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IBM Research
Metal Spintronics | Stuart Parkin © 2005 IBM Corporation
Magnetic Race-Track Memory:
Current Induced Domain Wall
Motion!
Stuart ParkinIBM Fellow
IBM Almaden Research CenterSan Jose, California
IBM Research
Metal Spintronics | Stuart Parkin © 2005 IBM Corporation
Two main types of digital data storage
Random access memoryHierarchy of memoriesSRAM- fast but expensiveDRAM- less fast and less expensiveHighly reliable but volatileFlash: non-volatile, less expensive, very slow, limited endurance
Hard disk drivesMassive storageNon-volatileVery cheap Very slowLess reliable!
Digital data storage
IBM Research
Metal Spintronics | Stuart Parkin © 2005 IBM Corporation
MRAM (Magnetic Random Access Memory)
non-volatilehigh performancecheap compared to
other solid state memoriesexpensive compared to
hard disk drives
VAM2
JAMT
MA
M1
V1
CAPC
Challenge: can we build a solid state device with the same cost as a hard disk drive but the performance and reliability of solid state memory?
Magnetic tunnel junction storage elements
IBM-IFX 16 MbitMRAM chip
IBM Research
Metal Spintronics | Stuart Parkin © 2005 IBM Corporation
Emerging memory technologies
Flow
Spin
Quantity FRAM
PCRAM
MRAM
PFRAM SiC Bipolar
PMC
Molecular
Polymer Perovskite
NanoX’tal
3DROM
IBM Research
Metal Spintronics | Stuart Parkin © 2005 IBM Corporation
Storage-Class Memory: Domain-Wall Magnetic Shift RegisterPhilosophy
Want a solid-state memory with no moving parts which is very cheap and of moderate to high performanceMain approaches
Make extremely small cellsRequires significant engineering developmentsCurrent roadmaps suggest that f<30nm will be possible within 5 years, thus
making this approach extremely challengingAccess multiple bits from one set of logic
Similar philosophy used in conventional storage drives and in millipedeHowever we want a solid state memory with no moving parts
Recent developments in magnetic materials makes this approach viable and attractive by storing information in domain walls (spatially varying order parameter in homogeneous material)
Lots of new science: Spin currents and torque, domain wall fringing fields
IBM Research
Metal Spintronics | Stuart Parkin © 2005 IBM Corporation
Vortex and transverse domain wall structures
Increasing width, thickness
Transverse wall
R.D. McMichael and M.J. Donahue, IEEE Trans. Magn. 33, 4167 (1997)
Vortex wall
Magnetostatic energy dominates:(shape anisotropy)
• magnetic moments along the wire• head-to-head domain walls
Micromagnetic simulationsLLG Micromagnetics SimulatorMike ScheinfeinCell size 5 x 5 x 5 nm3
width 250nm thickness 10 nm
width 140nm thickness 5 nm
Larger structures : more complex DW structures (double vortex,…)
IBM Research
Metal Spintronics | Stuart Parkin © 2005 IBM Corporation
∂ ∂≠ =
∂ ∂0, 0t t
θ φCurrent torque on DW
(Magnetic field pressure on DW, )∂ ∂≠ ≠
∂ ∂0, 0t t
θ φ
Massless motion!!
Domain wall motion
From Sadamichi Maekawa
IBM Research
Metal Spintronics | Stuart Parkin © 2005 IBM Corporation
Theory of current-driven domain wall motionAdiabatic vs non adiabatic spin torque
electron spins follows local magnetization (wide wall limit)
electron spins lags behind magnetization (narrow wall limit)
The adiabatic spin-torque is like a damping term (dissipative)the critical current in intrinsic (related to DW properties)no motion occurs below the critical currentturbulent motion occurs above the critical current – high velocity
The non-adiabatic spin-torque is like a magnetic field (precessional)DW velocity is non-zero for ideal wiresthe critical current is related to defects (roughness)
DW velocity vs current
damping α=0.01DW width 50 nmHp=1000 Oe
Spin torque amplitude : u (1 m/s ≡ J=106 A/cm2)Non-adiabatic contribution ~ β u
0
200
400
600
800
1000
0 200 400 600 800 1000
Vel
ocity
(m/s
)
u (m/s)
β=0β=α/5β=αβ=5α
IBM Research
Metal Spintronics | Stuart Parkin © 2005 IBM Corporation
DW motion under field and current
Magnetic fieldH=-200 Oe
Negative currentPositive current
time
time
Field-driven dynamics Current-driven dynamics
1280 x 140 x 15 nm3
Strong damping a=1
Field-driven dynamics : neighboring walls move in opposite directionsCurrent driven dynamics : neighboring walls move in the same direction
H= 0
IBM Research
Metal Spintronics | Stuart Parkin © 2005 IBM Corporation
Magnetic Race-track Memory
Current pulses move domains along “racetrack” shift registerTMR sensor to read bit patternSpecial current pulse-driven element to re-write a bit
A novel three-dimensional spintronic storage class memory The capacity of a hard disk drive butthe reliability and performance of solid state memory- a disruptive technology based on recent developments in spintronicmaterials and physics
Parkin, US patents 6834005, 6898132, 6920062
IBM Research
Metal Spintronics | Stuart Parkin © 2005 IBM Corporation
Magnetic Shift Register Memory
Writing a bit – current pulse on special write element
Parkin, US patents 6834005, 6898132, 6920062
IBM Research
Metal Spintronics | Stuart Parkin © 2005 IBM Corporation
Magnetic Race-Track Memory: Domain-Wall Magnetic Shift Register
Information stored as domain walls in vertical “race track”Reading and writing carried out along bottom of race trackElectronics built under race track using conventional CMOSDomains moved around track using nanosecond long pulses of current
- Data stored in the third dimension in tall columns of magnetic material
- Domains “race” around track for reading and writing
- 10 to 100 times the storage capacity of conventional solid state memory
- Could displace flash memory and hard disk drives for many applications
Alternating layers of two ferromagnetic
materials to pin domain walls
domain wall
IBM Research
Metal Spintronics | Stuart Parkin © 2005 IBM Corporation
Domain wall positions in race-track pinned by “notches” in walls of magnetic columns
Magnetic Shift Register Concept
IBM Research
Metal Spintronics | Stuart Parkin © 2005 IBM Corporation
Magnetic Shift Register Memory
Shift current pushes domains through stack
Write Element Read Element
Nanosecond long current pulses push domain walls around race-trackdue to a spin torque from transfer of spin angular momentum
Writing device
Reading device
IBM Research
Metal Spintronics | Stuart Parkin © 2005 IBM Corporation
Magnetic Shift Register Memory
Magnetic race-tracks can be connected in seriesMany other configurations possible
IBM Research
Metal Spintronics | Stuart Parkin © 2005 IBM Corporation
DRAM trench
Top
Mid
Bottom
DRAM trench: ~10 μm tall0.09 μm wide
IBM Research
Metal Spintronics | Stuart Parkin © 2005 IBM Corporation
Fabrication of race-track- prototype race-tracks under development- trench with notches demonstrated- plating with magnetic material a major challenge
IBM Research
Metal Spintronics | Stuart Parkin © 2005 IBM Corporation
Magnetic fringing field from moving domain wall writes bits
Domains in racetrack
form part of magnetic tunneling junction
Writing bits into race-track
Reading bits in race-track
Current moves domain wall in nearby wire
IBM Research
Metal Spintronics | Stuart Parkin © 2005 IBM Corporation
Writing with Domain Wall Fringing Fields: Simulation
Scheme of the experiment: (a) Small ferromagnetic (Py) element (10 nm thick ellipse) is placed 10 nm above Co stripe (500 nm long, 100 nm wide, and 10 nm thick); (b) Bx component of the domain wall fringing field as a function of x 10 nm above the stripe; (c,d) Domain color maps (top view) illustrate that sweeping a domain wall across the stripe results in the reversal of the Py magnetization: (b) –before, and (c) – after the reversal. The magnetization direction is color coded to the color wheel.
(a)
(b)
(c)
(d)
-50
0
50
B x (m
T)
500 nm
100
nm
10 nm10 nm
(a)
(b)
(c)
(d)
-50
0
50
B x (m
T)
500 nm
100
nm
10 nm10 nm
IBM Research
Metal Spintronics | Stuart Parkin © 2005 IBM Corporation
Micromagnetic Simulations of a Racetrack
Spacing between notches ~ 1.1 μmWire is smooth between the notches (no roughness)Notches are made from SEM images of real wires (slightly different from one another)
-30
-20
-10
0
10
20
30
0 5 10 15
I (m
A)
t (ns)
10.8 μm x 210 nm x 10nm
20mA = 109 A/cm2
+I
2.8ns pulse
4 domain walls, located next to one anotherbipolar pulses, 2.8 ns long
IBM Research
© 2005 IBM CorporationMetal Spintronics | Stuart Parkin
Current induced domain wall motion in magnetic nanowires
Domain Wall (DW) race track memory
pin and depin DWs controllably
Current driven DW motion
Critical current to depin DW– - vs pinning strength– - vs DW structure
Current
IBM Research
© 2005 IBM CorporationMetal Spintronics | Stuart Parkin
Fabrication of magnetic nano-wires
1) Sputter deposition (shadow mask)2) Coarse FIB
Py Py
SiOx
Focused Ion Beam (FIB)
3) Fine FIB
SiOx
Py
Py
Electron beam lithography
Pt
SiOxAu
Py
4 μm
IBM Research
© 2005 IBM CorporationMetal Spintronics | Stuart Parkin
E-beam patterned magnetic nanowire to explore motion of DWs
Pointed end to preventDW nucleation
Pad for DW injectionNotches for DW trapping
gold contacts
4 μm
Wedged injector L-shaped structure
4 μm
IBM Research
© 2005 IBM CorporationMetal Spintronics | Stuart Parkin
Probing DW structure using MFM
2.0µm
Vortex Domain Wall
Tranverse Domain Wall
2.0µm
AFM
MFM
AFM MFM M profile div(M)
Micromagnetic simulationsTopography
Domain Wall
300nm
Magnetic
IBM Research
© 2005 IBM CorporationMetal Spintronics | Stuart Parkin
Structure of domain wall at a single notchStructure of DWs trapped at a notch
Micromagnetic simulations
300nm
AFM MFM M profile div(M)
Vortex wall
Transverse wall
Metastable states: Seven different structures at a given notch
MFM imaging micromag simulations (divM)
• The energetics of a domain wall trapped at a notch is complex
• Many metastable states are observed depending upon the history
• Magnetic states must be well controlled to ensure reproducibility
IBM Research
© 2005 IBM CorporationMetal Spintronics | Stuart Parkin
Energy of metastable DW structures
1.4 10-9
1.6 10-9
-0.4 -0.3 -0.2 -0.1 0 0.1 0.2 0.3 0.4
Ener
gy (e
rg)
DW Position (μm)
vortex counter-clockwise
vortex clockwise vortex clockwise
vortex counter-clockwise
transverse
transverse
transverse
transverse
IBM Research
© 2005 IBM CorporationMetal Spintronics | Stuart Parkin
Patterning permalloy nanowires
Pointed end to assist DW annihilation
Notches for DW trapping (pinning center)
- Ni81Fe19 (thickness 10 nm) blank film deposited on Si substrates- E-beam lithography- Gold/Rhodium contact pads
Au pads
w: 40-300nm
t: 10-40nm
4um
IBM Research
© 2005 IBM CorporationMetal Spintronics | Stuart Parkin
Injecting DW into nanowire
1. Saturate magnetization of nanowire
M
Hsat
IBM Research
© 2005 IBM CorporationMetal Spintronics | Stuart Parkin
Injecting DW into nanowire
2. Apply local magnetic field
- Local field created by passing current through contact wire
-1.5 -1.0 -0.5 0.0 0.5 1.0 1.5Position (um)
Local field (HI)
Total field
MI
HI
IBM Research
© 2005 IBM CorporationMetal Spintronics | Stuart Parkin
M
IHI
Injecting DW into nanowire
2. Apply local and global magnetic field- If the total field is larger than the nucleation field
reversed domain created below the contact
-1.5 -1.0 -0.5 0.0 0.5 1.0 1.5Position (um)
Required nucleation field(~100 to 500 Oe) DW formed!
Hbias
IBM Research
© 2005 IBM CorporationMetal Spintronics | Stuart Parkin
M
Injecting DW into nanowire
3. Apply global magnetic field
- global field (Hbias) applied by Helmholtz coil
reversed domain expands and DW propagates under Hbias
Hbias
IBM Research
© 2005 IBM CorporationMetal Spintronics | Stuart Parkin
M
Injecting DW into nanowire
3. Apply global magnetic field
if Hbias is moderately small (i.e. smaller than the pinning field of the notch)DW pinned at the notch
Controlled nucleation and pinning of DW
IBM Research
© 2005 IBM CorporationMetal Spintronics | Stuart Parkin
Probing the existence of DW in the nanowire
- Anisotropic Magneto-Resistance (AMR)
High resistance
Low resistance
R
- Resistance difference is proportional to the volume of transverse magnetizatoin
ΔR
IBM Research
© 2005 IBM CorporationMetal Spintronics | Stuart Parkin
-0.35 -0.30 -0.25 -0.20 -0.15 -0.100
50
100
Cou
nts
ΔR (Ohms)
0 50 100 150-0.3
-0.2
-0.1
0.0
687.3
687.4
687.5
687.6
ΔR (O
hms)
Number Experiment
Resistance (O
hms)
Successive resistance measurement of DW states
Hsat
HbiasI
- meta-stable states at the notch
Hbias
IBM Research
© 2005 IBM CorporationMetal Spintronics | Stuart Parkin
-0.30 -0.25 -0.20 -0.15
0
2
4
6
8
1042.9
31.3
19.8
8.4
6.1
5.0
0.8
3.9
2.8
Cou
nts
(a. u
.)
R(DW)-R(sat)
1.7
Meta-stable states at the notch
Hbias
Vortex wall
-0.23 ~ -0.25
Transverse wall
-0.16 ~ -0.21
t=10nm, w=200nm, notch depth=0.4w
DW at notch
DW in wire
Can create different DW states
R(DW)-R(Sat) calculated from simulation
IBM Research
© 2005 IBM CorporationMetal Spintronics | Stuart Parkin
Depinning DW from a notch with magnetic field
M
Notch pinning potentialDW particle in a well
IBM Research
© 2005 IBM CorporationMetal Spintronics | Stuart Parkin
Depinning from a notch with magnetic field
0.0
0.2
0.4
0.6
0.8
1.0
0 20 40 60 80
H (Oe)
Depinning P
robabilityH=0
IBM Research
© 2005 IBM CorporationMetal Spintronics | Stuart Parkin
Depinning from a notch with magnetic field
0.0
0.2
0.4
0.6
0.8
1.0
0 20 40 60 80
H (Oe)
Depinning P
robabilityH=20
IBM Research
© 2005 IBM CorporationMetal Spintronics | Stuart Parkin
Depinning from a notch with magnetic field
0.0
0.2
0.4
0.6
0.8
1.0
0 20 40 60 80
H (Oe)
Depinning P
robabilityH=40
IBM Research
© 2005 IBM CorporationMetal Spintronics | Stuart Parkin
Depinning from a notch with magnetic field
0.0
0.2
0.4
0.6
0.8
1.0
0 20 40 60 80
H (Oe)
Depinning P
robabilityH=43
IBM Research
© 2005 IBM CorporationMetal Spintronics | Stuart Parkin
Depinning from a notch with magnetic field
0.0
0.2
0.4
0.6
0.8
1.0
0 20 40 60 80
H (Oe)
Depinning P
robability
Hdepin=45 Oe
H=80
IBM Research
© 2005 IBM CorporationMetal Spintronics | Stuart Parkin
Depinning from a notch with magnetic field
0.0
0.2
0.4
0.6
0.8
1.0
0 20 40 60 80
H (Oe)
Depinning P
robabilityVortex wall
45 Oe
IBM Research
© 2005 IBM CorporationMetal Spintronics | Stuart Parkin
0.0
0.2
0.4
0.6
0.8
1.0
0 20 40 60 80
H (Oe)
Depinning P
robability
Depinning from a notch with magnetic field
Vortex wall
Transverse wall
45 Oe
62 Oe
Same notch
different pinning potential for different wall structures
IBM Research
© 2005 IBM CorporationMetal Spintronics | Stuart Parkin
Depinning DW from a notch with current
Trap DW at the notch
(Apply magnetic field)
Pass voltage pulse
Study the presence of DW by AMR
R
IBM Research
© 2005 IBM CorporationMetal Spintronics | Stuart Parkin
Depinning DW from a notch with current
Trap DW at the notch
(Apply magnetic field)
Pass voltage pulse
Study the presence of DW by AMR
H
IBM Research
© 2005 IBM CorporationMetal Spintronics | Stuart Parkin
Depinning DW from a notch with current
Voltage pulse
Trap DW at the notch
(Apply magnetic field)
Pass voltage pulse (~nanoseconds)
Study the presence of DW by AMR
H
IBM Research
© 2005 IBM CorporationMetal Spintronics | Stuart Parkin
Depinning DW from a notch with current
Trap DW at the notch
(Apply magnetic field)
Pass voltage pulse
Study the presence of DW by AMR
R
IBM Research
© 2005 IBM CorporationMetal Spintronics | Stuart Parkin
0 20 40 60 800
2
4
6
8S2245-w3c2 J83, 030805, 4ns, L2
J C (A
/cm
2 ) x10
8
Field (Oe)
Depinning DW from a notch with current
Vortex wall
tpulse=4ns
- Fit to an analytical model
(LLG equation based)
IBM Research
© 2005 IBM CorporationMetal Spintronics | Stuart Parkin
0 20 40 60 800
2
4
6
8S2245-w3c2 J83, 030805, 4ns, L2
J C (A
/cm
2 ) x10
8
Field (Oe)
Depinning DW from a notch with current
Vortex wall
Transverse wall
5.2 x 108 (A/cm2)
tpulse=4ns
IBM Research
© 2005 IBM CorporationMetal Spintronics | Stuart Parkin
Joule heating
Vin
oscillo
Vout
R
Time-resolved resistance measurement
0 20 40 60 80 1001.0
1.5
2.0
2.5
3.54V 3.15V 2.51V 1.99V 1.58V 1.00V 0.50V
050705
R(t)
/R(0
)
Time (ns)
S2245-w1c5 J41 short GSGB2
0 1 2 3 4 51.0
1.5
2.0
2.5 SiOx sub Si sub
R(V
)/R(0
)
Voltage (V)
S2245-w3 300nm/w4c3 300nm
@ 10ns
IBM Research
© 2005 IBM CorporationMetal Spintronics | Stuart Parkin
Depinning DW from a notch with current
Transverse wallVortex wall
0 20 40 60 800
2
4
6
8S2245-w3c2 J83, 030805, 4ns, L2
J C (A
/cm
2 ) x10
8
Field (Oe)
Without heating considered (Resistance at static level, low R high Jc)
5.2 x 108 (A/cm2)
tpulse=4ns
IBM Research
© 2005 IBM CorporationMetal Spintronics | Stuart Parkin
0 20 40 60 800
2
4
6
8S2245-w3c2 J83, 030805, 4ns, L2
J C (A
/cm
2 ) x10
8
Field (Oe)
Depinning DW from a notch with current
Transverse wallVortex wall
With heating considered (Resistance during pulse, high R low Jc)
tpulse=4ns
3.1 x 108 (A/cm2)
IBM Research
© 2005 IBM CorporationMetal Spintronics | Stuart Parkin
0 20 40 60 800
2
4
6
8S2245-w3c2 J83, 030805, 4ns, L2
J C (A
/cm
2 ) x10
8
Field (Oe)
Depinning DW from a notch with current
Transverse wallVortex wall
During heating
3.1 x 108
0 20 40 60 800
2
4
6
8S2245-w3c2 J83, 030805, 4ns, L2
J C (A
/cm
2 ) x10
8
Field (Oe)
5.2 x 108
Before heating (t<1ns)
JC for DW depinning before heating starts JC for DW depinning during or after heating
Same JC for the two states at low field
IBM Research
© 2005 IBM CorporationMetal Spintronics | Stuart Parkin
Critical current vs pinning potential
0 20 40 60 80 100 120012345678
100nm 200nm 300nm
J C (A
/cm
2 ) x10
8
Pinning Field (Oe)
pulse duration 4ns
Theory1D model: Luc Thomas, Yaroslaw BazaliyG. Tatara et. al., Phys. Rev. Lett., 92, (2004) 086601-1
JC is independent of pinning potential depthas long as VPIN < 107 erg/cm3
0.0 0.1 0.2 0.3 0.4 0.5
0
20
40
60
80
100
120
0
2
4
6
8
wire width (nm) 300 200 100
Dep
inni
ng F
ield
(Oe)
Notch depth/wire width
Potential (erg/cm
3) x104
Atomic point contact type constraint
IBM Research
Spintronics | Domain wall motion © 2005 IBM Corporation
x
yzΨ
q
Depinning of a domain wall : theoretical description
0
40
80
120
160
0 20 40 60 80
u c (m
/s)
H (Oe)
Calculatedα=0.01
One dimensional model for DW motion:
Discovery of two regimes for current-driven de-pinning - field-like behavior:
critical current depends upon field and pinning strength- current-driven depinning:
critical current essentially independent on field and pinning strengthExcellent agreement with experiments
Critical current vs magnetic field
Analytical:Field-like regime
Analytical:current-like regime
DW is described by two dynamic variables- position- momentum (tilt away from equilibrium direction)
IBM Research
© 2005 IBM CorporationMetal Spintronics | Stuart Parkin
Seven consecutive identical current pulses in the MFM-Pulse: I=7mA J ~ 1 108 A/cm2 / 5 to 10 ms
e-
Domain Wall structure is modified by current pulses - both transverse and vortex walls are observed
Current induced change in DW structure
IBM Research
Metal Spintronics | Stuart Parkin © 2005 IBM Corporation
Current induced Domain wall motion - summaryOscillatory depinning• DW motion driven by current pulses
Complex dependence upon pulse length and amplitudeOscillatory dependence upon pulse length Period between 3.5 and 7 ns
• ModelOscillatory motion of the DW within a pinning potential Oscillatory depinning reproduced - both with 1D model and micromagnetics
for both vortex and transverse wallsDepinning occurs when the pulse length in in sync with the DW oscillationsDW inertia : motion after the end of the pulse
Notch potential dependenceField driven DW depinning depends on DW statesCurrent driven DW depinning
weak dependence on DW statesweak dependence on pinning potential
important for applications
IBM Research
Metal Spintronics | Stuart Parkin © 2005 IBM Corporation
Spintronics– new sensor and memory devices based on manipulating spin
polarized current spin-valve and MTJ devices
Hard disk drives>400 fold increase in storage capacity
Magnetic Random Access Memorypromises a solid state memory which is non-volatile, high
performance and cheap
Magnetic race-track memorypromises a novel data storage device with the capacity and
cost of a hard disk drive but with the performance and reliability of solid state memory
IBM Research
Metal Spintronics | Stuart Parkin © 2005 IBM Corporation
Hype cycle for emerging technologies!
Source: Gerstner, August 2005
IBM Research
Metal Spintronics | Stuart Parkin © 2005 IBM Corporation
SpinAps TeamMasamitsu Hayashi, Luc Thomas, Rai Moriya, Charles Rettner