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Dublin April 2007 1
Chapter 14: Spin Electronics and Magnetic Recording
1. Spin currents
2. Sensors
3. Memory
4. Logic
5. Spin transistors
6. Magnetic recording
Comments and corrections please: jcoey@tcd.ie
Dublin April 2007 2
Further reading
• Michael Ziese and Martin Thornton (editors), Spin Electronics, Springer, Berlin 2001, 493 pp.A multiauthor volume which treats topics at an introductory level, with some emphasis on oxide spin electronics.
• Uwe Hartmann (editor) Magnetic Multilayers and Giant Magnetoresistance, Springer, Berlin 1999,321pp.Readable articles focussed on magnetic multilayers and giant magnetoresistance.
• Mark Johnson (editor), Magnetoelectronics, Elsevier Amsterdam 2004, 396 pp.Covers magnetoelectronics in a series of articles, from an introduction to chapters on logic, tunelling and biochips.
• Sadamichi Maekawa (editor), Concepts in Spin Electronics, Oxford 2006, 398 pp.A monograph with a focus on theoretical aspects.
• Lawrence Comstock, Introduction to Magnetism and Magnetic Recording, Wiley-Interscience 1999,485 pp.A n extensive and useful introduction for engineers.
• M. L. Plumer, J. van Eck and D. Weller (editors) The Physics of Ultra-high Density Magnetic Recording,Springer, Berlin 1999, 355 pp.A series of articles covering micromagnetic and dynamic aspects of recording with a focus on media.
Dublin April 2007 3
Modern Electronics
Logic; CMOS - Complementary Metal-Oxide Semiconductor.
Uses p and n type silicon, carriers are electrons or holes in FETs. It consumes power only
when switching, and it is scalable.
NAND gate
n-type
p-type
Memory; SRAM - Static Random-Access Memory. 6T Volatile
DRAM - Dynamic Random-Access Memory 1T Volatile, refreshed every few ms.
FLASH - Nonvolatile; limited rewritability
Dublin April 2007 4
Dublin April 2007 5
It also has quantized angular momentum ms! where ms = ±1/2
spin up ! or spin down "
The associated magnetic moment is m = e!/2m = 1 Bohr magneton (µB).
Information can be coded into the ! and " channels
• Manipulate the ! and " electrons independently
• Exploit magnetic and electric fields
Conventional electronics has ignored the spin in the electron:
The electron is a mobile particle with a charge e = -1.6 10-19 C
Dublin April 2007 6
! Pure charge currents; charge flow
!Spin-polarized charge currents charge and angular momentum flow
! Pure spin currents angular momentum flow
Charge is conserved; Spin is not
14.1 Spin Currents
Dublin April 2007 7
Modes of electron transport in solids:
! Ballistic; transport in a conductor with no scattering
! Diffusive; transport in a conductor with multiple scattering
! Tunneling; transport across an insulator or vacuum by chance
Conductors have electrons in extended states: # = eik.r
Insulators have electrons in localised states: # = e-ix/x0
Charge transport
Dublin April 2007 8
Ballistic transportBallistic transport
lead
contact
conductor
L
L << $# = eik.r
Dublin April 2007 9
Diffusive transportDiffusive transport
lead
contact
conductor
L
L >> $
lsd >> $
lsd
# = eik.r
l = (De%sf)1/2
D = (1/3) vF $ l sd = ((1/3) !" 2)1/2
≈ 100
Dublin April 2007 10
TunnellingTunnelling
leadcontact
insulator
t
#
t ! x0
# = e-ix/x0
Dublin April 2007 11
ConductivityConductivity
nm
k-1 0.07
x0 ~0.1
$! 20 $" $"$" 20
$sd 200
Conduction in Cu is by the s electrons. The mean free path $! = $" "20 nm. The spin diffusion length $sd is much longer, " 200 nm
Cu
Ener
gy (
eV)
EFs - electrons
d - electrons
Cu
Ener
gy (
eV)
EFs - electrons
d - electrons
& = 1.7 10-8 'm
& = &0 + &(T) &0 " 10-8 'm %-1
Dublin April 2007 12
Length scalesLength scales
nm
k-1 0.07
x0 ~0.1
$! 5$" 1
$sd 30
Ni
d - electrons
s - electrons
Ener
gy (
eV) EF
Conduction is mainly by the s electrons. The s" electrons are stronglyscattered by the large d" electron density at EF. Hence the mean freepath $! > $". The conductivity ratio (= )!/)" " 5
The spin diffusion length $sd is much longer.
& = 7.0 10-8 'm
Mott two-currentmodel
lsd$
Dublin April 2007 13
Spin-polarised charge transport Spin-polarised charge transport
Source of spin-polarized electrons
Medium with longspin-diffusion length
spin-sensitivedetector
$sd
TWO-TERMINAL DEVICES; MagnetoresistorsB
Ferromagnetic metal; Ferromagnetic metal
NiFe, CoFe NiFe, CoFe
Normal metal; Cu
Dublin April 2007 14
How spin-polarised ?How spin-polarised ?
What is the degree of spin polarization of common ferromagnetic metals?
P can be determined from the calculated density of states, but it usually has to beweighted by the Fermi velocity, or the square of the Fermi velocity.
Values for an amorphous AlOx tunnel barrier are obtained by tunneling intosuperconducting Al. Andreev reflection can be used at a ballistic point contact
51Fe50Co50
48Fe20Ni80
33Ni
45Co
44Fe
P %
J Moodera, G MathonJMMM 200 248
P = (N!v!n - N"v"n)/(N!v!n + N"v"n)
n = 0 for photoemission n =1 for ballistic transport n = 2for diffusive or tunneling transport
P depends on materials combination andbias
IMAlOx
Al
H
Dublin April 2007 15
First-generation spin electronics
First-generation spin electronics has been built on spin-valves – sandwichstructures using GMR or TMR with a pinned layer and a free layer.
These can serve as very sensitive field sensors, or as bistable memory elements
Iaf
Free
pinned
af
I
GMR spin valve planar magnetic tunnel junction
free
pinned
One layer in the sandwich has its magnetization direction pinned by exchangecoupling with an antiferromagnet – exchange bias.
Dublin April 2007 16
GMR spin valve
Iaf
Free
pinned
spin valve
-100 -50 0 50 100
0
2
4
6
8
10
!R
/R%
Field(mT)
Magnitude of the effect " 10 %
5 108 sensorsper year —read heads
5 nm Ta
5 nm Ta
10 nm IrMn
2.9 nm Cu2.5 nm CoFe
1.5 nm CoFe
3.5 nm NiFe
5 nm Ta
5 nm Ta
10 nm IrMn
2.9 nm Cu2.5 nm CoFe
1.5 nm CoFe
3.5 nm NiFe
Dublin April 2007 17
Single MgO Tunnel Junctions
100 200 *
R/R
%
µ0H (mT)
CoFeB 3
/MgO t/
CoFeB 4
Ta5
Ru50
Ta5
NiFe5
IrMn10
CoFe2Ru0.85CoFeB4
MgO2.5
CoFeB3Ta 5
Cu 50
+ Artificialantiferromagnet
Dublin April 2007 18
TMR Spin valves
af
I
planar magnetic tunneljunction
free
pinned
1016 per yearfor MRAM ?
355%
Ikeda 2006
1970 1980 1990 2000 20100
100
200
300
Year
TM
R (
% )
Jullier
14%
(4.2
K) G
eOM
aeka
wa
2.5%
(2.5
K) N
iO
Sue
zaw
a 1%
NiO
Miy
azak
i 2.7
%(R
T)
Miy
azak
i 18%
(RT)
Moo
dera
22%
(RT)
Sou
sa 3
7%(R
T)
Nak
ashi
o 55
% (R
T)
Wan
g 70
% (R
T) C
oFeB
Bow
en27
%(R
T)
MgO
AlOx
Others Par
kin
220%
(R
T)
Yua
sa 1
88%
(R
T)
First-generation devices use a nanolayer of
disordered aluminium oxide as the tunnel
barrier, giving TMR of up to 70% (dark blue).
Crystalline MgO barriers improve the sensitivity
of the device by a factor of three (red),
changing MRAM architecture.SSP Parkin et al, Nature Materials 3, 862
(2004). H. Ohno, J.App. Phys. (2996(.
Dublin April 2007 19
Transmission through an MgO barrier
WH Butler et al Phys Rev B 63 054416 (2001)
•Majority channel
tunneling is dominated
by the transmissionthrough a #1 state
•!1 state decays rapidly
in anti-parallel
configuration
Dublin April 2007 20
Bias-dependence
AlOx tunnel junction; Signal 180 mV
Dublin April 2007 21
14.2 Sensors
>1 billion magnetic sensors of all types are produced every year; half of them for magnetic recording.
also in permanent magnet motors to control electronic commutation (classical MR in semiconductors)
and in proximity sensors.
Dublin April 2007 22
Anisotropic magnetoresistance (AMR)
I
thin film
Discovered by W. Thompson in 1857
& = & 0 + *&cos2,
Magnitude of the effect *&/& < 3% Theeffect is usually positive; &||> &-
Maximum sensitivity d&/d, occurs when ,= 45°. Hence the ’barber-pole’configuration used for devices.
AMR is due to spin-orbit s-d scattering
H
$ M
0 2 4 µ0H(T)
2.5 %
A sensor is most useful if it has a linear response to applied field.
Some sensors are inherently linear; - coil, Hall generator, NMR. Others must be specially prepared.
Dublin April 2007 23
Giant magnetoresistance (GMR) and tunnel magnetoresistance (MR)
Discovered by A. Fert in 1988
MR = Csin2./2
Sensitivity is maximum when . = //2
The bottom layer is pinned by exchangebias. The free layer has a weak easy axisat . = //2.
I
magnetic tunnel junction: tunnel magnetoresistance TMR
.
Easy axis H
Dublin April 2007 24
14.2.1 Noise
Four types of noise; 0 Johnson (thermal) noise
0 Shot noise
0 1/f (flicker) noise
0 Random telegraph noise
Dublin April 2007 25
log f
0 1 2
log SV
-6
-7
Thermal noise
1/f noise
Shot noise
Random telegraph noise
0 Johnson (thermal) noise.
SV(f) = 4kBTR
There are voltage fluctuations with no imposed current:
<V2> = 4kBTR#f
Dublin April 2007 26
0 Shot noise. A non-equilibrium effect associated with electric current
SI(f) = 2eI
There are current fluctuations, first seen in vacuum tubes
Ishot = (2eI#f)1/2
Operating a TMR sensor at a high bias, to increase the signal also increases the noise.
Dublin April 2007 27
0 1/f noise. A ubiquitous and remarkable effect exhibited by many natural and man-made
phenomena - heartbeat (< 0.3 Hz); water level of the Nile; pop music stations
SV(f) = Cf( ( ≈ -1
The power spectral density is
SV(f) = 1Hva/Nef
Hooge constant
1H = 10-3 for pure metals and semiconductors.
It can be as high as 103 in some magnetic films
1/f noise in CrO2
Dublin April 2007 28
0 Random telegraph noise.
Fluctuations between two distinct levels.
The noise presents itself as a broad
peak in the noise spectrum.
Dublin April 2007 29
Noise in a CoFe/AlOx/CoFe MTJ; Currents range from 0 to 36 microamps.
Modulate the signal at ! 10 kHz to avoid the 1/f noise.
1 10 100
Dublin April 2007 30
Magnetic Random Access Memory
400 bit ferrite corehalf-select memory(1965)
bit linesword lin
es
Hy
Hx
Freescale 4MbitMRAM(2006)
14.3 Memory
Dublin April 2007 31
Magnetic Random Access Memory
Stoner-Wohlfarth asteroid
Dublin April 2007 32
Toggle-switching
Hy
Hx
Dublin April 2007 33
Spin transfer torque
Electron current 2
Transverse spin component absorbed
Electron current 2
Torque exerted as electrons cross F2
F1 F2Electron current 3
F1 F2
Backscattered electrons exert torque on F2
L. Berger Phys Rev B 54 9353 (1996) J.Slonczewski JMMM 159 L1 (1996)
Dublin April 2007 34
Spin transfer torque
mB
Torque on a single-domainnanomagnet of moment m
" produce magnetization reversal
" move domain walls
" emit microwaves
damping
spin torque
4m/4t = %m&B - 'm&(m&B)Favourable scaling: Rate of transfer of angularmomentum from electron current 5 /r2j!/e;
change of angular momentum on flipping freelayer is 2m = 2/r2tM
Dublin April 2007 35
Competing memory technology.
PCRAM
Medium
Dublin April 2007 36
Data storage
Magnetic Disk
Optical Disc
Magnetic Tape
Capacity
Price Performance
Decision criteria:# Access time
# Frequency of use
# Concurrent access
# Archive requirements
# Permanent media
# Cost per megabyte
# Capacity
Source: IBM
Storage Hierarchy10000
1000
100
10
1
Source: Beerenberg Bank/Singulus Technology
The Comparison of Storage Media
Costs
(U
SD
/GB
)
1,0E+00 1,0E+01 1,0E+02 1,0E+03 1,0E+04 1,0E+05 1,0E+06Access time (ns)
Flash
SRAM
FRAM
OUM
HD
DVD RAM
DRAM
DR
AM
& C
o.
MR
AM
Semiconductor Memory
MRAM
W Maas, Singulus
Dublin April 2007 37
Vertically stacked memory
Magnetic Race-track Memory ‘Japanese car-park’
!Current pulses move domains along “racetrack” shift register!TMR sensor to read bit pattern!Special current pulse-driven domain wall element to re-write a bit
A novel 3-dimensional spintronicstorage class memory
- The capacity of a hard disk drive but the reliability and performance of solid state
memory - A disruptive technology based on recent
developments in spintronic materials andphysics
S.S.P.Parkin, US patents 6834005, 6898132,
6920062, 7031178
Dublin April 2007 38
M. Johnson, IEEE Trans Magn 36 2758 (2000)
A ferromagnetic element with a square hysteresisloop is an ideal bistable logic and memory element.
M/V
H/I
+Mr
-Mr
V
V+
V-
I I+ I-
B 6 W
t
2deg InAs Hall sensor R= 170'| |-1, RH = CoFe350 'T-1
+++
---
-+
Equivalent surface pole density, M Am/m2
Line of poles $=Mt A.
H = $/2/r = Mt/2/r If r=2t, H = M/4/
If M = 1 MAm-1, H = 80 kAm-1 (100 mT)regardless of scale.
14.4 Logic
Dublin April 2007 39
Logic
Generic logic device
0
1
IR
output
A C B
Inputs A and B set the state of the magnetic layer !(0)or "(1). The state of the element is read out at another
terminal with a current pulse IR which produces avoltage V0 or V1.
A clock pulse is applied at control terminal C. All fourlogic operations AB, A+B, AB, A+B are complete in twoclock cycles (reset/evaluate)
The normalised write current has one of two values Iw (5mT) or Iw
’((10 mT) and either polarity + or -
Nonvolatileswitch
Dublin April 2007 40
Domain wall logic
Logic elements made
from transistors
Interconnect
made from
copper /
aluminium
Data represented by magnetisation
direction.
SPINTRONIC
MEMORY
ELECTRONIC
LOGIC
AND
NAND
NOT
Interconnect made
from permalloy
Magnetic dw
logic elements.
C Allwood, R Cowburn et al. Science 309, 1688(2005)
Dublin April 2007 41
4-element domain wall circuit
B (
mT
)K
err
sig
na
l
Time (sec)
0.250.1250
AND
NOT
Fan Fan
Cross
I
II
III
IV
Dublin April 2007 42
Ultimate computing technology?
non volatile, fast, error resistance, low power, easy to integrate, low cost
MagneticLogic
MTJ“s-signal-stability-switching
Dublin April 2007 43
Perspectives
! System-on-a-chip. Sensing + signal processing.
! Digital signal processing
! Nonvolatile switches 2 programmable gate arrays; ASICs
! Integration of memory and logic
a) MRAM + CMOS
b) Universal magnetoelectronic device – memory and logic, with thepossibility of flipping between them,
Dublin April 2007 44
A new generation?
First-generation spin electronics was based on passive 2-terminal devices –magnetoresistors – for sensors and memory.
CMOS dominates 99 % of the world semiconductor market:
! Circuits have sufficient gain to permit fanout
! Inputs are tolerant of fluctuations
! High signal/noise ratio
! Output isolated from input
! Fast, scaleable and cheap. BUT
! Charge leaks away; memory is volatile and needs refreshing 100 times s-1
! Quiescent power requirement
Dublin April 2007 45
Hall Probe
Magnetic Gradiometer(bridge)
Wheatsone Bridge
2-gate
MOSFET
Tetrode
Multiplier
4 / 4+
Spin transistorsMagnetic switch (MTJ)
Magneto-resistor
Magnetic
Photodiode
Spin
Switch
Spin ElectronicDevices
Transistor
Filter
Amplifier
Photodiode
Varistor
Switch
Resistor
Diode
Classical
Devices
3 / 3+2+2Number of
Terminals\\\\
Spin
Diode
Dublin April 2007 46
Spin diffusion lengths (nm)
>500 >50semimetals
>2000 200semiconductors
305.0 0.9d-band metals
300 30s-band metals
$sd
(nm)
$! $" (nm)
$sd
Dublin April 2007 47
Mobility of semiconductors, semimetals and metals
0.2860Fe3O4
1.4392CrO2Half Metals
16628Ni
121380Co
201044Fe
48-Au
44-CuMetals
180000-Bi
2000-GraphiteSemimetals
10170(GaMn)As
8000-GaAs
30000-InSb
1400-SiSemiconductors
Mobility
(cm2V-1s-1)
Curie Point (K)
Dublin April 2007 48
Magnetic semiconductors
! Curie temperature > 500 K
! Ferromagnetism should be coupled to the carriers
! p or n type conductivity – spin-polarized electrons or holes
! Useful spin diffusion length and mobility
! Magnetoresistance in heterostructures
! Anomalous Hall effect
! Magneto-optic Faraday effect; MCD
Desiderata for a magnetic semiconductor
cb
vb
! "
Dublin April 2007 49
Magnetic semiconductors - overview
EuO Tc=69-180 K
(GaMn)AsTc < 175 K
Spin-split conduction band Spin-split valence band Spin-split impurity band
ZnO:Co Tc
> 400 K
Dublin April 2007 50
$sd
I
F1 N F2
V
Johnson transistor. Metal-base transistor whereconditions at $ and %determine &. Collectoris floating. It samples µ!or µ". No power gain. V
!- V" " nanovolts.
emitter $
%base
&collector
14.5 Spin Transistors
Dublin April 2007 51
Datta Das transistor
Spin-polarized electrons are injected into the channel, made of a two-dimensional electrongas, where $ > L (ballistic transport). They are subject to an electric field on passing under
the gate, which looks like a magnetic field from the viewpoint of the relativistic electron(Rashba effect) E = ev7B/c2. The spin precesses, and by adjusting the electric field, the
electron arrives with its spin parallel (or antiparallel) to the drain. The drain may be abistable magnetic element.
S. Datta and B. Das, Appl. Phys Letters 56 665 (1990)
source drain
gate
L
Dublin April 2007 52
Hot-electron spin transistors
Monsma transistor. Injects hotelectrons via a Schottkybarrier. Different energy-lossprocesses in the GMR baselead to a field-contollableemitter current.
parallel, ! passesantiparallel
Theemitter/collectorcurrent ratio ( is
very small in thesedevices.
Magnetic tunnel transistor
Parkin
Dublin April 2007 53
Spin MOSFET
Similar to ordinary field effect transistor, but with
ferromagnetic source and drain
Why? It combines
1) power amplification (semiconductor)
2) memory (ferromagnets)
Ferromagnet
Tunnel barrier
Silicon
v v
SOI suspended membranedemonstratoroxide
Gate Ferromagnet
Vg
Ferromagnet
Source Drain
J. F. Gregg et al JMMM 175 1 (1997)
Dublin April 2007 54
Bipolar transistor
p-njunctions
I Zuticv
Dublin April 2007 55
Single-electron spin transistor
Dublin April 2007 56
Pure spin currents
Is it possible in principle to separate and mainpulate spin currents independently ofcharge currents?
If so, electronics might avoid resistive losses.
Spin Hall Effect
due to spin-orbit scattering.
I
Kerr effect imageof a 500 x 100micron n-GaAssample at 30 K.
Kato et al Science306 1910 (2004)
I
Spin waves.
Dublin April 2007 57
! 1st generation passive devicesMRAM scaleup
Integrated sensors – magnetic biochips
Magnetically reprogrammable gate arrays
! 2nd generation active devicesComponents with spin or field-dependent power gain
Integration of memory and logic
Dynamic reconfiguration between memory and logic.
! Coming later?Magnetically-generated microwave chip/chip communication
Logic with spin currenta
Magnetic quantum computing
14.6 Prospects
Dublin April 2007 58
Hard disc drives
8 Gbit 1” drive forcameras 160 Gbit 2.5” perpendicular drive for laptops
Spindle motor
Voice-coil actuator
Read-write head
Magnetic medium
14.7 Magnetic Recording
Dublin April 2007 59
Technology Timeline
1950 1960 1970 1980 1990 2000 2010
RAMAC - firsthard-disc drive;inductive head
TMRdiscovered
AMR head
Spin-valvehead (CIP)
TMRhead
AMR discovered(1857)
In-plane perpendicular
GMR discovered….. Spin valve
180002.5” 1160 Gb2005
120024”50x2 40 Mb1955
rpmsizeplatterscaapcityyear
Dublin April 2007 60
A magnetic exponential - Recording
Superparamagnetic Limit
Magnetization blocked whenKV/kT > 40
V > 300 nm3
If record is on 100 grains,medium is 5 nm thick,area/bit is 6 10 -3 µm2 8100Gbit in2. (155 bit µ
m-2) .
1µm2
GMR
TMR
AMR
AMR
perpendicular
1 µm2
Dublin April 2007 61
Scaling
Why does magnetism lend itself to miniaturization ?
m a
A H = (m/4/r3)[2cos,er + sin,e,] HA =2Ma3/4/r3;
If a = 0.1m, r = 2a, M = 1 MAm-1 HA =M/16/ = 20 kAm-1 (~25 mT)
Magnet-generated fields are limited byM. Scale-independent
•A
I
H = I/2/r = 8jr H ~ r
Current-generated fields arelimited by j. Scaling is poor
Dublin April 2007 62
More transistors and magnets are produced in fabs
Than grains of rice are grown in paddy fields
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