Fundamental concepts of spintronics

Fundamental concepts of
spintronics
Jaroslav Fabian
Institute for Theoretical Physics
University of Regensburg
Stara Lesna, 24. 8. 2008 SFB 689

:outline:
• what is spintronics?
• spin injection
• spin-orbit coupling in solids (next lecture)
• spin devices
• conclusions: challenges
I. Zutic, J. Fabian, and S. Das Sarma,
Spintronics: Fundamentals and applications, Rev. Mod. Phys. 76, 323 (2004)
J. Fabian, A. Matos-Abiague, C. Ertler, P. Stano, and I. Zutic,
Semiconductor spintronics, Acta Phys. Slov, 57, 566 (2007)

what is spintronics?
narrow (device):
electronics with spin
broad:
umbrella for electron spin
phenomena in solids

spintronics drive
technology
fundamental
discoveries

The Nobel Prize in Physics 2007
The Royal Swedish Academy of Sciences has decided to award the
Nobel Prize in Physics for 2007 jointly to
Albert Fert
Unité Mixte de Physique CNRS/THALES,
Université Paris-Sud, Orsay, France
Peter Grünberg
Forschungszentrum Jülich, Germany,
"for the discovery of Giant Magnetoresistance".

Giant MagnetoResistance
P. Grunberg et al. (1988), A. Fert et al. (1988)
small resistance
large resistance
multilayers 30 - 40% at RT

GMR hard disk read heads
From: IBM web site

SPINTRONICS GOALS
spin control of electrical properties
(I-V characteristics)
electrical control of spin
(magnetization)

SPINTRONICS’ 3 REQUIREMENTS
• EFFICIENT SPIN
INJECTION
F
N
• SLOW SPIN
RELAXATION @
SPIN CONTROL
• RELIABLE SPIN
DETECTION
Silsbee-Johnson spin-charge coupling

:(electrical) spin injection:

Johnson-Silsbee spin injection experiment
Silsbee: emf appears in the proximity of a ferromagnetic metal and spinpolarized
nonmagnetic metal (inverse of spin injection)
R. Silsbee, Bull. Mag. Reson. 2, 284 (1980)
M. Johnson and R. H. Silsbee, Phys. Rev. Lett. 55, 1790 (1985).
μ 0
E E
δM
N (E) N (E) N (E) N (E)
spin injection
spin detection

visualizing spin injection
S. A. Crooker et al., JAP, 101,081716 (2007)
S. A. Crooker at al., Science 309, 2191 (2005)

spin injection into silicon
I. Appelbaum et al, Nature 447, 295 (2007)
I. Zutic and J. Fabian, Nature (NW) 447, 269 (2007)

spin injection into graphene
single-layer on a SiO 2 substrate, room temperature
N. Tombros, C. Jozsa, M. Popinciuc, H. T. Jonkman, and B. J. van Wees
Electronic spin transport and spin precession in single graphene layers at room temperature,
Nature 448, 571 (2007)
N. Tombros, S. Tanabe, A. Veligura, C. Jozsa, M. Popinciuc, H. T. Jonkman, and B. J. van Wees
Anisotropic spin relaxation in graphene, arXiv:0802.2892

Zincblende band structure (GaAs)
optical orientation transitions
(a)
S 1/2
E
CB
(b)
P 3/2
P 1/2
Γ6
Γ 8
Γ 7
E g
Δ
so
HH
LH
SO
0 k
m j
σ +
−1/2 1/2
3 1 1 3
σ +
CB
σ− σ−
−3/2 −1/2 1/2 3/2
2 2
−1/2 1/2
SO
HH,LH
From: I. Zutic, J. Fabian, S. Das Sarma, Rev. Mod. Phys. 76, 323 (2004)

:spin relaxation:

:key concepts:
spin relaxation and dephasing
B
Fe
t=0, spin imbalance
t=T 1 , spin balance
impurity
phonon
spin-orbit
coupling

:key concepts:
spin relaxation and dephasing
Bloch eqs

Time-resolved Faraday rotation
Source: web site of Awschalom’s group
ZnCdSe QW

mechanisms of spin relaxation
Elliott-Yafet mechanism
elemental metals and semiconductors
Dyakonov-Perel mechanism
Semiconductors without center of inversion
symmetry
Bir-Aronov-Pikus mechanism
Heavily p-doped semiconductors
Hyperfine interaction
Electrons bound on impurity sites or confined
In quantum dots
J. Fabian, A. Matos-Abiague, C. Ertler, P. Stano, and I. Zutic,
Semiconductor spintronics, Acta Physica Slovaca, 57, 565 (2007)

spin relaxation in bulk n-GaAs
relaxation tim e(ns)
τ
τ
τ
τ
τ
τ
R. I. Dzhioev et al., Phys. Rev. B 66, 245204 (2002)

spin relaxation in bulk n-Si
100
spin relaxation time T 1
[ns]
80
60
40
20
0
7.4 10 14
3.7 10 15
4.5 10 15
7.8 10 15
2.7 10 16
8.0 10 16
0 50 100 150 200 250 300
Temperature [K]
D. Lepine, Phys. Rev. B 6, 436 (1972)
J. Fabian, A. Matos-Abiague, C. Ertler, P. Stano, and I. Zutic, Acta Physica Slovaca, 57, 565 (2007)

:spin devices:
(spin detection)

:semiconductor spintronics devices:
• spin resonant diodes
• spin field-effect transistors
• magnetic semiconductor tunnel junction devices
• magnetic bipolar junction diodes and transistors
• spin optoelectronic devices
• spin galvanics devices
• spin Hall polarizeds
• spin-polarized semiconductor lasers
• spin pumping batteries
• spin-torque devices
• spin quantum computers
• ...

J. Fabian, A. Matos-Abiague, C. Ertler, and P. Stano,
Semiconductor spintronics, Acta Phys. Slov, 57, 566 (2007)

International Technology Roadmap
2004
for Semiconductors:
Emerging Research Logic Devices
RSFQ
1-D
structures
resonant
tunneling SET molecular QCA
spin
transistor
risk
2005, 2006

International Technology Roadmap
2004
for Semiconductors:
Emerging Research Logic Devices
RSFQ
1-D
structures
resonant
tunneling SET molecular QCA
spin
transistor
risk
2007

detour: material case study:
GaMnAs
• 5-15 % Mn
• p-doped (Mn replaces Ga)
• degenerate: p = 10 20 -10 21 /cm 3
• Tc = 170 K
• ferromagnetism and carrier density coupled
• kλ about 3 (localization?)
• impurity or valence band?
• quantum coherence effects observed
GaMnAs, from Jungwirth et al, Rev. Mod. Phys. 78, 809 (2006)

Where does GaMnAs fit?
No good answer yet

magnetic Resonant Tunnel Diodes
A. Slobodskyy et al, Phys. Rev. Lett. 90, 246601 (2003)
C. Ertler and J. Fabian, Appl. Phys. Lett. 89, 193507 (2006)
C. Ertler and J. Fabian, Phys. Rev. B 75 195323 (2007)
ZnSe
ZnSe
BeZnSe
BeZnSe
ZnMnSe
ZnMnSe
b)
ZnSe
ZnSe
Current (0-150 μA)
8% Mn
T=1.3K
0T
3T
6T
B
1.3 K
a)
Voltage (0-0.2 V)
• efficient spin filtering
• spin detection
• fast switching times
• coherence issues
• RT operation?
Current Density (A/cm 2 )
x 105 3
Δ E = 0
Δ E = 5 meV
Δ E = 10 meV
Δ E = 15 meV
2.5
T = 4.2 K
Δ E = 20 meV
Δ E = 25 meV
Δ E = 40 meV
2
out
ΔV 3
1.5
1
100
50
0.5
out
ΔV 0
2 0 10 20 30
z (nm)
0 out
0 ΔV 0.05 0.1 0.15 0.2 1
Voltage (V)
0.25
Energy (meV)

:selfsustained magneto-electric
oscillations in MRTDs:
C. Ertler and J. Fabian, Phys. Rev. Lett. 101, 077202 (2008)
Intrinsic bistability leads to temporal oscillations in
the current, magnetizaion, and particle density
(a)
x 10 15
j max
(b)
20
j (a.u.)
(c)
10
5
I
j min
0
0 10 20 30
Voltage (mV)
x 10 15
II
j tot
Δ (meV)
(d)
15
10
5
I
Δ max
Δ min
0
0 10 20 30
Voltage (mV)
II
n tot
14 x 1011 Time (t*)
j (a.u.)
10
5
0
j ↑
j ↓
50 100 150 200
Time (t*)
n (1/cm 2 )
12
10
8
6
4
2
n ↑
n ↓
50 100 150 200

:nanospintronics:
spin-based quantum information processing
D. Loss and D. P. DiVincenzo, PRA 57, 120 (1998)
• single and few spins manipulation and detection
• spin relaxation and decoherence
• entanglement control (EDAP: Fabian and Hohenester, PRB 72, 201304 (R) 2005)

closing: challenges in spintronics
• room-temperature ferromagnetic semiconductors, n and p type,
identification of mechanisms for ferromagnetic long-range order
• magnetic heterostructures: ferromagnetic quantum wells and quantum
dots
• spin-polarized transport through magnetic interfaces and inhomogeneities,
accurate determination of spin polarization of ferromagnets
• development of silicon (Si, Si:Ge) spintronics:
spin injection, spin relaxation, magnetism (?), quantum dots
• demonstration of semiconductor spin transistors--power gain and
magnetologic:
spin FETs, bipolar spin transistors
• niche devices for GaMnAs or other dilute magnetic semiconductors,
specific functionalities

closing: challenges in spintronics
• control of ferromagnetism by gating or current injection, spin-transfer
torque
• spin dynamics and spin pumping phenomena in spin transport
• control of spin-orbit coupling by gate and doping, interface properties
• single channel devices
• Spin transport in carbon nanotubes, graphene
• spin quantum information processing:
single and few spin manipulation, relaxation and decoherence, spin
entanglement control