Chapter 14: Spin Electronics and Magnetic RecordingDublin April 2007 1 Chapter 14: Spin Electronics...

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: [email protected]

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

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! 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

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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

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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


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


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$


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


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


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.


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


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


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(.


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


Bias-dependence

AlOx tunnel junction; Signal 180 mV


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.


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.


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


14.2.1 Noise

Four types of noise; 0 Johnson (thermal) noise

0 Shot noise

0 1/f (flicker) noise

0 Random telegraph noise


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


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.


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


0 Random telegraph noise.

Fluctuations between two distinct levels.

The noise presents itself as a broad

peak in the noise spectrum.


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


Magnetic Random Access Memory

400 bit ferrite corehalf-select memory(1965)

bit linesword lin

es

Hy

Hx

Freescale 4MbitMRAM(2006)

14.3 Memory


Magnetic Random Access Memory

Stoner-Wohlfarth asteroid


Toggle-switching

Hy

Hx


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)


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


Competing memory technology.

PCRAM

Medium


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


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


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


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


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)


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


Ultimate computing technology?

non volatile, fast, error resistance, low power, easy to integrate, low cost

MagneticLogic

MTJ“s-signal-stability-switching


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,


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


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


Spin diffusion lengths (nm)

>500 >50semimetals

>2000 200semiconductors

305.0 0.9d-band metals

300 30s-band metals

$sd

(nm)

$! $" (nm)

$sd


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)


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

! "


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


$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


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


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


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)


Bipolar transistor

p-njunctions

I Zuticv


Single-electron spin transistor


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.


! 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


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


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


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


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


More transistors and magnets are produced in fabs

Than grains of rice are grown in paddy fields

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