Chapter 14: Electronics and Magnetic Recording

1. Spin currents 2. Sensors 3. Memory 4. Logic 5. Spin transistors 6. Magnetic recording

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Dublin April 2007 1 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 , 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 2 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.

p-type

NAND gate

n-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 3 Dublin April 2007 4 Conventional electronics has ignored the spin in the electron :

The electron is a mobile particle with a charge e = -1.6 10-19 C

! It also has quantized angular momentum ms where m s = ±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

Dublin April 2007 5 14.1 Spin Currents

! 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

Dublin April 2007 6 Charge transport

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/x 0

Dublin April 2007 7 BallisticBallistic transporttransport

lead contact

conductor L

# = eik.r L << $

Dublin April 2007 8 DiffusiveDiffusive transporttransport

1/2 l = (De%sf) D = (1/3) v l = ((1/3) !" 2)1/2 lead F $ sd ≈ 100 contact

lsd

conductor L

L >> $ # = eik.r lsd >> $

Dublin April 2007 9 TunnellingTunnelling

insulator lead contact

#

t -ix/x # = e 0 ! t x0

Dublin April 2007 10 ConductivityConductivity

nm

-1 k 0.07 s - electrons Cu s - electrons EF Cu EF x0 ~0.1

$! 20 $" $"

Energy (eV) Energy d - electrons d - electrons $" 20 (eV) Energy & = 1.7 10-8 'm $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

" -8 -1 & = &0 + &(T) &0 10 'm %

Dublin April 2007 11 LengthLength scalesscales $ lsd nm k-1 0.07 Ni s - electrons EF x0 ~0.1

d - electrons $! 5 (eV) Energy $ 1 " & = 7.0 10-8 'm

$sd 30 Conduction is mainly by the s electrons. The s" electrons are strongly " scattered by the large d electron density at E F. Hence the mean free path > . The conductivity ratio = / " 5 Mott two-current $! $" ( )! )" model The spin diffusion length $sd is much longer.

Dublin April 2007 12 Spin-polarisedSpin-polarised chargecharge transporttransport

B TWO-TERMINAL DEVICES; Magnetoresistors Source of spin- Medium with long spin-sensitive polarized electrons spin-diffusion length detector $sd

Normal metal; Cu Ferromagnetic metal; Ferromagnetic metal NiFe, CoFe NiFe, CoFe

Dublin April 2007 13 HowHow spin-polarisedspin-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 be weighted by the Fermi velocity, or the square of the Fermi velocity.

Values for an amorphous AlO x tunnel barrier are obtained by tunneling into superconducting Al. Andreev reflection can be used at a ballistic point contact

P % P = (N!v!n - N"v"n)/(N!v!n + N"v"n) I Fe 44 M Co 45 AlO x n = 0 for photoemission n = Al Ni 33 1 for ballistic transport n = 2 H for diffusive or tunneling transport Fe20Ni80 48 J Moodera, G Mathon Fe Co 51 P depends on materials combination and JMMM 200 248 50 50 bias Dublin April 2007 14 First-generation spin electronics

First-generation spin electronics has been built on spin-valves – sandwich structures 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

Free I pinned free

pinned af I af GMR spin valve planar magnetic tunnel junction

One layer in the sandwich has its magnetization direction pinned by exchange coupling with an antiferromagnet – .

Dublin April 2007 15 GMR spin valve

10 Free 5 108 sensors pinned 8 per year — read heads af I 6

spin valve R/R% ! 4

5 nm Ta 5 nm Ta 2 3.5 nm NiFe 10 nm IrMn 1.5 nm CoFe 2.9 nm Cu 2.5 nm CoFe 2.5 nm CoFe 2.9 nm Cu 0 1.5 nm CoFe -100 -50 0 50 100 10 nm IrMn 3.5 nm NiFe Field(mT) 5 nm Ta 5 nm Ta Magnitude of the effect " 10 %

Dublin April 2007 16 Single MgO Tunnel Junctions

Cu 50 R/R% Ta 5 * CoFeB 3 CoFeB3 /MgO t/ MgO2.5 CoFeB4 CoFeB 4 Artificial Ru0.85 antiferromagnet CoFe2 + 200 100 IrMn10 NiFe5 Ta5 Ru50

Ta5 µ0H (mT)

Dublin April 2007 17 TMR Spin valves

355% Ikeda 2006 300 (RT) I % 220 n free Others Parki AlOx pinned af MgO

200 RT) planar magnetic tunnel ( junction

uasa 188% Y FeB o 16 C ) ) 10 per year TMR ( % ) ) % (RT) O NiO T ) 55 for MRAM ? 100 e 2%(RT 0% (RT G K) 2 o 5 7 shi iO ng .7%(RT) dera N 2 i 18%(R Wa % ki oo ak Naka First-generation devices use a nanolayer of 2.5%(2. 1 M 14%(4.2K) a ousa 37%(RT S iyaza Miyaz disordered aluminium oxide as the tunnel lier M ul uezawa J aekaw barrier, giving TMR of up to 70% (dark blue). M S (RT) Bowen Crystalline MgO barriers improve the sensitivity 0 27% of the device by a factor of three (red), 1970 1980 1990 2000 2010 changing MRAM architecture. Year SSP Parkin et al , Nature Materials 3, 862 Dublin April 2007 (2004). H. Ohno, J.App. Phys. (2996(. 18 Transmission through an MgO barrier

•Majority channel tunneling is dominated by the transmission through a #1 state ! • 1 state decays rapidly in anti-parallel configuration

WH Butler et al Phys Rev B 63 054416 (2001) Dublin April 2007 19 Bias-dependence

AlO x tunnel junction; Signal 180 mV

Dublin April 2007 20 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 21 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.

Anisotropic magnetoresistance (AMR)

$ M Discovered by W. Thompson in 1857 2 I & = & 0 + *& cos , thin film Magnitude of the effect *&/& < 3% The effect is usually positive; &||> &- Maximum sensitivity d&/d, occurs when , = 45°. Hence the ’barber-pole’ 2.5 % configuration used for devices. AMR is due to spin-orbit s-d scattering

0 2 4 µ0H(T)

H

Dublin April 2007 22 Giant magnetoresistance (GMR) and (MR)

Discovered by A. Fert in 1988 MR = Csin 2./2 Sensitivity is maximum when . = //2

H The bottom layer is pinned by exchange Easy axis bias. The free layer has a weak easy axis at . = //2. . I

magnetic tunnel junction: tunnel magnetoresistance TMR

Dublin April 2007 23 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 24 log SV

-6 1/f noise

Random telegraph noise -7

Thermal noise Shot noise

0 1 2 log f

0 Johnson (thermal) noise.

SV(f) = 4kBTR There are voltage fluctuations with no imposed current:

2 = 4kBTR#f

Dublin April 2007 25 0 Shot noise. A non-equilibrium effect associated with electric current

SI(f) = 2eI There are current fluctuations, first seen in vacuum tubes 1/2 Ishot = (2eI#f)

Operating a TMR sensor at a high bias, to increase the signal also increases the noise.

Dublin April 2007 26 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

-3 1H = 10 for pure metals and semiconductors. It can be as high as 103 in some magnetic films

1/f noise in CrO2

Dublin April 2007 27 0 Random telegraph noise.

Fluctuations between two distinct levels. The noise presents itself as a broad peak in the noise spectrum.

Dublin April 2007 28 1 10 100

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.

Dublin April 2007 29 14.3 Memory

Magnetic Random Access Memory

400 bit ferrite core half-select memory (1965)

Freescale 4 Mbit MRAM (2006) Hy bit l ines d lines wor

Dublin AprilHx 2007 30 Magnetic Random Access Memory

Stoner-Wohlfarth asteroid

Dublin April 2007 31 Toggle-switching

Hy

Hx

Dublin April 2007 32 Spin transfer torque

Electron current 2

Transverse spin component absorbed

Electron current 2 Torque exerted as electrons cross F2

F1 F2 Electron current 3 Backscattered electrons exert torque on F2

F1 F2 L. Berger Phys Rev B 54 9353 (1996) J. Slonczewski JMMM 159 L1 (1996)

Dublin April 2007 33 Spin transfer torque

Torque on a single-domain nanomagnet of moment m damping " spin torque produce magnetization reversal " move domain walls B m " emit microwaves

Favourable scaling: Rate of transfer of angular 4m/4t = %m&B - 'm&(m&B) momentum from electron current 5 /r2j!/e; change of angular momentum on flipping free layer is 2m = 2/r2tM

Dublin April 2007 34 Competing memory technology.

PCRAM

Medium

Dublin April 2007 35 Data storage

Storage Hierarchy The Comparison of Storage Media Decision criteria: 10000 # Access time Semiconductor # Frequency of use Memory # Concurrent access # Archive requirements Flash # Permanent media 1000 # Cost per megabyte SRAM # Capacity MRAM MRAM DRAM & Co. 100 Magnetic Disk Price Performance DRAM FRAM

Optical Disc Costs (USD/GB) 10 OUM HD

Magnetic Tape 1 DVD RAM

1,0E+00 1,0E+01 1,0E+02 1,0E+03 1,0E+04 1,0E+05 1,0E+06 Capacity Access time (ns) Source: IBM Source: Beerenberg Bank/Singulus Technology

W Maas, Singulus Dublin April 2007 36 Vertically stacked memory

Magnetic Race-track Memory ‘Japanese car-park’

A novel 3-dimensional spintronic storage 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 and physics

S.S.P.Parkin, US patents 6834005, 6898132, 6920062, 7031178 !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

Dublin April 2007 37 14.4 Logic

M/V M. Johnson, IEEE Trans Magn 36 2758 (2000) +M A ferromagnetic element with a square hysteresis r loop is an ideal bistable logic and memory element.

H/I

V+

-M r I I+ I- + - B 6 + - + - W V Equivalent surface pole density, M Am/m 2 2deg InAs Hall sensor R Line of poles $=Mt A. -1 V = 170'| | , RH = CoFe - 350 'T-1 H = $/2 /r = M t/2 /r If r=2 t, H = M/4 / + - t If M = 1 MAm -1, H = 80 kAm -1 (100 mT) regardless of scale.

Dublin April 2007 38 Logic

A C B IR Nonvolatile switch 0 1 output

Generic logic device A clock pulse is applied at control terminal C. All four Inputs A and B set the state of the magnetic layer !(0) logic operations AB, A+B, AB, A+B are complete in two or "(1). The state of the element is read out at another clock cycles (reset/evaluate) terminal with a current pulse IR which produces a The normalised write current has one of two values Iw (5 voltage V0 or V1. ’( mT) or Iw (10 mT) and either polarity + or -

Dublin April 2007 39 Domain wall logic

Interconnect made from copper / aluminium AND Logic elements made ELECTRONIC NAND from transistors LOGIC Interconnect made from permalloy Data represented by magnetisation direction. NOT Magnetic dw SPINTRONIC logic elements. MEMORY

C Allwood, R Cowburn et al. Science 309 , 1688 Dublin April 2007(2005 ) 40 4-element domain wall circuit

IV AND II

III (mT) B

NOT Cross

Fan Fan Kerr signal I

0 0.125 0.25 Time (sec) Dublin April 2007 41 Ultimate computing technology?

Magnetic Logic

MTJ“s -signal -stability -switching non volatile, fast, error resistance, low power, easy to integrate, low cost

Dublin April 2007 42 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 the possibility of flipping between them,

Dublin April 2007 43 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 44 Number of 2 2+ 3 / 3+ 4 / 4+ Terminals\\\\

Classical Switch Photodiode Transistor Wheatsone Bridge Devices 2-gate Resistor Filter MOSFET Varistor Tetrode Diode Amplifier Multiplier

Spin Electronic Spin Magnetic switch (MTJ) Spin transistors Hall Probe Devices Switch Magneto-resistor Magnetic Gradiometer (bridge) Spin Magnetic Diode Photodiode

Dublin April 2007 45 Spin diffusion lengths (nm)

$! $" $sd ( nm) (nm )

semiconductors 200 >2000

semimetals >50 >500

s-band metals 30 300

d-band metals 5.0 0.9 30

$sd

Dublin April 2007 46 Mobility of semiconductors, semimetals and metals

Curie Point (K) Mobility (cm2V-1s-1) Semiconductors Si - 1400

InSb - 30000

GaAs - 8000

(GaMn)As 170 10

Semimetals Graphite - 2000

Bi - 180000

Metals Cu - 44

Au - 48

Fe 1044 20

Co 1380 12

Ni 628 16

Half Metals CrO 2 392 1.4

Fe 3O4 860 0.2

Dublin April 2007 47 Magnetic semiconductors

Desiderata for a magnetic semiconductor

! Curie temperature > 500 K ! should be coupled to the carriers cb ! p or n type conductivity – spin-polarized electrons or holes ! Useful spin diffusion length and mobility vb ! Magnetoresistance in heterostructures ! Anomalous Hall effect ! " ! Magneto-optic Faraday effect; MCD

Dublin April 2007 48 Magnetic semiconductors - overview

EuO T =69- (GaMn)As c ZnO:Co T 180 K T < 175 K c c > 400 K

Spin-split conduction band Spin-split valence band Spin-split impurity band

Dublin April 2007 49 14.5 Spin Transistors

F1 N F2 emitter $ &collector

V %base $sd I Johnson transistor. Metal-base transistor where conditions at $ and %determine &. Collector

is floating. It samples µ!or µ". No power gain. V " !- V" nanovolts.

Dublin April 2007 50 Datta Das transistor

L gate

source drain

Spin-polarized electrons are injected into the channel, made of a two-dimensional electron gas, 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 a bistable magnetic element. S. Datta and B. Das, Appl. Phys Letters 56 665 (1990)

Dublin April 2007 51 Hot-electron spin transistors

Monsma transistor. Injects hot electrons via a Schottky barrier. Different energy-loss processes in the GMR base lead to a field-contollable parallel, ! passes antiparallel emitter current.

Magnetic tunnel transistor Parkin

The emitter/collector current ratio ( is very small in these devices.

Dublin April 2007 52 Spin MOSFET

Source Drain Vg Ferromagnet SOI suspended membrane Ferromagnet Gate Ferromagnet Tunnel barrier oxide demonstrator

Silicon v v

Similar to ordinary field effect transistor, but with ferromagnetic source and drain

Why? It combines

1) power amplification (semiconductor) 2) memory (ferromagnets)

J. F. Gregg et al JMMM 175 1 (1997) Dublin April 2007 53 Bipolar transistor

p-n junctions

I Zutic v

Dublin April 2007 54 Single-electron spin transistor

Dublin April 2007 55 Pure spin currents

Is it possible in principle to separate and mainpulate spin currents independently of charge currents?

If so, electronics might avoid resistive losses.

Spin waves. Spin Hall Effect Kerr effect image of a 500 x 100 micron n-GaAs sample at 30 K. I I Kato et al Science 306 1910 (2004)

due to spin-orbit scattering.

Dublin April 2007 56 14.6 Prospects

! 1st generation passive devices MRAM scaleup Integrated sensors – magnetic biochips Magnetically reprogrammable gate arrays

! 2nd generation active devices Components 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

Dublin April 2007 57 14.7 Magnetic Recording Hard disc drives

Magnetic medium

Read-write head

Spindle motor

Voice-coil actuator

8 Gbit 1” drive for cameras 160 Gbit 2.5” perpendicular drive for laptops

Dublin April 2007 58 Technology Timeline

In-plane perpendicular 1950 1960 1970 1980 1990 2000 2010 AMR discovered AMR head TMR (1857) TMR GMR discovered head discovered ….. Spin valve Spin-valve RAMAC - first head (CIP) hard-disc drive; inductive head

year caapcity platters size rpm

1955 40 Mb 50x2 24” 1200

2005 160 Gb 1 2.5” 18000

Dublin April 2007 59 A magnetic exponential - Recording

perpendicular AMR TMR

11µ µmm22 GMR Superparamagnetic Limit AMR Magnetization blocked when KV/kT > 40 V > 300 nm3 If record is on 100 grains, medium is 5 nm thick, area/bit is 6 10 -3 µm2 8100 Gbit in2. (155 bit µ m-2) . Dublin April 2007 60 Scaling Why does magnetism lend itself to miniaturization ? H 3 A = ( m/4/r )[2cos ,er + sin ,e,] HA = 2Ma3/4/r3;

-1 If a = 0.1m, r = 2a, M = 1 MAm HA = M/16/ = 20 kAm -1 (~25 mT) Magnet-generated fields are limited by M. Scale-independent a m •A I H = I /2/r = 8jr H ~ r Current-generated fields are limited by j. Scaling is poor

Dublin April 2007 61 More transistors and magnets are produced in fabs

Than grains of rice are grown in paddy fields

Dublin April 2007 62