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