Semiconductor spintronicsSemiconductor spintronics
Tomáš 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, Kamil OlejníkJan Kučera, Karel Výborný, Jan Zemen, et al.
University of Texas Texas A&M Univ.
Allan MacDonald, Qian Niu et al. Jairo Sinova, et al.
NERCSWAN
1.1. Basic Basic physical principles of the operation of spintronic devicesphysical principles of the operation of spintronic devices
2.2. Current metal Current metal sspipintronics in HDD read-heads and memory chipsntronics in HDD read-heads and memory chips
3.3. Research in semiconductor Research in semiconductor sspintronipintronics cs
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
1.1. Basic Basic physical principles of the operation of spintronic devicesphysical principles of the operation of spintronic devices
2.2. Current metal Current metal sspipintronics in HDD read-heads and memory chipsntronics in HDD read-heads and memory chips
3.3. Research in semiconductor Research in semiconductor sspintronipintronics cs
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. Basic Basic physical principles of the operation of spintronic devicesphysical principles of the operation of spintronic devices
2.2. Current metal Current metal sspipintronics in HDD read-heads and memory chipsntronics in HDD read-heads and memory chips
3.3. Research in semiconductor Research in semiconductor sspintronipintronics cs
4. Summary4. Summary
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
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
EF
DO
S
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 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
see J. Zemen 12:05, T2
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
• Combines electrical transistor action with magnetic storage
• Switching between p-type and n-type transistor by M programmable logic
CBAMR SET
SpintronSpintronics in non-magnetic semiconductorsics 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
_
+ + + + + + + + + + + + +
_ _ _ _ _ _ _ _ _ _ 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
intrinsic skew scattering
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
V
BBeffeff
pss
Spin-orbit coupling
n
n
p
SHE microchip, 100A superconducting 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
Wunderlich et al. PRL 05
1.1. Basic Basic physical principles of the operation of spintronic devicesphysical principles of the operation of spintronic devices
2.2. Current metal Current metal sspipintronics in HDD read-heads and memory chipsntronics in HDD read-heads and memory chips
3.3. Research in semiconductor Research in semiconductor sspintronipintronics cs
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:
& 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 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
(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