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SPINTRONICS. Tom áš Jungwirth. Universit y of Nottingham. Fyzikální ústav AVČR. 1. Current s pi ntronics in HDD read-heads and memory chips 2. Physical principles of operation of current spintronic devices 3. Research at the frontiers of s pintroni cs 4. Summary.

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slide1

SPINTRONICS

Tomáš Jungwirth

University of Nottingham

Fyzikální ústav AVČR

slide2

1. Current spintronics in HDD read-heads and memory chips

2.Physical principles of operation of current spintronic devices

3. Research at the frontiers of spintronics

4. Summary

slide3

Current spintronics applications

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

1 bit: 1mm x 1mm

MByte

From PC hard drives ('90)

to micro-discs - spintronic read-heads

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

GByte

slide5

HARD DISK DRIVE READ HEADS

spintronic read heads

horse-shoe read/write heads

slide6

Anisotropic magnetoresistance (AMR) read head

1992 - dawn of spintronics

Appreciable sensitivity, simple design,

scalable, cheap

slide8

MEMORY CHIPS

.DRAM(capacitor) - high density, cheep x slow,

high power, volatile

.SRAM(transistors) - low power, fast x low density,

expensive, volatile

.Flash (floating gate) - non-volatilex slow, limited life,

expensive

Operation through electron chargemanipulation

slide9

MRAM – universal memory

fast, small, non-volatile

First commercial 4Mb MRAM

Tunneling magneto-resistance effect (TMR)

RAM chip that won't forget

instant on-and-off computers

slide11

MRAM – universal memory

fast, small, non-volatile

First commercial 4Mb MRAM

Tunneling magneto-resistance effect (TMR)

RAM chip that won't forget

instant on-and-off computers

slide12

1. Current spintronics in HDD read-heads and memory chips

2.Physical principles of current spintronic devices operation

3. Research at the frontiers of spintronics

4. Summary

slide13

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

slide14

quantum mechanics & special relativity  particles/antiparticles & spin

Dirac eq.

E=p2/2m

E ih d/dt

p -ih d/dr

. . .

E2/c2=p2+m2c2

(E=mc2 for p=0)

high-energy physics

solid-state physics

and microelectronics

slide15

e-

Resistor

classical

spintronic

external manipulation of

charge & spin

internal communication between

charge & spin

slide16

total wf antisymmetric =

* spin wf symmetric

(aligned)

orbital wf antisymmetric

e-

FERO

MAG

NET

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

Pauli exclusion principle & Coulomb repulsionFerromagnetism

  • Robust(can be as strong as bonding in solids)
  • Strong coupling to magnetic field
  • (weak fields = anisotropy fields needed
  • only to reorient macroscopic moment)
slide17

p

s

V

e-

Beff

Relativistic "single-particle"

Spin-orbit coupling

(Dirac eq. in external field V(r) & 2nd-order

in v /c around non-relativistic limit)

Produces

an electric field

Ingredients: - potential V(r)

- motion of an electron

E

In the rest frame of an electron

the electric field generates and

effective magnetic field

- gives an effective interaction with the electron’s

magnetic moment

  • Current sensitive to magnetization
  • direction
slide18

ky

kx

p

s

V

e-

Beff

Spintronics

Ferromagnetism

Coulomb repulsion & Pauli exclusion principle

Spin-orbit coupling

Dirac eq. in external field V(r) & 2nd-order

in v /c around non-relativistic limit

Fermi surfaces

~(k . s)2

+ Mx . sx

~(k . s)2

~Mx . sx

FM without SO-coupling

SO-coupling without FM

FM & SO-coupling

slide19

ky

kx

Fermi surfaces

~(k . s)2

+ Mx . sx

~(k . s)2

~Mx . sx

FM & SO-coupling

FM without SO-coupling

SO-coupling without FM

AMR

M

Ferromagnetism: sensitivity to magnetic field

SO-coupling: anisotropies in Ohmic transport

characteristics; ~1-10% MR sensor

scattering

ky

kx

M

ky

kx

hot spots for scattering of states moving  M

 R(M  I)> R(M || I)

slide20

Diode

classical

spin-valve

TMR

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

no (few) spin-up DOS available at EF

large spin-up DOS available at EF

slide21

1. Current spintronics in HDD read-heads and memory chips

2.Physical principles of current spintronic devices operation

3. Research at the frontiers of spintronics

4. Summary

slide23

EXTERNAL MAGNETIC FIELD

problems with integration - extra wires, addressing neighboring bits

slide24

Current (instead of magnetic field) induced switching

Angular momentum conservation  spin-torque

slide25

magnetic field

current

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

local, reliable, but fairly

large currents needed

Likely the future of MRAMs

slide26

Spintronics in the footsteps of classical electronics

from resistors and diodes to transistors

slide27

AMR based diode

- TAMR sensor/memory elemets

TAMR

TMR

no need for exchange biasing

or spin coherent tunneling

Au

FM

AFM

Simpler design without exchange-biasing

the fixed magnet contact

slide28

Spintronic transistor based on AMR type of effect

Huge, gatable, and hysteretic MR

Single-electron transistor

Two "gates": electric and magnetic

slide29

M

[010]

[110]

F

Q

VD

[100]

Source

Drain

[110]

[010]

Gate

VG

Q0

Q0

e2/2C

Spintronic transistor based on CBAMR

magnetic

electric &

SO-coupling 

(M)

control of Coulomb blockade oscillations

slide30

CBAMR SET

  • 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

In principle feasible but difficult

to realize at room temperature

slide32

p

s

V

Beff

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

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

slide33

Ga

Mn

As

Mn

Ferromagnetic semiconductors – all semiconductor spintronics

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

GaAs - standard semiconductor

Mn - dilute magnetic element

(Ga,Mn)As - ferromagnetic

semiconductor

slide34

Ga

Mn

As

Mn

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

ferromagnetic semiconductor

  • 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
slide35

Ga

Mn

As

Mn

(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

slide36

Ga

Mn

As

Mn

Mn–hole spin-spin interaction

As-p

Mn-d

hybridization

Hybridization  like-spin level repulsion  Jpd SMn shole interaction

slide37

Mn

As

Ga

Ferromagnetic Mn-Mn coupling mediated by holes

heff = Jpd <SMn> || x

Hole Fermi surfaces

Heff = Jpd <shole> || -x

slide38

No apparent physical barriers for achieving room Tc in III-Mn-V

or related functional dilute moment ferromagnetic semiconductors

Need to combine detailed understanding of physics and technology

Impurity-band holes

short-range coupl.

Delocalized holes

long-range coupl.

Weak hybrid.

Strong hybrid.

InSb, InAs, GaAs

d5

GaP

slide39

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
slide40

Spintronics in non-magnetic semiconductors

way around the problem of Tc in ferromagnetic semiconductors & back to exploring spintronics fundamentals

slide41

V

Spintronics relies on extraordinary magnetoresistance

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

B

anisotropic magnetoresistance

_ _ _ _ _ _ _ _ _ _

_

FL

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

I

V

_

_

FSO

M

_

I

e.g. ordinary (quantum) Hall effect

and anomalous Hall effect

Known for more than 100 years

but still controversial

slide42

_

_

_

FSO

_

non-magnetic

FSO

I

majority

_

_

_

FSO

_

FSO

I

minority

V

V=0

Anomalous Hall effect in ferromagnetic conductors:

spin-dependent deflection & more spin-ups  transverse voltage

skew scattering

side jump

intrinsic

Spin Hall effect in non-magnetic conductors:

spin-dependent deflection  transverse edge spin polarization

slide43

n

p

n

Cu

Spin Hall effect detected optically

in GaAs-based structures

Same magnetization achieved

by external field generated by

a superconducting magnet

with 106 x larger dimensions &

106 x larger currents

SHE mikročip, 100A

supravodivý magnet, 100 A

SHE edge spin accumulation can be

extracted and moved further into the circuit

SHE detected elecrically in metals

slide44

1. Current spintronics in HDD read-heads and memory chips

2.Physical principles of current spintronic devices operation

3. Research at the frontiers of spintronics

4. Summary

slide45

Downscaling approach about to expire

currently ~ 30 nm feature size

interatomic 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)
slide46

Ferro

Magnetization

Current

Anisotropic magneto-resistance

sensor

Electromagnet

  • Information reading

  • Information reading & storage

Tunneling magneto-resistance sensor and memory bit

  • Information reading & storage & writing

Current induced magnetization rotation

slide47

Ga

Mn

As

Mn

  • Information reading & storage & writing & processing

Spintronic single-electron transistor:

magnetoresistance controlled by gate voltage

  • New materials

Dilute moment ferromagnetic semiconductors

  • Spintronics fundamentals

AMR, anomalous and spin Hall effects