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baconossifiedMechanics

Oct 29, 2013 (3 years and 11 months ago)

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Semiconductor spintronics

Tom
áš

Jungwirth



Universit
y of Nottingham




Bryan Gallagher, Tom Foxon,


Richard Campion,
et al.



Hitachi Cambridge


Jorg Wunderlich
,
David Williams, et a
l
.

Institute of Physics ASCR, Prague



Sasha

Shick
, Jan Ma
šek,

Vít Novák, Kamil Olejník

Jan
Kučera, Karel Výborný, Jan Zemen,
et al.

University of
Texas


Texas A&M

Univ.




Allan MacDonald
, Qian Niu et al.
Jairo Sinova, et al.

NERC

SWAN


1.

Basic
physical principles of the operation of spintronic devices




2.

Current metal
s
pi
ntronics in HDD read
-
heads and memory chips



3.

Research in semiconductor
s
pintroni
cs



4. 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 equation





E=p
2
/2m

E


ih d/dt

p


-
ih d/dr

. . .

E
2
/c
2
=p
2
+m
2
c
2

(E=mc
2

for p=0)

high
-
energy physics

solid
-
state physics

and microelectronics

Resistor

classical

spin
tronic

e
-

external manipulation of

charge & spin

internal communication between


charge & spin

Pauli exclusion principle & Coulomb repulsion



Ferromagnetism

total wf antisymmetric =

orbital wf antisymmetric

* spin wf symmetric

(aligned)

FERO

MAG

NET

e
-



Robust

(can be as strong as bonding in solids)




Strong coupling to magnetic field



(weak fields = anisotropy fields needed


only to reorient macroscopic moment)

many
-
body

e
-

relativistic single
-
particle


V

B
eff

p

s

Spin
-
orbit coupling


(Dirac eq. in external field

V(
r
)
& 2nd
-
order

in
v
/c

around non
-
relativistic limit
)



Current sensitive to magnetization


direction


1.

Basic
physical principles of the operation of spintronic devices




2.

Current metal
s
pi
ntronics in HDD read
-
heads and memory chips



3.

Research in semiconductor
s
pintroni
cs



4. Summary

Current spintronics applications

First hard disc

(1956)
-

classical electromagnet for read
-
out

From PC hard drives ('90)

to m
i
cro
-
discs

-

spintroni
c read
-
heads

MB
’s

10’s
-
100’s
GB
’s

1 bit: 1mm x 1mm

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

Anisotropic magnetoresistance (AMR) read head

1992
-

dawn of spintronics

Appreciable sensitivity, simple design, scalable, cheap

Giant magnetoresistance (GMR) read head
-

1997

High sensitivity



and


are almost on and off states:


“1” and “0” & magnetic


memory bit

MEMORY CHIPS

.
DRAM

(capacitor)

-

high

density,

cheep

x



high

power,

volatile


.
SRAM

(transistors)

-

l ow

power,

fast

x

low

density,



expensive,

volatile


.
Flash

(floating

gate)

-

non
-
volatile

x

slow,

limited

lifetime,



expensive

Operation through electron
charge

manipulation

MRAM


universal memory


fast, small, low
-
power, durable, and

non
-
volatile

2006
-

First commercial 4Mb MRAM

RAM chip that actually won't forget


instant on
-
and
-
off computers

Based on Tunneling Magneto
-
Resistance (similar to GMR but insulating spacer)

RAM chip that actually won't forget


instant on
-
and
-
off computers

Based on Tunneling Magneto
-
Resistance (similar to GMR but insulating spacer)


1.

Basic
physical principles of the operation of spintronic devices




2.

Current metal
s
pi
ntronics in HDD read
-
heads and memory chips



3.

Research in semiconductor
s
pintroni
cs



4. Summary

Dilute moment nature of ferromagnetic semiconductors

Ga

As

Mn

Mn

10
-
100x smaller M
s

One

Current induced switching

replacing external field

Tsoi et al. PRL 98, Mayers Sci 99

Key problems with increasing MRAM capacity (bit density):


-

Unintentional dipolar cross
-
links

-

External field addressing neighboring bits

10
-
100x weaker dipolar fields

10
-
100x smaller currents for switching

Sinova et al., PRB 04,
Yamanouchi et al. Nature 04

Mn

Ga

As

Mn

Fe
rromagnetic semiconductors

GaAs
-

standard III
-
V semiconductor


Group
-
II
Mn
-

dilute
magnetic

moments


& holes


(Ga,Mn)As
-

fe
r
romagnetic


semiconductor



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

Mn
-
d
-
like local

moments

As
-
p
-
like holes

Mn

Ga

As

Mn

E
F

DOS

Energy

spin


spin


GaAs:Mn


extrinsic p
-
type semiconductor

with 5 d
-
electron local moment

on the Mn impurity


valence band As
-
p
-
like holes

As
-
p
-
like holes localized on Mn acceptors

<< 1% Mn

onset of ferromagnetism near MIT

Jungwirth et al. RMP ‘06

~1% Mn

>2% Mn

One

Dipolar
-
field
-
free current induced switching nanostructures

Micromagnetics (magnetic anisotropy) without dipolar fields (shape anisotropy)

~100 nm

Domain wall

Strain controlled magnetocrystalline (SO
-
induced) anisotropy

Can be moved by ~100x

smaller currents than in

metals

Humpfner et al. 06,

Wunderlich et al. 06

see J. Zemen 12:05, T2

electric
&

magnetic

control of CB oscillations

Coulomb blockade AMR spintronic transistor

Wunderlich et al. PRL 06

Source

Drain

Gate

V
G

V
D

Q

[
010
]

F

M

[
110
]

[
100
]

[
110
]

[
010
]

Anisotropic chemical

potential





Combines electrical transistor action


with magnetic storage





Switching between p
-
type and n
-
type


transistor by
M


programmable logic

CBAMR SET

Spintron
ics in non
-
magnetic semiconductors



way around the problem of low Curie T in ferromagnetic semiconductors &

back to exploring spintronics fundamentals

Spintronics relies on extraordinary magnetoresistance

B

V

I

_

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

_ _ _ _ _ _ _ _ _ _

F
L

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

_

F
SO

_

_

V

and anomalous
Hall effect

anisotropic
magnetoresistance

M

Known for more than 100 years

intrinsic

skew scattering

I

_

F
SO

F
SO

_

_

_

majority

minority

V

Anomalous Hall effect in ferromagnetic conductors:

spin
-
dependent deflection & more spin
-
ups


transverse voltage


I

_

F
SO

F
SO

_

_

_

V=0

non
-
magnetic

Spin Hall effect in non
-
magnetic conductors
:

spin
-
dependent deflection


transverse edge spin polarization



V

B
eff

p

s

Spin
-
orbit coupling



n

n

p

SHE microchip, 100

A

sup
erconducting

magnet
, 100

A

Spin Hall effect detected optically

in GaAs
-
based structures

Same magnetization achieved

by external field generated by

a superconducting magnet

with
10
6
x
larger dimensions &

10
6

x
larger currents

Cu

SHE detected elecrically in metals

SHE edge spin accumulation can be

extracted and moved further into the circuit

Wunderlich et al. PRL 05


1.

Basic
physical principles of the operation of spintronic devices




2.

Current metal
s
pi
ntronics in HDD read
-
heads and memory chips



3.

Research in semiconductor
s
pintroni
cs



4. Summary



Information
reading



Magnetization



Current



Information
reading

&
storage


Tunneling magneto
-
resistance sensor and memory bit



Information
reading & storage

&
writing


Current induced magnetization switching



Information
reading & storage & writing

&
processing

Spintronic single
-
electron transistor
:

magnetoresistance controlled by gate voltage



Materials:
Dilute moment

ferromagnetic semiconductors

Mn

Ga

As

Mn

Spintronics explores new avenues for:

& non
-
magnetic


spin Hall effect

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


(Li / Zn stoichiometry)

In (Ga,Mn)As T
c

~ #Mn
Ga

(T
c
=170K for 6% MnGa)


But the SC refuses to accept many group
-
II Mn

on the group
-
III Ga sublattice

Materials research of DMSs

Masek et al. PRL 07

(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

As

Mn

Mn

Ga

As

Mn

Mn

hole spin
-
spin interaction

hybridization

Hybridization


like
-
spin level repulsion


J
pd
S
Mn



s
hole

interaction

Mn
-
d

As
-
p

H
eff

=
J
pd

<
s
hole
> ||
-
x

Mn

As

Ga

h
eff

=
J
pd

<
S
Mn
> || x

Hol e Fermi surfaces

Ferromagnetic Mn
-
Mn coupling mediated by holes