Spintronics and magnetic semiconductors Tomas Jungwirth University of Nottingham Bryan Gallagher,...

Spintronics and magnetic semiconductorsSpintronics and magnetic semiconductors

Tomas Jungwirth

University of Nottingham

Bryan Gallagher, Tom Foxon, Richard Campion, et al.

Hitachi Cambridge

Jorg Wunderlich, David Williams, et al.

Institute of Physics ASCR, Prague

Sasha Shick, Jan Mašek, Vít Novák, et al.

University of Texas Texas A&M Univ.

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

NERCSWAN

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

2.2. Basic Basic physical principles of the operation of spintronic devices physical principles of the operation of spintronic devices

3.3. Semiconductor Semiconductor sspintronipintronics researchcs research

4. Summary4. Summary

Current spintronics applications Current spintronics applications

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

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

MBMB’s’s

10’s-100’s 10’s-100’s GBGB’s’s

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

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

Anisotropic magnetoresistance (AMR) read headAnisotropic magnetoresistance (AMR) read head1992 - dawn of spintronics1992 - dawn of spintronics

Appreciable sensitivity, simple design, scalable, cheap

Giant magnetoresistance (GMR) read head - 1997Giant magnetoresistance (GMR) read head - 1997

High sensitivity

and are almost on and off states:

“1” and “0” & magnetic memory bit

MEMORY CHIPSMEMORY CHIPS

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

high power, volatile

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

expensive, volatile

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

expensive

Operation through electron chargecharge manipulation

MRAM – universal memoryMRAM – 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)

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

2.2. Basic Basic physical principles of the operation of spintronic devices physical principles of the operation of spintronic devices

3.3. Semiconductor Semiconductor sspintronipintronics researchcs research

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 equation

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)

many-body

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

Conventional ferromagnetic metals

itinerant 4s:no exch.-split

no SO

localized 3d:exch. split

SO coupled

ss sd

sdss

Mott’s model of transportAb initio Kubo (CPA) formula forAMR and AHE in FeNi alloys

difficult to connect models and microscopics

Banhart&Ebert EPL‘95Khmelevskyi ‘PRB 03Mott&Wills ‘36

AMR AHE

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

2.2. Basic Basic physical principles of the operation of spintronic devices physical principles of the operation of spintronic devices

3.3. Semiconductor Semiconductor sspintronipintronics researchcs research

4. Summary4. Summary

Mn

Ga

AsMn

FeFerromagnetic semiconductorsrromagnetic semiconductors

GaAs - GaAs - standard III-V semiconductorstandard III-V semiconductor

Group-II Group-II Mn - Mn - dilute dilute magneticmagnetic moments moments & holes& holes

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

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

Mn-d-like localmoments

As-p-like holes

Mn

Ga

AsMn

- carriers with both strong SO carriers with both strong SO coupling coupling and exchange splitting, yet simpleand exchange splitting, yet simple semiconductor-like bandssemiconductor-like bands

- Mn 3d5 (S=5/2, L=0): no SO coupling just help to stabilize ferromagnetism

Favorable systems for exploring physical origins of old spintronics effects and for finding new ones

FM without SO-couplingSO-coupling without FM

FM & SO-coupling

~(k . s)2

~(k . s)2 + Mx . sx

ky

kx

kx

k y

M

kx

k y

M

Enhanced interbandscattering near degeneracy

~Mx . sx

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

AMR: a reflection of Fermi surface spin textures in transportAMR: a reflection of Fermi surface spin textures in transport

Family of new AMR effects: TAMR – anisotropic TDOSFamily of new AMR effects: TAMR – anisotropic TDOS

TAMR – discovered in GaMnAs

AuGaMnAs

AuAlOx Au

predicted and observed in metals

[100]

[010]

[100]

[010]

[100]

[010]

Gould, et al., PRL'04, Brey et al. APL’04,Ruster et al.PRL’05, Giraud et al. APL’05, Saito et al. PRB’05,

[010]

M[110]

[100]

[110][010]

Shick et al.PRB'06, Bolotin et al. PRL'06, Viret et al. EJP’06, Moser et al. 06, Grigorenko et al. ‘06

Res

ista

nce

TAMR spintronic dTAMR spintronic diodiodee

classicalclassical

sspipintronic TMRntronic TMR

Au

No need for exchange biased fixed magnetor spin coherent tunneling

sspipintronic TAMRntronic TAMR

Au

TMR

electric && magneticmagnetic

control of CB oscillations

Coulomb blockade AMR spintronic transistorCoulomb blockade AMR spintronic transistor

Wunderlich et al. PRL 06

Source Drain

GateVG

VDQ

[010]

M[110]

[100]

[110][010]

Anisotropic chemical potential

• Generic effect in FMs with SO-coupling (predicted higher-T CBAMR for metals)

• Combines electrical transistor action with magnetic storage

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

CBAMR SET

Dilute moment nature of ferromagnetic semiconductorsDilute moment nature of ferromagnetic semiconductors

GaAs Mn

Mn

10-100x smaller Ms

One

Current induced switchingreplacing 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

One

Dipolar-field-free current induced switching nanostructuresDipolar-field-free current induced switching nanostructures

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

~100 nm

(b)

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

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 Tc ~ #MnGa (Tc=170K for 6% MnGa)

But the SC refuses to accept many group-II Mnon the group-III Ga sublattice

Materials research of DMSsMaterials research of DMSs

Masek et al. PRL 07

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

2.2. Basic Basic physical principles of the operation of spintronic devices physical principles of the operation of spintronic devices

3.3. Semiconductor Semiconductor sspintronipintronics researchcs research

4. Summary4. Summary

• Information reading

Ferro

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 momentferromagnetic semiconductors

Mn

GaAs Mn

Spintronics explores new avenues for:

(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

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