SPINTRONICSSPINTRONICS

Tomáš Jungwirth

Fyzikální ústav AVČR University of Nottingham

1.1. Current Current sspipintronics in HDD read-heads and memory chipsntronics in HDD read-heads and memory chips

2.2. Physical principles of operation of current spintronic devices Physical principles of operation of current spintronic devices

3.3. Research at the frontiers of Research at the frontiers of sspintronipintronicscs

4. Summary4. Summary

Current spintronics applications Current spintronics applications

First hard discFirst hard disc (1956) (1956) - - classical electronics for read-outclassical electronics for read-out

From PC hard drives ('90)From PC hard drives ('90)to mto miicro-discscro-discs - spintroni - spintronic read-headsc read-heads

MByteMByte

GByteGByte

1 bit: 1mm x 1mm1 bit: 1mm x 1mm

1 bit: 101 bit: 10-3-3mm x 10mm x 10-3-3mmmm

HARD DISKSHARD DISKS

HARD DISK DRIVE READ HEADSHARD DISK DRIVE READ HEADS

horse-shoe read/write heads

spintronic read heads

Anisotropic magnetoresistance (AMR) read headAnisotropic magnetoresistance (AMR) read head

1992 - dawn of spintronics1992 - dawn of spintronics

Appreciable sensitivity, simple design, scalable, cheap

Giant magnetoresistance (GMR) read headGiant magnetoresistance (GMR) read head

19971997

High sensitivity

MEMORY CHIPSMEMORY CHIPS

.DRAMDRAM (capacitor) - high density, cheephigh density, cheep x slow,

high power, volatile

.SRAMSRAM (transistors) - low power, fastlow power, fast x low density,

expensive, volatile

.Flash (floating gate) - non-volatilenon-volatile x slow, limited life,

expensive

Operation through electron chargecharge manipulation

MRAM – universal memoryMRAM – universal memory fast, small, non-volatile

RAM chip that won't forget

↓

instant on-and-off computers

Tunneling magneto-resistance effect (TMR)

First commercial 4Mb MRAM

MRAM – universal memoryMRAM – universal memory fast, small, non-volatile

RAM chip that won't forget

↓

instant on-and-off computers

Tunneling magneto-resistance effect (TMR)

First commercial 4Mb MRAM

1.1. Current Current sspipintronics in HDD read-heads and memory chipsntronics in HDD read-heads and memory chips

2.2. Physical principles of current spintronic devices operationPhysical principles of current spintronic devices operation

3.3. Research at the frontiers of Research at the frontiers of sspintronipintronicscs

4. Summary4. Summary

Electron has a charge (electronics) and

spin (spintronics)

Electrons do not actually “spin”,they produce a magnetic moment that is equivalent to an electron spinning clockwise or anti-clockwise

quantum mechanics & special relativity particles/antiparticles & spin Dirac eq.

E=p2/2mE ih d/dtp -ih d/dr. . .

E2/c2=p2+m2c2

(E=mc2 for p=0)

high-energy physics solid-state physicsand microelectronics

ResistorResistor

classicalclassical

spinspintronic tronic

ee--

external manipulation ofexternal manipulation ofcharge & spincharge & spin

internal communication between internal communication between charge & spincharge & spin

Pauli exclusion principle & Coulomb repulsionPauli exclusion principle & Coulomb repulsion FerromagnetismFerromagnetism

total wf antisymmetric = orbital wf antisymmetric * spin wf symmetric (aligned)

FEROFERO MAGMAG NETNET

ee--

• RobustRobust (can be as strong as bonding in solids)(can be as strong as bonding in solids)

• Strong coupling to magnetic fieldStrong coupling to magnetic field (weak fields = anisotropy fields needed (weak fields = anisotropy fields needed only to reorient macroscopic moment)only to reorient macroscopic moment)

Non-relativistic (except for the spin) many-body

Ingredients: - potential V(r)

- motion of an electron

Producesan electric field

In the rest frame of an electronthe electric field generates and effective magnetic field

- gives an effective interaction with the electron’s magnetic moment

E

)(1

rVe

E

ee--

Relativistic "single-particle"

effSO BsH

p)V(cm2

1B

22eff

V

BBeffeff

pss

Spin-orbit couplingSpin-orbit coupling (Dirac eq. in external field V(r) & 2nd-order in v /c around non-relativistic limit)

• Current sensitive to magnetizationCurrent sensitive to magnetization directiondirection

Spin-orbit couplingSpin-orbit coupling Dirac eq. in external field V(r) & 2nd-order in v /c around non-relativistic limit

ee--

effSO BsH

p)V(cm2

1B

22eff

V

BBeffeff

pss

SpintronicsSpintronics

FerromagnetismFerromagnetism Coulomb repulsion & Pauli exclusion principle

~(k . s)2

ky

kx~Mx . sx

Fermi surfaces

FM without SO-coupling SO-coupling without FM FM & SO-coupling

~(k . s)2

+ Mx . sx

FM without SO-coupling SO-coupling without FM FM & SO-coupling

~(k . s)2 ~(k . s)2

+ Mx . sx

ky

kx

kx

kx

k y

k y

M

M

scattering

~Mx . sx

Fermi surfaces

AMR Ferromagnetism: sensitivity to magnetic field

SO-coupling: anisotropies in Ohmic transportcharacteristics; ~1-10% MR sensor

hot spots for scattering of states moving M R(M I)> R(M || I)

DiodDiodee

classicalclassical

spinspin-valve-valve

TMRBased on ferromagnetism only; ~100% MR sensor or memory

no (few) spin-up DOS available at EF large spin-up DOS available at EF

1.1. Current Current sspipintronics in HDD read-heads and memory chipsntronics in HDD read-heads and memory chips

2.2. Physical principles of current spintronic devices operationPhysical principles of current spintronic devices operation

3.3. Research at the frontiers of Research at the frontiers of sspintronipintronicscs

4. Summary4. Summary

Removing external magnetic fields (down-scaling problem)Removing external magnetic fields (down-scaling problem)

EXTERNAL MAGNETIC FIELDEXTERNAL MAGNETIC FIELD

problems with integration - extra wires, addressing neighboring bits

Current (instead of magnetic field) induced switching

Angular momentum conservation spin-torque

current

magnetic field

local, reliable, but fairlylarge currents needed

Myers et al., Science '99; PRL '02

Likely the future of MRAMsLikely the future of MRAMs

SpintronSpintronics in the footsteps of classical electronicsics in the footsteps of classical electronicsfrom resistors and diodes to transistors

TAMR

Au

TMR

- TAMR sensor/memory elemets

no need for exchange biasing or spin coherent tunneling

AMR based diode

FM

AFM

Simpler design without exchange-biasingthe fixed magnet contact

Single-electron transistor

Two "gates": electric and magnetic

Spintronic transistor based on AMR type of effect

Huge, gatable, and hysteretic MR

GMMGG0

20

C

C

e

)M(V&)]M(VV[CQ&

C2

)QQ(U

electric && magneticmagnetic

control of Coulomb blockade oscillations

n-1 n n+1 n+2n-1 n n+1 n+2

EC

QQindind = = nnee

QQindind = (= (n+1/2)n+1/2)eeQ0

Q0

e2/2C

Q

0

'D

'

e

)M(Q)Q(VdQU

[010]

M[110]

[100]

[110][010]

SO-coupling (M)

Spintronic transistor based on CBAMR

Source Drain

GateVG

VDQ

• Generic effect in FMs with SO-coupling

• Combines electrical transistor action with magnetic storage

• Switching between p-type and n-type transistor by M programmable logic

CBAMR SET

In principle feasible but difficultto realize at room temperature

SpintronSpintronics in the footsteps of classical electronicsics in the footsteps of classical electronicsfrom metals to semiconductors

Spin FET – spin injection from ferromagnet & SO coupling in semiconductor

V

BBeffeff

pss

Difficulties with injecting spin polarized currents from metal ferromagnets to semiconductors, with spin-coherence, etc. not yet realized

FeFerromagnetic semiconductors – all semiconductor spintronicsrromagnetic semiconductors – all semiconductor spintronics

GaAs - GaAs - standard semiconductorstandard semiconductor

Mn - Mn - dilute dilute magneticmagnetic element element

(Ga,Mn)As - fe(Ga,Mn)As - ferrromagneticromagnetic semiconductorsemiconductor

Mn

Ga

AsMn

More tricky than just hammering an iron nail in a silicon wafer

(Ga,Mn)As (and other III-Mn-V)ferromagnetic semiconductor

Mn

Ga

AsMn

• compatible with conventional III-V semiconductors (GaAs)

• dilute moment system e.g., low currents needed for writing

• Mn-Mn coupling mediated by spin-polarized delocalized holes spintronics

• tunability of magnetic properties as in the more conventional semiconductor electronic properties.

• strong spin-orbit coupling magnetic and magnetotransport anisotropies

• Mn-doping (group II for III substitution) limited to ~10%

• p-type doping only

• maximum Curie temperature below 200 K

(Ga,Mn)As material(Ga,Mn)As material

5 d-electrons with L=0 S=5/2 local moment

moderately shallow acceptor (110 meV) hole

- Mn local moments too dilute (near-neghbors cople AF)

- Holes do not polarize in pure GaAs

- Hole mediated Mn-Mn FM coupling

Mn

Ga

AsMn

Mn

Ga

AsMn

Mn–hole spin-spin interaction

hybridization

Hybridization like-spin level repulsion Jpd SMn shole interaction

Mn-d

As-p

Heff

= Jpd

<shole> || -x

MnAs

Ga

heff

= Jpd

<SMn> || x

Hole Fermi surfaces

Ferromagnetic Mn-Mn coupling mediated by holes

No apparent physical barriers for achieving room Tc in III-Mn-Vor related functional dilute moment ferromagnetic semiconductors

Need to combine detailed understanding of physics and technology

Weak hybrid.Delocalized holeslong-range coupl.

InSb, InAs, GaAsd5

Strong hybrid.Impurity-band holesshort-range coupl.

GaP

And look into related semiconductor host families like e.g. I-II-V’s

III = I + II Ga = Li + Zn

GaAs and LiZnAs are twin SC

(Ga,Mn)As and Li(Zn,Mn)As

should be twin ferromagnetic SC

But Mn isovalent in Li(Zn,Mn)As

no Mn concentration limit

possibly both p-type and n-type ferromagnetic SC

SpintronSpintronics in non-magnetic semiconductorsics in non-magnetic semiconductorsway around the problem of Tc in ferromagnetic semiconductors & back to exploring spintronics fundamentals

Spintronics relies on extraordinary magnetoresistance

B

V

I

_

+ + + + + + + + + + + + +

_ _ _ _ _ _ _ _ _ _ FL

Ordinary magnetoresistance:response in normal metals to external magnetic field via classical Lorentz force

Extraordinary magnetoresistance:response to internal spin polarization in ferromagnets often via quantum-relativistic spin-orbit coupling

e.g. ordinary (quantum) Hall effect

I

_ FSO__

Vand anomalous Hall effect

anisotropic magnetoresistance

M

Known for more than 100 years but still controversial

intrinsic skew scattering side jump

I

_ FSO

FSO

_ __majority

minority

V

Anomalous Hall effect in ferromagnetic conductors:spin-dependent deflection & more spin-ups transverse voltage

I

_ FSO

FSO

_ __

V=0

non-magnetic

Spin Hall effect in non-magnetic conductors:spin-dependent deflection transverse edge spin polarization

n

n

p

SHE mikročip, 100A supravodivý magnet, 100 A

Spin Hall effect detected optically in GaAs-based structures

Same magnetization achievedby external field generated bya superconducting magnet with 106 x larger dimensions & 106 x larger currents

Cu

SHE detected elecrically in metals SHE edge spin accumulation can beextracted and moved further into the circuit

1.1. Current Current sspipintronics in HDD read-heads and memory chipsntronics in HDD read-heads and memory chips

2.2. Physical principles of current spintronic devices operationPhysical principles of current spintronic devices operation

3.3. Research at the frontiers of Research at the frontiers of sspintronipintronicscs

4. Summary4. Summary

Downscaling approach about to expire

currently ~ 30 nm feature sizeinteratomic distance in ~20 years

Spintronics: from straighforward downscaling to more "intelligent" device concepts:

• simpler more efficient realization for a given functionality (AMR sensor)

• multifunctional (integrated reading, writing, and processing) • new materials (ferromagnetic semiconductors)

• fundamental understanding of quantum-relativistic electron transport (extraordinary MR)

• Information reading

Electromagnet Anisotropic magneto-resistance sensor

Ferro Magnetization

Current

• Information reading & storage

Tunneling magneto-resistance sensor and memory bit

• Information reading & storage & writing

Current induced magnetization rotation

• Information reading & storage & writing & processing

Spintronic single-electron transistor::magnetoresistance controlled by gate voltage

• New materialsDilute moment ferromagnetic semiconductors

Mn

Ga

AsMn

• Spintronics fundamentals

AMR, anomalous and spin Hall effects