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Spintronics and magnetic semiconductors
Tomas Jungwirth
Institute of Physics ASCR, Prague
Sasha Shick, Jan Mašek, Vít Novák, et al.
University of Nottingham
Bryan Gallagher, Tom Foxon,
Richard Campion, et al.
University of Texas
Texas A&M Univ.
Hitachi Cambridge
Allan MacDonald, Qian Niu et al.
Jairo Sinova, et al.
Jorg Wunderlich, David Williams, et al.
NERC
SWAN
1. Current spintronics in HDD read-heads and memory chips
2. Basic physical principles of the operation of spintronic devices
3. Semiconductor spintronics research
4. Summary
Current spintronics applications
First hard disc (1956) - classical electromagnet for read-out
1 bit: 1mm x 1mm
MB’s
From PC hard drives ('90)
to micro-discs - spintronic read-heads
1 bit: 10-3mm x 10-3mm
10’s-100’s GB’s
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) - low 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
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)
RAM chip that actually won't forget  instant on-and-off computers
1. Current spintronics in HDD read-heads and memory chips
2. Basic physical principles of the operation of spintronic devices
3. Semiconductor spintronics research
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
E=p2/2m
E ih d/dt
p -ih d/dr
...
E2/c2=p2+m2c2
(E=mc2 for p=0)
high-energy physics
Dirac equation
solid-state physics
and microelectronics
Resistor
classical
spintronic
external manipulation of
charge
&
spin
internal communication between
charge & spin
e-
many-body
e-
Pauli exclusion principle & Coulomb repulsion  Ferromagnetism
total wf antisymmetric
FERO
= orbital wf antisymmetric * spin wf symmetric
(aligned)
MAG
• Robust (can be as strong as bonding in solids)
• Strong coupling to magnetic field
(weak fields = anisotropy fields needed
only to reorient macroscopic moment)
NET
relativistic single-particle
Spin-orbit coupling
(Dirac eq. in external field V(r) & 2nd-order
in v /c around non-relativistic limit)



Beff 

1
( V )  p
2 2
2m c
 
H SO  s  Beff
e-
V
Beff
• Current sensitive to magnetization
direction
s
p
Conventional ferromagnetic metals
Mott’s model of transport
ss
sd
ss
sd
itinerant 4s:
no exch.-split
no SO
Ab initio Kubo (CPA) formula for
AMR and AHE in FeNi alloys
AMR
Mott&Wills ‘36
AHE
Khmelevskyi ‘PRB 03 Banhart&Ebert EPL‘95
localized 3d:
exch. split
SO coupled
difficult to connect models and microscopics
1. Current spintronics in HDD read-heads and memory chips
2. Basic physical principles of the operation of spintronic devices
3. Semiconductor spintronics research
4. Summary
Ferromagnetic semiconductors
More tricky than just hammering an iron nail in a silicon wafer
Ga
Mn
As
Mn
GaAs - standard III-V semiconductor
Group-II Mn - dilute magnetic moments
& holes
(Ga,Mn)As - ferromagnetic
semiconductor
Ga
As-p-like holes
Mn
As
Mn
Mn-d-like local
moments
- carriers with both strong SO coupling
and exchange splitting, yet simple
semiconductor-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
AMR: a reflection of Fermi surface spin textures in transport
ky
~(k . s)2
SO-coupling without FM
~Mx . sx
kx
FM without SO-coupling
Enhanced interband
scattering near degeneracy
M
~(k . s)2 + Mx . sx
FM & SO-coupling
M
Hot spots for scattering of states moving  M
 R(M  I)> R(M || I)
Family of new AMR effects: TAMR – anisotropic TDOS
TAMR – discovered in GaMnAs
Au
GaMnAs
AlOx

predicted and observed in metals
Au Au
Resistance
[100]
[100]
[010]
[100]
[010]
[010]
[110]
[010] M
F
[100]
[110]
[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,
Shick et al.PRB'06, Bolotin et al. PRL'06,
Viret et al. EJP’06, Moser et al. 06,
Grigorenko et al. ‘06
TAMR spintronic diode
classical
spintronic TMR
spintronic TAMR
TMR
Au
Au
No need for exchange biased fixed magnet
or spin coherent tunneling
Coulomb blockade AMR spintronic transistor
Source
Q VD
Drain
Anisotropic chemical
potential
[110]
Gate
VG
[010] M
F
[100]
[110]
electric
& magnetic
control of CB oscillations
Wunderlich et al. PRL 06
[010]
CBAMR SET
• 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
Dilute moment nature of ferromagnetic semiconductors
Key problems with increasing MRAM capacity (bit density):
- Unintentional dipolar cross-links
- External field addressing neighboring bits
One
10-100x weaker dipolar fields
Current induced switching
replacing external field
10-100x smaller Ms
Tsoi et al. PRL 98, Mayers Sci 99
Ga
As
Mn
10-100x smaller currents for switching
Sinova et al., PRB 04, Yamanouchi et al. Nature 04
Mn
Dipolar-field-free current induced switching nanostructures
Micromagnetics (magnetic anisotropy) without dipolar fields (shape anisotropy)
One
Domain wall
(b)
~100 nm
Can be moved by ~100x
smaller currents than in
metals
Humpfner et al. 06,
Wunderlich et al. 06
Strain controlled magnetocrystalline (SO-induced) anisotropy
Materials research of DMSs
In (Ga,Mn)As Tc ~ #MnGa (Tc=170K for 6% MnGa)
But the SC refuses to accept many group-II Mn
on the group-III Ga sublattice
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
Masek et al. PRL 07
 no Mn concentration limit
 possibly both p-type and n-type ferromagnetic SC
(Li / Zn stoichiometry)
1. Current spintronics in HDD read-heads and memory chips
2. Basic physical principles of the operation of spintronic devices
3. Semiconductor spintronics research
4. Summary
Magnetization
Spintronics explores new avenues for:
• Information reading


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
Ga
As
• Materials: Dilute moment
ferromagnetic semiconductors
Mn
Mn
(Ga,Mn)As material
Ga
Mn
As
Mn
- Mn local moments too dilute
(near-neghbors cople AF)
- Holes do not polarize
in pure GaAs
- Hole mediated Mn-Mn
FM coupling
5 d-electrons with L=0
 S=5/2 local moment
moderately shallow
acceptor (110 meV)
 hole
Mn–hole spin-spin interaction
Ga
Mn
As
Mn
As-p
Mn-d
hybridization
Hybridization  like-spin level repulsion  Jpd SMn  shole interaction
Ferromagnetic Mn-Mn coupling mediated by holes
heff = Jpd <SMn> || x
Hole Fermi surfaces
Mn
As
Ga
Heff = Jpd <shole> || -x
Spintronics in non-magnetic semiconductors
way around the problem of Tc in ferromagnetic semiconductors &
back to exploring spintronics fundamentals
Spintronics relies on extraordinary magnetoresistance
Ordinary magnetoresistance:
response in normal metals to external
magnetic field via classical Lorentz force
B
Extraordinary magnetoresistance:
response to internal spin polarization in ferromagnets
often via quantum-relativistic spin-orbit coupling
anisotropic
magnetoresistance
_ _ _ _ _ _ _ _ _ _
_
FL
+++++++++++++
V
I
M
__
FSO
I
e.g. ordinary (quantum)
Hall effect
V
and anomalous
Hall effect
Known for more than 100 years
but still controversial
Anomalous Hall effect in ferromagnetic conductors:
spin-dependent deflection & more spin-ups  transverse voltage
skew scattering
intrinsic
side jump
majority
_
__
FSO
FSO
I
_
minority
FSO
__
FSO
non-magnetic
V
I
V=0
Spin Hall effect in non-magnetic conductors:
spin-dependent deflection  transverse edge spin polarization
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
p
n
n
SHE mikročip, 100A
SHE detected elecrically in metals
Cu
supravodivý magnet, 100 A
SHE edge spin accumulation can be
extracted and moved further into the circuit