Spin injection
concept and technology

Ron Jansenャンセン ロン

Spintronics Research Center
National Institute of Advanced Industrial Science and Technology(AIST), Tsukuba, Japan

Spin injection
Transfer of spin angular momentum
from a ferromagnet to a non-ferromagnetic material

Normal metal
Semiconductor
Inorganic / organic
Carbon-based

Non-equilibrium spin polarization

Semiconductors – Electronics
Computing and amplification
Processor
chip

First
transistor

Very powerful technology
Present everywhere in today’s society
Workhorse – silicon field-effect transistor

Magnets – information storage
Magnetization is vector quantity
Preferred axis - binary information

Compass

Magnetic needle

Combining the best of both worlds
Ferromagnets

Semiconductors- Stable memory- Fast switching- High ordering temp.
- Spin transport

Bandgap engineering Carrier density & type Electrical gating Long spin lifetime -- Technology base(magnetic recording)Technology base (electronics)

Semiconductor Spintronics
Can we develop spin-basedtransistors, switches, andlogic circuits ?

How to create, control andpropagate spin informationin semiconductor structures?

Outline
Part I
Introduction and brief overview
Key methods and devices for spin injection

Part IIElectrical spin devices

Part III
Hot topics

Injection
Detection
Transport
Manipulation

Spin relaxation time in Si
Magnitude spin accumulation
Interface states
Doping concentration

Creation of spin polarizationin non-magnetic materials
Injection of spin-polarized charge current- Hot-electron spin filtering- From ferromagnetic tunnel contact

Injection of pure spin current (no charge)- Spin pumping by magnetization dynamics- Spin + thermoelectric effects (spin caloritronics)

Optical injection
Electric field + spin-orbit interaction- Spin Hall effect etc.

Not in this talkFirst electrical spin injection - 1992
current injection across vacuum tunnel barrier
Nickel
STM tip

circularly
polarized
light emission

I

UHV cleaved GaAs(001)
Observation of spin-polarized-electron tunneling from a ferromagnet into GaAs
S.F. Alvarado and P. Renaud, PRL 68, 1387 (1992).
Tunneling potential barrier dependence of electron spin polarization,
S.F. Alvarado, PRL 75, 513 (1995).

Solid state devices for spin injection/detection
Optical detection in spin-LED
FM

Hot-electron
spin filtering

LED
UndopedSilicon3-terminal, single interface

4-terminal, non-local

Source current

Non-local voltage

VSpin
injectorFMSpin

FM detector

Semiconductor
Semiconductor

Key developments since 2007
Spin injection in Si spin-LED (low T)
Jonker & Co. (2007)

2007

Spin transport in Si (hot electrons, low T)
Appelbaum & Co. (2007)

Electrical spin injection & detection
in GaAs (low T). Crowell & Co. (2007)

2008
Electric-field control of spin in Si 2DEG
(without S-O interaction, low T, large B)
Jansen & Co. (2010)2011

Spin injection & detection in Si at 300 K
reproduced. Jonker & Co. (2011)(error in data interpretation)

2010

Non-local spin-valve Si channel, 300 K
TDK/Osaka group (2011)2009

Electrical spin injection & detectionin Si at 300 K (n- and p-type)
Jansen & Co. (2009)

Spin accumulation / transport in n-Ge
Shin & Co. KAIST (2011) &Wang & Co. UCLA (2011).
Spin accumulation in p-Ge/MgO/Fe (low T)
Saito & Co. AIST (2011).

Spin injection into silicon by tunnelingSpin-polarized electrons
Recombination with holesCircularly polarized emission- Si n-i-p LED- top layer 80 nm
n-type Si (21018 cm-3)

Circular light polarization (%)

Optical detection via electroluminescence
T = 50 K, 80 K

Magnetic field (T)Jonker et al., Nature physics 3, 542 (2007), see also
Li & Dery, PRL 105, 037204 (2010) for theory of spin-dependent optical transitions in Si

Electrical spin injection & detection in GaAs
non-local geometry
Source current

Non-local voltage

VSpin
injectorFMSpin

FM detector

GaAs
semiconductor
Semiconductor

Spin-valveAP

T = 50 K

P

Lou et al. Nature Physics 3, 197 (2007)

Hanle

Electrical spin injection & detection in Si
non-local geometry
Source current

Non-local voltage

VSpin
injectorSpin-valve
responseFMSpin

T = 10 K
n-Si (31018 cm-3)

FM detector

SiliconSemiconductor

van ‘t Erve et al. APL 91, 212109 (2007)

Hanle
spin precession

Outline
Part I
Introduction and brief overview
Key methods and devices for spin injection

Part IIElectrical spin devices

Part III
Hot topics

Injection
Detection
Transport
Manipulation

Spin relaxation time in Si
Magnitude spin accumulation
Interface states
Doping concentration

Building blocks for electrical spin devices
Example: the canonical spin-FET

Building blocks:
1) Injection of spin-polarized electrons into semiconductor
2) Transport spin information through the semiconductor3) Detection of spin polarization
4) Control of spin polarization by gate electric fieldCreation of spin polarization
by electrical injection from a ferromagnetic tunnel contact
Transfer of spin informationby spin-polarized tunneling
Creates spin accumulation

 =   - Supply of spins by current- Spin relaxation in SiJs

Conductivity mismatch in direct contact
fundamental obstacle for diffusive spin injection ….

Depolarization of spin current predominantly in ferromagnet
Almost no spin accumulation in semiconductor
 resistivitysd spin-diffusion length

Schmidt et al. PRB 62, R4790 (2000).

Spin injection using hot-electron spin filtering
Hot-electron transport driven
by electron kinetic energy.
No conductivity mismatch !!

semiconductor

ferromagnet

Proposed in 2003 as a way to circumvent the conductivity mismatch
R. Jansen, J. Phys. D 36, R289 (2003).
Experimental demonstration in 2007 by Appelbaum et al. Nature 447, 295 (2007).

Spin transport in silicon using “hot” electrons
undoped Si, low T
Very long spin coherence time
(undoped Si at low T)

Spin signal (pA)

T < 150 K

UndopedSiliconTransverse magnetic field (Oe)
FM 1

FM 2

- not a very practical geometry- small current levels

Appelbaum et al. Nature 447, 295 (2007)& PRL 99, 177209 (2007).

Intermezzo: Spin-polarized tunnelingFe

MgO Tunnel barrier

Majority
spin

Fe

Fe

MgO Tunnel barrier

Minority
spin

Minority spin states of Fe decay much faster into
the MgO barrier:

Tunnel current from the ferromagnet acrossthe thin insulator is highly spin-polarizedFeSpin injection by tunneling

Conductivity mismatchspin-dependent interface resistanceSourceFerromagnete-

Insulator

Semiconductor

Spin current
created at
the interface

Spin injection into a semiconductor
Schottky tunnel contacts
Fe

GaAs
Grown by MBE

Tunneling across the narrow Schottkyenergy barrier between metal (Fe) andsemiconductor (AlGaAs)

Optical spin detection: Hanbicki et al. APL 80,1240 (2002)

Spin injection via Schottky tunnel contact
three-terminal geometry, single contacts

Spinpolarization
detected via
Hanle effectThree-terminal measurement
Probes spins under asingle magnetic contact

n-type GaAs,
Lou et al. PRL 96, 176603 (2006)

Spin manipulation by the Hanle effectPrecession of spins in transverse magnetic field

Hanle
curve

Bz
spin
Transverse magnetic field BZ
 Spin precession
 Suppression of spin
accumulation

Hanle line-width inversely proportional to spin lifetime

Spin polarization in n-type silicon at 300 K
Silicon / Al2O3 / Ni80Fe20
T = 300 K

0.2
Lorentzian fit= 142 psV (mV) =1.2 meV0.1

n = 1.8 1019 cm¯3
100 x 200 m2 contacts
3-terminal geometry

0.0-1600 -8000

B (Oe)

800

1600

S.P. Dash et al.
Nature 462, 491 (2009)

Spin polarization in n-type Si at 300 K
Reproduced with different barrier materials
NiFe / SiO2 / n-Si
Li / Erve / Jonker, Nat. Comm. 2, 245 (2011)

CoFe / MgO / n-Si
Jeon et al. APL 98, 262102 (2011)

Spin polarization in n-type siliconControl experiment with Yb or Au nanolayerV (mV)

0.20

300 K

0.15

P

0%
3
=

0.10

0
=
P

0.05

Standard
Silicon / Al2O3 / Ni80Fe20
Control device
Silicon / Al2O3 / Yb (2 nm) / Ni80Fe20

0.00

V (mV)

0.20

300 K

0.15

P

0%
3
=

0.10

0
=
P

0.05

Control device
Silicon / Al2O3 / Au (3 nm) / Ni80Fe20

Proof that signal is due tospin injection by tunneling

0.00
-1600

Standard
Silicon / Al2O3 / Ni80Fe20

-800

0

B (Oe)

800

1600

Patel et al. JAP 106, 016107 (2009)

Outline
Part I
Introduction and brief overview
Key methods and devices for spin injection

Part IIElectrical spin devices

Part III
Hot topics

Injection
Detection
Transport
Manipulation

Spin relaxation time in Si
Magnitude spin accumulation
Interface states
Doping concentration

Electrical detection of spin polarization
Ferromagnetic tunnel contact - resistance is proportional to 

Hanle signal at constanttunnel current:V = P * /2

p.s.
Applies to non-local detection (I = 0)
and to 3-terminal detection (I  0)

Tunnel spin polarization:

P=G  GG  GSpin detection in two-terminal geometry
criteria for observing magnetoresistance
MR = RP/RAP - 1
Calculation

After Fert and Jaffrès
PRB 64, 184420 (2001)

Cutoff when average timeelectron spends in channel(dwell) becomes > s
Determined by:- spin relaxation time
- channel length- mobility

Spin lifetime and high-frequency operation
Two-terminal MR device
dwell = L2/D  25 ps
Dwell time (s)Magnetoresistance (%)

1E-11

1E-10

1E-9

1E-8

1E-7

n = 1.8 1019 cm¯3
L = 100 nm
Pure diffusion

40
30

100
GHz

10
GHz

470 nm
600 ps

20
10
0
1E-12

 = 19 m
 = 1 s

230 nm
150 ps
1E-11

1E-10

1E-9

1.5 m
6 ns
1E-8

1E-7

2

Contact resistance ( m )dwell = sOutline
Part I
Introduction and brief overview
Key methods and devices for spin injection

Part IIElectrical spin devices

Part III
Hot topics

Injection
Detection
Transport
Manipulation

Spin relaxation time in Si
Magnitude spin accumulation
Interface states
Doping concentration

Spin transport
Diffusion, drift, relaxation and precession of spin
Time evolution of spin density S

spin
precession

spindiffusion

spin
drift

Larmor precession frequency

spin
relaxation

Spin transport
Spin-diffusion and spin-diffusion length
Spin-diffusion length (LSD):
Distance spin polarization can diffuse during spin lifetime

Spin density

Spin lifetime

Diffusion constantDiffusion

Diffusion

Positionp.s.
diffusion constant for spinand charge can be different
see Yu and Flatte,
PRB 66, 235302 (2002).

Spin transport
Measurement of spin-diffusion length – (1) direct
Direct: measure spin signal as a function of separation dbetween 2 FM contacts (multiple devices needed).

T=8K

Suzuki et al.
APEX 4, 023003 (2011)

n-type Si~1019 cm¯3

Sasaki et al.
APL 98, 012508 (2011)

Spin transport
Measurement of spin-diffusion length – (2) indirect
Indirect: Measure the Hanle curve in a transport configuration
Fit with equation for spin-precession/diffusion

Lou et al.
Nat. Physics 3,
197 (2007)

Hanle
non-local devicen-type GaAs

+ integration
over width of
injector anddetector (x1, x2)

Spin transport
Measurement of spin-diffusion length – (2) indirect

Salis et al. PRB 81, 205323 (2010)

n-type GaAs

Spin transport
Measurement of spin-diffusion length – (3) indirect
Indirect:- Measure Hanle curve in 3-terminal device  s
- Measure charge mobility + use Einstein relation  D
Silicon / Al
/ Ni80Fe20 T = 300 K
T=
300
2O
3K
0.2
Lorentzian fit= 142 psV (mV)



 =

0.1

1.2 meV

0.1

0.0- Fast (single device)- Good SNR (large area contact)- Probes spin-diffusion under contact

LSD = 230 nm-1600 -8000

B (Oe)

800

1600

Outline
Part I
Introduction and brief overview
Key methods and devices for spin injection

Part IIElectrical spin devices

Part III
Hot topics

Injection
Detection
Transport
Manipulation

Spin relaxation time in Si
Magnitude spin accumulation
Interface states
Doping concentration

Operation of the spin-FET
Datta and Das

No rotation of spin

Spin rotation by 180 degrees
Large conductanceSmall conductanceOn-state

Off-stateThis gate action requires spin-orbit interaction- Too weak in silicon, organics, graphene etc.
- Doesn’t scale well (precession angle per unit length,
given by spin-orbit strength)Electric field control of spin polarizationin silicon quantum wells- Control spin polarization magnitude- No spin-orbit interaction involved

TMR =I (9T) - I (0T)
I (0T)

R. Jansen et al.
Nature Materials 9, 133 (2010)

Outline
Part I
Introduction and brief overview
Key methods and devices for spin injection

Part IIElectrical spin devices

Part III
Hot topics

Injection
Detection
Transport
Manipulation

Spin relaxation time in Si
Magnitude spin accumulation
Interface states
Doping concentration

Spin lifetime in n-type Si at room temperature
No or very low doping
7 – 10 ns at 300 K
Elliott-Yafet mechanism
Spin-orbit + phonons
heavily doped (1019 cm-3)
1 ns at 300 K (ESR),
Elliott-Yafet mechanism
Spin-orbit + impurity
scattering
Cheng, Wu, Fabian, PRL 104, 016601 (2010).

Spin lifetime in n-type Si from Hanle effectConsistent results for different tunnel barriers
T = 10 K
NiFe / SiO2 / n-Si
Li, van ‘t Erve & Jonker,
Nature Comm. 2, 245 (2011).

CoFe / MgO / n-Si
Jeon et al. APL 98,
262102 (2011).
NiFe / Al2O3 / n-Si
Dash et al. Nature
462, 491 (2009)

Spin lifetime extracted from Hanle effect
3-terminal (3T) and non-local (NL) devices

Extrinsic contributions to spin relaxation
proximity of magnetic tunnel contact

Paramagnetic
interface states
and barrier defects

Spin precession in local magnetostatic
fields arising from roughness

Spin decoherence
in exchange fieldof ferromagnet

Spin relaxation near magnetic tunnel interface

spin-RA product (normalized)role of ferromagnetic electrode

1.0

Hanle
Co
80 ps

Ni80Fe20
142 psNi285 ps0.5

Injected spins feelpresence of theferromagnet !
 Apparent reductionof spin lifetime
Ni80Fe20  0Msat = 0.9 T
Co  0Msat = 1.8 T
Fe  0Msat = 2.2 T

0.0
-2000

-1000

0

B (Oe)

1000

2000

T = 300 K
n-Si = 1.8 1019 cm¯3 (As)Extrinsic contributions to spin relaxation
spin precession in local magnetostatic fields

Inhomogeneous spin precession axis and frequency

Hanle effect and inverted Hanle effect20

Fe electrodeS2

spin-RA product (km )18

Bexternal
Bms

S

16
14
in-plane

12

Bms
Spin precession in Bms
 reduced at Bexternal = 0

10
8
6
4

30 %
Hanle

2
0
-8000-4000 04000 8000

B (Oe)

Bexternal in-plane, parallel tomagnetization and injected
spins, reduces precession
 recovery of 
http://arxiv.org/abs/1101.1691
Poster WP – 52 (Wednesday)Inverted Hanle effect – a generic feature
p-Ge / MgO / Fe, 10 K
Sol. St. Comm. 151, 1159 (2011)n-Si / MgO / CoFe
T = 300 K
APL 98,262102 (2011)

Bias-voltage (mV)

83.8

83.7

83.6

83.5
-0.4

-0.2

0.0

0.2

spin-RA (normalized)

Magnetic field ( T )GaAs / Al2O3 / Co

1.0

Arxiv:1101.1691

0.8
inverted
Hanle

0.6
0.4
0.2

Hanle
0.0
-6

-4

-2

0

B (kOe)2

46n-Ge / Al2O3 / NiFe
T = 10 K
ArXiv:1107.3510

0.4Extracting spin lifetime from Hanle effectBe careful !
Concluded that trendof spin lifetime vs.
doping density is as
expected.
Or……
What if this is not
the spin lifetime,
but just a lower limit ?

NiFe / SiO2 / n-Si
Li, van ‘t Erve & Jonker,
Nature Comm. 2, 245 (2011).

Outline
Part I
Introduction and brief overview
Key methods and devices for spin injection

Part IIElectrical spin devices

Part III
Hot topics

Injection
Detection
Transport
Manipulation

Spin relaxation time in Si
Magnitude spin accumulation
Interface states
Doping concentration

Magnitude of the spin accumulation
spin resistanceJs

Spin current:

Use Einstein relation:

Spin resistance:2

rs =  Lsd
Resistivity

in m3spin-diffusion lengthSpin resistance of semiconductor – large contact

Spin resistance:

rS =

2
 Lsd

WX WY Lsd

Areal spin resistance:(in )

rS =  Lsd

 contact area W X W Y(in m2)

Spin resistance of semiconductor – small contact

Spin resistance:

rS =

2
 Lsd

Lsd WY Lsd

Areal spin resistance:()rS =  Lsd
Large contact

WX
Lsd

(m2)
Geometriccorrection < 1Magnitude of the spin accumulation
Comparison with standard model for spin resistance6

Area spin resistance instandard model
rs =  Lsd ~ 0.01 km2

300K

2

Spin-RA (km )5
4
3

3 mcm> 230 nm21Standard model does
not match with theexperimental data

0-1-1600 -8000

B (Oe)

800

1600

Magnitude of the spin accumulation
Reproduced in different systems
NiFe / SiO2 / n-Si
Li / Erve / Jonker, Nat. Comm. 2, 245 (2011)

Spin-RA ~ 1.2 km2

CoFe / MgO / n-Si
Jeon et al. APL 98, 262102 (2011)

Spin-RA ~ 4 - 5 km2

Also larger than theory: rs =  Lsd ~ 0.01 km2

Size of spin accumulation – experiment vs theory
From: Li / Erve / Jonker, Nature Comm. 2, 245 (2011)

Geometric correction (750 !) cannot be used forlarge area contact with Wx, Wy >> LSD.
Correct value r =  Lsd ~ 10 m2
Theory and experiment not in agreement !

Large spin accumulation - possible explanations
1) Local magnetostatic fields (roughness) Hanle data underestimates s and thus Lsd

2) Inhomogeneous tunnel current density
 real tunnel area << geometric area

3) Two-step tunneling via localized interface states- proposed by Tran et al. - PRL 102, 036601 (2009)- already ruled out in Dash et al. Nature 462, 491 (2009)

Outline
Part I
Introduction and brief overview
Key methods and devices for spin injection

Part IIElectrical spin devices

Part III
Hot topics

Injection
Detection
Transport
Manipulation

Spin relaxation time in Si
Magnitude spin accumulation
Interface states
Doping concentration

Two-step tunneling via localized interface states
Potential source of enhanced spin accumulation
Proposal by Tran et al.
PRL 102, 036601 (2009):
FM

oxide

1

Tunnelbarrier

Semiconductor-

Two-step (sequential) tunneling via
localized interface states-

Spin accumulation in localizedstates can be larger than insemiconductor channel-

Only if bulk bands and interface
states are well separated by a large
resistance (Schottky) barrier

2

Depletion
regionp.s. to date no experimental confirmation of this mechanism in spin injection

Transport and localized states
Two-step tunneling

Direct tunnelingTran et al.
PRL102, 036601(2009)

1

2

Enhancement(specific conditions)
Thermionic emission

Resonant / off-resonant tunneling

Jansen and Min
PRL 99, 246604(2007)

1

2

No enhancement
but reduction

Reduction of
Spin polarization

Two-step tunneling via localized interface states
Basic equations and parameters

 rb

rls Spin resistance of localizedstatesrch Spin resistance ofsemiconductor channel
rb Resistance between localizedstates and semiconductor
Enhancement factor: spin
accumulation in localizedstates (ls) can be larger thanin semiconductor channel (ch)No detrimental effect on spincurrent into semiconductor if
rls is larger than Schottky
barrier resistance rb.

Spin accumulation in interface states
Upper limit to the enhancement set by Schottky barrier

rls
Js

Schottkybarrier
resistance

rch

rb
ch

LS

If there is enhancement ( rLS  rch )then:lsTran et al.


rs =
Js

 rb

PRL102,036601(2009)

Test of spin accumulation in interface states
Schottky barrier reduction by Cs
Schottky barrierB ~ 0.7 - 0.8 eV
Narrow width (heavily doped Si)

no Cs

Strongly reduced Schottky barrierB ~ 0.2 - 0.3 eV
Width reduced correspondinglywith Cs

Cs
Effect of Cs: see R. Jansen et al., PRB 82, 241305(R) (2010).

Ruling out spin accumulation in interface states
Dash et al. Nature 462, 491 (2009)62

Spin-RA (km )

5

300K

172 mVbias

no Cs

1) No significant change with Cs
2) With Cs still large spin-RA in
1-10 km2 range.

4with Cs

3

But with Cs the rb is muchsmaller than this (rb should be
40 m2 at best)

2
1
0
-1

-1600 -8000
B (Oe)

800 1600

Data with Cs cannot be
explained by enhancement
due to surface states

Outline
Part I
Introduction and brief overview
Key methods and devices for spin injection

Part IIElectrical spin devices

Part III
Hot topics

Injection
Detection
Transport
Manipulation

Spin relaxation time in Si
Magnitude spin accumulation
Interface states
Doping concentration

The carrier depletion issue
Metal / Semiconductor contact

Depletion region:
1) Diode – small current– large dwell time

Tunnel barrier

Min et al. Nature Mater 5, 817 (2006)

2) Spin transport  tunneling& thermionic emission

Electron spin injection

Jansen et al. PRL 99, 246604 (2007)

Schottky barrier

B
Conduction bandV3) Two-step tunneling via
interface states
Tran et al. PRL 102, 036601 (2009)

Ferromagnetic
metal

SiliconValence band

1
Depletion region
10 – 1000 nm

2

Reduced depletion in FM/Semiconductor contacts
heavily doped surface region

Fe

GaAs
Grown by MBE

Used by many groups
for GaAs and more recently for Si.

Tuning of the FM/Si tunnel contact resistancewith low work function ferromagnets
108

spin-detection

spin-injection
300 K

2

Current density (A/m )Si/Al2O3/Gd106

Gd 0.7 nm

10sub-nm Gd

Gd 0.8 ~ 5 nm

Ni80Fe20

4

Gd 0.5 nm

Al2O3

10

2

10

0

Gd 0.4 nm
Gd 0.3 nm

Si

Si/Al2O3/Gd/Ni80Fe20
Si/Al2O3/Ni80Fe20

10

-2

-0.4

-0.2

0.0

Voltage (V)

0.2

0.4
B.C. Min et al,
Nature Materials 5, 817 (2006)

Engineering spin tunnel contacts to silicon by Cs
Ni80Fe20
Al2O3Si

Cs exposed
Si surface(< 1 ML)

heavily doped
19-3
1.8 x 10 cm

4

10

4

with Cs

2

10

0

10

no Cs

10
no Cs

2

10

0

with Cs

-2

10

6

10

10

-2

-0.4 -0.2 0.0 0.2 0.4-0.2 -0.1 0.0 0.1 0.2

VSi - VFM (V)VSi - VFM (V)

10

2

6

10

moderately doped
18-3
1.5 x 10 cm

Current density (A/m )2

Current density (A/m )EF-TEM

R. Jansen
et al.,
PRB 82,241305(R)(2010).

Electrical creation of spin polarization in Si
medium donor impurity concentration100V (V)

80

Electrical Hanle
18-3
n = 1.5 x 10 cm (Sb)
T = 300 K, with Cs

60

Without Cs:- Strong diode- No Hanle signal
With Cs:- Schottky barrier
suppressed- Hanle signal !

40
20
0
-20
-1600 -800 0 800 1600
Magnetic field (Oe)Suppression of carrier
depletion enables
spin injection & detection

R. Jansen et al., PRB 82, 241305(R) (2010).

Summary

Exciting progress- In GaAs, recently also in Si & Ge and at 300 K

Basic understanding of spin injection, but- Many aspects poorly or not understood- Theories not yet confirmed experimentally

Outlook

Challenges:- Electric field control without spin-orbit- Device design

Opportunities by new approaches to spin injection- Spin pumping from magnetization dynamics- Thermal spin current (spin caloritronics)