cr/B content)
450˚C
30nm
0.006
No
600˚C
50nm
0.002
No
400˚C
1.36nm
(CFA/MgO)
0.012
Yes
(1.9 x 106)
Cr seeding/
400˚C
10nm
0.033
3.4%
6.7%
1010
521
MgO interface required
Yes
(5.34 x 106)
proper seeding required
Memory Comparison (Qualcomm (2009)
Intermag 2012
Spin Torque Nano-Oscillators
Switching in response to a 10 mA current pulse
1.0
Easy Axis Magnetization
Spin-Current Switched MRAM
I
Tunnel
junction
0.5
High-speed
switching
0.0
-0.5
simulation
-1.0
50 nm
0
Spin Transfer Nano-Oscillators
50
100
150
200
Time (ps)
0.7 T, q = 10o
8 mA 8.5 mA
7 mA
0.4
7.5 mA
Power (pW)
I
Au
NiFe
CoFe
Cu
0.3
data
6.5 mA
0.2
9 mA
6 mA
0.1
5.5 mA
0.0
1 mm
Simulations: OOMMF math.nist.gov/oommf/
9.6
9.7
9.8
Frequency (GHz)
9.9
10.0
Tunable
High Q
oscillator
(2 GHz –
100 GHz)
Summary of Present Status
19
Frequency (GHz)
Field Tunable
30
28 GHz/T
20
10
0
0.0
Current Tunable
18
0.5 GHz/mA
17
16
6
0.2
0.4
0.6
0.8
1.0
10000
•Oscillators are tunable over a
wide range of frequencies via
applied field or current
•Output is narrow band with Q
values > 10,000
•Voltage outputs in the mV regime
8
Current (mA)
10
Narrow Band
Field (T)
Power (nV2/Hz)
Frequency (GHz)
40
8000
6000
f = 17.052 GHz
f = 3.00 MHz
4000
2000
0
17.025
17.050
Frequency (GHz)
17.075
Phase Locking 500 nm Spaced Contacts
(nV)2/Hz
0
3.200E-12
6.400E-12
9.600E-12
1.280E-11
1.600E-11
1.920E-11
2.240E-11
2.560E-11
2.880E-11
3.200E-11
3.520E-11
3.840E-11
4.160E-11
4.480E-11
4.800E-11
5.120E-11
5.440E-11
5.760E-11
6.080E-11
6.400E-11
6.720E-11
7.040E-11
7.360E-11
7.680E-11
8.000E-11
A
14.6
14.4
f (GHz)
Spin valve
3.7
Locked
IA
A
B
500 nm
13.3
14.2
IB
14.0
B
When phase locked
power increases & linewidth decreases
13.8
0
5
10
15
PT  PA  PB  2 PAPB cos( )
IB (mA)
A biased at 11.5 mA; B swept 0 – 15 mA
10
60
2
0
40
20
14.0
14.2
14.4
f (GHz)
14.6
14.8
B
20
A
10
0
0
13.8
30
2
2
A
4
Locked
PSD (nV) /Hz
6
PSD (nV) /Hz
B
Power (pW)
80
2
PSD (nV) /Hz
8
13.8
14.0
14.2
14.4
14.6
14.8
8
6
4
2
0
13.8
f (GHz)
Kaka et al, Nature, Sept. 2005
14.0
14.2
14.4
f (GHz)
14.6
14.8
0
90
180
270
Phase Shift (deg)
Pattern Recognition Using Coupled Nano-Oscillators
a
b
c
(a): An pattern recognition circuit using an array of electrically coupled DMTJ spin-torque
oscillators. The DMTJ creates a harmonic oscillation which is read out by the analyzer layer
(topmost layer). The frequency of oscillation of these oscillator is tuned using the two transistors on
the branch. These transistors modulate the current based the on the level of match between an
input and a reference image.
(b): The parallel connection causes the different oscillators to cohere with each other and converge
on a common frequency of oscillation. In this case, the signals converge to two frequencies
representing high and low match signals.
(c): The time domain signals show that the signals converge two oscillating frequencies--all of
them in-phase with each other.
Proposed Double Junction Structure
Pattern Recognition
Reference
Input
Coherence Time
Signal Amplitude
For exact matches with different
patterns, the frequency and the
output power of the total signal
are nearly the same.
Pattern Recognition
Reference Input
Coherence Time
Signal Amplitude
Frequencies of best match are
shown in dashed box. These can
be filtered using BPF.
Amplitude shows ‘6’ is a better
match than ‘2’.
Pattern Recognition
Reference
Input
Coherence Time
Signal Amplitude
For exact matches with different
patterns, the frequency and the
output power of the total signal
are nearly the same.
Reconfigurable Array of Magnetic Automata
• Objective
– To develop a new, fully reprogrammable,
ultra-low power (10 zeptojoules/operation)
spin logic and memory architecture based on
a Reconfigurable Array of Magnetic Automata
(RAMA) operated at room temperature.
Reconfigurable Array of Magnetic Automata (RAMA)
Top potion of cross-bars
Anti-ferromagnetically ordered array
Bottom potion of cross-bar
anatomy
Piezo electric matrix
+
Magnetic nano-pillar
+
Collossal Magneto-Capacitive
Layer (CMC)
Nanomagnetic array fabricated by E-beam
lithography
47.81 nm
35
30
Z[nm]
25
20
15
10
5
0
0
0.00 nm
0.5
1
1.5
X[µm]
Diameter increases to ~75 nm FWHM and pillars are somewhat
conical
2
29/30
Self-Assembled CFO Pillars
Diblock copolymer line patterns for metallization
45 kg/mol
Domain period D ~ N2/3
slope ~
0.627
34 nm period
nm period, 9 nm
1217
kg/mol
linewidth W lines
16 kg/mol
70 nm
17 nm period
15 nm period
30
BCP patterns
can be scaled to sub-10 nm dimensions, templated into complex
C. Ross MIT
patterns and transferred into metal wires