1. USING SPIN IN (FUTURE) ELECTRONIC DEVICES
Tomas Jungwirth
University of Nottingham
Bryan Gallagher, Kevin Edmonds
Tom Foxon, Richard Campion, et al.
IP ASCR, Prague
Jan Mašek,Alexander Shick
Jan Kučera, František Máca
Texas A&M
Jairo Sinova, et al.
University of Wuerzburg
Laurens Molenkamp, Charles Gould et al.
University of Texas
Allan MaDonald, Qian Niu, et al.
Hitachi Cambridge
Jorg Wunderlich, Bernd Kaestner et al.

2. OUTLINE - Current and future (???) spintronic devices
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
OUTLINE
- Current and future (???) spintronic devices
- Challenges for spintronics  research topics
- Electrical manipulation of spin in normal semiconductors
(Spin Hall effect)
- Ferromagnetic semiconductors - materials and devices

3. CURRENT SPINTRONIC DEVICES

4. HARD DISKS

5. HARD DISK DRIVE READ HEADS
spintronic read heads
horse-shoe read/write heads

6. Anisotropic magnetoresistance (AMR) read head
dawn of spintronics
Ferromagnetism  large response (many spins) to small magnetic fields
Spin-orbit coupling  spin response detected electrically

7. Giant magnetoresistance (GMR) read head
1997
GMR

8. Operation through electron charge manipulation
MEMORY CHIPS
.DRAM (capacitor) - high density, cheep x slow,
high power, volatile
.SRAM (transistors) - low power, fast x low density,
expensive, volatile
.Flash (floating gate) - non-volatile x slow, limited life,
expensive
Operation through electron charge manipulation

9. MRAM – universal memory
(fast, small, non-volatile)
Tunneling magneto-resistance effect
RAM chip that won't forget ↓
instant on-and-off computers

11. MRAM – universal memory
(fast, small, non-volatile)
Tunneling magneto-resistance effect
RAM chip that won't forget ↓
instant on-and-off computers

12. FUTURE (? or ???) SPINTRONIC DEVICES

13. PROCESSORS Low-dissipation microelectronics
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April 24 — We have electronic gizmos for just about every part of our daily lives, from brushing our teeth to staying in touch no matter where we are. Our swollen houses are stuffed with TVs, computers, and ever-larger and more complicated appliances.
PROCESSORS
Low-dissipation microelectronics
Long spin-coherence times → information carried by spin-currents
Instead of electrical currents. Functionality based on spin-dynamics,
e.g., domain wall motion
NOT gate
Allwood et al., Science ’02

14. QUANTUM COMPUTERS 1 a + b Classical bit massive quantum Q-bit
Classical bit
massive quantum
parallelism
Q-bit a
+ b

15. CHALLENGES FOR SPINTRONICS

16. EXANGE-BIAS FM AFM fails when scaled down to ~10 nm dimensions
Look for other MR concepts

17. EXTERNAL MAGNETIC FIELD
problems with integration - extra wires, addressing neighboring bits

18. Current (insted of magnetic field) induced switching
Buhrman & Ralph, NNUN ABSTRACTS '02
Slonczewski, JMMM '96; Berger, PRB '96
Angular momentum conservation  spin-torque

19. local, reliable, but fairly large currents needed
magnetic field
current
Myers et al., Science '99; PRL '02
local, reliable, but fairly
large currents needed
Likely the future of MRAMs

20. INTEGRATION WITH SEMICONDUCTOR ELECTRONICS
 Spin-valve transistor
 Metal ferromagnet to
semiconductor spin-injector
 All-semiconductor spintronics
- electrical manipulation of spins (no external magnetic field)
- making semiconductors ferromagnetic

21. ELECTRICAL MANIPULATION OF SPINS IN NORMAL
SEMICONDUCTORS - SPIN HALL EFFECT

22. Ordinary Hall effect B I V
Lorentz force deflect charged-particles towards the edge B V I _
_ _ _ _ _ _ _ _ _ _ FL
Detected by measuring transverse voltage

23. Spin Hall effect
Spin-orbit coupling “force” deflects like-spin particles I _
FSO
V=0
non-magnetic
Spin-current generation in non-magnetic systems
without applying external magnetic fields
Spin accumulation without charge accumulation
excludes simple electrical detection
Kato, Myars, Gossard, Awschalom, Science
Wunderlich, Kaestner, Sinova, Jungwirth, PRL '04

24. Spin-orbit coupling (relativistic effect)
Produces
an electric field
Ingredients: - potential V(r)
- motion of an electron E
In the rest frame of an electron
the electric field generates and
effective magnetic field
- gives an effective interaction with the electron’s
magnetic moment
Since I am the first talk and the topic of spin-orbit coupling will come up often let me introduce its basic notion. The spin orbit coupling interactions is nothing but an effective magnetic field interaction felt by a moving charge due to a changing electric field due to its relative motion in a potential generating such an electric field. Maxwell’s equations show us that this relative motion of the charge and the electric field induces an effective magnetic field proportional to the orbital momentum of the quasiparticle.
As illustrated in the equation shown the spin quantization axis for such a E
Beff k

25. Skew scattering off impurity potential

26. SO-coupling from host atoms in a perfect crystal
l=0 for electrons  weak SO
l=1 for holes  strong SO
Enhanced in asymmetric QW

27.  z-component of spin due to precession in effective "Zeeman" field
Classical dynamics in k-dependent (Rashba) field:
LLG equations for small drift  adiabatic solution: 

28. Novel co-planar spin-LED
Conventional
vertical spin-LED
Novel co-planar spin-LED
Y. Ohno et al.: Nature 402, 790 (1999)
R. Fiederling et al.: Nature 402, 787 (1999)
B. T. Jonker et al.: PRB 62, 8180 (2000)
X. Jiang et al.: PRL 90, (2003)
R. Wang et al.: APL 86, (2005) …
● No hetero-interface along the LED current
● Spin detection directly in the 2DHG
● Light emission near edge of the 2DHG
● 2DHG with strong and tunable SO
2DHG
2DEG
Spin polarization detected through circular polarization of emitted light

29. EXPERIMENT
Spin Hall Effect
2DHG
2DEG VT VD

30. Spin Hall Effect Device
Experiment “A”
Experiment “B”
This micrograph shows our device used for studying the SHE. A p-channel containing a 2DHG is bordered by two n-regions containing the 2DEG, therefore upper and lower quasi-lateral pn-junctions are defined at both sides of the channel.

31. Experiment “A” Experiment “B”
Opposite perpendicular polarization for opposite Ip currents
or opposite edges  SPIN HALL EFFECT

32. FERROMAGNETIC SEMICONDUCTORS

33. MnGa As Ga (Ga,Mn)As diluted magnetic semiconductor
Low-T MBE - random but uniform Mn distribution
up to ~ 10% doping
MnGa As Ga
5 d-electrons with
L=0, S=5/2
moderately shallow
acceptor

34. Jpd = + 0.6 meV nm3 Theoretical descriptions
Microscopic: atomic orbitals & Coulomb correlation of d-electrons & hopping
Jpd = meV nm3
Jpd SMn.shole
Effective magnetic:
Coulomb correlation of d-electrons & hopping AF kinetic-exchange coupling

35. Intrinsic properties of (Ga,Mn)As: Tc linear in MnGa local moment
Jungwirth, Wang, et al.
cond-mat/
Intrinsic properties of (Ga,Mn)As: Tc linear in MnGa local moment
concentration; falls rapidly with decreasing hole density in more than
50% compensated samples; nearly independent of hole density
for compensation < 50%.

36. Extrinsic effects: Interstitial Mn - a magnetism killer
Interstitial Mn is detrimental to magnetic order:
compensating double-donor – reduces carrier density
couples antiferromagnetically to substitutional Mn even in
low compensation samples  smaller effective number of Mn moments Blinowski PRB ‘03, Mašek, Máca PRB '03 Mn As
Yu et al., PRB ’02:
~10-20% of total Mn concentration is incorporated as interstitials
Increased TC on annealing corresponds to removal of these defects.

37. Tc as grown and annealed samples
Tc=173K
8% Mn
Open symbols as grown. Closed symbols annealed
Jungwirth, Wang, et al.
cond-mat/

38. Number of holes per Mneff
Tc/xeff vs p/Mneff
High (>40%) compensation
Number of holes per Mneff
Jungwirth, Wang, et al.
cond-mat/

39. Generation of Mnint during growth
Theoretical linear dependence of Mnsub on total Mn confirmed experimentally
Mnsub
MnInt
Jungwirth, Wang, et al.
cond-mat/

40. Prospects of (Ga,Mn)As based materials with room Tc
- Concentration of uncompensated MnGa moments has to reach ~10%
only 6.2% in the current record Tc=173K sample
- Charge compensation not so important unless > 40%
- No indication from theory or experiment that the problem is other
than technological - better control of growth-T, stoichiometry;
new growth or chemical composition strategies to incorporate more
MnGa local moments or enhance p-d coupling

41. Tunneling anisotropic magnetoresistance no exchange-bias needed
Giant magneto-resistance
(Ga,Mn)As Au Au
no exchange-bias needed
[100]
[010]
Single magnetic layer
sensor or memory
Gould, Ruster, Jungwirth,
et al., PRL '04

42. Spin-orbit coupling and anisotropies
M || <111>
M || <100>
spin-split bands at M≠0
Dietl et al., Science '00
(Abolfath, Jungwirth et al., PRB '01
Magnetization orientation dependences
Hole total energy over Fermi volume
→ magnetic anisotropy
Group velocities at the Fermi surface and density of states for scattering
→ in plane magneto-resistance anisotropy
Density of states at the Fermi energy
→ anisotropic tunnel magneto-resistance

43. GaMnAs Nanocontact TAMR
5nm thick 2% Mn GaMnAs Hall bars & nanoconstrictions
Current [110]
30nm
Constriction
30nm constriction
Tunnelling conduction at low temperatures & voltages
Giddings, Khalid, Jungwirth, Sinova et al. PRL '05

44. Landauer-Büttiker tunnelling probabilites
Wavevector dependent tunnelling probabilityT (ky, kz)
Red high T; blue low T. y x jt z
Magnetisation in plane
Magnetization perpendicular to plane
Magnetization in plane x z y jt
constriction:
strong z-confinement
(ultra-thin film)
less strong y –confinement (constriction)

45. Very large TAMR in single nanocontacts
30nm constriction
Very large TAMR in single nanocontacts
1400%

46. 3m bar 30nm constriction B || z B|| y B || y B || x AMR & TAMR
AMR in unstructured bar
TAMR in constriction
MR response of constricted device and bar are very similar
in character but largely enhanced in the tunnel constriction

47. Final remark: spintronics in footsteps of electronics
Spintronic nano-transistor
field-controlled MR device
Spintronic wire
AMR device
Spintronic diode
GMR, TMR, TAMR device